Land-atmosphere interactions

Irrigation is a widely used water management practice that is often poorly parameterized in land surface and climate models. Previous studies have addressed this issue via use of irrigation area, applied water inventory data, or soil moisture content. These approaches have a variety of drawbacks including data latency, accurately prescribing irrigation intensity, and a lack of conservation of water volume for models using a prescribed soil moisture approach. In this study, we parameterize irrigation fluxes using satellite observations of evapotranspiration (ET) compared to ET from a suite of land surface models without irrigation. We then incorporate the irrigation flux into the Community Land Model (CLM) and use a systematic trial-and-error procedure to determine the ground- and surface-water withdrawals that are necessary to balance the new irrigation flux. The resulting CLM simulation with irrigation produces ET that matches the magnitude and seasonality of observed satellite ET well, with a mean difference of 6.3 mm month−1 and a correlation of 0.95. Differences between the new CLM ET values and satellite-observed ET values are always less than 30 mm month−1 and the differences show no pattern with respect to seasonality. The results reinforce the importance of accurately parameterizing anthropogenic hydrologic fluxes into land surface and climate models to assess environmental change under current and future climates and land management regimes.

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.

Large-scale cultivation of perennial bioenergy crops (e.g., miscanthus and switchgrass) offers unique opportunities to mitigate climate change through avoided fossil fuel use and associated greenhouse gas reduction. Although conversion of existing agriculturally intensive lands (e.g., maize and soy) to perennial bioenergy cropping systems has been shown to reduce near-surface temperatures, unintended consequences on natural water resources via depletion of soil moisture may offset these benefits. The hydroclimatic impacts associated with perennial bioenergy crop expansion over the contiguous United States are quantified using the Weather Research and Forecasting Model dynamically coupled to a land surface model (LSM). A suite of continuous (2000–09) medium-range resolution (20-km grid spacing) ensemble-based simulations is conducted using seasonally evolving biophysical representation of perennial bioenergy cropping systems within the LSM based on observational data. Deployment is carried out only over suitable abandoned and degraded farmlands to avoid competition with existing food cropping systems. Results show that near-surface cooling (locally, up to 58C) is greatest during the growing season over portions of the central United States. For some regions, principal impacts are restricted to a reduction in near-surface temperature (e.g., eastern portions of the United States), whereas for other regions deployment leads to soil moisture reduction in excess of 0.15–0.2 m 3 m 23 during the simulated 10-yr period (e.g., western Great Plains). This reduction (;25%–30% of available soil moisture) manifests as a progressively decreasing trend over time. The large-scale focus of this research demonstrates the long-term hydroclimatic sustainability of large-scale deployment of perennial bioenergy crops across the continental United States, revealing potential hot spots of suitable deployment and regions to avoid.

Saharan dust is shown to enter the Central Amazon Basin (CAB) in bursts which accompany major wet season rain systems. Low-level horizontal convergence feeding these rain systems draws dust from plumes which have crossed the tropical Atlantic under the large-scale circulation fields. Mass exchange of air between the surface and 4km over the eastern Amazon basin is calculated using rawinsonde data collected during storm events. Mean concentrations of dust observed by aircraft over the western tropical Atlantic are used to calculate the amount of dust injected into the Basin. Individual storm events inject some 480,000 tons of dust into the north-eastern Amazon Basin. Storm and dust climatology suggest that the annual importation of dust is in the order of 13 Mtons. In the north-eastern basin, this may amount to as much as 190kg ha<img src="/entityImage/script/2212.gif" alt="-" border="0" style="font-weight: bold"></img>1 yr<img src="/entityImage/script/2212.gif" alt="-" border="0" style="font-weight: bold"></img>1. Deposition of trace species, such as phosphate, associated with this dust ranges from 1-4kg ha<img src="/entityImage/script/2212.gif" alt="-" border="0" style="font-weight: bold"></img>1 yr<img src="/entityImage/script/2212.gif" alt="-" border="0" style="font-weight: bold"></img>1. Uncertainties in these estimates are not believed to be greater than ± 50% and may be as low as±20%. The deposition fluxes from Saharan dust are essentially identical to the CAB wet deposition fluxes from precipitation in the wet season; a result that implies that the major ionic composition of rain water in the CAB wet season may be strongly influenced by inputs of material originating on the African continent nearly 5000km away. The total amount of Saharan dust calculated to enter the Amazon basin is 1/2 to 1/3 of that estimated to cross 60°W longitude between 10° and 25°N latitude. We conclude that part of the productivity of the Amazon rain forest is dependent upon critical trace elements contained in the soil dust originating in the Sahara/Sahel. This dependence should be reflected by expansions and contractions of the Amazon rain forest in direct relationship to expansions and contractions of the Sahara/Sahel. Turnover rates for nutrient species deposited with Saharan dust in the Amazon Basin suggest a time scale of 500 to 20,000years. We believe the dependence of one large ecosystem upon another separated by an ocean and coupled by the atmosphere to be fundamentally important to any view of how the global system functions. Any strategy designed to preserve the Amazonian rain forest or any part thereof should equally concern itself with the inter-relationship between the rain forest, global climate and arid zones well removed from Amazonia.

