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| Titel |
Updraft and downdraft characterization with Doppler lidar: cloud-free versus cumuli-topped mixed layer |
| VerfasserIn |
A. Ansmann, J. Fruntke, R. Engelmann |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 10, no. 16 ; Nr. 10, no. 16 (2010-08-24), S.7845-7858 |
| Datensatznummer |
250008724
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| Publikation (Nr.) |
 copernicus.org/acp-10-7845-2010.pdf |
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| Zusammenfassung |
| 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 zi (boundary-layer depth zi),
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 zi–0.18 zi 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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