Quarterly Journal of the Royal Meteorological Society, Dec 1, 2021
Cities are a source of complex land-atmosphere interactions. Spatial differences in the energy ba... more Cities are a source of complex land-atmosphere interactions. Spatial differences in the energy balance and enhanced surface roughness interact with the atmosphere to alter clouds and precipitation. Here, we explore how London (UK) alters cloud formation during the spring and summer. The Met Office's high-resolution operational forecasts predict enhanced cloud cover over the city, as found in observations, but underpredicts the intensity. During low wind speeds, cloud enhancement over the city is strongest and linked to an urban-induced thermal circulation. These circulations advect moist air from the city edge inwards, transporting it upwards with a large moisture convergence over the urban area. At around 1,000 m above the surface, the turbulent moisture flux takes over the moisture transport to the cloud layer. A relative humidity budget shows the moisture flux in the upper boundary layer to be the largest contribution to the urban-rural differences in relative humidity.
Quarterly Journal of the Royal Meteorological Society, Oct 1, 2021
A recently developed, height-distributed urban drag parametrization is tested with the London Mod... more A recently developed, height-distributed urban drag parametrization is tested with the London Model, a sub-kilometre resolution version of the Met Office Unified Model over Greater London. The distributed-drag parametrization requires vertical morphology profiles in the form of height-distributed frontal-area functions, which capture the full extent and variability of building heights. London's morphology profiles are calculated and parametrized by an exponential distribution with the ratio of maximum to mean building height as the parameter. A case study evaluates the differences between the new distributed-drag scheme and the current London Model setup using the MORUSES urban land-surface model. The new drag parametrization shows increased horizontal spatial variability in total surface stress, identifying densely built-up areas, high-rise building clusters, parks, and the river. Effects on the wind speed in the lower levels include a lesser gradient and more heterogeneous wind profiles, extended wakes downwind of the city centre, and vertically growing perturbations that suggest the formation of internal boundary layers. The surface sensible heat fluxes are underpredicted, which is attributed to difficulties coupling the distributed momentum exchange with the surface-based heat exchange. K E Y W O R D S drag parametrization, MORUSES, regional and mesoscale modelling, urban canopy model, urban meteorology, urban morphology analysis 1 INTRODUCTION Urban environments alter aerodynamic, radiative, thermal, and hydrological processes, which can intensify heat waves, flash floods, and air pollution. Accurate urban models are necessary for better warnings of severe weather hazards and to improve weather forecasts and services in the most populated areas in the world. Moreover, urban climate models are crucial for planning how to adapt cities for more extreme weather and how to transform This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Air quality in cities is influenced not only by emissions and chemical transformations but also b... more Air quality in cities is influenced not only by emissions and chemical transformations but also by the physical state of the atmosphere which varies both temporally and spatially. Increasingly, tall buildings (TB) are common features of the urban landscape, yet their impact on urban air flow and dispersion is not well understood, and their effects are not appropriately captured in parameterisation schemes. Here, hardware models of areas within two global mega-cities (London and Beijing) are used to analyse the impact of TB on flow and transport in isolated and cluster settings. Results show that TB generate strong updrafts and downdrafts that affect street-level flow fields. Velocity differences do not decay monotonically with distance from the TB, especially in the near-wake region where the flow is characterised by recirculating winds and jets. Lateral distance from an isolated TB centreline is crucial, and flow is still strongly impacted at longitudinal distances of several TB heights. Evaluation of a wake-flow scheme (ADMS-Build) in the isolated TB case indicates important characteristics are not captured. There is better agreement for a slender, shorter TB than a taller non-cuboidal TB. Better prediction of flow occurs horizontally further away and vertically further from the surface. TB clusters modify the shape of pollutant plumes. Strong updrafts generated by the overlapping wakes of TB clusters lift pollutants out of the canopy, causing a much deeper tracer plume in the lee of the cluster, and an elevated plume centreline with maximum concentrations around the TB mean height. Enhanced vertical spread of the pollutants in the near-wake of the cluster results in overall lower maximum concentrations, but higher concentrations above the mean TB height. These results have important implications for interpreting observations in areas with TB. Using real world ceilometer observations in two mega-cities (Beijing and Paris), we assess the
Journal of Atmospheric and Oceanic Technology, Apr 1, 2017
To investigate the boundary layer dynamics of the coastal megacity Shanghai, China, backscatter d... more To investigate the boundary layer dynamics of the coastal megacity Shanghai, China, backscatter data measured by a Vaisala CL51 ceilometer are analyzed with a modified ideal curve fitting algorithm. The boundary layer height z i retrieved by this method and from radiosondes compare reasonably overall. Analyses of mobile and stationary ceilometer data provide spatial and temporal characteristics of Shanghai's boundary layer height. The consistency between when the ceilometer is moving and stationary highlights the potential of mobile observations of transects across cities. An analysis of 16 months of z i measured at the Fengxian site in Shanghai reveals that the diurnal variation of z i in the four seasons follows the expected pattern; for all seasons z i starts to increase at sunrise, reflecting the influence of solar radiation. However, the boundary layer height is generally higher in autumn and winter than in summer and spring (mean hourly averaged z i for days with low cloud fraction at 1100-1200 local time are 900, 654, 934, and 768 m for spring, summer, autumn, and winter, respectively). This is attributed to seasonal differences in the dominant meteorological conditions, including the effects of a sea breeze at the near-coastal Fengxian site. Given the success of the retrieval method, other ceilometers installed across Shanghai are now being analyzed to understand more about the spatial dynamics of z i and to investigate in more detail the effects of prevailing mesoscale circulations and their seasonal dynamics.
