GISS Lunch Seminar
Speaker: Mark Miller (Rutgers Univ.)
Title: Mesoscale Organization in Summertime Transition Stratocumulus over the Eastern North Atlantic

Abstract:
Observations and modeling studies indicate that cloudiness over the Eastern North Atlantic (ENA) has declined over the past three decades. The reasons for this decline may involve modifications to the complex marine boundary layer cloud morphology that exists there. Boundary layer cloudiness over the ENA typically evolves from single layer stratocumulus in the high latitudes to broken cumulus in the mid-latitudes with a transition between in which cumulus form beneath and rise into stratocumulus, a morphology referred to as cumulus-coupled stratocumulus. The onset of cumulus-coupling often leads to mesoscale organization in the form of Marine Boundary Layer Convective Complexes (MBLCC). Appearing as mini cumulonimbus visually and on radar, we have identified distinct scale-dependent configurations of MBLCC, often spanning tens of kilometers and featuring a vigorous, drizzling stratocumulus "anvil," sometimes with defined edges.

We utilize four years of summertime data from the US Department of Energy's ENA Observatory and high-resolution Weather Research and Forecasting (WRF) model simulations to study MBLCC. These tools enable us to examine variations in cloud morphology as a function of scale, and to document differences in thermodynamics and precipitation. Our findings indicate that the smallest scales of MBLCC in cumulus-coupled stratocumulus, ranging from 5-20 km, occur when winds and wind shear are weak and there are minimal evaporation-driven downdrafts. As the scale increases to 20-50 km, wind speed and precipitation intensify, and evaporative downdrafts originating near the base of cumulus reach the ocean surface. At larger scales, 50-100 km, stronger wind speeds and greater wind shear accompany downdrafts originating from deeper within the cloud layer.