8/26/2023 0 Comments Show slope on logger pro 3.11![]() ![]() André and Mahrt (1982) concluded that “turbulent heat flux divergence and clear-air radiative cooling contribute about equally to the development of nocturnal surface inversions.” André and Mahrt (1982) also found that the inversion layer continues to grow because of radiative cooling effects but its lowest part, which is turbulent and strongly stably stratified, remains rather shallow throughout the night. Higher wind speeds also make the EET less pronounced because increased turbulent kinetic energy prevents the surface layer from decoupling from the boundary layer. Turbulence decays faster in valleys, sinkholes, gullies, other small local depressions, or regions sheltered by vegetation, whereas it typically persists longer on hill tops and other well-exposed areas. As turbulent mixing decreases, the surface layer becomes decoupled from the boundary layer and the air confined within the shallow surface layer cools very rapidly. ![]() Effects of surface heterogeneities on meteorological quantities can become more pronounced as a result of the limited turbulent mixing and related small eddy sizes within this stable surface layer. In particular, the role of a katabatic flow in CP development is not always clear-cut, and it might actually have a negative feedback that stops CP intensification in some cases.Īcevedo and Fitzjarrald (2001) concluded that the rates of change of mean quantities during the early evening transition (EET)-the first two hours after sunset-are “many times larger than they are subsequently until sunrise.” In that study, the EET is described as the period in which the surface energy budget changes rapidly and a shallow stable boundary layer develops. However, there does not appear to be a unique physical–dynamical process producing the CP in those studies. One common finding of these studies is that large spatial variations in near-surface temperatures develop shortly after sunset under clear-sky conditions with weak synoptic forcings. 2003) as well as in shallow gullies ( Mahrt et al. Measurement campaigns were further conducted to study nocturnal temperature distributions and drainage flows in basins and limestone sinkholes in mountainous terrain (e.g., Whiteman et al. 2003 Mahrt 2006), and the role of trees and forests on nocturnal cold-pool (CP) formation has also been discussed ( Gustavsson et al. Since then, the variability of near-surface temperatures in terrain of different complexity has been investigated in a number of studies ( Mahrt and Heald 1983 Tabony 1985 Thompson 1986 LeMone et al. As stated in Geiger’s (1965) review of early studies, names such as “cold island,” “frost hole,” “cold air pool,” or “frost hollow,” are often used to describe the phenomenon of cold-air buildup in low-lying areas. The spatial variability of nighttime (NT) temperatures and correlation between low temperatures and topographic depressions has been widely discussed in the literature. Observations that significant temperature changes can occur even with elevation changes on the order of 6 m have important implications in agriculture as well as in data assimilation. An analysis of Richardson numbers demonstrates that cold-pool formation frequently occurs under strongly stable conditions that tend to suppress vertical turbulent mixing in the surface layer. Since such warming was not observed, it is concluded that the katabatic jets did not actually penetrate the cold pool but likely flowed over it. However, a heat budget analysis for these nights suggested that the katabatic flows were associated with warm-air advection along the slope and that if katabatic jets had penetrated the cold pool, they would have produced substantial warming in the region of the cold pool. The wind data also show signatures of katabatic flow for about 50% of the strong cold-pool events. ![]() Wind data collected with sonic anemometers on a 15-m-tall tower at the micronet for a period of three months (spring 2005) suggest that flow sheltering by vegetation plays an important role in the cold-pool formation. The strength of each cold-pool event was classified according to a cold-pool index based on average nocturnal temperature perturbations within the cold-pool region. ![]() Cold pools frequently formed at the base of a gentle slope in a small depression of only ∼6-m depth that is also sheltered by trees. Spatial analyses were performed for a 2-yr-long temperature record from 26 temperature/humidity surface stations, deployed across a 120 m × 320 m micronet located in a region of gently sloped terrain with maximum elevation changes of ∼25 m. Temperature and wind data from a rural micronet and nearby site of the Oklahoma Mesonet are analyzed to study the frequency, strength, and formation processes of cold-pool events in a region with gentle terrain. ![]()
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