Late-Morning Mixed Layer Temperature Budgets over a Midwest Watershed: CASES 97

The Cooperative Atmosphere-Surface Exchange Study (CASES, see http//:www.jos.ucar.edu/cases) is a grass-roots collaboration to study of the interaction between the atmosphere and surface over a watershed for periods of seconds to years. The Walnut River Watershed east of Wichita, Kansas, was selected for this purpose because of its size, its location within a dense meteorological observing network, its long stream-gauge record, and its location in a region with a strong east-west gradient in precipitation and land use. The Atmospheric Boundary Layer Experiments array run by Argonne National Laboratory provides long-term observations of the boundary layer and surface carbon-dioxide exchange in the lower part of the watershed; the Hydrologic Science Team of Oregon State University has a dense raingauge network and soil-moisture measurements in the Whitewater Subbasin. These long-term observations are supplemented by episodic intensive field programs, two of which have now been held, in 1997 and 1999.

The first episodic field program, CASES-97 was held from 21 April -- 17 June, 1997, and included soil-moisture scaling and precipitation estimation as well as fair-weather boundary-layer goals. Here, we present hourly budgets of moisture and sensible heat over the watershed, from 1000 to 1300 LST, under cloudless, fair-weather conditions for two of the five intensive observing periods. One period had patchy surface moisture and vegetation conditions and was two days after a moderate rain event; the second day had more uniform vegetation conditions and a wet surface due to heavy rains the day before. The budget computations use data from 3 wind profilers, 90-minute serial radiosondes at the profiler sites, surface meteorological and flux stations, and two aircraft with flux measuring capability. We show how the data from several different platforms are combined to estimate the budgets, present the results including the apparent influence of surface processes, and discuss some lessons learned from CASES-97.