Most conventional numerical schemes for soil ground heat flux estimation rely on the knowledge of... more Most conventional numerical schemes for soil ground heat flux estimation rely on the knowledge of the temporal evolution of soil temperature. Here we propose and test a novel scheme, which requires no information on soil temperatures to supplement the flux plate measurement. The proposed method is based on the fundamental solution of the one-dimensional heat equation and Duhamel's principle for the incorporation of inhomogeneous boundary conditions. Being completely independent of the soil temperature, the new scheme therefore avoids a potential source of error in measurements and in heat storage calculation. The only thermal property involved in the new scheme is the thermal diffusivity of the soil, which is a weak function of soil water content and can be approximated as constant with reasonable accuracy. For validation, the proposed method is compared to the conventional approach using a canonical one-dimensional heat conduction problem, as well as real field measurements. Results of the comparison highlight that the new model is robust and capable of preserving the good accuracy of the conventional approach with reduced input information. In addition, the effect of inclusion of the heat storage term in the ground heat flux is evaluated in the context of surface energy balance closure for field measurements.
Renewable and Sustainable Energy Reviews, 47, 830-843, Apr 2015
Studies on urban heat island (UHI) have been more than a century after the phenomenon was first d... more Studies on urban heat island (UHI) have been more than a century after the phenomenon was first discovered in the early 1800s. UHI emerges as the source of many urban environmental problems and exacerbates the living environment in cities. Under the challenges of increasing urbanization and future climate changes, there is a pressing need for sustainable adaptation/mitigation strategies for UHI effects, one popular option being the use of reflective materials. While it is introduced as an effective method to reduce temperature and energy consumption in cities, its impacts on environmental sustainability and large-scale non-local effect are inadequately explored. This paper provides a synthetic overview of potential environmental impacts of reflective materials at a variety of scales, ranging from energy load on a single building to regional hydroclimate. The review shows that mitigation potential of reflective materials depends on a set of factors, including building characteristics, urban environment, meteor- ological and geographical conditions, to name a few. Precaution needs to be exercised by city planners and policy makers for large-scale deployment of reflective materials before their environmental impacts, especially on regional hydroclimates, are better understood. In general, it is recommended that optimal strategy for UHI needs to be determined on a city-by-city basis, rather than adopting a “one-solution- fits-all” strategy.
Urbanization modifies surface energy and water budgets, and has significant impacts on local and ... more Urbanization modifies surface energy and water budgets, and has significant impacts on local and regional hydroclimate. In recent decades, a number of urban canopy models have been developed and implemented into the Weather Research and Forecasting (WRF) model to capture urban land-surface processes. Most of these models are inadequate due to the lack of realistic representation of urban hydrological processes. Here, we imple- ment physically-based parametrizations of urban hydrological processes into the single layer urban canopy model in the WRF model. The new single-layer urban canopy model features the integration of, (1) anthropogenic latent heat, (2) urban irrigation, (3) evaporation from paved surfaces, and (4) the urban oasis effect. The new WRF–urban modelling system is evaluated against field measurements for four different cities; results show that the model performance is substantially improved as compared to the current schemes, especially for latent heat flux. In particular, to evaluate the performance of green roofs as an urban heat island mitigation strategy, we integrate in the urban canopy model a multilayer green roof system, enabled by the physical urban hydrological schemes. Simulations show that green roofs are capable of reducing surface temperature and sensible heat flux as well as enhancing building energy efficiency.
We couple a single column model (SCM) to a cutting-edge single-layer urban canopy model (SLUCM) w... more We couple a single column model (SCM) to a cutting-edge single-layer urban canopy model (SLUCM) with realistic representation of urban hydrological processes. The land-surface transport of energy and moisture parametrized by the SLUCM provides lower boundary conditions to the overlying atmosphere. The coupled SLUCM–SCM model is tested against field measurements of sensible and latent heat fluxes in the surface layer, as well as vertical profiles of temperature and humidity in the mixed layer under convective conditions. The model is then used to simulate urban land–atmosphere interactions by changing urban geometry, surface albedo, vegetation fraction and aerodynamic roughness. Results show that changes of landscape characteristics have a significant impact on the growth of the boundary layer as well as on the distributions of temperature and humidity in the mixed layer. Overall, the proposed numerical framework provides a useful stand-alone modelling tool, with which the impact of urban land-surface conditions on the local hydrometeorology can be assessed via land–atmosphere interactions.
Geophysical Research Letters, 41: 8348-58, Dec 2014
The partitioning of solar energy into sensible, latent, and ground heat over the land surface is ... more The partitioning of solar energy into sensible, latent, and ground heat over the land surface is responsible for changes of state variables in the soil-atmosphere system. Recent research enables the reconstruction of the land surface temperature and ground heat flux using Green’s function approach, as well as the estimate of the distribution of available energy into latent and sensible heat fluxes based on linear stability analysis. Combining the Green’s function approach and linear stability analysis, we propose a new physically-based numerical procedure to estimate the land surface energy partitioning in this paper. The new method is capable of predicting all surface energy budgets using a single depth soil measurement; the model reliability is evaluated with comparisons to flux tower measurements. Results of this study deepen our insight into the implicit link between surface energy partition and subsurface soil dynamics, and how the link can be employed to related research areas.
Land surface energy balance in a built environment is widely modelled using urban canopy models w... more Land surface energy balance in a built environment is widely modelled using urban canopy models with representation of building arrays as a big street canyon. Modification of this simplified geometric representation, on the other hand, leads to challenging numerical difficulties in improving physical parameterization schemes that are deterministic in nature. In this paper, we develop a stochastic algorithm to estimate view factors between canyon facets in the presence of shade trees based on Monte Carlo simulation, where an analytical formulation is inhibited by the complex geometry. The model is validated against analytical solutions of benchmark radiative problems as well as field measurements in real street canyons. In conjunction with the matrix method resolving infinite number of reflections, the proposed model is capable of predicting the radiative exchange inside the street canyon with good accuracy. Modeling of transient evolution of thermal filed inside the street canyon using the proposed method demonstrate the potential of shade trees in mitigating canyon surface temperatures as well as saving of building energy use. This new numerical framework also deepens our insight into the fundamental physics of radiative heat transfer and surface energy balance for urban climate modeling.
