H11A-0711
Spectral Behavior of a Linearized Land-Atmosphere Model: Applications to Hydrometeorology
The present study develops an improved version of the linearized land-atmosphere model first introduced by
Lettau (1951). This model is used to investigate the spectral response of land-surface variables to a daily
forcing of incoming radiation at the land-surface. An analytical solution of the problem is found in the form of
temporal Fourier series and gives the atmospheric boundary-layer and soil profiles of state variables
(potential temperature, specific humidity, sensible and latent heat fluxes). Moreover the spectral dependency
of surface variables is expressed as function of land-surface parameters (friction velocity, vegetation height,
aerodynamic resistance, stomatal conductance).
This original approach has several advantages:
First, the model only requires little data to work and perform well: only time series of incoming radiation at the
land-surface, mean specific humidity and temperature at any given height are required. These inputs being
widely available over the globe, the model can easily be run and tested under various conditions.
The model will also help analysing the diurnal shape and frequency dependency of surface variables and
soil-ABL profiles. In particular, a strong emphasis is being placed on the explanation and prediction of
Evaporative Fraction (EF) and Bowen Ratio diurnal shapes. EF is shown to remain a diurnal constant under
restricting conditions: fair and dry weather, with strong solar radiation and no clouds. Moreover, the EF
pseudo-constancy value is found and given as function of surface parameters, such as aerodynamic
resistance and stomatal conductance.
Then, application of the model for the conception of remote-sensing tools, according to the temporal
resolution of the sensor, will also be discussed.
Finally, possible extensions and improvement of the model will be discussed.
H11A-0712
The Atmospheric Effects of Selective Logging in the Amazon – An LES Case Study
The recently developed regional atmospheric modeling system (RAMS)-based forest large-eddy simulation (RAFLES) was dynamically coupled with the Ecosystem Demography model (ED2) as a transpiration, surface energy, light attenuation and CO2 flux module. This modeling system is used to explore the effects of micro- scale structural changes to canopy structure due to selective logging in Tapajos Park in the Amazon. 3-D high resolution structure maps of undisturbed-control and selectively logged regions are interpreted from ICONOS images and ground observations. Large eddy simulations of two case studies at 5x5x4 m resolution are conducted: one represents a case with common dry season sheared boundary layer; the other represents a hypothetical purely convective boundary layer. In each case, simulation results using observed canopy structures representing a 2x2 km section of undisturbed and selectively logged forests are compared with artificial homogeneous canopies with the same mean leaf densities. Micro-scale changes to the canopy structure impact the domain averaged roughness properties, the effective drag coefficients, and the displacement height. These results can be used to parameterize the effects of selective logging on the surface parameterization and particularly the surface-layer resistance to transpiration water flux in regional and global models, which cannot explicitly resolve these effects due to their coarse resolution. Selective logging also leads to higher-order effects pertaining to the spatial statistics of the ejection-sweep cycle and lateral advection of scalars. By modifying the correlations between micro-scale canopy features and flow statistics and fluxes, and by causing increased lateral scalar advection at a small localized scale associated with particular structural features, micro-scale changes to canopy structure may pose a challenge for eddy- flux measurements in disturbed areas. LES simulation of disturbed canopies can be used to evaluate the potential error due to crown-scale regions of preferred ejection and sweep activity and the resulting spatial patterns of advection of water and CO2.
H11A-0713
Relationship between the vegetation cover and the water balance in non-humid regions of China
The growth of vegetation is affected by intermittent water availability, while it also feeds back to influence regional water balance. A better understanding of the relationship between vegetation state and water balance will be useful for the explanation of the complicated interactions among climate changes, vegetation dynamics, and water cycles. In the present study, the effect of regional climate on vegetation coverage and average annual water balance is analyzed under the framework of the Budyko's curve for the 99 catchments in the non-humid regions of China, including the Inland River basin, the Haihe River basin, and the Yellow River basin. The distribution of vegetation coverage on the Budyko curve is analyzed, and it is found that a wetter environment (higher P/E0) has higher vegetation coverage (M) associated with higher evapotranspiration efficiency (E/E0). A positive correlation between water balance component (E/P) and vegetation coverage is found for most of the Yellow River basin and the Inland River basin, while a negative correlation of M~E/P is found in the Hai River basin. Using the simplified Eagleson's annual water balance model, it is shown that the mean vegetation coverage in the study areas is well correlated with the meteorological factors, thus confirming the Eagleson's equation. Investigation of the relationship between mean vegetation coverage and soil moisture is attempted through water balance analysis. The result indicates that only 1% of the soil moisture difference can result in about 3.2% difference in vegetation coverage for the study areas. The present research also attempts to build up the leakage between the Budyko curve and the Eagleson's annual water balance equation for better understanding the vegetation- climate-water cycle interaction and predicting change of vegetation coverage due to the climate change and human activity.
