Characterizing variability of synoptic and cloud regimes over the ARM Azores site and northeast Atlantic

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Description

The Azores lie in a region of substantial variability in cloud and synoptic properties, as demonstrated during a 20-month deployment deployment of the ARM Mobile Facility (AMF) on Graciosa Island. We explore this variability using the clustering technique of self-organizing maps (SOMs). The SOM algorithm finds twenty-five characteristic synoptic states based on monthly 500-mb geopotential height anomalies from ERA–Interim (reanalysis). Once these characteristic states are determined, other fields from reanalysis, satellite data, and surface observations are projected onto these characteristic states. For every month, the clustering algorithm consistently identifies four archetypical synoptic patterns based on the relationship of the 500-mb geopotential pattern relative to the Azores: prefrontal, trough, postfrontal, and ridge. June is the month of largest low cloud fraction but is characterized by a 500-mb ridge pattern only ~35% of the time. Synoptic transitions (prefrontal and postfrontal regimes) and synoptic intrusions from higher latitudes (trough regime) comprise the other 65% of the time. The location and persistence of low clouds are strongly related to the location and intensity of the Bermuda High. Postfrontal and ridge patterns are the most conducive to liquid-phase clouds in the vicinity of the Azores and are accompanied by surface-level high pressure and subsidence.