Urban-rural contrast is central to many urban environmental problems, a prominent example being t... more Urban-rural contrast is central to many urban environmental problems, a prominent example being the urban heat island effect. To ameliorate urban thermal stress, extensive research work has been focused on the mitigation of critical environmental temperatures, while the timing of excessive heating was largely overlooked. Advection of soil water flux plays a critical role in determining the soil thermal field, which in turn modulates surface energy fluxes and land–atmosphere interactions. In this paper, we formulate the wave phase differences between soil temperatures and soil heat fluxes due to soil water advection based on harmonic function method. It has been found that phase lags, viz. hysteresis effects, exist among all land surface energy budgets and the land surface
temperature. More generally, the difference of phase and time evolution of the land surface temperature and the ground heat flux is also manifested in annual cycles. In the context of urban–rural differences, the temporal difference in peak surface temperatures and peak turbulent fluxes has profound implication to human thermal comfort and building energy efficiency.
The accurate estimation of soil thermal field is crucially important because soil temperature is ... more The accurate estimation of soil thermal field is crucially important because soil temperature is a key parameter widely used in many related fields. This paper proposed an improved heat-conduction-equation (HCE) method to reconstruct soil thermal field using two-depth easurements of soil temperature. The revised HCE method employed a direct approach as well as an indirect approach to estimate both the daily average soil temperature (DST) and the instantaneous soil temperature (IST), during which the annual temperature cycle and the diurnal temperature cycle are combined. Two validation experiments (i.e., Test-1 and Test-2) were performed with soil temperature measurements at five stations chosen from the Soil Climate Analysis Network (SCAN). The results show that the revised HCE method improves the accuracy of modeling soil thermal field in comparison to its traditional form. The root mean square errors (RMSEs) of the ISTs estimated by the traditional HCE method range from 1.0 to 2.1 oC (2.4 to 4.8 oC) in Test-1 (Test-2); while the errors of the revised HCE method by the indirect approach are reduced to less
than 1.0 oC in both Test-1 and Test-2; the errors by the direct approach are further reduced to less than 0.7 oC in both Test-1 and Test-2. The improved method has further potential to estimate soil heat flux, a variable that can be inferred using soil temperature gradients associated with soil apparent thermal conductivity.
Journal of Applied Meteorology and Climatology, 53, 2114-29, 2014
Urban facets—the walls, roofs, and ground in built-up terrain—are often conceptualized as homogen... more Urban facets—the walls, roofs, and ground in built-up terrain—are often conceptualized as homogeneous surfaces, despite the obvious variability in the composition and material properties of the urban fabric at the subfacet scale. This study focuses on understanding the influence of this subfacet heterogeneity, and the associated influence of different material properties, on the urban surface energy budget. The Princeton Urban Canopy Model, which was developed with the ability to capture subfacet variability, is evaluated at sites of various building densities and then applied to simulate the energy exchanges of each subfacet with the atmosphere over a densely built site. The analyses show that, although all impervious built surfaces convert
most of the incoming energy into sensible heat rather than latent heat, sensible heat fluxes from asphalt pavements and dark rooftops are 2 times as high as those from concrete surfaces and light-colored roofs.
Another important characteristic of urban areas—the shift in the peak time of sensible heat flux in comparison
with rural areas—is here shown to be mainly linked to concrete’s high heat storage capacity as well as to
radiative trapping in the urban canyon. The results also illustrate that the vegetated pervious soil surfaces that
dot the urban landscape play a dual role: during wet periods they redistribute much of the available energy
into evaporative fluxes but when moisture stressed they behave more like an impervious surface. This role reversal, along with the direct evaporation of water stored over impervious surfaces, significantly reduces the overall Bowen ratio of the urban site after rain events.
Rapid urbanization has emerged as the source of many adverse environmental effects and brings cit... more Rapid urbanization has emerged as the source of many adverse environmental effects and brings cities to a vulnerable situation under future climate challenges. Green roofs are proven to be an effective solution to alleviate these effects by field observations nder a wide range of climate conditions. Recent advances in modeling urban land-atmosphere interactions provide a useful tool in capturing the dynamics of coupled transport of water and energy in urban conies, thus bridge the gap of modeling at city to regional scales. The performance of urban hydrological models depends heavily on the accurate determination of the input parameter space, where uncertainty is ubiquitous. In this paper, we use an advanced Monte Carlo approach, viz. the Subset Simulation, to quantify the sensitivity of urban hydrological modeling to parameter uncertainties. Results of the sensitivity analysis reveal that green roofs exhibit markedly different thermal and hydrological behavior as compared to conventional roofs, due to the modification of the surface energy portioning by well-irrigated vegetation. In addition, statistical predictions of critical responses of green roofs (extreme surface temperature, heat fluxes, etc.) have relatively weak dependence on climatic conditions. The statistical quantification of sensitivity provides guidance for future development of urban hydrological models with practical applications such as urban heat island mitigation.
Elastomer-based composites embedded with thermally-responsive material (TRM) and a liquid-phase J... more Elastomer-based composites embedded with thermally-responsive material (TRM) and a liquid-phase Joule heater are capable of reversibly changing their elastic rigidity by up to four orders of magnitude. At room temperature, the TRM layer is rigid and prevents the surrounding elastomer from elastically bending or stretching. When activated, the embedded Joule heater softens or melts the TRM, which leads to a dramatic reduction in the elastic rigidity of the composite. In this manuscript, we examine the activation of these composites by performing analytical, numerical, and experimental studies of the temperature distribution, thermal history, and phase transition. We consider both low melting point (LMP) metal alloys (e.g. Field’s metal) and shape memory polymer (SMP). An analytical solution using the Galerkin Based Integral (GBI) method is derived for the cases where no phase change is involved, while a numerical scheme using the Latent Heat Accumulation (LHA) method is utilized to probe scenarios where phase change has a central role in the elastic rigidity change. The analytical and numerical studies predict a temperature history that is in good agreement with experimental measurements obtained with an IR thermometer. Analysis of the internal temperature distribution leads to scaling laws for determining the required activation time and allowable input power rate for composites containing either LMP alloys or SMP. These scaling laws could potentially be used to inform the design of rigidity tunable composites (RTC) used in assistive wearable technologies and biologically-inspired soft-matter robotics.
The hysteresis effect in diurnal cycles of net radiation Rn and ground heat flux G0 has been obse... more The hysteresis effect in diurnal cycles of net radiation Rn and ground heat flux G0 has been observed in many studies, while the governing mechanism remains vague. In this study, we link the phenomenology of hysteresis loops to the wave phase difference between the diurnal evolutions of various terms in the surface energy balance. Rn and G0 are parameterized with the incoming solar radiation and the surface temperature as two control parameters of the surface energy partitioning. The theoretical analysis shows that the vertical water flux W and the scaled ratio A_s/A_T (net shortwave radiation to outgoing longwave radiation) play crucial roles in shaping hysteresis loops of Rn and G0. Comparisons to field measurements indicate that hysteresis loops for different land covers can be well captured by the theoretical model, which is also consistent with Camuffo-Bernadi formula. This study provides insight into the surface partitioning and temporal evolution of the energy budget at the land surface.
