Isolating contributions to the diurnal cycle in southeast-Pacific stratocumulus

Observations from the VOCALS field campaign documented a complicated diurnal signal in southeast Pacific stratocumulus (SEP) behavior. Longwave cooling at cloud top drives turbulent boundary layer flow during the nighttime hours, and during the day shortwave radiation suppresses turbulent moisture transports and drizzle. The large-scale vertical motion field is more complicated than in other stratocumulus regions, however. Superimposed on the mean large-scale subsidence is a periodic vertical velocity oscillation termed the "upsidence wave," a westward-propagating gravity-wave response to the diurnal heating of the continent. Thus, the large-scale vertical motion in the SEP varies diurnally and by longitude. The upsidence wave can modulate boundary layer depth, which in turn affects peak cloud liquid-water content and thus precipitation production. These two effects — the diurnal cycle of radiation and the upsidence wave — are sometimes additive and sometimes cancelling. We present results from numerical simulations that isolate the effects of these two factors in determining outcomes of cloud and turbulence properties.