International Journal of Biometeorology, Feb 6, 2016
The effect of variations in land cover on mean radiant surface temperature (Tmrt) is explored thr... more The effect of variations in land cover on mean radiant surface temperature (Tmrt) is explored through a simple scheme developed within the radiation model SOLWEIG. Outgoing longwave radiation is parameterised using surface temperature observations on a grass and an asphalt surface, whereas outgoing shortwave radiation is modelled through variations in albedo for the different surfaces. The influence of surface materials on Tmrt is small compared to the effects of shadowing. Nevertheless, altering ground surface materials could contribute to a reduction on Tmrt to reduce the radiant load during heatwave episodes in locations where shadowing is not an option. Evaluation of the new scheme suggests that despite its simplicity it can simulate the outgoing fluxes well, especially during sunny conditions. However, it underestimates at night and in shadowed locations. One grass surface used to develop the parameterisation, with very different characteristics compared to an evaluation grass site, caused Tmrt to be underestimated. The implications of using high resolution (e.g. 15 minutes) temporal forcing data under partly cloudy conditions are demonstrated even for fairly proximal sites.
Despite inter-building longwave radiative exchanges playing an important role in determining buil... more Despite inter-building longwave radiative exchanges playing an important role in determining building energy and environmental performance, simulation tools (e.g. EnergyPlus) simplify this by assuming the surface temperature of surrounding buildings to be equal to the air temperature, and therefore cause bias. Here we propose a ‘spin-up’ approach to update building external surface temperature using either air or the isolated building temperatures. Neighbourhoods with different plan area fraction of buildings (λP) are analysed to assess the impact on building external surface temperatures, cooling and heating energy demand as well as indoor overheating degree hours. Using the default EnergyPlus method causes a large bias in all metrics in a dense urban area (λP = 0.6) and climates assessed (cf. the new method): external wall temperature (3 °C less, midday median), annual energy demand for cooling (17.1% less) and heating (6.2% higher), annual overheating degree hours during the day (> 28 °C, 24.5% less) and night (> 26 °C, 60.1% less). These biases are larger at lower latitudes. Thus, neglecting the surroundings influence on inter-building longwave radiation impacts critical design considerations of building energy and thermal performance in dense urban areas
Observations at the Mauna Loa Observatory, Hawaii, established the systematic increase of anthrop... more Observations at the Mauna Loa Observatory, Hawaii, established the systematic increase of anthropogenic CO 2 in the atmosphere. For the same reasons that this site provides excellent globally averaged CO 2 data, it may provide temperature data with global significance. Here, we examine hourly temperature records, averaged annually for 1977-2006, to determine linear trends as a function of time of day. For night-time data (22:00 to 06:00, LST (local standard time)) there is a near-uniform warming of 0.040 • C y −1. During the day, the linear trend shows a slight cooling of −0.013 • C y −1 at 12:00 (noon, LST). Overall, at Mauna Loa Observatory, there is a mean warming trend of 0.021 • C y −1. The dominance of night-time warming results in a relatively large annual decrease in the diurnal temperature range (DTR) of −0.050 • C y −1. These trends are consistent with the observed increases in the concentrations of CO 2 and its role as a greenhouse gas, and indicate the possible relevance of the Mauna Loa temperature measurements to global warming.
The integrated wind risk warning model for rail transport presented has four elements: Background... more The integrated wind risk warning model for rail transport presented has four elements: Background wind data, a wind field model, a vulnerability model, and a risk model. Background wind data uses observations in this study. Using the wind field model with effective surface roughness lengths, the background wind data are interpolated to a 30-m resolution grid. In the vulnerability model, the aerodynamic characteristics of railway vehicles are analyzed with CFD (Computational Fluid Dynamics) modelling. In the risk model, the maximum value of three aerodynamic forces is used as the criteria to evaluate rail safety and to quantify the risk level under extremely windy weather. The full model is tested for the Shanghai Metro Line 16 using wind conditions during Typhoon Chan-hom. The proposed approach enables quick quantification of real-time safety risk levels during typhoon landfall, providing sophisticated warning information for rail vehicle operation safety.
Often the meteorological forcing data required for urban hydrological models are unavailable at t... more Often the meteorological forcing data required for urban hydrological models are unavailable at the required temporal resolution or for the desired period. Although reanalysis data can provide this information, the spatial resolution is often coarse relative to cities, so downscaling is required prior to use as realistic forcing. In this study, WATCH WFDEI reanalysis data are used to force the Surface Urban Energy and Water balance Scheme (SUEWS). From sensitivity tests in two cities, Vancouver and London with different orography, we conclude precipitation is the most important meteorological variable to be properly downscaled to obtain reliable surface hydrology results, with relative humidity being the second most important. Overestimation of precipitation in reanalysis data at the three sites gives 6-21 % higher annual modelled evaporation, 26-39 % higher runoff at one site and 4 % lower value at one site when compared to modelled values using observed forcing data. Application of a bias correction method to the reanalysis precipitation reduces the model bias compared to using observed forcing data, when evaluated using eddy covariance evaporation measurements.
