Characterizing the Impact of Entrainment Rate in Stratocumulus from ARM Observations and Large-Eddy Simulations
Poster PDF
Authors
David B. Mechem — University of Kansas
Virendra Prakash Ghate — Argonne National Laboratory
Lucas McMichael — University of Kansas
Jordan Eissner — University of Kansas
Michael Jensen — Brookhaven National Laboratory
Scott Giangrande — Brookhaven National Laboratory
Category
Warm low clouds, including aerosol interactions
Description
Entrainment is a fundamental measure of the interaction of clouds and their environment but is a mechanism that is highly parameterized (rather than resolved) in Earth System Models (ESMs). Improved formulations of entrainment rely on constraints provided by observations and process models. To this end, we explore entrainment behavior using mixed-layer model (MLM) budgets applied to Atmospheric Radiation Measurement Program (ARM) observations and large-eddy simulation (LES) for several cases of stratocumulus-topped boundary layers.
Observations made during the MAGIC deployment of the ARM Mobile Facility were collocated with observations from the Geostationary Operational Environmental Satellite (GOES-15) and European Center for Medium-range Weather Forecasting (ECMWF) reanalysis model. From these data, hourly estimates of entrainment velocities in closed cellular stratocumulus cloud conditions were calculated from the mixed-layer mass budget equation, modified to accommodate observations sampled from a moving platform. Entrainment velocity estimates from MAGIC are highly variable and on average exhibit no pronounced diurnal cycle nor any dependence on longitude. Furthermore, large-scale vertical motion was found to be highly variable and included periods of ascent. These results suggest that mechanistic evaluation of low-cloud behavior in ESMs cannot be obtained from climatological mean estimates alone.
Large-eddy simulation of mid-day coastal stratocumulus, analyzed in an MLM framework, identified the relative importance of different mechanisms to cloud thinning/thickening. The analysis demonstrated that entrainment fluxes remained active, even in the presence of strong shortwave heating. The cloud thinned but demonstrated unexpected resiliency, with the MLM analysis indicating that the relative contribution of long-wave cooling over the afternoon, as shortwave flux decreases, can become large enough to offset entrainment warming/drying. This approach is extended to a case of daytime drizzling stratocumulus observed over the Eastern North Atlantic (ENA) ARM site during the ACE–ENA field campaign.