H11A-0714
An Investigation of the Effects of Vegetation on Radio Signal Propagation Through a Corn Field
As part on of an ongoing research effort, seven rain gauge platforms are deployed in a 1×1-km corn field, near Ames, Iowa. Each site transmits its data to The University of Iowa via a cellular modem operating at 2.4 GHz, at 15-minunte intervals. To monitor communications quality, the system records the received signal at each site in the field every 15 minutes. Though not collected for this purpose, the signal strength data, gathered between mid May and the end of October 2007 provide an opportunity to investigate how vegetation and water moisture in the atmospheric boundary layer affect radio signal propagation. The signal strength data are quite noisy, yet one can clearly identify a long-term trend where the signal strength decreases as the growing season progresses and the plants progressively obscure the radios' view to the cell towers. At the end of the growing season, as the plants dry out and are finally harvested, the signal strength recovers. One can explain this recovery in terms of the water-content in the plants. This is supported by the measured plant water content at different times and selected locations in the field. Spectral and other analysis of the signal strength data also reveal a subtle, yet clear diurnal variation where the signal strength from all sites in the field reaches a minimum between 1 p.m. and 2 p.m. local time. This may be explained by water vapor in the atmospheric boundary layer, probably driven by plant transpiration. However, the maximum signal strength occurs at night, but at different times for different stations. This is still being investigated.
H11A-0715
Changing pan evaporation and its attribution in China
Pan evaporation is an indicator of evaporation demand, which has declined in many regions over the past several decades. It is important to understand why and what dominant factors controlling pan evaporation are. We collected daily climate data from 122 meteorological stations across China for the past 50 years, including pan evaporation (with a diameter of 20cm), solar irradiance and etc. We approached pan evaporation using Penman equation, and quantified effect of climatic factors (namely radiation, temperature, humidity and wind speed) on pan evaporation using its partial derivations. We have a primary analysis according to the data from 9 stations, namely Harbin in Northeast, Aletai and Ruoqiang in Northwest, Germu and Changdu in Qinghai-Tibet Plateau, Taiyuan in North China, Chengdu in Southwest, Hangzhou in East China, and Guangzhou in South China. The result shows that pan evaporation has a downward trend (except Chengdu station), especially Aletai, Changdu and Guangzhou at a rate of over -10mm/yr2. Regarding the factors controlling pan evaporation change, declining wind speed is the major factor in 3 north stations (Harbin, Ruoqiang and Germu), at about -3~-5mm/yr2; change in vapor press deficit is most important in Aletai and Hangzhou, at about -3mm/yr2 and 3mm/yr2 respectively; decreasing solar irradiance is the key factor in Taiyuan, Changdu, Chengdu and Guangzhou, at about -4~-5mm/yr2. Generally, changing pan evaporation is controlled by weakening wind speed in the western part of China, and by declining solar irradiance in the eastern part. Further analysis on other stations will help to reveal regional variation in climatic drives of changing hydrologic cycle. In addition, comparing with the observed data, we found that decrease in net shortwave radiation was underestimated using empirical formula recommended in the Irrigation and Drainage Paper 56 by FAO. It is speculated that changing albedo and aerosols concentration have been altering regional energy balance.