British Journal of Environment and Climate Change, 3(1): 86-102, 2013, Mar 2013
Aims: In this paper, we aim to assess different parameterization schemes for quantifying the surf... more Aims: In this paper, we aim to assess different parameterization schemes for quantifying the surface energy portioning process, in particular, the latent and sensible heat fluxes, and their applicability to various surface cover types.
Study design: This study intercompares theoretical models that predict the relative efficiency of the latent heat (evapotranspiration) with respect to the sensible heat flux. Model predictions are compared with field measurements over surface covers with different physical characteristics and soil water availability.
Place and Duration of Study: This study was carried out at the Arizona State University, Tempe, AZ, between August 2012 and December 2012.
Methodology: Three theoretical models for prediction of the relative efficiency of the latent heat were investigated, based on the lumped heat transfer (Priestley), the linear stability analysis (LSA) and the maximum entropy principle (MEP), respectively. Model predictions were compared against field measurements over three different land cover types, viz. water, grassland and suburban surfaces. An explicit moisture availability parameter β is incorporated in the MEP model, to facilitate direct comparison against the LSA and field measurements. Standard post-processing and quality control were applied to field measured turbulent fluxes using the eddy-covariance (EC) technique. To be consistent with the premise of all theoretical models, diurnal series of sensible and latent heat fluxes were filtered such that only data points under convective conditions were selected.
Results: Among all three models, the application of Priestley model is restricted to saturated land surfaces, and generally overestimates the relative efficiency of the latent heat for water-limited surfaces. The LSA and MEP models predict similar β ranges, i.e., 0.05-0.3 in summer and 0.1-0.7 in winter over suburban area, and 0.1 to 0.5 over lake surface. Over vegetated surfaces, the MEP model predicts a reasonable β range around unity by taking transpiration into consideration, while the LSA model consistently underestimated the relative efficiency.
Conclusion: Moisture availability plays an essential role in regulating the surface energy partitioning process. The introduction of the moisture availability parameter enables versatile theoretical models for latent heat (and evapotranspiration) predictions over a wide range of land cover types. This study provides a physical insight into the thermodynamics mechanism governing the surface energy balance, and the potential to develop novel surface energy parameterization schemes based on the concept of relative efficiency. The MEP model is found to have the greatest potential in terms of future theoretical model development.
In this study, the Princeton Roof Model (PROM) is developed and used to simulate the hygrothermal... more In this study, the Princeton Roof Model (PROM) is developed and used to simulate the hygrothermal dynamics of green roof systems. PROM is embedded within the framework of the Princeton Urban Canopy Model, with a multi-layer spatially-analytical heat transfer scheme and an improved hydrological module. The model is validated by comparing simulated surface temperature and soil moisture to the measurements at two experimental sites, one in Beijing, China and the other in New Jersey, USA. The results demonstrate that PROM is able to capture the diurnal cycle of roof temperatures and the soil moisture dynamics of green roofs with reasonable accuracy. Driven by a 30-day summertime meteorological forcing from July 2001, PROM is used to investigate the green roof thermal improvement to the urban indoor and outdoor environments, compared to conventional roofs. The impact of green roofs is significant in reducing surface temperatures, and outdoor and indoor heat fluxes during this summer period. To quantify this thermal improvement, three indices related to surface temperature, outdoor heat flux and indoor heat flux, are introduced; and the dependence of these indices on hydrological and meteorological conditions is investigated. The results indicate that incoming solar radiation, air temperature, wind speed and medium layer moisture are the main determinants of the green roof performance
We propose a new surface exchange scheme, coupling the transport of energy and water in urban can... more We propose a new surface exchange scheme, coupling the transport of energy and water in urban canopies. The new model resolves the sub-facet heterogeneity of urban surfaces, which is particularly useful for capturing surface exchange processes from vegetated urban surfaces, such as lawns or green roofs. We develop detailed urban hydrological models for surfaces consisting of either natural (soil and vegetation) or engineered materials with water-holding capacity. The coupling of energy and water transport enables us to parameterize surface evaporation from different urban facets including soils, vegetation and water-holding engineered surfaces. The new coupled model is evaluated using field measurement data obtained through a wireless sensor network deployed over the Princeton University campus. Comparison of model prediction and measured results shows that the proposed surface exchange scheme is able to predict widely-varying surface temperatures for each heterogeneous sub-facet with good accuracy. Different weather conditions and seasonal variability are found to have insignificant effect on the model performance. The new model is also able to capture the subsurface hydrological processes with reasonable accuracy, particularly for urban lawns. The proposed model is then applied to assess different mitigation strategies of the urban heat island effect.
Effects of the urban length scale and the wind speed on the urban heat island effect are quantifi... more Effects of the urban length scale and the wind speed on the urban heat island effect are quantified through time-dependent energy balance. The heating of the urban surfaces during the daytime sets the initial temperature, and this overheating is cooled during the night-time through mean convection motion over the urban surface, resulting in an exponential decay in the temperature. The solution to the time-dependent energy balance equation accurately reproduces this temporal decay, with the main factors being the length scale of the urban area and the wind speed. The temporal data for Phoenix, Arizona are reasonably accurately traced by this model, when the urban length scale is varied from 1983 to 2010. The minimum temperature reached at the end of this cooling period then corresponds to the UHI intensity, which increases with increasing urban length scale and decreasing wind speed. The wind speed effect is also accurately re-traced using this method; however, different correction factors are required for different cities, indicating the effects of the urban surface heat content, structural morphology and density. Thus, using a small number of readily available data for the urban length scale and the wind speed, the UHI intensity can be described with possible projections for future trends.
Soil field experiments usually consist of measurements of soil temperatures, heat fluxes and soil... more Soil field experiments usually consist of measurements of soil temperatures, heat fluxes and soil water contents. Accurate determination of the soil thermal field, in particular, prediction of the soil surface temperature and the ground heat, contains the signature to the surface energy partitioning, and is therefore critical to the surface energy balance closure problem. In this paper, we develop a numerical procedure to reconstruct the entire soil thermal field from a single depth measurement of either temperature or heat flux. The new algorithm is based on Green’s function approach by using the fundamental solution of heat conduction in semi-infinite soils and Duhamel’s integral for incorporation of general boundary conditions. It is highlighted that the new approach is capable of accurately reproducing results of some existing numerical approaches, with a more general setting and treatment of the heat diffusion problem, and hence provides a possible unified framework for the estimation of thermal field in soils.
