Microphysical variability in southeast Pacific Stratocumulus clouds: Synoptic conditions and radiative response

Abstract

Synoptic and satellite-derived cloud property variations for the southeast Pacific stratocumulus region associated with changes in coastal satellite-derived cloud droplet number concentrations (Nd ) are explored. MAX and MIN Nd composites are defined by the top and bottom terciles of daily area-mean Nd values over the Arica Bight, the region with the largest mean oceanic Nd , for the five October months of 2001, 2005, 2006, 2007 and 2008. The ability of the satellite retrievals to capture composite differences is assessed with ship-based data. Nd and ship-based accumulation mode aerosol concentrations (Na) correlate well (r =0.65), with a best-fit aerosol activation value d lnN d/d lnNa of 0.56 for pixels with Nd >50 cm?3. The adiabatically-derived MODIS cloud depths also correlate well with the ship-based cloud depths (r =0.7), though are consistently higher (mean bias of almost 60 m). The MAX-Nd composite is characterized by a weaker subtropical anticyclone and weaker winds both at the surface and the lower free troposphere than the MIN-Nd composite. The MAX-N d composite clouds over the Arica Bight are thinner than the MIN-Nd composite clouds, have lower cloud tops, lower near-coastal cloud albedos, and occur below warmer and drier free tropospheres (as deduced from radiosondes and NCEP Reanalysis). CloudSat radar reflectivities indicate little near-coastal precipitation.The co-occurrence of more boundary-layer aerosol/higher N d within a more stable atmosphere suggests a boundary layer source for the aerosol, rather than the free troposphere. The MAX-Nd composite cloud thinning extends offshore to 80°W, with lower cloud top heights out to 95°W. At 85°W, the top-of-atmosphere shortwave fluxes are significantly higher (≃50%) for the MAX-Nd composite, with thicker, lower clouds and higher cloud fractions than for the MIN-Nd composite. The change in Nd at this loca-tion is small (though positive), suggesting that the MAXMIN Nd composite differences in radiative properties primarily reflects synoptic changes. Circulation anomalies and a one-point spatial correlation map reveal a weakening of the 850 hPa southerly winds decreases the free tropospheric cold temperature advection. The resulting increase in the static stability along 85°W is highly correlated to the increased cloud fraction, despite accompanying weaker free tropospheric subsidence. © 2010 Author(s).