H11A-0716
Processes investigation on evapotranspiration on steppe in semi-arid region, Mongolia
Mongolia territory, locating in semi-arid at mid-latitude, is covered dominantly by grassland more than 80%. On such semi-arid grassland, land surface is anticipated directly to affects the atmosphere at the local to regional scale primarily through the energy and water vapor transferred to the atmosphere. These fluxes are governed most strongly by vegetation and surface moisture and therefore are extremely sensitive to changes in terrestrial hydrologic processes. Since July 2002, intensive observation, including micrometeorology, phenology, heat budget and soil heat/water condition, has been conducted at site on sparse grassland in Mongolia. In this work, variability of evapotranspiration was investigated by assessing to soil water and atmospheric status. The parameterization of soil evaporation using soil surface moisture provide useful tool to partitioning soil evaporation and transpiration to evapotranspiration. Verification by micro-Lysimeter observation on calculating soil evaporation and evapotranspiration shows the partitioning was reasonable in study region. The seasonality of evapotranspiration on such semi-arid sparse grassland was briefly straiten by soil moisture relating snow melt and precipitation events. In the observation period from July 2002 to June 2006 consisting two cycle of cross pre-growth to senescence period, partition of transpiration was 22%. Corresponding to heat fluxes seasonality, evapotranspiration sensitive to precipitation (ground surface moisture). Ratio of transpiration to evapotranspiration was averaged to be 14% in the moister grass-growing period of 2003, but was 38.5% in drier early-growth of 2004 and was 40.9% in drier peak growth of 2002, which is anticipated to determined by high leaf conductance in drier period and lower leaf-to-air specific deficit in moister period. During soil drought events, the magnitude of the decrease in transpiration by phreatophytes typically is slight because they are deep rooted and obtain their water from near the water table rather than from the overlying soil zone.
H11A-0717
Estimates of Flood Inundation and Evaporation in the Niger Inland Delta Region using the JULES land-surface model
Observed river gauging data indicate significant evaporative losses from the land and water surface in the Niger Inland delta. These losses indicate an important potential feedback between the land-surface and atmosphere. Moreover, the reduction of flow downstream of the wetland has clear implications for water management in the region and beyond. Here we have modelled the land-atmosphere coupling in the Niger Inland Delta by adding an overbank flow parameterization to the Joint UK Land-Environment Simulator (JULES) land-surface model (Blyth et al., 2002). Our hydrological model comprises a probability-distributed model of soil moisture and runoff production (PDM; Moore, 2007) coupled with a discrete approximation to the 1D kinematic wave equation to route river water downslope (G2G; Bell et al., 2007). The model was driven using data from the ALMIP experiment (Boone et al., 2006). The model simulates the broad features of the observed river flow pattern, including a downstream attenuation of the flood-wave through the wetland region. The model results illustrate significant evaporative losses from the inundated region leading to a ~10 percent reduction in river flow. The greatest relative decreases in river flow occur during spring and summer low flows. Moisture flux from the inundated region is greatly increased, accounting for up to 50 percent of the total land-atmosphere water flux during periods of maximum flooding. Moreover, a surface temperature anomaly of up to -8 K was observed in the inundated region Further work is planned to use sub-grid-resolution topographic data to improve the representation of overbank flow in the model; to compare the extents of modelled and observed inundated areas using satellite microwave remote sensing; and to include wetland evaporation process in online climate simulations to investigate land-atmosphere feedbacks.
H11A-0718
Soil Heat Flow Model
The Penman-Monteith method for estimating evapotranspiration (ET) has been recommended by FAO. This method requires measures of temperature, wind speed, relative humidity and heat flow in the soil. This last variable is rarely available. Soil heat flow is generally small compared to the net radiation, and many times is ignored in the energy balance. Nevertheless, the addition or subtraction of this amount in the energy balance equation should be considered for evapo-transpiration calculation. Penman-Monteith method suggests approximate estimates of soil heat flows as the difference between the maxima and minimum daily temperatures multiplied by a convenient coefficient. However, such approach ignores important variations in this parameter occurring during the day, and could influence the accuracy of the result. This work proposes to estimate soil heat flows by means of a mathematical model that includes the estimate of soil temperatures profiles and heat flows as a function of thermal properties of the soil, such as difussivity and conductivity coefficients. The model calculates soil heat flows in three stages. The first estimates hourly air temperature based on the average daily temperature and Fourier series coefficients. The obtained hourly air temperature constitutes an input variable for the second stage of the model. Surface soil temperature is assumed to be equal to air temperature. The second stage, applies heat transfer principles, using the thermal properties of the soil in order to obtain the soil temperature profile in a one meter depth soil stratum. Finally, the results of the second stage are used to calculate the hourly heat flow in the soil and compare this estimate with other methods and with measured values. Calculated hourly temperatures reproduced observed values closely. Correlation coefficients between observed and calculated values for the three summer months are 0.98, 0.96 and 0.97. Hourly soil heat fluxes are also closely estimated, showing clear diurnal variations. Correlation coefficient for the entire study period between observed and estimated values is 0.96.
