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Land-Surface Processes
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G. Land-Surface/Atmospheric Interactions and Their Modeling
[Background] [IHOP/NCAR
Soil Moisture]
[Land
Surface Model Development]
[High
resolution land data assimilation system]
1. Background
RAP's objectives in this area are to understand, through theoretical and observational studies, the complex interactions (including biophysical, hydrological, and bio-geochemical interactions) between the land-surface and the atmosphere at micro- and meso-scales, and to improve land-surface models. The ultimate goal is to integrate such knowledge into numerical mesoscale weather prediction and regional climate models in order to improve prediction of the impacts of land-surface processes on regional weather, climate, and hydrology.
2. IHOP/NCAR Soil Moisture, Soil Temperature, and Vegetation Observation Network
The overall goal of International H2O Project (IHOP 2002) is to understand the distribution and variability of water vapor in time and space. A critical component of this mission to is investigate: 1) the variability of water vapor near the surface as function of land-use type and the vegetation growing cycle, and 2) its influence on the thermodynamic structure of boundary layer.
Land surface conditions (including terrain, soil, land-use, soil moisture, albedo) affect the partition of surface radiation flux into latent and sensible heat flux, which in turn affects the deep convection initiation and development. However, it remains a difficult task to model soil hydrology and vegetation in land surface models, and to treat the transport of water vapor in the boundary layer. To support the IHOP boundary layer mission, a joint observation effort was formed among RAP, MMM, ATD, University of Colorado, Oregon State University, and North Carolina State University research groups. Nine NCAR surface flux stations plus one additional flux station, operated by the University of Colorado were delayed for the IHOP 2002 atmospheric boundary layer mission in the Southern Great Plains during May and June 2002.
These ten flux-tower stations were strategically located along three pre-planned boundary-layer-mission flight tracks (western, central, and eastern flight legs, as seen on Figure 1, and over various land-use types. In addition to the differences in land-use types, the prolonged drought in the Oklahoma panhandle intensified the gradient in surface evaporation between the dry and sparsely-vegetated regions in eastern Kansas.
Figure 1. Location of HOP/NCAR/CU flux-tower stations.
The maximum rainfall accumulation was close to 270 mm at Station 8 and only about 50 mm at Stations 2 and 3 (see Fig. 2). In general, the western leg was drier than the central and eastern leg during the IHOP field experiment. A large variation in rainfall and evaporation was evident at stations located along the same flight leg. Even though the soil was wet at three eastern stations, significant evaporation only occurred after late-May (for Station 9) and mid-June (for Stations 7 and 8). This type of surface evaporation evolution may be largely due to the seasonal variation of vegetation characteristics.
Figure 2. Comparison of accumulated rain and surface evaporation measured at stations 1-9. Note that results shown here are based on raw data, which have not been quality controlled.
Figure 3 shows the weekly measurement of leaf area index (LAI) at a wheat site (Station 5) and at a grass site (Station 8). LAI was measured along a transect, which is typically 50 to 100 feet long at each station. The wheat was at its peak growing season at the beginning of the experiment. Grass was dormant at the beginning but experienced a rapid greening process in late May, explaining the late surge of evaporation at Station 8. However, other processes, in addition to rainfall and vegetation greening, may also influence the evolution of surface evaporation and plant transpiration.
Figure 3: Leaf area index (LAI) measured at stations 5 and 6. The line represents the averaged along a transect at the station, while the dots represent the standard deviation of these measurements along the transect for a given time.