Tower-based measurements from within and above the urban canopy in two cities are used to evaluat... more Tower-based measurements from within and above the urban canopy in two cities are used to evaluate several existing approaches that parametrize the vertical profiles of wind speed and temperature within the urban roughness sublayer (RSL). It is shown that current use of Monin-Obukhov similarity theory (MOST) in numerical weather prediction models can be improved upon by using RSL corrections when modelling the vertical profiles of wind speed and friction velocity in the urban RSL using MOST. Using anisotropic building morphological information improves the agreement between observed and parametrized profiles of wind speed and momentum fluxes for selected methods. The largest improvement is found when using dynamically-varying aerodynamic roughness length and displacement height. Adding a RSL correction to MOST, however, does not improve the parametrization of the vertical profiles of temperature and heat fluxes. This is expected since sources and sinks of heat are assumed uniformly distributed through a simple flux boundary condition in all RSL formulations, yet are highly patchy and anisotropic in a real urban context. Our results can be used to inform the choice of surface-layer representations for air quality, dispersion, and numerical weather prediction applications in the urban environment. Keywords Roughness sublayer • Temperature profile • Urban canopy • Wind profile 1 Introduction When modelling urban meteorological processes, it is crucial to represent the exchange of momentum and scalars such as temperature and humidity between the surface and overly-B Natalie E. Theeuwes
Urban areas are a hotspot for the interactions between the built environment, its inhabitants, an... more Urban areas are a hotspot for the interactions between the built environment, its inhabitants, and weather. Unlike the impact of temperatures through the well-known urban heat island effect, urban effects on cloud formation remain unknown. In this study we show observational evidence of a systematic enhancement of cloud cover in the afternoon and evening over two large metropolitan areas in Europe (Paris and London). Long-term measurements in and around London show that during late-spring and summer, even though less moisture is available at the surface and the atmosphere is drier, low clouds can persist longer over the urban area as vertical mixing of the available moisture is maintained for a longer period of time, into the evening transition. Our findings show that urban impacts on weather extend beyond temperature effects. These prolonged clouds over the city might enhance the urban heat island via night-time radiative forcing.
Quarterly Journal of the Royal Meteorological Society, Jul 1, 2018
The use of Automatic Lidars and Ceilometers (ALC) is increasingly extended beyond monitoring clou... more The use of Automatic Lidars and Ceilometers (ALC) is increasingly extended beyond monitoring cloud base height to the study of atmospheric boundary layer (ABL) dynamics. Therefore, long-term sensor network observations require robust algorithms to automatically detect the mixed layer height (Z ML). Here, a novel automatic algorithm CABAM (Characterising the Atmospheric Boundary layer based on ALC Measurements) is presented. CABAM is the first non-proprietary mixed layer height algorithm specifically designed for the commonly deployed Vaisala CL31 ceilometer. The method tracks Z ML , takes into account precipitation, classifies the ABL based on cloud cover and cloud type, and determines the relation between Z ML and cloud base height. CABAM relies solely on ALC measurements. Results perform well against independent reference (AMDAR: Aircraft Meteorological Data Relay) measurements and supervised Z ML detection. AMDAR-derived temperature inversion heights allow Z ML evaluation throughout the day. Very good agreement is found in the afternoon when the mixed layer height extends over the full ABL. However, during night or the morning transition the temperature inversion is more likely associated with the top of the residual layer. From comparison with SYNOP reports, the ABL classification scheme generally correctly distinguishes between convective and stratiform boundary-layer clouds, with slightly better performance during daytime. Applied to 6 years of ALC observations in central London, Kotthaus and Grimmond (2018), a companion paper, demonstrate CABAM results are valuable to characterise the urban boundary layer over London, United Kingdom, where clouds of various types are frequent.
The process of coupling the Surface Urban Energy and Water Scheme (SUEWS) into the Weather resear... more The process of coupling the Surface Urban Energy and Water Scheme (SUEWS) into the Weather research and forecasting (WRF) model is presented, including pre-processing of model parameters to represent spatial variability of surface characteristics. Fluxes and mixed layer height observations in the southern UK are used to evaluate a two-week period in each season. Mean absolute errors, based on all periods, are smaller in residential Swindon than central London for turbulent sensible and latent heat fluxes (Q H , Q E) with greater skill on clear days at both sites (for incoming and outgoing short-and 5 longwave radiation, Q H and Q E). Clear seasonality is seen in the model performance: with better absolute skill for Q H and Q E in autumn and winter, when there is a higher frequency of clear days, than in spring and summer. As the WRF-modelled incoming shortwave radiation has large errors, we apply a bulk transmissivity derived from local observations to reduce the incoming shortwave radiation input to the land surface scheme-this could correspond to increased presence of aerosols in cities. We use the coupled WRF-SUEWS system to investigate impacts of the anthropogenic heat flux emissions on boundary 10 layer dynamics by comparing areas with contrasting human activities (central-commercial and residential areas) in Greater London-larger anthropogenic heat emissions not only elevate the mixed layer heights but also lead to a warmer and drier near-surface atmosphere.
