Boundary-Layer Controls on Shallow Cumulus Convection at the ARM SGP Site

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Description

The relationship between shallow cumulus clouds and the structure of the convective boundary layer (CBL) is examined using observations from 119 shallow cumulus days during the warm seasons (Apr.-Sept.) 2011-2015. The data are from the Southern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) site. A key tool in the study is a vertically staring Doppler lidar, which is used to quantify the vertical velocity variance in the CBL, cloud base updraft fraction, updraft mass flux, and sub-cloud circulations. Surface flux, radiosonde, AERI, and microwave profiler data are used to further contextualize these lidar observations. From these data we show that a dimensionless “cloud control” variable (√CIN/Wcb) explains most of the observed variation in cloud base updraft fraction, updraft mass flux, and cloud liquid water path. In this relationship CIN is the convective inhibition to reach cloud base and Wcbis the cloud base vertical velocity. The “cloud control” variable is subsequently used to stratify our boundary-layer observations, revealing key differences in the structure of the CBL between days with active and forced convective clouds. These results provide an improved observational basis for shallow cumulus parameterization schemes used in numerical weather prediction and climate models. This work is performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.LLNL-ABS-698949