Most conventional numerical schemes for soil ground heat flux estimation rely on the knowledge of... more Most conventional numerical schemes for soil ground heat flux estimation rely on the knowledge of the temporal evolution of soil temperature. Here we propose and test a novel scheme, which requires no information on soil temperatures to supplement the flux plate measurement. The proposed method is based on the fundamental solution of the one-dimensional heat equation and Duhamel’s principle for the incorporation of inhomogeneous boundary conditions. Being completely independent of the soil temperature, the new scheme therefore avoids a potential source of error in measurements and in heat storage calculation. The only thermal property involved in the new scheme is the thermal diffusivity of the soil, which is a weak function of soil water content and can be approximated as constant with reasonable accuracy. For validation, the proposed method is compared to the conventional approach using a canonical one-dimensional heat conduction problem, as well as real field measurements. Results of comparison highlight that the new model is robust and capable of preserving the good accuracy of the conventional approach with reduced input information. In addition, the effect of inclusion of the heat storage term in the ground heat flux is evaluated in the context of surface energy balance closure for field measurements.
Single-layer physically-based urban canopy models (UCM) have gained popularity for modeling urban... more Single-layer physically-based urban canopy models (UCM) have gained popularity for modeling urban-atmosphere interactions, especially the energy transport component. For an urban canopy model to capture the physics of conductive, radiative, and turbulent advective transport of energy, it is important to provide it with an accurate parameter space, including both mesoscale meteorological forcing and microscale surface inputs. While field measurement of all input parameters of an urban canopy model is rarely possible, understanding the model sensitivity to individual parameters is essential to determine the relative importance of parameter uncertainty for model performance. In this paper, we use an advanced Monte Carlo approach, namely Subset Simulation, to quantify the impact of the uncertainty of surface input parameters on the output of an offline modified version of WRF-UCM. Based on the conditional sampling technique, the importance of surface parameters is determined in terms of their impact on critical model responses. It is found that model outputs (both critical energy fluxes and surface temperatures) are highly sensitive to uncertainties in urban geometry, while variations in emissivities and building interior temperatures are relatively insignificant. In addition, the sensitivity of the model to input surface parameters is also shown to be very weakly dependent on meteorological parameters. The statistical quantification of the model’s sensitivity to input parameters has practical implications, such as surface parameter calibrations in UCM and guidance for urban heat island mitigation strategies.
Recently, Gao et al. [2010] (hereafter referred to as GHL) presented a heat conduction analysis a... more Recently, Gao et al. [2010] (hereafter referred to as GHL) presented a heat conduction analysis aimed to address the well known energy balance closure problem [see, e.g. Foken, 2008], by attributing the energy residue (Rn - G0) - (H + LE) partially to the phase difference between T0 and G0. Here Rn is the net radiation incidental on soil surface, H, LE and G are the sensible, latent and soil (conductive) heat fluxes respectively, with subscript 0 denoting the soil surface. We commend GHL’s consideration and characterization on the often neglected phase difference between soil surface temperature and heat flux in the context of surface energy balance closure. However, it is found that their heat conduction model was based on an ill-defined boundary value problem (BVP). This comment highlights the flaw that GHL overlooked the phase difference between soil surface temperature and the incidental (forcing) heat flux.
Most conventional numerical schemes for soil ground heat flux estimation rely on the knowledge of... more Most conventional numerical schemes for soil ground heat flux estimation rely on the knowledge of the temporal evolution of soil temperature. Here we propose and test a novel scheme, which requires no information on soil temperatures to supplement the flux plate measurement. The proposed method is based on the fundamental solution of the one-dimensional heat equation and Duhamel's principle for the incorporation of inhomogeneous boundary conditions. Being completely independent of the soil temperature, the new scheme therefore avoids a potential source of error in measurements and in heat storage calculation. The only thermal property involved in the new scheme is the thermal diffusivity of the soil, which is a weak function of soil water content and can be approximated as constant with reasonable accuracy. For validation, the proposed method is compared to the conventional approach using a canonical one-dimensional heat conduction problem, as well as real field measurements. Results of the comparison highlight that the new model is robust and capable of preserving the good accuracy of the conventional approach with reduced input information. In addition, the effect of inclusion of the heat storage term in the ground heat flux is evaluated in the context of surface energy balance closure for field measurements.
Renewable and Sustainable Energy Reviews, 47, 830-843, Apr 2015
Studies on urban heat island (UHI) have been more than a century after the phenomenon was first d... more Studies on urban heat island (UHI) have been more than a century after the phenomenon was first discovered in the early 1800s. UHI emerges as the source of many urban environmental problems and exacerbates the living environment in cities. Under the challenges of increasing urbanization and future climate changes, there is a pressing need for sustainable adaptation/mitigation strategies for UHI effects, one popular option being the use of reflective materials. While it is introduced as an effective method to reduce temperature and energy consumption in cities, its impacts on environmental sustainability and large-scale non-local effect are inadequately explored. This paper provides a synthetic overview of potential environmental impacts of reflective materials at a variety of scales, ranging from energy load on a single building to regional hydroclimate. The review shows that mitigation potential of reflective materials depends on a set of factors, including building characteristics, urban environment, meteor- ological and geographical conditions, to name a few. Precaution needs to be exercised by city planners and policy makers for large-scale deployment of reflective materials before their environmental impacts, especially on regional hydroclimates, are better understood. In general, it is recommended that optimal strategy for UHI needs to be determined on a city-by-city basis, rather than adopting a “one-solution- fits-all” strategy.
Urbanization modifies surface energy and water budgets, and has significant impacts on local and ... more Urbanization modifies surface energy and water budgets, and has significant impacts on local and regional hydroclimate. In recent decades, a number of urban canopy models have been developed and implemented into the Weather Research and Forecasting (WRF) model to capture urban land-surface processes. Most of these models are inadequate due to the lack of realistic representation of urban hydrological processes. Here, we imple- ment physically-based parametrizations of urban hydrological processes into the single layer urban canopy model in the WRF model. The new single-layer urban canopy model features the integration of, (1) anthropogenic latent heat, (2) urban irrigation, (3) evaporation from paved surfaces, and (4) the urban oasis effect. The new WRF–urban modelling system is evaluated against field measurements for four different cities; results show that the model performance is substantially improved as compared to the current schemes, especially for latent heat flux. In particular, to evaluate the performance of green roofs as an urban heat island mitigation strategy, we integrate in the urban canopy model a multilayer green roof system, enabled by the physical urban hydrological schemes. Simulations show that green roofs are capable of reducing surface temperature and sensible heat flux as well as enhancing building energy efficiency.