Journal of Advances in Modeling Earth Systems, Aug 1, 2020
Urban land surface processes need to be represented to inform future urban climate and building e... more Urban land surface processes need to be represented to inform future urban climate and building energy projections. Here, the single layer urban canopy model Town Energy Balance (TEB) is coupled to the Weather Research and Forecasting (WRF) model to create WRF-TEB. The coupling method is described generically, implemented into software, and the code and data are released with a Singularity image to address issues of scientific reproducibility. The coupling is implemented modularly and verified by an integration test. Results show no detectable errors in the coupling. Separately, a meteorological evaluation is undertaken using observations from Toulouse, France. The latter evaluation, during an urban canopy layer heat island episode, shows reasonable ability to estimate turbulent heat flux densities and other meteorological quantities. We conclude that new model couplings should make use of integration tests as meteorological evaluations by themselves are insufficient, given that errors are difficult to attribute because of the interplay between observational errors and multiple parameterization schemes (e.g., radiation, microphysics, and boundary layer). Plain Language Summary With increasing urbanization and climate change, estimates of possible future urban climate and building energy scenarios are needed. Weather models (e.g., Weather Research and Forecasting; WRF) provide the state of the atmosphere, and urban land surface models (e.g., Town Energy Balance; TEB) allow the interactions and feedbacks of people-buildings-atmosphere to be investigated. WRF-TEB is a free and open-source model aimed at the urban climate and energy community to investigate applications such as the energy consumption associated with air conditioners, or the generation of solar energy in cities. 1. Introduction With increasing urbanization (United Nations, 2019) and climate change (Collins et al., 2013), the study of urban atmospheric phenomena such as the spatial variation of temperature (Arnfield, 2003), or the impact of the urban environment on moisture (
Use of wind pressure coefficients to simulate natural ventilation and building energy for isolate... more Use of wind pressure coefficients to simulate natural ventilation and building energy for isolated and surrounded buildings. Building and Environment, 230. 109951.
The impact of heterogeneous uptake of HO 2 on aerosol surfaces on radical concentrations and the ... more The impact of heterogeneous uptake of HO 2 on aerosol surfaces on radical concentrations and the O 3 production regime in Beijing in summertime was investigated. The uptake coefficient of HO 2 onto aerosol surfaces, γ HO 2 , was calculated for the AIRPRO campaign in Beijing, in summer 2017, as a function of measured aerosol soluble copper concentration, [Cu 2+ ] eff , aerosol liquid water content, [ALWC], and particulate matter concentration, [PM]. An average γ HO 2 across the entire campaign of 0.070 ± 0.035 was calculated, with values ranging from 0.002 to 0.15, and found to be significantly lower than the value of γ HO 2 = 0.2, commonly used in modelling studies. Using the calculated γ HO 2 values for the summer AIRPRO campaign, OH, HO 2 and RO 2 radical concentrations were modelled using a box model incorporating the Master Chemical Mechanism (v3.3.1), with and without the addition of γ HO 2 , and compared to the measured radical concentrations. The rate of destruction analysis showed the dominant HO 2 loss pathway to be HO 2 + NO for all NO concentrations across the summer Beijing campaign, with HO 2 uptake contributing < 0.3 % to the total loss of HO 2 on average. This result for Beijing summertime would suggest that under most conditions encountered, HO 2 uptake onto aerosol surfaces is not important to consider when investigating increasing O 3 production with decreasing [PM] across the North China Plain. At low [NO], however, i.e. < 0.1 ppb, which was often encountered in the afternoons, up to 29 % of modelled HO 2 loss was due to HO 2 uptake on aerosols when calculated γ HO 2 was included, even with the much lower γ HO 2 values compared to γ HO 2 = 0.2, a result which agrees with the aerosol-inhibited O 3 regime recently proposed by Ivatt et al. (2022). As such it can be concluded that in cleaner environments, away from polluted urban centres where HO 2 loss chemistry is not dominated by NO but where aerosol surface area is high still, changes in PM concentration and hence aerosol surface area could still have a significant effect on both overall HO 2 concentration and the O 3 production regime. Using modelled radical concentrations, the absolute O 3 sensitivity to NO x and volatile organic compounds (VOCs) showed that, on average across the summer AIRPRO campaign, the O 3 production regime remained VOC-limited, with the exception of a few days in the afternoon when the NO mixing ratio dropped low enough for the O 3 regime to shift towards being NO x-limited. The O 3 sensitivity to VOCs, the dominant regime during the summer AIRPRO campaign, was observed to decrease and shift towards a NO x-sensitive regime both when NO mixing ratio decreased and with the addition of aerosol uptake. This suggests that if [NO x ] continues to decrease in the future, ozone reduction policies focussing solely on NO x reductions may not be as efficient as expected if [PM] and, hence, HO 2 uptake to aerosol surfaces continue to decrease. The addition of aerosol uptake into the model, for both the γ HO 2 calculated from measured data and when using a fixed value of γ HO 2 = 0.2, did not have a significant effect on the overall O 3 production regime across the campaign. While not important for this campaign, aerosol uptake could be important for areas of lower NO concentration that are already in a NO x-sensitive regime.
Journal of Applied Meteorology and Climatology, 2021
Urban heat island (UHI) and sea–land-breeze systems are well-known and important characteristics ... more Urban heat island (UHI) and sea–land-breeze systems are well-known and important characteristics of the climate of coastal cities. To model these, the accurate estimation of the surface energy balance (SEB) is a key factor needed to improve local-scale simulations of thermodynamic and dynamic boundary circulations. The Weather Research and Forecasting Model with a single-layer urban canopy model (WRF/SLUCM), with parameters derived from MODIS and local GIS information, is used to investigate the UHI and sea-breeze circulations (SBC) in the megacity of Shanghai. The WRF/SLUCM can reproduce observed urban radiation and SEB fluxes, near-surface meteorological variables, and the evolution of the UHI and SBC. Simulations for an August period show the maximum UHI tends to drift northwest in the afternoon, driven by the prevailing southeast wind. The sea breeze lasts for about 4 h and is strongest between 1200 and 1400 local time (UTC + 8 h). The interaction between UHI and SBC is evident ...
