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 to150 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 derivativeTheta(2m)/ partial derivativez(e) = Theta(ze) to the ambient wind and thermal stratification partial derivativeTheta/partial derivativez = Theta,(z) during fair-weather nights. As in many previous studies, predawn Theta(2m) varies linearly with z(e), and Theta,(ze); 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. The 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 - Vaisala 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 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) approximate to 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 0.5Theta,(z) in contrast to the predawn relationship. However, Theta,(ze) approximate to 0 for ln Fr = 3.7, a value similar to that just before sunrise. A heuristic Lagrangian parcel model is used to explain the horizontal uniformity of time-evolving Theta(2m) when the surface flow is coupled with the ambient wind, as well as both the linear variation of Theta(2m) with elevation and the time required to reach maximum values of sigma(Theta) under drainage-flow conditions.