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 _e1∼ 150 m) southeast of Wichita, Kansas, are used to study the relationship of the change of the 2-m potential temperature Θ_2m with station elevation Z_e, _pt∂Θ_2m/_pt∂Z_e, ≡ Θ_.ze to the ambient wind and thermal stratification _pt∂Θ/_pt∂Z ≡ Θ_.z, during fair-weather nights. As in many previous studies, predawn Θ_2m varies linearly with Z_e, and Θ_.ze _e1∼ Θ_.z over a depth h that represents the maximum elevation range of the stations. Departures from the linear Θ_2m-elevation relationship (Θ_.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-V_da⊣is̈al̈a frequency. The near-surface wind is coupled to the ambient flow for Fr = 3.3, based on where the straight line relating Θ_.ze to In Fr intersects the In Fr axis. Under these conditions, Θ_2m is constant horizontally even though Θ_.z > 0, suggesting that near-surface air moves up- and downslope dry adiabatically. However, Θ_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 Θ_2m horizontal variability σ_Θ through the night is also a function of the predawn Froude number. For the nights with the lowest Fr, the σ_Θ maximum occurs in the last 1-3 h before sunrise. For nights with Fr 3.3 Θ_.ze _ 0 and for intermediate values, σ_Θ peaks 2-3 h after sunset. The standard deviations relative to the Θ_.ze line reach their lowest values in the last hours of darkness. Thus, it is not surprising that the relationships of Θ_.ze to Fr and Θ_.z based on data through the night show more scatter, and Θ_.ze ; 0.5Θ_.z in contrast to the predawn relationship. However, Θ_.ze 0 for In 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 Θ_2m when the surface flow is coupled with the ambient wind, as well as both the linear variation of Θ_2m with elevation and the time required to reach maximum values of σ_Θ under drainage-flow conditions.