We couple a single column model (SCM) to a cutting-edge single-layer urban canopy model (SLUCM) w... more We couple a single column model (SCM) to a cutting-edge single-layer urban canopy model (SLUCM) with realistic representation of urban hydrological processes. The land-surface transport of energy and moisture parametrized by the SLUCM provides lower boundary conditions to the overlying atmosphere. The coupled SLUCM–SCM model is tested against field measurements of sensible and latent heat fluxes in the surface layer, as well as vertical profiles of temperature and humidity in the mixed layer under convective conditions. The model is then used to simulate urban land–atmosphere interactions by changing urban geometry, surface albedo, vegetation fraction and aerodynamic roughness. Results show that changes of landscape characteristics have a significant impact on the growth of the boundary layer as well as on the distributions of temperature and humidity in the mixed layer. Overall, the proposed numerical framework provides a useful stand-alone modelling tool, with which the impact of urban land-surface conditions on the local hydrometeorology can be assessed via land–atmosphere interactions.
Geophysical Research Letters, 41: 8348-58, Dec 2014
The partitioning of solar energy into sensible, latent, and ground heat over the land surface is ... more The partitioning of solar energy into sensible, latent, and ground heat over the land surface is responsible for changes of state variables in the soil-atmosphere system. Recent research enables the reconstruction of the land surface temperature and ground heat flux using Green’s function approach, as well as the estimate of the distribution of available energy into latent and sensible heat fluxes based on linear stability analysis. Combining the Green’s function approach and linear stability analysis, we propose a new physically-based numerical procedure to estimate the land surface energy partitioning in this paper. The new method is capable of predicting all surface energy budgets using a single depth soil measurement; the model reliability is evaluated with comparisons to flux tower measurements. Results of this study deepen our insight into the implicit link between surface energy partition and subsurface soil dynamics, and how the link can be employed to related research areas.
Land surface energy balance in a built environment is widely modelled using urban canopy models w... more Land surface energy balance in a built environment is widely modelled using urban canopy models with representation of building arrays as a big street canyon. Modification of this simplified geometric representation, on the other hand, leads to challenging numerical difficulties in improving physical parameterization schemes that are deterministic in nature. In this paper, we develop a stochastic algorithm to estimate view factors between canyon facets in the presence of shade trees based on Monte Carlo simulation, where an analytical formulation is inhibited by the complex geometry. The model is validated against analytical solutions of benchmark radiative problems as well as field measurements in real street canyons. In conjunction with the matrix method resolving infinite number of reflections, the proposed model is capable of predicting the radiative exchange inside the street canyon with good accuracy. Modeling of transient evolution of thermal filed inside the street canyon using the proposed method demonstrate the potential of shade trees in mitigating canyon surface temperatures as well as saving of building energy use. This new numerical framework also deepens our insight into the fundamental physics of radiative heat transfer and surface energy balance for urban climate modeling.
Urban-rural contrast is central to many urban environmental problems, a prominent example being t... more Urban-rural contrast is central to many urban environmental problems, a prominent example being the urban heat island effect. To ameliorate urban thermal stress, extensive research work has been focused on the mitigation of critical environmental temperatures, while the timing of excessive heating was largely overlooked. Advection of soil water flux plays a critical role in determining the soil thermal field, which in turn modulates surface energy fluxes and land–atmosphere interactions. In this paper, we formulate the wave phase differences between soil temperatures and soil heat fluxes due to soil water advection based on harmonic function method. It has been found that phase lags, viz. hysteresis effects, exist among all land surface energy budgets and the land surface
temperature. More generally, the difference of phase and time evolution of the land surface temperature and the ground heat flux is also manifested in annual cycles. In the context of urban–rural differences, the temporal difference in peak surface temperatures and peak turbulent fluxes has profound implication to human thermal comfort and building energy efficiency.
The accurate estimation of soil thermal field is crucially important because soil temperature is ... more The accurate estimation of soil thermal field is crucially important because soil temperature is a key parameter widely used in many related fields. This paper proposed an improved heat-conduction-equation (HCE) method to reconstruct soil thermal field using two-depth easurements of soil temperature. The revised HCE method employed a direct approach as well as an indirect approach to estimate both the daily average soil temperature (DST) and the instantaneous soil temperature (IST), during which the annual temperature cycle and the diurnal temperature cycle are combined. Two validation experiments (i.e., Test-1 and Test-2) were performed with soil temperature measurements at five stations chosen from the Soil Climate Analysis Network (SCAN). The results show that the revised HCE method improves the accuracy of modeling soil thermal field in comparison to its traditional form. The root mean square errors (RMSEs) of the ISTs estimated by the traditional HCE method range from 1.0 to 2.1 oC (2.4 to 4.8 oC) in Test-1 (Test-2); while the errors of the revised HCE method by the indirect approach are reduced to less
than 1.0 oC in both Test-1 and Test-2; the errors by the direct approach are further reduced to less than 0.7 oC in both Test-1 and Test-2. The improved method has further potential to estimate soil heat flux, a variable that can be inferred using soil temperature gradients associated with soil apparent thermal conductivity.
Journal of Applied Meteorology and Climatology, 53, 2114-29, 2014
Urban facets—the walls, roofs, and ground in built-up terrain—are often conceptualized as homogen... more Urban facets—the walls, roofs, and ground in built-up terrain—are often conceptualized as homogeneous surfaces, despite the obvious variability in the composition and material properties of the urban fabric at the subfacet scale. This study focuses on understanding the influence of this subfacet heterogeneity, and the associated influence of different material properties, on the urban surface energy budget. The Princeton Urban Canopy Model, which was developed with the ability to capture subfacet variability, is evaluated at sites of various building densities and then applied to simulate the energy exchanges of each subfacet with the atmosphere over a densely built site. The analyses show that, although all impervious built surfaces convert
most of the incoming energy into sensible heat rather than latent heat, sensible heat fluxes from asphalt pavements and dark rooftops are 2 times as high as those from concrete surfaces and light-colored roofs.
Another important characteristic of urban areas—the shift in the peak time of sensible heat flux in comparison
with rural areas—is here shown to be mainly linked to concrete’s high heat storage capacity as well as to
radiative trapping in the urban canyon. The results also illustrate that the vegetated pervious soil surfaces that
dot the urban landscape play a dual role: during wet periods they redistribute much of the available energy
into evaporative fluxes but when moisture stressed they behave more like an impervious surface. This role reversal, along with the direct evaporation of water stored over impervious surfaces, significantly reduces the overall Bowen ratio of the urban site after rain events.
