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 (N_d) are explored. MAX and MIN N_d composites are defined by the top and bottom terciles of daily area-mean N_d values over the Arica Bight, the region with the largest mean oceanic N_d, 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. N_d and ship-based accumulation mode aerosol concentrations (N_a) correlate well (r = 0.65), with a best-fit aerosol activation value dln N_ddln N_a of 0.56 for pixels with N_d>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-N_d composite is characterized by a weaker subtropical anticyclone and weaker winds both at the surface and the lower free troposphere than the MIN-N_d composite. The MAX-N_d composite clouds over the Arica Bight are thinner than the MIN-N_d 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-N_d 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-N_d composite, with thicker, lower clouds and higher cloud fractions than for the MIN-N_d composite. The change in N_d at this location is small (though positive), suggesting that the MAX-MIN N_d 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.