Quarterly Journal of the Royal Meteorological Society, Dec 1, 2021
Cities are a source of complex land-atmosphere interactions. Spatial differences in the energy ba... more Cities are a source of complex land-atmosphere interactions. Spatial differences in the energy balance and enhanced surface roughness interact with the atmosphere to alter clouds and precipitation. Here, we explore how London (UK) alters cloud formation during the spring and summer. The Met Office's high-resolution operational forecasts predict enhanced cloud cover over the city, as found in observations, but underpredicts the intensity. During low wind speeds, cloud enhancement over the city is strongest and linked to an urban-induced thermal circulation. These circulations advect moist air from the city edge inwards, transporting it upwards with a large moisture convergence over the urban area. At around 1,000 m above the surface, the turbulent moisture flux takes over the moisture transport to the cloud layer. A relative humidity budget shows the moisture flux in the upper boundary layer to be the largest contribution to the urban-rural differences in relative humidity.
Quarterly Journal of the Royal Meteorological Society, Oct 1, 2021
A recently developed, height-distributed urban drag parametrization is tested with the London Mod... more A recently developed, height-distributed urban drag parametrization is tested with the London Model, a sub-kilometre resolution version of the Met Office Unified Model over Greater London. The distributed-drag parametrization requires vertical morphology profiles in the form of height-distributed frontal-area functions, which capture the full extent and variability of building heights. London's morphology profiles are calculated and parametrized by an exponential distribution with the ratio of maximum to mean building height as the parameter. A case study evaluates the differences between the new distributed-drag scheme and the current London Model setup using the MORUSES urban land-surface model. The new drag parametrization shows increased horizontal spatial variability in total surface stress, identifying densely built-up areas, high-rise building clusters, parks, and the river. Effects on the wind speed in the lower levels include a lesser gradient and more heterogeneous wind profiles, extended wakes downwind of the city centre, and vertically growing perturbations that suggest the formation of internal boundary layers. The surface sensible heat fluxes are underpredicted, which is attributed to difficulties coupling the distributed momentum exchange with the surface-based heat exchange. K E Y W O R D S drag parametrization, MORUSES, regional and mesoscale modelling, urban canopy model, urban meteorology, urban morphology analysis 1 INTRODUCTION Urban environments alter aerodynamic, radiative, thermal, and hydrological processes, which can intensify heat waves, flash floods, and air pollution. Accurate urban models are necessary for better warnings of severe weather hazards and to improve weather forecasts and services in the most populated areas in the world. Moreover, urban climate models are crucial for planning how to adapt cities for more extreme weather and how to transform This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Air quality in cities is influenced not only by emissions and chemical transformations but also b... more Air quality in cities is influenced not only by emissions and chemical transformations but also by the physical state of the atmosphere which varies both temporally and spatially. Increasingly, tall buildings (TB) are common features of the urban landscape, yet their impact on urban air flow and dispersion is not well understood, and their effects are not appropriately captured in parameterisation schemes. Here, hardware models of areas within two global mega-cities (London and Beijing) are used to analyse the impact of TB on flow and transport in isolated and cluster settings. Results show that TB generate strong updrafts and downdrafts that affect street-level flow fields. Velocity differences do not decay monotonically with distance from the TB, especially in the near-wake region where the flow is characterised by recirculating winds and jets. Lateral distance from an isolated TB centreline is crucial, and flow is still strongly impacted at longitudinal distances of several TB heights. Evaluation of a wake-flow scheme (ADMS-Build) in the isolated TB case indicates important characteristics are not captured. There is better agreement for a slender, shorter TB than a taller non-cuboidal TB. Better prediction of flow occurs horizontally further away and vertically further from the surface. TB clusters modify the shape of pollutant plumes. Strong updrafts generated by the overlapping wakes of TB clusters lift pollutants out of the canopy, causing a much deeper tracer plume in the lee of the cluster, and an elevated plume centreline with maximum concentrations around the TB mean height. Enhanced vertical spread of the pollutants in the near-wake of the cluster results in overall lower maximum concentrations, but higher concentrations above the mean TB height. These results have important implications for interpreting observations in areas with TB. Using real world ceilometer observations in two mega-cities (Beijing and Paris), we assess the
Journal of Atmospheric and Oceanic Technology, Apr 1, 2017
To investigate the boundary layer dynamics of the coastal megacity Shanghai, China, backscatter d... more To investigate the boundary layer dynamics of the coastal megacity Shanghai, China, backscatter data measured by a Vaisala CL51 ceilometer are analyzed with a modified ideal curve fitting algorithm. The boundary layer height z i retrieved by this method and from radiosondes compare reasonably overall. Analyses of mobile and stationary ceilometer data provide spatial and temporal characteristics of Shanghai's boundary layer height. The consistency between when the ceilometer is moving and stationary highlights the potential of mobile observations of transects across cities. An analysis of 16 months of z i measured at the Fengxian site in Shanghai reveals that the diurnal variation of z i in the four seasons follows the expected pattern; for all seasons z i starts to increase at sunrise, reflecting the influence of solar radiation. However, the boundary layer height is generally higher in autumn and winter than in summer and spring (mean hourly averaged z i for days with low cloud fraction at 1100-1200 local time are 900, 654, 934, and 768 m for spring, summer, autumn, and winter, respectively). This is attributed to seasonal differences in the dominant meteorological conditions, including the effects of a sea breeze at the near-coastal Fengxian site. Given the success of the retrieval method, other ceilometers installed across Shanghai are now being analyzed to understand more about the spatial dynamics of z i and to investigate in more detail the effects of prevailing mesoscale circulations and their seasonal dynamics.
