Direct and Bulk Entrainment Estimates in Individually Tracked Boundary Layer Clouds

We investigate the dependence of entrainment and detrainment rates on shallow cloud core and environmental properties using large-eddy simulations of two well-documented cases. The simulations were performed with the System for Atmospheric Modeling (SAM; Kharioutdinov et at. 2009) large-eddy simulation model with direct entrainment and detrainment calculations based on Dawe and Austin (2011a). We analyze equilibrium boundary layer simulations for three CGILs (Blossey et al. 2013) cases at 3 different sea surface temperatures, conducted on a 9.6 x 9.6 x 4.8 km domain at 25 m resolution, as well as a 12-hour simulation of deep tropical marine convection for a GATE (Kharioutdinov et al., 2010) case on a 86 x 86 x 25 km domain at 50 m resolution. We track the histories of individual clouds and their sub-cloud plumes using the 3-dimensional feature-tracking algorithm of Dawe and Austin (2012). Preliminary results indicate that the entrainment rate of the shallow clouds depends most strongly on buoyancy and vertical velocity, more weakly on environmental stability. Based on the analysis of the shallow cumulus regime, we find power-law relationships between the vertical mass-flux distributions and the entrainment rates as a function of characteristic cloud length scale, and show that the cumulative behaviour of the individual entraining clouds can be related to the bulk entrainment rates estimated by the in-cloud moist static energy.

Blossey et al., 2013:: J. Adv. Model. Earth Syst., 5, 234–258, doi:10.1002/jame.20025.

Dawe and Austin, 2011: Mon. Wea. Rev., 139, 444–456. doi: 10.1175/2010MWR3473.1

Dawe and Austin, 2012: Atmos. Chem. Phys., 12, 1101-1119, doi:10.5194/acp-12-1101-2012

Khairoutdinov and Randall, 2003: J. Atmos. Sci., 60, 607–625 doi: 10.1175/1520-0469(2003)060<0607:CRMOTA>2.0.CO;2

Khairoutdinov et al., 2009: J. Adv. Model. Earth Syst., 1, 15, doi:10.3894/JAMES.2009.1.15