Horizontal Variability of 2-m Temperature at Night during CASES-97

Surface-station, radiosonde, and Doppler minisodar data from the Cooperative Atmosphere-Surface Exchange Study-1997 (CASES-97) field project, collected in a 60-km-wide array in the lower Walnut River watershed (terrain variation similar to 150 m) southeast of Wichita, Kansas, are used to study the relationship of the change of the 2-m potential temperature Theta 2m with station elevation z e , partial differential Theta 2m/ partial differential z e identical with Theta ,ze to the ambient wind and thermal stratification partial differential Theta / partial differential z identical with Theta ,z during fair-weather nights. As in many previous studies, predawn Theta 2m varies linearly with z e , and Theta ,ze similar to Theta ,z over a depth h that represents the maximum elevation range of the stations. Departures from the linear Theta 2m-elevation relationship ( Theta ,ze line) are related to vegetation (cool for vegetation, warm for bare ground), local terrain (drainage flows from nearby hills, although a causal relationship is not established), and the formation of a cold pool at lower elevations on some days. theta he near-surface flow and its evolution are functions of the Froude number Fr = S /(Nh ), where S is the mean wind speed from the surface to h, and N is the corresponding Brunt-Vaeisaelae frequency. The near-surface wind is coupled to the ambient flow for Fr = 3.3, based on where the straight line relating Theta ,ze to ln Fr intersects the ln Fr axis. Under these conditions, Theta 2m is constant horizontally even though Theta ,z > 0, suggesting that near-surface air moves up- and downslope dry adiabatically. However, Theta 2m cools (or warms) everywhere at the same rate. The lowest Froude numbers are associated with drainage flows, while intermediate values characterize regimes with intermediate behavior. The evolution of Theta 2m horizontal variability sigma Theta through the night is also a function of the predawn Froude number. For the nights with the lowest Fr, the sigma Theta maximum occurs in the last 1-3 h before sunrise. For nights with Fr similar to 3.3 ( Theta ,ze approximately 0) and for intermediate values, sigma Theta peaks 2-3 h after sunset. The standard deviations relative to the Theta ,ze line reach their lowest values in the last hours of darkness. Thus, it is not surprising that the relationships of Theta ,ze to Fr and Theta ,z based on data through the night show more scatter, and Theta ,ze similar to 0.5 Theta ,z in contrast