International Journal of Biometeorology, Feb 6, 2016
The effect of variations in land cover on mean radiant surface temperature (Tmrt) is explored thr... more The effect of variations in land cover on mean radiant surface temperature (Tmrt) is explored through a simple scheme developed within the radiation model SOLWEIG. Outgoing longwave radiation is parameterised using surface temperature observations on a grass and an asphalt surface, whereas outgoing shortwave radiation is modelled through variations in albedo for the different surfaces. The influence of surface materials on Tmrt is small compared to the effects of shadowing. Nevertheless, altering ground surface materials could contribute to a reduction on Tmrt to reduce the radiant load during heatwave episodes in locations where shadowing is not an option. Evaluation of the new scheme suggests that despite its simplicity it can simulate the outgoing fluxes well, especially during sunny conditions. However, it underestimates at night and in shadowed locations. One grass surface used to develop the parameterisation, with very different characteristics compared to an evaluation grass site, caused Tmrt to be underestimated. The implications of using high resolution (e.g. 15 minutes) temporal forcing data under partly cloudy conditions are demonstrated even for fairly proximal sites.
Despite inter-building longwave radiative exchanges playing an important role in determining buil... more Despite inter-building longwave radiative exchanges playing an important role in determining building energy and environmental performance, simulation tools (e.g. EnergyPlus) simplify this by assuming the surface temperature of surrounding buildings to be equal to the air temperature, and therefore cause bias. Here we propose a ‘spin-up’ approach to update building external surface temperature using either air or the isolated building temperatures. Neighbourhoods with different plan area fraction of buildings (λP) are analysed to assess the impact on building external surface temperatures, cooling and heating energy demand as well as indoor overheating degree hours. Using the default EnergyPlus method causes a large bias in all metrics in a dense urban area (λP = 0.6) and climates assessed (cf. the new method): external wall temperature (3 °C less, midday median), annual energy demand for cooling (17.1% less) and heating (6.2% higher), annual overheating degree hours during the day (> 28 °C, 24.5% less) and night (> 26 °C, 60.1% less). These biases are larger at lower latitudes. Thus, neglecting the surroundings influence on inter-building longwave radiation impacts critical design considerations of building energy and thermal performance in dense urban areas
Observations at the Mauna Loa Observatory, Hawaii, established the systematic increase of anthrop... more Observations at the Mauna Loa Observatory, Hawaii, established the systematic increase of anthropogenic CO 2 in the atmosphere. For the same reasons that this site provides excellent globally averaged CO 2 data, it may provide temperature data with global significance. Here, we examine hourly temperature records, averaged annually for 1977-2006, to determine linear trends as a function of time of day. For night-time data (22:00 to 06:00, LST (local standard time)) there is a near-uniform warming of 0.040 • C y −1. During the day, the linear trend shows a slight cooling of −0.013 • C y −1 at 12:00 (noon, LST). Overall, at Mauna Loa Observatory, there is a mean warming trend of 0.021 • C y −1. The dominance of night-time warming results in a relatively large annual decrease in the diurnal temperature range (DTR) of −0.050 • C y −1. These trends are consistent with the observed increases in the concentrations of CO 2 and its role as a greenhouse gas, and indicate the possible relevance of the Mauna Loa temperature measurements to global warming.
The integrated wind risk warning model for rail transport presented has four elements: Background... more The integrated wind risk warning model for rail transport presented has four elements: Background wind data, a wind field model, a vulnerability model, and a risk model. Background wind data uses observations in this study. Using the wind field model with effective surface roughness lengths, the background wind data are interpolated to a 30-m resolution grid. In the vulnerability model, the aerodynamic characteristics of railway vehicles are analyzed with CFD (Computational Fluid Dynamics) modelling. In the risk model, the maximum value of three aerodynamic forces is used as the criteria to evaluate rail safety and to quantify the risk level under extremely windy weather. The full model is tested for the Shanghai Metro Line 16 using wind conditions during Typhoon Chan-hom. The proposed approach enables quick quantification of real-time safety risk levels during typhoon landfall, providing sophisticated warning information for rail vehicle operation safety.
Often the meteorological forcing data required for urban hydrological models are unavailable at t... more Often the meteorological forcing data required for urban hydrological models are unavailable at the required temporal resolution or for the desired period. Although reanalysis data can provide this information, the spatial resolution is often coarse relative to cities, so downscaling is required prior to use as realistic forcing. In this study, WATCH WFDEI reanalysis data are used to force the Surface Urban Energy and Water balance Scheme (SUEWS). From sensitivity tests in two cities, Vancouver and London with different orography, we conclude precipitation is the most important meteorological variable to be properly downscaled to obtain reliable surface hydrology results, with relative humidity being the second most important. Overestimation of precipitation in reanalysis data at the three sites gives 6-21 % higher annual modelled evaporation, 26-39 % higher runoff at one site and 4 % lower value at one site when compared to modelled values using observed forcing data. Application of a bias correction method to the reanalysis precipitation reduces the model bias compared to using observed forcing data, when evaluated using eddy covariance evaporation measurements.
