One of the continuing issues in climate studies is the degree to which heterogeneous surface conditions affect the circulations in the atmospheric boundary layer. During the past several years, we have carried out observational and modeling studies of boundary layer structure at the U.S. DOE's Cloud and Radiation Testbed (CART) site in the Southern Great Plains (SGP). As part of these studies we are using data from the extensive measurement networks located in the SGP CART to construct near-surface fields of wind velocities and temperatures for multiple-month periods over several years. The spatial resolution of the interpolated fields is 6.25 km, the temporal resolution is 30 minutes, and the fields are computed over a domain approximately 300 by 360 km in extent. In this paper we use these data to search for evidence of secondary circulations induced by the significant temperature contrasts found over extended areas of the SGP site. These contrasts arise, in turn, from the significant differences in agricultural practices in the region. In particular, large areas of the CART are planted in winter wheat, whose growing cycle differs significantly from the natural grasses and other crops grown in the area. In the late winter and spring the winter wheat areas are substantially greener and cooler than the rest of the CART; conversely, in mid- to late summer the wheat has been harvested and those areas are drier and warmer than the non-wheat sections. These contrasting patterns are readily discernible from satellite NDVI data.
In previous investigations of individual case study days it has been difficult to identify unambiguously the presence of secondary circulations in the observed wind fields because any circulations are typically masked by synoptic-scale flows. By compositing the mean fields by time of day over several months, however, we are able to minimize the masking effects of synoptic conditions. Moreover, by contrasting results obtained over two different portions of the growing cycle, late winter and spring compared with mid- and late summer, we are able to look for seasonal changes in the diurnal variations of the wind fields. Finally, by relying on data rather than models, our results are independent of modeling assumptions regarding soil moisture and simulated sensible or latent heat fluxes. We present the results of our analysis and discuss the implications it has for the importance of secondary circulations in the Southern Great Plains.