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- Title
Horizontal Variability of 2-m Temperature at Night during CASES-97.
- Authors
Lemone, Margaret A.; Ikeda, Kyoko; Grossman, Robert L.; Rotach, Mathias W.
- 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 Θ[sub 2m] with station elevation z[sub e] , ∂Θ[sub 2m] /∂z[sub e] ≡ Θ[sub ,ze] to the ambient wind and thermal stratification ∂Θ/∂z ≡ Θ[sub ,z] during fair-weather nights. As in many previous studies, predawn Θ[sub 2m] varies linearly with z[sub e] , and Θ[sub ,ze] ∼ Θ[sub ,z] over a depth h that represents the maximum elevation range of the stations. Departures from the linear Θ[sub 2m] –elevation relationship (Θ[sub ,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[sub ,ze] to ln Fr intersects the ln Fr axis. Under these conditions, Θ[sub 2m] is constant horizontally even though Θ[sub ,z] > 0, suggesting that near-surface air moves up- and downslope dry adiabatically. However, Θ[sub 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[sub 2m] horizontal variability σ[sub Θ] through the night is also a function of the predawn Froude number. For the nights with the lowest Fr, the σ[sub Θ] maximum occurs in the last 1–3 h before sunrise. For nights with Fr ∼ 3.3 (Θ[sub ,ze] ≈ 0) and for intermediate values, σ[sub Θ] peaks 2–3 h after sunset. The standard deviations relative to the Θ[sub ,ze] line reach their lowest values in the last hours of darkness. Thus, it is not surprising that the relationships of Θ[sub ,ze] to Fr and Θ[sub ,z] based on data through the night show more scatter, and Θ[sub ,ze] ∼ 0.5Θ[sub ,z] in contrast to the predawn relationship. However, Θ[sub ,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 Θ[sub 2m] when the surface flow is coupled with the ambient wind, as well as both the linear variation of Θ[sub 2m] with elevation and the time required to reach maximum values of σ[sub Θ] under drainage-flow conditions.
- Subjects
KANSAS; UNITED States; TEMPERATURE; ATMOSPHERE; NIGHT; WIND speed
- Publication
Journal of the Atmospheric Sciences, 2003, Vol 60, Issue 20, p2431
- ISSN
0022-4928
- Publication type
Industry Overview
- DOI
10.1175/1520-0469(2003)060<2431:HVOMTA>2.0.CO;2