Tower-based measurements from within and above the urban canopy in two cities are used to evaluat... more Tower-based measurements from within and above the urban canopy in two cities are used to evaluate several existing approaches that parametrize the vertical profiles of wind speed and temperature within the urban roughness sublayer (RSL). It is shown that current use of Monin-Obukhov similarity theory (MOST) in numerical weather prediction models can be improved upon by using RSL corrections when modelling the vertical profiles of wind speed and friction velocity in the urban RSL using MOST. Using anisotropic building morphological information improves the agreement between observed and parametrized profiles of wind speed and momentum fluxes for selected methods. The largest improvement is found when using dynamically-varying aerodynamic roughness length and displacement height. Adding a RSL correction to MOST, however, does not improve the parametrization of the vertical profiles of temperature and heat fluxes. This is expected since sources and sinks of heat are assumed uniformly distributed through a simple flux boundary condition in all RSL formulations, yet are highly patchy and anisotropic in a real urban context. Our results can be used to inform the choice of surface-layer representations for air quality, dispersion, and numerical weather prediction applications in the urban environment. Keywords Roughness sublayer • Temperature profile • Urban canopy • Wind profile 1 Introduction When modelling urban meteorological processes, it is crucial to represent the exchange of momentum and scalars such as temperature and humidity between the surface and overly-B Natalie E. Theeuwes
Urban areas are a hotspot for the interactions between the built environment, its inhabitants, an... more Urban areas are a hotspot for the interactions between the built environment, its inhabitants, and weather. Unlike the impact of temperatures through the well-known urban heat island effect, urban effects on cloud formation remain unknown. In this study we show observational evidence of a systematic enhancement of cloud cover in the afternoon and evening over two large metropolitan areas in Europe (Paris and London). Long-term measurements in and around London show that during late-spring and summer, even though less moisture is available at the surface and the atmosphere is drier, low clouds can persist longer over the urban area as vertical mixing of the available moisture is maintained for a longer period of time, into the evening transition. Our findings show that urban impacts on weather extend beyond temperature effects. These prolonged clouds over the city might enhance the urban heat island via night-time radiative forcing.
Quarterly Journal of the Royal Meteorological Society, Jul 1, 2018
The use of Automatic Lidars and Ceilometers (ALC) is increasingly extended beyond monitoring clou... more The use of Automatic Lidars and Ceilometers (ALC) is increasingly extended beyond monitoring cloud base height to the study of atmospheric boundary layer (ABL) dynamics. Therefore, long-term sensor network observations require robust algorithms to automatically detect the mixed layer height (Z ML). Here, a novel automatic algorithm CABAM (Characterising the Atmospheric Boundary layer based on ALC Measurements) is presented. CABAM is the first non-proprietary mixed layer height algorithm specifically designed for the commonly deployed Vaisala CL31 ceilometer. The method tracks Z ML , takes into account precipitation, classifies the ABL based on cloud cover and cloud type, and determines the relation between Z ML and cloud base height. CABAM relies solely on ALC measurements. Results perform well against independent reference (AMDAR: Aircraft Meteorological Data Relay) measurements and supervised Z ML detection. AMDAR-derived temperature inversion heights allow Z ML evaluation throughout the day. Very good agreement is found in the afternoon when the mixed layer height extends over the full ABL. However, during night or the morning transition the temperature inversion is more likely associated with the top of the residual layer. From comparison with SYNOP reports, the ABL classification scheme generally correctly distinguishes between convective and stratiform boundary-layer clouds, with slightly better performance during daytime. Applied to 6 years of ALC observations in central London, Kotthaus and Grimmond (2018), a companion paper, demonstrate CABAM results are valuable to characterise the urban boundary layer over London, United Kingdom, where clouds of various types are frequent.
The process of coupling the Surface Urban Energy and Water Scheme (SUEWS) into the Weather resear... more The process of coupling the Surface Urban Energy and Water Scheme (SUEWS) into the Weather research and forecasting (WRF) model is presented, including pre-processing of model parameters to represent spatial variability of surface characteristics. Fluxes and mixed layer height observations in the southern UK are used to evaluate a two-week period in each season. Mean absolute errors, based on all periods, are smaller in residential Swindon than central London for turbulent sensible and latent heat fluxes (Q H , Q E) with greater skill on clear days at both sites (for incoming and outgoing short-and 5 longwave radiation, Q H and Q E). Clear seasonality is seen in the model performance: with better absolute skill for Q H and Q E in autumn and winter, when there is a higher frequency of clear days, than in spring and summer. As the WRF-modelled incoming shortwave radiation has large errors, we apply a bulk transmissivity derived from local observations to reduce the incoming shortwave radiation input to the land surface scheme-this could correspond to increased presence of aerosols in cities. We use the coupled WRF-SUEWS system to investigate impacts of the anthropogenic heat flux emissions on boundary 10 layer dynamics by comparing areas with contrasting human activities (central-commercial and residential areas) in Greater London-larger anthropogenic heat emissions not only elevate the mixed layer heights but also lead to a warmer and drier near-surface atmosphere.
