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

Abstract

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 ?150 m) southeast of Wichita, Kansas, are used to study the relationship of the change of the 2-m potential temperature Θ2m with station elevation ze, ?Θ2m/?ze ≡ Θ,ze to the ambient wind and thermal stratification ?Θ/?z ≡ Θ,z during fair-weather nights. As in many previous studies, predawn Θ2m varies linearly with ze, and Θ,ze ? Θ,z over a depth h that represents the maximum elevation range of the stations. Departures from the linear Θ2m?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?Väisälä 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 ln Fr intersects the ln Fr axis. Under these conditions, Θ2m is constant horizontally even though Θ,z > 0, suggesting that near-surface air moves up- and downslope dry adiabatically. However, Θ2m 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 Θ2m horizontal variability σΘ through the night is also a function of the predawn Froude number. For the nights with the lowest Fr, the σΘ maximum occurs in the last 1?3 h before sunrise. For nights with Fr ? 3.3 (Θ,ze ≈ 0) and for intermediate values, σΘ 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 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 Θ2m when the surface flow is coupled with the ambient wind, as well as both the linear variation of Θ2m with elevation and the time required to reach maximum values of σΘ under drainage-flow conditions.