Momentum roughness and view-angle dependent heat roughness at a Southern Great Plains test-site

Surface roughness parameters were determined for harvested wheat fields over level terrain at the US Department of Energy's Cloud and Radiation Testbed (CART) site in north-central Oklahoma. Measurements of wind speed and temperature were made by radiosondes and instruments mounted on 2 and 10 m towers during neutral and unstable atmospheric conditions in the atmospheric surface layer. Surface temperatures were measured radiatively over 750 m trajectories. Roughness heights were calculated for the region using the Monin-Obukhov similarity theory. The scalar roughness and the local momentum roughness were determined using wind speed measurements at 10 m and temperature measurements at 2 m combined with eddy correlation measurements for u(*). The scalar roughness z(oh) was determined to be 0.0021 and 0.0038 m for the nadir and the off-nadir viewing angle, respectively. It was estimated that the displacement height d is negligible. A regional momentum surface roughness of z(o) = 0.15 m was determined by means of the radiosonde profiles. Good agreement (r = 0.92) between measured and calculated sensible heat flux values was found using an independent data set of radiosonde profiles. (C) 1998 Elsevier Science B.V. All rights reserved.