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

Abstract

Abstract. For the first time, a comprehensive, heightresolved 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.

Strong aerosol–cloud interaction in altocumulus during updraft periods: lidar observations over central Europe

“…This statement is in agreement with the study of Shinozuka et al (2015). In agreement with the extended discussion in the literature, it seems to be obvious that satellite observations, focusing on ACI (with values mainly below 0.4), may not provide us with a realistic view on the aerosol effect on the microphysical properties of liquid-water clouds, so that the open question remains of how reliable satellite-based estimates of the indirect aerosol effect are on global climate (Quaas et al, 2009;Ban-Weiss et al, 2014;Ma et al, 2014).…”

“…The Doppler wind lidar (WILI) of TROPOS operates at a wavelength of 2022 nm (Engelmann et al, 2008;Ansmann et al, 2010). Vertical and temporal resolutions are 70 m and 2 s, respectively.…”

Observed spatiotemporal variability of boundary-layer turbulence over flat, heterogeneous terrain

“…Later, radar wind profiler (e.g., Eymard and Weill, 1988;Angevine et al, 1994;Eng et al, 2003) and more recently, Doppler lidar measurements (e.g. Lothon et al, 2009;Hogan et al, 2009;Ansmann et al, 2010;Lenschow et al, 2012) became available for studying vertical velocity characteristics in the CBL. Both in situ aircraft measurements and ground-based remote sensing have advantages and disadvantages: as aircraft observations are expensive, data are usually available for a small number of flight levels only.…”

A Probabilistic Bulk Model of Coupled Mixed Layer and Convection. Part II: Shallow Convection Case

“…Furthermore, in PBCM we are only resolving the positively buoyant part of the distribution (updrafts), and we do not consider the downdrafts, which are often thought to impose a bimodal structure to the pdf (Golaz et al 2002). Quantifying the actual distributions of turbulent variables remains an active area of research (e.g., Ansmann et al 2010;Couvreux et al 2010).…”