Escoamento de drenagem na camada limite planetária estável

Abstract

The main goal of the work is to investigate the cold air drainage flow events over a grassland with gently undulating terrain, the typical landscape of the Pampas. Drainage flows happen over a wide range of scales of topographic features under stable conditions of the atmosphere. Under these conditions, it is possible for cold air to accumulate over sloped surfaces, and because its weight is not completely balanced by the vertical ambient pressure gradient, the parcel becomes negative buoyant and flows down the slope. In the work, wind, temperature, and radiation data collected at a micrometeorological tower and four surroundings small masts during 32 days of autumn of 2015 and only at the tower during the year of 2017 are analyzed. The results indicate that drainage events of cold air do occur over this type of landscape (Pampas). Strong cooling rates at the surface and the presence of a horizontal temperature gradient and weak large-scale (synoptic) winds are necessary conditions for these types of flow to be observed. These drainage flows have typical depth of 1 to 2 meters and typical speeds that do not exceed 1 m/s. When the synoptic wind vector is weak and points in the downslope direction, it adds up to the drainage flow, and flow speeds above 1 m/s are observed. The turbulent kinetic energy values associates to these events are small, and the Richardson number reveals that such flows are intermittent. It is worth to mention that the local site had a surface of only 2 to 4 degrees of inclination with respect to the true horizontal.