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Updraft and downdraft characterization with Doppler lidar: Cloud-free versus cumuli-topped mixed layer

by A. Ansmann, J. Fruntke, R. Engelmann
Atmospheric Chemistry and Physics ()

Abstract

For the first time, a comprehensive, height-resolved Doppler lidar study of updrafts and downdrafts in the mixing layer is presented. The Doppler lidar measurements were performed at Leipzig, Germany, in the summer half year of 2006. The conditional sampling method is applied to the measured vertical velocities to identify, count, and analyze significant updraft and downdraft events. Three cases of atmospheric boundary-layer (ABL) evolution with and without fair-weather cumuli formation are discussed. Updrafts occur with an average frequency of 1-2 per unit length z(i) (boundary-layer depth z(i)), downdrafts 20-30% more frequently. In the case with cumuli formation, the draft occurrence frequency is enhanced by about 50% at cloud level or near cloud base. The counted updraft events cover 30-34%, downdrafts 53-57% of the velocity time series in the central part of the ABL (subcloud layer) during the main period of convective activity. By considering all drafts with horizontal extent > 36 m in the analysis, the updraft mean horizontal extent ranges here from 200-420 m and is about 0.16 z(i)-0.18 z(i) in all three cases disregarding the occurrence of cumulus clouds. Downdraft extents are a factor of 1.3-1.5 larger. The average value of the updraft mean vertical velocities is 0.5-0.7 m/s or 0.40 w(*)-0.45 w(*) (convective velocity scale w(*)), and the negative downdraft mean vertical velocities are weaker by roughly 10-20%. The analysis of the relationship between the size (horizontal extent) of the updrafts and downdrafts and their mean vertical velocity reveals a pronounced increase of the average vertical velocity in updrafts from 0.4-0.5 m/s for small thermals (100-200 m) to about 1.5 m/s for large updrafts (> 600 m) in the subcloud layer in the case with fair-weather cumuli. At cloudless conditions, the updraft velocities were found to be 20% smaller for the large thermals.

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