Journal of Advances in Modeling Earth Systems, Aug 1, 2020
Urban land surface processes need to be represented to inform future urban climate and building e... more Urban land surface processes need to be represented to inform future urban climate and building energy projections. Here, the single layer urban canopy model Town Energy Balance (TEB) is coupled to the Weather Research and Forecasting (WRF) model to create WRF-TEB. The coupling method is described generically, implemented into software, and the code and data are released with a Singularity image to address issues of scientific reproducibility. The coupling is implemented modularly and verified by an integration test. Results show no detectable errors in the coupling. Separately, a meteorological evaluation is undertaken using observations from Toulouse, France. The latter evaluation, during an urban canopy layer heat island episode, shows reasonable ability to estimate turbulent heat flux densities and other meteorological quantities. We conclude that new model couplings should make use of integration tests as meteorological evaluations by themselves are insufficient, given that errors are difficult to attribute because of the interplay between observational errors and multiple parameterization schemes (e.g., radiation, microphysics, and boundary layer). Plain Language Summary With increasing urbanization and climate change, estimates of possible future urban climate and building energy scenarios are needed. Weather models (e.g., Weather Research and Forecasting; WRF) provide the state of the atmosphere, and urban land surface models (e.g., Town Energy Balance; TEB) allow the interactions and feedbacks of people-buildings-atmosphere to be investigated. WRF-TEB is a free and open-source model aimed at the urban climate and energy community to investigate applications such as the energy consumption associated with air conditioners, or the generation of solar energy in cities. 1. Introduction With increasing urbanization (United Nations, 2019) and climate change (Collins et al., 2013), the study of urban atmospheric phenomena such as the spatial variation of temperature (Arnfield, 2003), or the impact of the urban environment on moisture (
Use of wind pressure coefficients to simulate natural ventilation and building energy for isolate... more Use of wind pressure coefficients to simulate natural ventilation and building energy for isolated and surrounded buildings. Building and Environment, 230. 109951.
The impact of heterogeneous uptake of HO 2 on aerosol surfaces on radical concentrations and the ... more The impact of heterogeneous uptake of HO 2 on aerosol surfaces on radical concentrations and the O 3 production regime in Beijing in summertime was investigated. The uptake coefficient of HO 2 onto aerosol surfaces, γ HO 2 , was calculated for the AIRPRO campaign in Beijing, in summer 2017, as a function of measured aerosol soluble copper concentration, [Cu 2+ ] eff , aerosol liquid water content, [ALWC], and particulate matter concentration, [PM]. An average γ HO 2 across the entire campaign of 0.070 ± 0.035 was calculated, with values ranging from 0.002 to 0.15, and found to be significantly lower than the value of γ HO 2 = 0.2, commonly used in modelling studies. Using the calculated γ HO 2 values for the summer AIRPRO campaign, OH, HO 2 and RO 2 radical concentrations were modelled using a box model incorporating the Master Chemical Mechanism (v3.3.1), with and without the addition of γ HO 2 , and compared to the measured radical concentrations. The rate of destruction analysis showed the dominant HO 2 loss pathway to be HO 2 + NO for all NO concentrations across the summer Beijing campaign, with HO 2 uptake contributing < 0.3 % to the total loss of HO 2 on average. This result for Beijing summertime would suggest that under most conditions encountered, HO 2 uptake onto aerosol surfaces is not important to consider when investigating increasing O 3 production with decreasing [PM] across the North China Plain. At low [NO], however, i.e. < 0.1 ppb, which was often encountered in the afternoons, up to 29 % of modelled HO 2 loss was due to HO 2 uptake on aerosols when calculated γ HO 2 was included, even with the much lower γ HO 2 values compared to γ HO 2 = 0.2, a result which agrees with the aerosol-inhibited O 3 regime recently proposed by Ivatt et al. (2022). As such it can be concluded that in cleaner environments, away from polluted urban centres where HO 2 loss chemistry is not dominated by NO but where aerosol surface area is high still, changes in PM concentration and hence aerosol surface area could still have a significant effect on both overall HO 2 concentration and the O 3 production regime. Using modelled radical concentrations, the absolute O 3 sensitivity to NO x and volatile organic compounds (VOCs) showed that, on average across the summer AIRPRO campaign, the O 3 production regime remained VOC-limited, with the exception of a few days in the afternoon when the NO mixing ratio dropped low enough for the O 3 regime to shift towards being NO x-limited. The O 3 sensitivity to VOCs, the dominant regime during the summer AIRPRO campaign, was observed to decrease and shift towards a NO x-sensitive regime both when NO mixing ratio decreased and with the addition of aerosol uptake. This suggests that if [NO x ] continues to decrease in the future, ozone reduction policies focussing solely on NO x reductions may not be as efficient as expected if [PM] and, hence, HO 2 uptake to aerosol surfaces continue to decrease. The addition of aerosol uptake into the model, for both the γ HO 2 calculated from measured data and when using a fixed value of γ HO 2 = 0.2, did not have a significant effect on the overall O 3 production regime across the campaign. While not important for this campaign, aerosol uptake could be important for areas of lower NO concentration that are already in a NO x-sensitive regime.
Journal of Applied Meteorology and Climatology, 2021
Urban heat island (UHI) and sea–land-breeze systems are well-known and important characteristics ... more Urban heat island (UHI) and sea–land-breeze systems are well-known and important characteristics of the climate of coastal cities. To model these, the accurate estimation of the surface energy balance (SEB) is a key factor needed to improve local-scale simulations of thermodynamic and dynamic boundary circulations. The Weather Research and Forecasting Model with a single-layer urban canopy model (WRF/SLUCM), with parameters derived from MODIS and local GIS information, is used to investigate the UHI and sea-breeze circulations (SBC) in the megacity of Shanghai. The WRF/SLUCM can reproduce observed urban radiation and SEB fluxes, near-surface meteorological variables, and the evolution of the UHI and SBC. Simulations for an August period show the maximum UHI tends to drift northwest in the afternoon, driven by the prevailing southeast wind. The sea breeze lasts for about 4 h and is strongest between 1200 and 1400 local time (UTC + 8 h). The interaction between UHI and SBC is evident ...
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Papers by Sue Grimmond