Using the CASES-97 Data to model land surface heterogeneity

To understand the effects of land-surface heterogeneity and the interactions between the land-surface and the planetary boundary layer at different scales, we developed a multi-scale data set based on the Cooperative Atmosphere-Surface Exchange Study (CASES-97) field observation conducted in the upper Walnut River watershed, Kansas. The domain covered by this data set is 74x71 km, which approaches the grid size used in many weather and climate models. The surface meteorological conditions obtained from nine surface stations, high-resolution S-Pol radar precipitation analysis, and 4-km hourly NCEP national precipitation analysis are interpolated/mapped into half-hourly, continuous gridded data with three resolutions of 1, 5, and 10 km. Also, the gridded surface boundary conditions are comprised of a 1x1 km STATSGO soil data, 1x1 USGS/EROS vegetation data, and 30 m Kansas land use map. The 1x1 km AVHRR products which give the NDVI was also utilized to specify the surface vegetation characterization.

We use this multi-scale surface forcing data set to drive three land-surface models to generate one-month-long gridded surface heat flux maps for the CASES-97 area. To ensure the quality of the uniformly distributed surface heat flux maps, the surface heat fluxes generated by the models are validated against 1) continuous surface-heat-fluxes measured at eight flux stations , and 2) heat fluxes obtained from two aircraft (the University of Wyoming King Air and the NOAA Twin Otter) at low levels during 5 IOP days. In general, the model results are favorably compared to these data. Simulating the rapid greening process of grassland (a dominant land-use type in the CASES-97 domain) is challenging and requires an accurate description of underlying vegetation characteristics and its effect on canopy evapotranspiration in models. Across the CASES-97 domain, the variability in surface heat fluxes typically ranges from 50-150 W m-2, induced by the differences in vegetation characteristics, soil moisture and surface radiation forcing. This variability tends to decrease when the grassland reaches its peak growing stage.