Rapid urbanization has emerged as the source of many adverse environmental effects and brings cit... more Rapid urbanization has emerged as the source of many adverse environmental effects and brings cities to a vulnerable situation under future climate challenges. Green roofs are proven to be an effective solution to alleviate these effects by field observations nder a wide range of climate conditions. Recent advances in modeling urban land-atmosphere interactions provide a useful tool in capturing the dynamics of coupled transport of water and energy in urban conies, thus bridge the gap of modeling at city to regional scales. The performance of urban hydrological models depends heavily on the accurate determination of the input parameter space, where uncertainty is ubiquitous. In this paper, we use an advanced Monte Carlo approach, viz. the Subset Simulation, to quantify the sensitivity of urban hydrological modeling to parameter uncertainties. Results of the sensitivity analysis reveal that green roofs exhibit markedly different thermal and hydrological behavior as compared to conventional roofs, due to the modification of the surface energy portioning by well-irrigated vegetation. In addition, statistical predictions of critical responses of green roofs (extreme surface temperature, heat fluxes, etc.) have relatively weak dependence on climatic conditions. The statistical quantification of sensitivity provides guidance for future development of urban hydrological models with practical applications such as urban heat island mitigation.
Elastomer-based composites embedded with thermally-responsive material (TRM) and a liquid-phase J... more Elastomer-based composites embedded with thermally-responsive material (TRM) and a liquid-phase Joule heater are capable of reversibly changing their elastic rigidity by up to four orders of magnitude. At room temperature, the TRM layer is rigid and prevents the surrounding elastomer from elastically bending or stretching. When activated, the embedded Joule heater softens or melts the TRM, which leads to a dramatic reduction in the elastic rigidity of the composite. In this manuscript, we examine the activation of these composites by performing analytical, numerical, and experimental studies of the temperature distribution, thermal history, and phase transition. We consider both low melting point (LMP) metal alloys (e.g. Field’s metal) and shape memory polymer (SMP). An analytical solution using the Galerkin Based Integral (GBI) method is derived for the cases where no phase change is involved, while a numerical scheme using the Latent Heat Accumulation (LHA) method is utilized to probe scenarios where phase change has a central role in the elastic rigidity change. The analytical and numerical studies predict a temperature history that is in good agreement with experimental measurements obtained with an IR thermometer. Analysis of the internal temperature distribution leads to scaling laws for determining the required activation time and allowable input power rate for composites containing either LMP alloys or SMP. These scaling laws could potentially be used to inform the design of rigidity tunable composites (RTC) used in assistive wearable technologies and biologically-inspired soft-matter robotics.
The hysteresis effect in diurnal cycles of net radiation Rn and ground heat flux G0 has been obse... more The hysteresis effect in diurnal cycles of net radiation Rn and ground heat flux G0 has been observed in many studies, while the governing mechanism remains vague. In this study, we link the phenomenology of hysteresis loops to the wave phase difference between the diurnal evolutions of various terms in the surface energy balance. Rn and G0 are parameterized with the incoming solar radiation and the surface temperature as two control parameters of the surface energy partitioning. The theoretical analysis shows that the vertical water flux W and the scaled ratio A_s/A_T (net shortwave radiation to outgoing longwave radiation) play crucial roles in shaping hysteresis loops of Rn and G0. Comparisons to field measurements indicate that hysteresis loops for different land covers can be well captured by the theoretical model, which is also consistent with Camuffo-Bernadi formula. This study provides insight into the surface partitioning and temporal evolution of the energy budget at the land surface.
British Journal of Environment and Climate Change, 3(1): 86-102, 2013, Mar 2013
Aims: In this paper, we aim to assess different parameterization schemes for quantifying the surf... more Aims: In this paper, we aim to assess different parameterization schemes for quantifying the surface energy portioning process, in particular, the latent and sensible heat fluxes, and their applicability to various surface cover types.
Study design: This study intercompares theoretical models that predict the relative efficiency of the latent heat (evapotranspiration) with respect to the sensible heat flux. Model predictions are compared with field measurements over surface covers with different physical characteristics and soil water availability.
Place and Duration of Study: This study was carried out at the Arizona State University, Tempe, AZ, between August 2012 and December 2012.
Methodology: Three theoretical models for prediction of the relative efficiency of the latent heat were investigated, based on the lumped heat transfer (Priestley), the linear stability analysis (LSA) and the maximum entropy principle (MEP), respectively. Model predictions were compared against field measurements over three different land cover types, viz. water, grassland and suburban surfaces. An explicit moisture availability parameter β is incorporated in the MEP model, to facilitate direct comparison against the LSA and field measurements. Standard post-processing and quality control were applied to field measured turbulent fluxes using the eddy-covariance (EC) technique. To be consistent with the premise of all theoretical models, diurnal series of sensible and latent heat fluxes were filtered such that only data points under convective conditions were selected.
Results: Among all three models, the application of Priestley model is restricted to saturated land surfaces, and generally overestimates the relative efficiency of the latent heat for water-limited surfaces. The LSA and MEP models predict similar β ranges, i.e., 0.05-0.3 in summer and 0.1-0.7 in winter over suburban area, and 0.1 to 0.5 over lake surface. Over vegetated surfaces, the MEP model predicts a reasonable β range around unity by taking transpiration into consideration, while the LSA model consistently underestimated the relative efficiency.
Conclusion: Moisture availability plays an essential role in regulating the surface energy partitioning process. The introduction of the moisture availability parameter enables versatile theoretical models for latent heat (and evapotranspiration) predictions over a wide range of land cover types. This study provides a physical insight into the thermodynamics mechanism governing the surface energy balance, and the potential to develop novel surface energy parameterization schemes based on the concept of relative efficiency. The MEP model is found to have the greatest potential in terms of future theoretical model development.
