Properties and evolution of open cellular convection over the Eastern North Atlantic

Abstract:

The evolution of boundary-layer clouds accompanying cold-air outbreaks remain an understudied phenomenon. A 36-hour period of open-cellular convection sampled by the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Eastern North Atlantic (ENA) site on Graciosa Island in the Azores is thoroughly analyzed using Lagrangian and Eulerian approaches. This case is characterized by strong surface fluxes (>300 W m^-2) and deep clouds (cloud-top height >4 km) accompanying advection of cold air over much warmer water. The Lagrangian analysis focuses on an ensemble of backward trajectories originating from the ENA site and moving backward in time, providing a history of the fluid parcels arriving at each of the 36 hours sampled at Graciosa Island. For the first 21 hours of the event, parcels originate near Greenland, whereas over the last 15 hours their origins lie farther to the east. This difference yields two distinct thermodynamic histories of the parcels eventually reaching the ENA site, although this is not obvious from the observations at ENA. Geostationary satellite data indicate a wide range of cloud-top temperatures, implying a wide variety of cloud depths. Mean cloud cover is between 80-100% for most of the trajectories, decreasing somewhat as the trajectories near the ENA site (68-72%). The Ka-band profiling cloud radar (KAZR) sampled shallow cumulus and deeper congestus clouds up to a height of ~5 km, which contained copious precipitation. This precipitation resulted in cold pools that display a pronounced signal in the surface temperature, humidity, and pressure measurements. Doppler lidar retrievals yield profiles of vertical velocity variance and mass flux in the subcloud layer, which can be used to validate cloud-base mass flux closures used in shallow cumulus parameterizations. The detailed measurements reveal several processes responsible for maintaining the cloud and precipitation structure.