Climate Effects/Controls on Mesoscale Processes
Other Agency – Large-Eddy Observations and LES of Liquid Stratus over the ARM Southern Great Plains Climate Research Facility
Mechem (primary – CIMMS at OU), Y. Kogan, Schultz
Funding Agency: U.S. DOE
Objectives
Employ high resolution Doppler cloud radar and large eddy simulation
to analyze the structure and dynamics of continental stratocumulus.
Accomplishments
Studies employing continuous years of low cloud observations over the
Southern Great Plains ARM Climate Research Facility (ACRF) emphasized
their climatological, microphysical, and radiative characteristics.
The recently enhanced capabilities of the ACRF cloud radar suite, specifically,
the higher sampling rate of the boundary layer, are ideal for exploring
the dynamic aspects of these cloud systems. These upgraded sensors
enable "large eddy observations" (LEOs; Kollias and Albrecht,
JAS 2000) -- the coherent sampling of boundary layer turbulence structures
responsible for most of the transport. High resolution cloud radar
observations (95 GHz W-band ARM Cloud Radar -- WACR) and large eddy
simulation (System for Atmospheric Modeling -- Explicit Microphysics
(SAMEX)) were employed to analyze the cloud structure and turbulent
quantities for a typical springtime postfrontal boundary layer stratocumulus
case. Preliminary observational and modeling analyses suggest both
similarities and differences relative to marine stratocumulus.
For clouds containing little or no precipitation, boundary layer turbulence structures sampled by the WACR are coherent in both time and in the vertical. Statistics from the WACR indicate a slightly subadiabatic cloud layer and an eddy structure dominated by updrafts and downdrafts of roughly similar properties. The slight negative skewness implies that convection driven top-down by cloud top longwave cooling weakly predominates. The overall magnitude of in-cloud and turbulence was relatively weak, compared to typical marine cases. LES captures the magnitude of in-cloud turbulence and skewness present in the WACR observations, which suggests it is representing the overall character of the flow reasonably. Relative to marine clouds, which are typically studied in a Lagrangian framework with relatively weak advective forcings, continental clouds associated with synoptic systems require highly constrained estimates of these advective terms.
This project is ongoing.
Publications
Mechem, D.B. and Y. L. Kogan, 2007: Large eddy observations and
LES of liquid stratus over the ACRF. Proc., Seventeenth Atmospheric Radiation
Measurement (ARM) Science Team Meeting, Monterey, CA, U.S. Dept. of Energy.
Characterization of stratocumulus associated with a midlatitude synoptic system passing over the Southern Great Plains ACRF. (a) GOES IR imagery from 0645 UTC 8 April 2006. Blue box indicates location of the ACRF; (b) Processed radar reflectivity and velocity data from the WACR; (c) Vertical velocity at three levels in the cloud corresponding to a nondimensional cloud-normalized height. The graphical distance between 0.50-0.75 corresponds to 1 m s-1; (d) Variance and skewness calculated from a large eddy simulation of this case. Statistics calculated from the WACR data are overlaid on the LES profiles.