Southeast Pacific Stratocumulus: High Frequency Variability and Meso-scale Structures over San Félix Island

Abstract

Stratocumulus cloud cover patterns and their relationship to drizzle were characterized at San Felix Island (SFI: 26.5°S, 80°W) in the Southeast Pacific. Small closed, large closed and open cells were identified in about 65% of the MODIS (Moderate Resolution Imaging Spectro-radiometer) satellite images during 2003. The MODIS imagery was combined with ceilometer and surface meteorological measurements, human observations of cloud types and drizzle, and large-scale meteorological analyses for January through June. We identified two drizzle regimes: a synoptically-quiescent summer (Jan-March) regime characterized by a strong anticyclone, large closed cells, and frequent drizzle, and an autumn (April-June) regime characterized by a weaker anticyclone, small closed cells and open cells, and precipitation that was mainly associated with synoptic activity. The large closed cells had higher mean cloud bases and tops than the small closed cells, accounted for 45% of the cumulus-under- stratocumulus reports, and 29% of the total drizzle and rain reports. Large closed cells occupied more intermittently-coupled boundary layers than did the small closed cells. Open cells also occurred in more decoupled conditions, but only accounted for 18% of the total reports of drizzle and rain. The atmospheric stability of large and small closed cells were similar, but large closed cells were more commonly associated with a strong anticyclone, and small closed cells with wave activity superimposed upon a weakened anticyclone. The increased drizzle and occurrence of cumulus-under-stratocumulus in the summer rather than autumn is consistent with higher nighttime liquid water paths. A contribution of this study is the documentation of the ways in which synoptic activity can affect stratocumulus decks.