In this study, the Princeton Roof Model (PROM) is developed and used to simulate the hygrothermal... more In this study, the Princeton Roof Model (PROM) is developed and used to simulate the hygrothermal dynamics of green roof systems. PROM is embedded within the framework of the Princeton Urban Canopy Model, with a multi-layer spatially-analytical heat transfer scheme and an improved hydrological module. The model is validated by comparing simulated surface temperature and soil moisture to the measurements at two experimental sites, one in Beijing, China and the other in New Jersey, USA. The results demonstrate that PROM is able to capture the diurnal cycle of roof temperatures and the soil moisture dynamics of green roofs with reasonable accuracy. Driven by a 30-day summertime meteorological forcing from July 2001, PROM is used to investigate the green roof thermal improvement to the urban indoor and outdoor environments, compared to conventional roofs. The impact of green roofs is significant in reducing surface temperatures, and outdoor and indoor heat fluxes during this summer period. To quantify this thermal improvement, three indices related to surface temperature, outdoor heat flux and indoor heat flux, are introduced; and the dependence of these indices on hydrological and meteorological conditions is investigated. The results indicate that incoming solar radiation, air temperature, wind speed and medium layer moisture are the main determinants of the green roof performance
We propose a new surface exchange scheme, coupling the transport of energy and water in urban can... more We propose a new surface exchange scheme, coupling the transport of energy and water in urban canopies. The new model resolves the sub-facet heterogeneity of urban surfaces, which is particularly useful for capturing surface exchange processes from vegetated urban surfaces, such as lawns or green roofs. We develop detailed urban hydrological models for surfaces consisting of either natural (soil and vegetation) or engineered materials with water-holding capacity. The coupling of energy and water transport enables us to parameterize surface evaporation from different urban facets including soils, vegetation and water-holding engineered surfaces. The new coupled model is evaluated using field measurement data obtained through a wireless sensor network deployed over the Princeton University campus. Comparison of model prediction and measured results shows that the proposed surface exchange scheme is able to predict widely-varying surface temperatures for each heterogeneous sub-facet with good accuracy. Different weather conditions and seasonal variability are found to have insignificant effect on the model performance. The new model is also able to capture the subsurface hydrological processes with reasonable accuracy, particularly for urban lawns. The proposed model is then applied to assess different mitigation strategies of the urban heat island effect.
Effects of the urban length scale and the wind speed on the urban heat island effect are quantifi... more Effects of the urban length scale and the wind speed on the urban heat island effect are quantified through time-dependent energy balance. The heating of the urban surfaces during the daytime sets the initial temperature, and this overheating is cooled during the night-time through mean convection motion over the urban surface, resulting in an exponential decay in the temperature. The solution to the time-dependent energy balance equation accurately reproduces this temporal decay, with the main factors being the length scale of the urban area and the wind speed. The temporal data for Phoenix, Arizona are reasonably accurately traced by this model, when the urban length scale is varied from 1983 to 2010. The minimum temperature reached at the end of this cooling period then corresponds to the UHI intensity, which increases with increasing urban length scale and decreasing wind speed. The wind speed effect is also accurately re-traced using this method; however, different correction factors are required for different cities, indicating the effects of the urban surface heat content, structural morphology and density. Thus, using a small number of readily available data for the urban length scale and the wind speed, the UHI intensity can be described with possible projections for future trends.
Soil field experiments usually consist of measurements of soil temperatures, heat fluxes and soil... more Soil field experiments usually consist of measurements of soil temperatures, heat fluxes and soil water contents. Accurate determination of the soil thermal field, in particular, prediction of the soil surface temperature and the ground heat, contains the signature to the surface energy partitioning, and is therefore critical to the surface energy balance closure problem. In this paper, we develop a numerical procedure to reconstruct the entire soil thermal field from a single depth measurement of either temperature or heat flux. The new algorithm is based on Green’s function approach by using the fundamental solution of heat conduction in semi-infinite soils and Duhamel’s integral for incorporation of general boundary conditions. It is highlighted that the new approach is capable of accurately reproducing results of some existing numerical approaches, with a more general setting and treatment of the heat diffusion problem, and hence provides a possible unified framework for the estimation of thermal field in soils.
Most conventional numerical schemes for soil ground heat flux estimation rely on the knowledge of... more Most conventional numerical schemes for soil ground heat flux estimation rely on the knowledge of the temporal evolution of soil temperature. Here we propose and test a novel scheme, which requires no information on soil temperatures to supplement the flux plate measurement. The proposed method is based on the fundamental solution of the one-dimensional heat equation and Duhamel’s principle for the incorporation of inhomogeneous boundary conditions. Being completely independent of the soil temperature, the new scheme therefore avoids a potential source of error in measurements and in heat storage calculation. The only thermal property involved in the new scheme is the thermal diffusivity of the soil, which is a weak function of soil water content and can be approximated as constant with reasonable accuracy. For validation, the proposed method is compared to the conventional approach using a canonical one-dimensional heat conduction problem, as well as real field measurements. Results of comparison highlight that the new model is robust and capable of preserving the good accuracy of the conventional approach with reduced input information. In addition, the effect of inclusion of the heat storage term in the ground heat flux is evaluated in the context of surface energy balance closure for field measurements.
Single-layer physically-based urban canopy models (UCM) have gained popularity for modeling urban... more Single-layer physically-based urban canopy models (UCM) have gained popularity for modeling urban-atmosphere interactions, especially the energy transport component. For an urban canopy model to capture the physics of conductive, radiative, and turbulent advective transport of energy, it is important to provide it with an accurate parameter space, including both mesoscale meteorological forcing and microscale surface inputs. While field measurement of all input parameters of an urban canopy model is rarely possible, understanding the model sensitivity to individual parameters is essential to determine the relative importance of parameter uncertainty for model performance. In this paper, we use an advanced Monte Carlo approach, namely Subset Simulation, to quantify the impact of the uncertainty of surface input parameters on the output of an offline modified version of WRF-UCM. Based on the conditional sampling technique, the importance of surface parameters is determined in terms of their impact on critical model responses. It is found that model outputs (both critical energy fluxes and surface temperatures) are highly sensitive to uncertainties in urban geometry, while variations in emissivities and building interior temperatures are relatively insignificant. In addition, the sensitivity of the model to input surface parameters is also shown to be very weakly dependent on meteorological parameters. The statistical quantification of the model’s sensitivity to input parameters has practical implications, such as surface parameter calibrations in UCM and guidance for urban heat island mitigation strategies.
Recently, Gao et al. [2010] (hereafter referred to as GHL) presented a heat conduction analysis a... more Recently, Gao et al. [2010] (hereafter referred to as GHL) presented a heat conduction analysis aimed to address the well known energy balance closure problem [see, e.g. Foken, 2008], by attributing the energy residue (Rn - G0) - (H + LE) partially to the phase difference between T0 and G0. Here Rn is the net radiation incidental on soil surface, H, LE and G are the sensible, latent and soil (conductive) heat fluxes respectively, with subscript 0 denoting the soil surface. We commend GHL’s consideration and characterization on the often neglected phase difference between soil surface temperature and heat flux in the context of surface energy balance closure. However, it is found that their heat conduction model was based on an ill-defined boundary value problem (BVP). This comment highlights the flaw that GHL overlooked the phase difference between soil surface temperature and the incidental (forcing) heat flux.
