Mesoscale stratocumulus cloud patterns around San Felix Island

Laser ceilometer and surface meteorological measurements during the first semester of 2003 at San Felix Island (SFI: 26.5°S, 80°W) provide a unique opportunity to characterize both in-situ and satellite –derived stratocumulus (SC) cloud cover patterns and their variability at and around SFI. Here, maximum SC cover peaks in austral summer, consistent with a southernmost location of the Southeast Pacific SC deck. The diurnal cycle (AM/PM) in SC frequency, as derived from ceilometer data, also peaks in austral summer (3.0) with an almost negligible diurnal cycle in winter. Synoptic-scale forcing of the SC cover was first assessed by means of the low-troposphere stability parameter (LTS = potential temperature difference between 700 hPa and the ocean surface) derived from in-situ and from the NCEP/NCAR reanalysis indicating a linear correlation (r = 0.6 ) with the morning (AM) frequency of SC ceilings below 2000 m and with mean morning cloud base heights (r = - 0.6), when low-passed with a 10-day cutoff filter, consistent with a peak in the spectral variance of these parameters. MODIS satellite imagery allowed for classification of SC into three main patterns that could be identified in about 50 % of the images obtained for 2003: small closed cells, large closed cells and open cells. A fourth category included all cloud patterns that could not fit into any of the previous groups. Large closed cells occur mainly (75%) in austral spring and summer, while the small ones prevail (81%) in austral fall and winter, when open cell patterns are also more frequent (68%). Large closed-cell decks show mean cloud bases about 1200 m with large (~ 400 m) standard deviation that includes drizzle events, in a largely decoupled MBL, as inferred from LCLs. In contrast, small closed-cell structures exhibit bases below 900 m with smaller deviations (~ 220 m), above an almost fully coupled MBL. Sequences of open- cell transitions to small closed- cell and vice versa were occasionally identified during austral winter, consistent with larger departures in the LTS.