Abstract
Synoptic variations associated with changes in satellite-derived cloud droplet number concentrations (Nd) for the southeast Pacific stratocumulus deck were examined using a composite analysis applied to daily values from the five October months of 2001, 2005, 2006, 2007 and 2008. MAX and MIN Nd composites were 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. Nd and ship-based accumulation mode aerosol concentrations (Na) correlate well (r=0.65), with a best-fit aerosol activation value dln Nddln Na of 0.53 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-Nd composite clouds over the Arica Bight are thinner than the MIN-Nd composite clouds, have lower cloud tops, and occur within warmer, drier free tropospheres (as deduced from radiosondes) that imply greater coastal subsidence. The cloud thinning compensates radiatively for increased reflectance from increases in Nd, most apparent near the coast. CloudSat radar reflectivities do not imply significant aerosol scavenging by precipitation near the coast, indicating that variability in wind transport contributes to the aerosol variability. The co-occurrence of more boundary-layer aerosol/higher Nd within a more stable atmosphere suggests a boundary layer source for the aerosol, rather than the free troposphere. Along 85° W, the top-of-atmosphere shortwave fluxes are significantly higher (~50%) for the MAX-Nd composite than for the MIN-Nd composite, with thicker clouds and higher cloud fractions. The change in Nd at this location is small (though positive), so that the composite difference primarily reflects synoptic changes. A one-point spatial correlation map reveals anomalous northerly winds at 850 hPa account for an anomalous warm temperature advection. The increase in the static stability along 85° W is highly correlated to the increased cloud fraction, despite accompanying weaker free tropospheric subsidence. This synoptic impact on offshore cloud properties is arguably our most radiatively important finding, and draws attention to the free tropospheric meridional flow as a meteorological control.