Cloud, Precipitation, and Aerosol Properties for Open Cellular Convection Associated with a Cold-Air Outbreak over the Eastern North Atlantic

Warm boundary-layer clouds are often organized in patterns of open cells, with clear areas surrounded by thin bands of clouds at the edges. Over the Eastern North Atlantic (ENA), these cloud patterns are regularly associated with cold air outbreaks, defined loosely as southward-moving cold air masses traversing relatively warmer ocean waters. This gradient together with strong winds leads to high sensible and latent heat fluxes that provide buoyancy leading to convection. We explore a case study (36 hours) of cloud behavior associated with a cold air outbreak over the ENA that produced convection ranging from 3-6 km in height and minute-average surface rain rates in the excess of 30 mm h^-1. The case is analyzed using data collected by instruments onboard polar orbiting (MODIS) and geostationary (SEVIRI) satellites, along with a suite of instruments at the Atmospheric Radiation Measurement (ARM) ENA site on Graciosa island. Satellite observations and scanning X-band radar characterize the spatial aspects of the cloud system, while profiling Doppler lidar and cloud radar provide insights into the vertical structure of individual clouds that pass over the instruments. The data from the vertically pointing instruments were combined to retrieve turbulence and micro-physical properties of precipitation below cloud base. These observations show strong precipitation and evidence of low-level cold pools that may be influencing convective organization. A series of idealized large-eddy simulations (LES) of the case provides insights into bulk entrainment rates, cloud-core dynamics, characteristic cell scale, and coalescence processing of cloud condensation nuclei. This case provides an interesting contrast to other open-cellular cloud systems that evolve from closed cellular cloud fields.