Modeling the Response of Coastal Stratocumulus Clouds to Sudden and
Gradual Variations of Surface Heat Fluxes
Abstract
Coastal stratocumulus clouds (Sc) have unique modeling challenges due to
their development over coastal land, one of them being the accurate
representation of surface fluxes. Unlike marine Sc, where the ocean can
store significant heat and release relatively constant surface heat
fluxes over the day, there are strong diurnal variations of both
sensible and latent heat fluxes over land. Moreover, land surface fluxes
have a strong feedback with cloud cover. Many modeling efforts have been
directed to improve the representation of surface fluxes through
developing more accurate land surface models with increasing complexity.
Regarding the boundary layer turbulence, for marine Sc, greater sensible
fluxes are known to intensify updrafts and increase entrainment, while
greater latent heat fluxes have been linked to decoupling. An example of
surface flux variations for Marine Sc is the transition of Sc to shallow
Cumulus along the trade winds, which occurs over a number of days. For
coastal land, changes of surface fluxes occur in a much shorter
timescale (hours), and the sensitivity of their dynamical response has
not been explored. In this work, we study the response of coastal Sc to
controlled variations of surface fluxes using Large Eddy Simulations.
Representative scenarios of diurnal profiles are generated using 12
years of surface flux measurements for cloudy days over southern
California, and then simulated under several configurations that
describe sudden and gradual changes of surface fluxes with varied timing
and magnitude. Sudden changes result in increased cloud thinning and
earlier dissipation times, although the timing of the sudden increase is
also important, in relation to the original dissipation time. The
response time to sudden changes of surface fluxes is evident in the
evolution of maximum vertical velocity and vertically integrated
Turbulent Kinetic Energy, with timescales of 1 and 2 eddy turnover
times, respectively.