Exploratory cloud-resolving simulations of boundary-layer Arctic stratus clouds: Part I: Warm-season clouds

Two-dimensional simulations of arctic stratus clouds (ASC) were conducted using a sophisticated cloud-resolving model with explicit microphysics and a two-stream radiative transfer model. The effects of varying cloud condensation nuclei (CCN) concentrations upon the subsequent cloud and its microphysical, radiative and dynamical structure were studied. In this study CCN concentrations were varied within the ranges found in warm-season arctic boundary layers (ABLs) to produce non-drizzling and weakly drizzling stratus decks. Experiments that included all model physics, no-drizzle, and no shortwave radiation were conducted to elucidate the effects of microphysics and radiation on the simulated stratus. Both simulations that did and that did not include the effects of drizzle showed that the higher CCN concentrations produced a cloud with larger reflectivity and absorptivity, but also produced eddies that were weaker than with lower CCN concentrations. Simulations that included the effects of drizzle showed a similar response to changes in CCN concentrations. Simulations with no drizzle produced more vigorous eddies than their drizzling counterparts because cooling due to evaporation below cloud tends to stabilize the ABL. The simulations without the effects of short-wave radiation produced very vigorous eddies that penetrated more deeply into the ABL. In this case, the simulation with higher CCN concentrations produced the most vigorous eddies. This resulted from a subtle interplay of microphysics, radiation, and dynamics.