Influence of Land Cover, Soil Moisture, and Terrain on the Horizontal Distribution of Sensible and Latent Heat Fluxesin Southeast Kansas during IHOP_2002 and CASES-97

Analysis of daytime fair-weather aircraft and surface-flux tower data from the May-June 2002 International H2O Project (IHOP_2002) and the April-May 1997 Cooperative Atmosphere Surface Exchange Study (CASES-97) shows the expected strong influence of vegetation cover on the horizontal distribution of sensible (H) and latent heat (LE) fluxes over a 45-km flight track located in southeast Kansas, with LE maxima over green vegetation and H maxima over dormant vegetation. Least-square best fit lines to plots of H vs LE from ½-hour averaged surface data from CASES-97 showed that the slope Delta_xyLE/Delta_xyH is negative on days with sufficient horizontal variability to define a slope. For each case, the line extended from points with the highest LE and lowest H (green vegetation) to points with the lowest LE and highest H (dormant vegetation or bare ground). Based on the surface energy balance, H+LE = Rnet – Gsfc, where Rnet is the net radiation and Gsfc the flux into the soil, an approximately –1 slope (or Rnet – Gsfc ~constant) seemed reasonable for days with no clouds, and a match to the surface slope was suggested as a way for evaluating the usefulness of aircraft data for testing land-surface models (LSMs); but actual slopes varied around that number. For IHOP_2002, aircraft data suggest a steeper slope (~ –2). Observations and LSM results are used to show that IHOP_2002 has a steeper Delta_xyLE/Delta_xyH slope than CASES-97 because of (a) more rain (wetter soils) in IHOP_2002, and (b) the combined effects of local terrain (changes in soil depth, downslope water transport) and plant phenology of the grasses associated with the ridges and the crops (winter wheat) associated with nearby valleys.