The coastal belt in the southwestern Bangladesh is facing a unique problem compared to other parts of the country because of the direct interaction of the saline surface water (SW) from the coastal rivers and nearby sea with the fresh groundwater (GW) in the underlying aquifers. In this study, an integrated river-aquifer model is developed for the shallow unconfined aquifer in the southwestern Bangladesh to simulate the GW flow characteristics and dynamic flow exchanges between the rivers and aquifer systems. Well-known USGS numerical code for GW flow simulation, MODFLOW and its river simulation package (RIV) are applied in the framework of GW Modeling System (GMS) software, which is satisfactorily calibrated against the observed water levels. Simulation result demonstrates that fluctuations of hydraulic heads are dependent on seasonal variations of recharge from precipitation and riverbed leakage. In addition, the river-aquifer relationships proved to be very responsive to water ta...

Generally, surface and air temperatures seem closely related but we show that they respond differently to evaporative conditions. We evaluate the temperature increase in response to solar radiation for different evaporative fractions, using observations from the Southern Great Plains. The warming rate of air temperature decreases only by 1.7 × 10 −3 K/(W m −2) from dry to moist conditions compared to a stronger reduction by 14 × 10 −3 K/(W m −2) for surface temperature. The weaker response of air temperature to evaporative fraction is explained by the larger growth of boundary layer on drier days, which suppresses the warming of air. Estimates based on this explanation reproduce the warming rate of air temperature in observations. Our results show that diurnal variations of surface temperatures contain imprints of evapotranspiration while air temperatures do not. These findings appear important to be considered when using, analyzing, or interpreting temperature data in studies dealing with climate change, hydrology, or land-atmosphere interactions. Plain Language Summary Surface and air temperatures are measured just 2 m apart, so it might seem that they carry the same information. Here we show that these temperature measurements respond rather differently to whether the surface evaporates or not. To show this, we use data from a well-equipped measurement site in the Central United States and calculate the rates by which the surface and the air warm in response to solar radiation. We found that the surface temperature responds about 8 times stronger to evaporation than air temperature. We explain this weaker response of air temperature by the stronger growth of the boundary layer without evaporation. This results in a deeper, well-mixed boundary layer that does not warm as strongly. What our results imply is that evaporation may be inferred from surface temperatures, but not from air temperatures.

Abstract Knowledge of soil–vegetation–atmosphere energy exchange processes is essential for examining the response of agriculture to changes in climate in both the short and long term. However, there are relatively few sites where all the flux measurements necessary for evaluating these responses are available; where they exist, data are often incomplete and/or of limited duration. At the same time, there is often an extensive observation network available that has gathered key meteorological data (sunshine, wind, rainfall, etc.) over decades. Simulating the terms of the surface energy balance (SEB) using available meteorological, soil and vegetation data can improve our understanding of how agricultural systems respond to climate and how this response will vary spatially. Here, we employ a physically-based scheme to simulate the SEB fluxes over a mid-latitude, maritime temperate environment using routine weather observations. The latent heat flux is a critical SEB term as it incorporates the response of the plant to environmental conditions including available energy and soil water. This response is represented in modeling schemes through surface resistance (rs), which is usually expressed as a function of near-surface water vapor alone. In this study, we simulate the SEB over two grassland sites, where eddy flux observations are available, representing imperfectly- and poorly- drained soils. We employ three different formulations of rs, representing varying degrees of sophistication, to estimate the surface fluxes. Due to differences in soil moisture characteristics between the sites, we ultimately focused our attention on an rs formulation that accounted for soil water retention capacity, based on the Jarvis conductance model; the results at both hourly and daily intervals are in good agreement, with RMSE values of ≈ 40 W m−2 for sensible and latent heat fluxes at both sites. The findings show the potential value of using routine weather observations to generate the SEB where flux observations are not available and the importance of soil properties in estimating surface fluxes. These findings could contribute to the assessment of past and future climate change on grassland ecosystems.

This study investigates land-atmosphere coupling over South Asia during the pre-monsoon and monsoon seasons. We conducted three ensemble simulations using the Weather Research and Forecasting (WRF) model (version 3.8.1) as a regional climate model (RCM) coupled with the Noah land surface model (LSM). The default WRF configuration overestimates the roughness length for heat over short vegetation and underestimates the same metric over tall vegetation, leading to a larger sensible heat flux over all South Asia in both aforementioned seasons. This overestimated (underestimated) the strength of the land-atmosphere coupling over larger (smaller) sensible heat flux regions. Dynamically updating the roughness length for heat as a function of canopy height in the LSM results in an error reduction for this variable for different vegetation types. This leads to weaker land-atmosphere coupling for certain regions, resulting in a lower sensible heat flux, in both the pre-monsoon and monsoon seasons. This reduction of sensible heating affects the formation of convection over those regions.