H11A-0719
A theoretical framework of the vertical discretized ground column for calculating skin temperature
The theoretical framework of the vertical discretization of a ground column for calculating Earth's skin temperature is presented. The suggested discretization is derived from the evenly heat-content discretization with the optimal effective thickness for layer-temperature simulation. For the same level number, the suggested discretization is more accurate in skin temperature as well as surface ground heat flux simulations than those used in some state-of-the-art models. A proposed scheme can reduce the normalized root-mean- square error (or RMSE/STD ratio) of the calculated surface ground heat flux of a cropland site significantly to 2% (or 0.9 W m-2), from 11% (or 5 W m-2) by a 5-layer scheme used in ECMWF, from 19% (or 8 W m-2) by a 5-layer scheme used in ECHAM, and from 74% (or 32 W m-2) by a single-layer scheme used in the UCLA GCM. Better accuracy can be achieved by including more layers to the vertical discretization. Similar improvements are expected for other locations with different land types since the numerical error is inherited into the models for all the land types. The proposed scheme can be easily implemented into state-of-the-art climate models for the temperature simulation of snow, ice and soil.
H11A-0720
Influence of Source/Sink Distributions to the Flux-Profile Relationships Observed in a Black Spruce Forest
The flux-profile relationships are fundamental to determine turbulent fluxes. In the surface layer, normalized gradients have been expressed as universal functions of the atmospheric stability. However, in the roughness sublayer, observed normalized gradients are systematically smaller than that expected from the universal functions, and there is no consensus to solve the problem. On the other hand, the normalized gradient becomes larger than that expected from the universal functions for the surface layer in the vicinity of source/sink (Coppin et al., 1986, Boundary-Layer Meteorol., 35, 167-191). Clarifying what affects the flux- profile relationships in the roughness sublayer is important to determine fluxes by the gradient method, especially for low concentration gasses. We examined seasonal variations of the flux-profile relationship using observed momentum, sensible heat, water vapor and carbon dioxide in a black spruce forest (64°52'N, 147°51'W) in Fairbanks, Alaska, USA. In this site, the source/sink distributions for water vapor and carbon dioxide changed from understory in early spring to overstory in summer, whereas those for momentum and sensible heat did not change significantly. The flux-profile relationships reflected the seasonal characteristics of the source/sink distributions of the canopy. The result suggests that when applying the gradient method to determine the flux, it is necessary to determine the turbulent transfer coefficient by conditionally selecting the data which reflected the same source/sink distributions.
H11A-0721
Turbulent Transport Simulation of Water Vapor over Non-homogeneous Terrain
This study presented a two-dimensional k-ε model in conjunction with the advection-diffusion equation to simulate the flow field and turbulent transport of water vapor over an irrigated non-uniform terrain (e.g., rice paddy) where a porous (forest) and impermeable (building) obstacle was encountered. In this study, porous obstacles were classified into three density levels: high, medium and low. The simulation results showed that the characteristics of flow field in high and medium porous density were obviously different from those in low density ones. The height of mixing layer and the accumulated value of water vapor concentration grew with the porous density. Furthermore, porous density levels also caused different turbulent transport effects in the flow field.
H11A-0722
Estimation of Regional-Scale Actual Evapotranspiration in Okayama prefecture in Japan using Complementary Relationship
It is important to estimate accurately a water balance in watershed for proposing a reuse of water resources and a proper settlement of water utilization. Evapotranspiration (ET) is an important factor of water balance. Therefore, it is needed to estimate accurately the actual ET. The objective of this study is to estimate accurately monthly actual ET in Yoshii, Asahi, and Takahashi River watersheds in Okayama prefecture from 1999 to 2000. The monthly actual ET was calculated by a Morton and a modified Brutsaert and Stricker (B&S) method, using Automated Meteorological Data Acquisition Systems (AMeDAS) in the basin. The actual ET was estimated using land covers which were classified in 11 categories. The land covers includes the effects of albedo. The actual ET was related to the elevation at each AMeDAS station. Using this relationship, the actual ET at the 1 or 5 km grid-interval mesh in the basin was calculated, and finally, the distribution of actual ET was mapped. The monthly ET estimated by the modified B&S method were smaller than that by Morton method which showed a same tendency as the Penman potential ET (PET). The annual values of Mortonfs ET, modified B&Sfs ET, and PET were estimated as 796, 645, and 800 mm, respectively. The ET by the modified B&S was larger in hilly and mountainous areas than in settlement or city. In general, it was a reasonable result because city or settlement areas were covered with concrete and asphalt and the ET was controlled.