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Papers by Zhihua Wang
temperature. More generally, the difference of phase and time evolution of the land surface temperature and the ground heat flux is also manifested in annual cycles. In the context of urban–rural differences, the temporal difference in peak surface temperatures and peak turbulent fluxes has profound implication to human thermal comfort and building energy efficiency.
than 1.0 oC in both Test-1 and Test-2; the errors by the direct approach are further reduced to less than 0.7 oC in both Test-1 and Test-2. The improved method has further potential to estimate soil heat flux, a variable that can be inferred using soil temperature gradients associated with soil apparent thermal conductivity.
most of the incoming energy into sensible heat rather than latent heat, sensible heat fluxes from asphalt pavements and dark rooftops are 2 times as high as those from concrete surfaces and light-colored roofs.
Another important characteristic of urban areas—the shift in the peak time of sensible heat flux in comparison
with rural areas—is here shown to be mainly linked to concrete’s high heat storage capacity as well as to
radiative trapping in the urban canyon. The results also illustrate that the vegetated pervious soil surfaces that
dot the urban landscape play a dual role: during wet periods they redistribute much of the available energy
into evaporative fluxes but when moisture stressed they behave more like an impervious surface. This role reversal, along with the direct evaporation of water stored over impervious surfaces, significantly reduces the overall Bowen ratio of the urban site after rain events.
Study design: This study intercompares theoretical models that predict the relative efficiency of the latent heat (evapotranspiration) with respect to the sensible heat flux. Model predictions are compared with field measurements over surface covers with different physical characteristics and soil water availability.
Place and Duration of Study: This study was carried out at the Arizona State University, Tempe, AZ, between August 2012 and December 2012.
Methodology: Three theoretical models for prediction of the relative efficiency of the latent heat were investigated, based on the lumped heat transfer (Priestley), the linear stability analysis (LSA) and the maximum entropy principle (MEP), respectively. Model predictions were compared against field measurements over three different land cover types, viz. water, grassland and suburban surfaces. An explicit moisture availability parameter β is incorporated in the MEP model, to facilitate direct comparison against the LSA and field measurements. Standard post-processing and quality control were applied to field measured turbulent fluxes using the eddy-covariance (EC) technique. To be consistent with the premise of all theoretical models, diurnal series of sensible and latent heat fluxes were filtered such that only data points under convective conditions were selected.
Results: Among all three models, the application of Priestley model is restricted to saturated land surfaces, and generally overestimates the relative efficiency of the latent heat for water-limited surfaces. The LSA and MEP models predict similar β ranges, i.e., 0.05-0.3 in summer and 0.1-0.7 in winter over suburban area, and 0.1 to 0.5 over lake surface. Over vegetated surfaces, the MEP model predicts a reasonable β range around unity by taking transpiration into consideration, while the LSA model consistently underestimated the relative efficiency.
Conclusion: Moisture availability plays an essential role in regulating the surface energy partitioning process. The introduction of the moisture availability parameter enables versatile theoretical models for latent heat (and evapotranspiration) predictions over a wide range of land cover types. This study provides a physical insight into the thermodynamics mechanism governing the surface energy balance, and the potential to develop novel surface energy parameterization schemes based on the concept of relative efficiency. The MEP model is found to have the greatest potential in terms of future theoretical model development.
temperature. More generally, the difference of phase and time evolution of the land surface temperature and the ground heat flux is also manifested in annual cycles. In the context of urban–rural differences, the temporal difference in peak surface temperatures and peak turbulent fluxes has profound implication to human thermal comfort and building energy efficiency.
than 1.0 oC in both Test-1 and Test-2; the errors by the direct approach are further reduced to less than 0.7 oC in both Test-1 and Test-2. The improved method has further potential to estimate soil heat flux, a variable that can be inferred using soil temperature gradients associated with soil apparent thermal conductivity.
most of the incoming energy into sensible heat rather than latent heat, sensible heat fluxes from asphalt pavements and dark rooftops are 2 times as high as those from concrete surfaces and light-colored roofs.
Another important characteristic of urban areas—the shift in the peak time of sensible heat flux in comparison
with rural areas—is here shown to be mainly linked to concrete’s high heat storage capacity as well as to
radiative trapping in the urban canyon. The results also illustrate that the vegetated pervious soil surfaces that
dot the urban landscape play a dual role: during wet periods they redistribute much of the available energy
into evaporative fluxes but when moisture stressed they behave more like an impervious surface. This role reversal, along with the direct evaporation of water stored over impervious surfaces, significantly reduces the overall Bowen ratio of the urban site after rain events.
Study design: This study intercompares theoretical models that predict the relative efficiency of the latent heat (evapotranspiration) with respect to the sensible heat flux. Model predictions are compared with field measurements over surface covers with different physical characteristics and soil water availability.
Place and Duration of Study: This study was carried out at the Arizona State University, Tempe, AZ, between August 2012 and December 2012.
Methodology: Three theoretical models for prediction of the relative efficiency of the latent heat were investigated, based on the lumped heat transfer (Priestley), the linear stability analysis (LSA) and the maximum entropy principle (MEP), respectively. Model predictions were compared against field measurements over three different land cover types, viz. water, grassland and suburban surfaces. An explicit moisture availability parameter β is incorporated in the MEP model, to facilitate direct comparison against the LSA and field measurements. Standard post-processing and quality control were applied to field measured turbulent fluxes using the eddy-covariance (EC) technique. To be consistent with the premise of all theoretical models, diurnal series of sensible and latent heat fluxes were filtered such that only data points under convective conditions were selected.
Results: Among all three models, the application of Priestley model is restricted to saturated land surfaces, and generally overestimates the relative efficiency of the latent heat for water-limited surfaces. The LSA and MEP models predict similar β ranges, i.e., 0.05-0.3 in summer and 0.1-0.7 in winter over suburban area, and 0.1 to 0.5 over lake surface. Over vegetated surfaces, the MEP model predicts a reasonable β range around unity by taking transpiration into consideration, while the LSA model consistently underestimated the relative efficiency.
Conclusion: Moisture availability plays an essential role in regulating the surface energy partitioning process. The introduction of the moisture availability parameter enables versatile theoretical models for latent heat (and evapotranspiration) predictions over a wide range of land cover types. This study provides a physical insight into the thermodynamics mechanism governing the surface energy balance, and the potential to develop novel surface energy parameterization schemes based on the concept of relative efficiency. The MEP model is found to have the greatest potential in terms of future theoretical model development.