Diurnal variations in turbulence and mass-transports in continental boundary layer stratocumulus clouds from mm-wavelength radar observations

Boundary layer stratocumulus clouds are an important factor in the earth's radiation budget due their high albedo and low cloud top height. They are closely coupled to the turbulence in the boundary layer and affect the surface energy and moisture budgets. The continental boundary layer stratocumulus clouds are intimately tied to the diurnal cycle of the boundary layer and hence need to be represented in climate model simulations. In this study, a 14-hour event of stratocumulus cloud is analyzed using the data the Atmospheric Radiation Measurement (ARM)'s Southern Great Plains (SGP) observing facility located at Lamont, Oklahoma. Observations from a vertically pointing Ka-band Doppler radar known as Millimeter Cloud Radar (MMCR) along with the observations from other instruments including a microwave radiometer, ceilometer, flux suite, radiosondes etc. were used to map the cloud and boundary layer structure. The high temporal (4 sec) and spatial (45 m) resolution Doppler velocity observations from MMCR were used as a surrogate for the vertical air motion during absence of precipitation. Hourly values of in-cloud vertical velocity variance, skewness, updraft fraction, downdraft fraction and mass-flux are reported.

The cloud liquid water content was retrieved using the radar-radiometer retrieval technique at a 20 sec temporal resolution. An attempt is made to retrieve the liquid water flux by combining the retrieved liquid water content with the vertical velocity on hourly time scales. The event started at 10:00 and ended at 24:00 local time on 25 March 2005. The daytime was characterized by high surface buoyancy flux and a surface coupled BL while the nighttime was characterized by a no surface buoyancy flux and a decoupled BL. Diurnal changes in the magnitude and vertical profile of in-cloud vertical velocity variance, skewness, updraft fraction are reported along with those of mass-flux and liquid water flux obtained from radar observations.