H11A-0723
Hydrologic Dynamics During the Dry Season In a Steep Forested Catchment
We have made coordinated continuous high-frequency observations of the near surface microclimate and soil moisture in a steep forested watershed in Northern California (Angelo Coast Range Reserve) for nearly a year, from a relative mild wet season into a dry summer. The observations reveal several surprises. Soil moisture by the stream decreased more rapidly than that upslope at the start of the dry season, and reached a value that was slightly higher than that upslope as the dry season progressed, even though the ABL was cooler and moister by the stream. The systematics of the spatial and temporal variations in soil moisture cannot be explained by variations in evaporation associated with ABL temperature and relative humidity. This paper presents an estimate of evaporation derived from soil moisture extraction from the catchment and a hypothesis of the processes that maintain soil moisture upslope.
H11A-0724
Seasonal and intraseasonal variations in evaporation and surface energy budget from eddy covariance measurements over an open water surface in Mississippi, U.S.A.
Understanding seasonal and intraseasonal variations in evaporation over lake/reservoir is important for water resource management as well as predicting variations in hydrology as a result of climate change. Since August of 2007, we have conducted a long-term eddy covariance measurement of evaporation and the surface energy budget over Ross Barnett Reservoir (32o26'N, 90o02'W) in Mississippi, USA. The fetch for eddy covariance system exceeds 2 km in all directions and the water depth is about 4 m around the flux tower. The tower with its height of 4 m stands over a stationary wood platform with its size of 3 m ¡Á 3 m and height of about 1 m above the water surface. Along with sensible and latent heat fluxes, microclimate data are also measured, including wind speed, wind direction, relative humidity, solar radiation, net radiation, air temperature at four levels, water surface temperature, and water temperature at eight depths down to about 4 m. Mississippi is subject to frequent influences of different synoptic weather systems in a year around. Incursions of these different systems bring in air masses with different properties in temperature and moisture. Cold fronts, for example, carry them with cold and dry air from north while warm fronts with warm and moist air. Our results indicate that synoptic weather variations play an important role in controlling evaporations and the surface energy budget. For example, daily H and LE (i.e., evaporation) during the passages of cold fronts are around 2¨C4 times those of normal days and these cold front events lead to an increase in the seasonal H by approximately 420 and LE by 160%. However, the warm weather systems suppress largely the turbulent exchanges of sensible and latent heat, leading to very small evaporation and sensible heat fluxes (even negative). These results imply that future potential changes in cold front activities (intensity, frequency, and duration) as a result of climate change may lead to substantial shifts in regional energy budget and hydrological balance in the southern regions with an abundance of open water bodies (e.g., lakes, reservoirs, swamps etc). Using these datasets, the daytime and nighttime evaporation rates are also analyzed and nighttime evaporative water losses are substantial, contributing a significant portion to the total evaporative water loss.
H11A-0725
Towards Estimating and Validating Evapotranspiration Estimates at Global Scale
The measurement of climatic variables from space is essential for understanding the role of terrestrial hydrosphere-biosphere in the Earth's climate system. Amongst the climatic variables surface latent heating (or Evapotranspiration - ET) is often considered as the climate linchpin variable since it plays a vital link between the global hydrological and the energy cycles through surface evaporation and also links the water and carbon cycle through vegetation transpiration. These linkages operate over a range of fine (diurnal) to coarser (seasonal) time scales. However, no observational driven global ET datasets exist yet. To that end, we developed a remote sensing based ET product at continental scales. In the current study, two different algorithms are used to estimate ET at the global scale. The first algorithm is based on the surface energy balance approach (SEBS) in which the gradient between air- and surface temperature serves as the core parameterization of the surface heat fluxes. The second algorithm uses the Peman-Monteith (PM) combination equation to estimate latent heat fluxes. When SEBS required inputs (primarily surface temperature) are not available because of cloud cover, a PM estimate is implemented instead. To assure the consistency and accuracy of the mixed-model ET output, the PM based algorithm is calibrated to best fit the climatology of the SEBS retrievals. Input for both the algorithms are obtained from multiple sensors onboard the NASA EOS based AQUA satellite. The blended approach is applied to estimate ET for 2003-2006 at 10-km spatial resolution over land areas of Earth using remotely sensed datasets that include CERES coarse resolution (25 km) surface radiation, AIRS surface meteorology and surface temperature (20 km), and MODIS and AVHRR based vegetation (8-km) and land-use (1 km) data, when available. To demonstrate the validity of the product, the estimates are compared with eddy covariance based datasets available from the FLUXNET regional network of towers.
