Synoptic regime classification at the Azores site and the corresponding cloud microphysical properties

Description

The ARM Mobile Facility (AMF) Azores site was located at the southern tier of the North Atlantic storm alley, thus experiencing frequent transitions between midlatitude and subtropical synoptic regimes that are related to the zonal placement of the North Atlantic storm track. Given that poleward shifts in midlatitude storm tracks constitute a substantial source of cloud and rain climate feedbacks, the Azores site serves as a promising location to examine relevant cloud properties and processes at scales ranging from synoptic to microphysical and to evaluate the ability of a suite of models to simulate the primary system behaviors.

Two different methods are used here to classify the synoptic regimes affecting the Azores site during the time of the AMF deployment. The first relies on a new climatology of midlatitude storminess that, in addition to locating the storm center, delineates the area of influence of a midlatitude storm. The second applies a clustering algorithm to Cloud Optical Thickness-Cloud Top Pressure histograms and derives the major weather states based on the morphology of the cloud field. Once the synoptic regimes derived by the two methods are identified and mapped and their relationship is examined, data from the AMF retrievals are compiled over regime-representative time periods for statistical evaluation of global model and large-eddy simulations. Preliminary analysis of output from GISS GCM current-climate simulations is performed to establish baseline model skill in simulating the observed regimes and the resulting cloud structures. In addition, recently derived microphysical retrievals of drizzle particle size distributions are statistically compared with existing regime-representative large-eddy simulations of low-lying marine clouds.