H11A-0726
Lake-atmosphere exchanges: the LATEX field experimental campaign
Understanding the interaction of the atmosphere with underlying water surfaces is of great importance for a wide range of scientific fields such as water resources management, climate simulations and change impact studies, and regional weather forecasting in coastal areas. However, atmospheric dynamics over water surfaces have generally received less attention than land-atmosphere interactions, partially due to logistical difficulties in operating in-situ field studies. The Lake-Atmosphere Turbulent EXchanges (LATEX) field measurement was designed to address the issues of air-water interactions over lakes. The experiment was performed over Lake Geneva (Switzerland) on a 10 meter high tower situated 100 meters offshore. The main instrumentation consisted of a vertical array of four sonic anemometers and four open path gas analyzers measuring wind speed, temperature, and humidity at 20 Hz. Additional supporting measurements included net radiation, water surface temperature, relative humidity and temperature of air, and wave height and speed. The diurnal trends of momentum, heat, and water vapor fluxes for the whole experimental period are presented and several evaporation models of varying complexity are tested. A new model based on the Penman approach and sensible heat flux measurement is also proposed and tested. The roughness lengths of the surface (for momentum, heat, and water vapor) are investigated. The focus is then turned to the coherent structures over the lake and results from a quadrant analysis for momentum, heat and water vapor fluxes are analyzed. Under neutral and stable stratification, ejections and sweeps contribute equally to the vertical fluxes; as the atmospheric boundary layer turns to unstable, ejections begin to clearly dominate.
H11A-0727
Raman lidar measurements of temperature and humidity Internal boundary layer profiles over Lake Seedorf, Switzerland
The Seedorf Evaporation Experiment was designed to measure internal boundary layers which form over changes in roughness, humidity and temperature surface conditions. Situated in an agricultural region with a small lake (300 m), a state of the art scanning raman LIDAR was deployed in the summer of 2008 along with scintillometers, sodar/rass, and turbulence flux measurements to observe internal boundary layer development. Presented are comparisons of first observations and model results.
H11A-0728
Formation of Surface Hoar in Alpine Terrain
The formation of surface hoar, which takes predominantly place during cold and clear nights over snow covered surfaces, is particularly important for avalanche formation since a buried surface hoar layer forms very often a significant weak layer. We present measurements and simulations of surface hoar formation at an Alpine study plot in the Swiss Alps. It is first shown, that surface hoar formation is quantitatively explained by measured latent heat fluxes. The heat fluxes are successfully modelled with the snow cover model SNOWPACK, which describes surface fluxes with a bulk approximation of Monin-Obukhov similarity theory. Using the model, it is then shown that larger wind speeds typically stop surface hoar formation because of increased sensible heat flux to the surface, which prevents significant latent heat flux to the surface. This corresponds to observations and offers an explanation different from the often cited mechanical destruction of surface hoar at higher winds. It is thus hypothesised that hoar formation over complex snow covered surfaces is often a result of local energy and moisture budgets. This explains the observed discontinuity of surface hoar layers.
H11A-0729
Isolating Effects of Water Table Dynamics, Terrain, and Soil Moisture Heterogeneity on the Atmospheric Boundary Layer Using Coupled Models
There is a growing body of literature recognizing the connection between atmospheric boundary layer processes and surface and subsurface heterogeneity and flow, but much remains unknown about the nature of these hydrologic feedbacks. In recent work, the three-dimensional, variably saturated groundwater model ParFlow (PF) was coupled to the three-dimensional mesoscale atmospheric model ARPS (Advanced Regional Prediction System). The coupled model, PF.ARPS, was used to demonstrate atmosphere-land surface-subsurface interactions for a watershed in Oklahoma. In the current work, this coupled model is used to study effects of water table dynamics and soil moisture heterogeneity on the development and behavior of the atmospheric boundary layer for a set of idealized test cases. Numerical experiments that isolate the effects of subsurface heterogeneity, terrain, soil moisture initialization, and atmospheric conditions are performed. Detailed soil moisture distributions from offline spinups using ParFlow coupled to the Common Land Model (PF.CLM) are used to initialize idealized PF.ARPS runs. Results indicate that the water table becomes more dynamic as subsurface heterogeneity increases. This is reflected in soil moisture profiles and thus energy fluxes and evaporation at the land surface. Our results also illustrate the role of terrain in inducing differential land surface heating and cooling which stimulates the development of convective circulations in addition to those induced from heterogeneous soil moisture. Subsequent effects on atmospheric boundary layer development are discussed.
H11A-0730
Coupled hydrologic modeling for soil moisture initialization of high-resolution atmospheric simulations over complex terrain
Atmospheric flow over steep, mountainous terrain is particularly affected by variations in soil moisture due to thermal heating effects. Previous work using the Advanced Regional Prediction System (ARPS) for simulations of flow over Owens Valley in California have demonstrated the model's sensitivity to soil moisture and even snow cover. Standard initialization procedures for mesoscale models often interpolate surface conditions from coarse grids. This can lead to misrepresentation of surface variables due to the inability of the coarse grid to resolve finer scale topographic features, an issue which is especially important in complex terrain. For example, 32 km North American Regional Reanalysis (NARR) does not resolve the terrain of Owens Valley (width ~12 km), so that soil moisture and snow cover from NARR do not reflect the potentially drastic differences in surface conditions on the mountain tops versus the valley floor. In addition to resolution issues, most land-surface models used in mesoscale atmospheric modeling do not allow for lateral flow of water from grid cell to grid cell. This isolation of cells from their neighbors prohibits such models from including the effects of topography on the distribution of soil moisture. Groundwater recharge is also often neglected but can be an important factor nonetheless in determining the soil moisture distribution over complex terrain. We have run a 3D coupled hydrologic model in an offline multi-year spin-up procedure to obtain a more accurate initial soil moisture distribution for ARPS. The coupled hydrologic model, PF.CLM, consists of ParFlow, a variably saturated groundwater model with integrated overland flow, coupled to the Common Land Model (CLM). Our case study site is a 2,500 km 2 segment of the Owens Valley, a region of complex terrain east of the Sierras in California and the site of the Terrain-Induced Rotor Experiment (T-REX) in March and April, 2006. In previous work, PF.CLM was run with very simple subsurface geology for the valley region. The current study employs more accurate subsurface geology, complex terrain including steep mountain slopes and alluvial valleys, and fine spatial resolution that includes surface soil and vegetation types derived from ARPS. Results from the coupled model are compared to observations of soil moisture measured during the T-REX field campaign as well as to standard soil moisture initialization data from NARR. Due to the horizontal extent of the simulation domain (50 km by 50 km at 350 m resolution), as well as the steepness and complexity of the terrain (~4 km vertical extent and ~1 m resolution), the PF.CLM computational domain is very large, and requires high-performance parallel computing. Additionally, the ARPS simulations are run in a horizontal nested grid configuration, making this study a multi-step project that includes not only the challenge of accurately representing diverse physical processes in complex terrain, but also high-performance computing challenges.
H11A-0731
Potential Linkages from the Water Table to Soil Moisture to ET to Precipitation and so on
We present observed and simulated water table, simulated soil moisture and ET over North America to explore their linkages and feedbacks. Our results show that the water table is shallow in large regions of the continent and can influence soil moisture, ET and convective precipitation, which in turn feeds back to soil moisture and water table. We will also present our preliminary results over South America where the shallow water table may have important implications to the Amazon water cycle and ecosystem dynamics.
H11A-0732
Estimating Shortwave Solar Radiation Using Net Radiation and Meteorological Measurements
Shortwave radiation has a wide variety of uses in land-atmosphere interactions research. Actual evapotranspiration estimation that involves stomatal conductance models like Jarvis and Ball-Berry require shortwave radiation to estimate photon flux density. However, in most weather stations, shortwave radiation and net radiation are not measured. Nevertheless, if one is measured, the other can be estimated. Net radiation is the difference between downward and upward radiative fluxes, including shortwave and long- wave radiation. The objective of this study was to develop and evaluate a shortwave radiation estimation model as a function of measured net radiation, air and soil temperatures, and relative humidity. The model was written in Visual Basic 6 to automate all calculations. A statistical comparison of measured and estimated shortwave radiation showed excellent agreement with R2 of 0.98 and a root mean square error of 4%.