Synoptic variability of the stratocumulus deck

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Atmospheric Chemistry and Physics Discussions This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract Synoptic variations associated with changes in satellite-derived cloud droplet number concentrations (N d) for the southeast Pacific stratocumulus deck were examined using a composite analysis 5 and bottom terciles of daily area-mean N d values over the Arica Bight, the region with the largest mean oceanic N d. N d and ship-based accumulation mode aerosol concentrations (N a) correlate well (r = 0.65), with a best-fit aerosol activation value d lnN d d lnN a of 0.53 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 10 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, and occur within warmer, drier free tropospheres (as deduced from radiosondes) that imply 15 greater coastal subsidence. The cloud thinning compensates radiatively for increased reflectance from increases in N d , most apparent near the coast. CloudSat radar re-flectivities 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 N d within a more stable atmosphere 20 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-N d composite than for the MIN-N d composite, with thicker clouds and higher cloud fractions. The change in N d at this location is small (though positive), so that the composite difference primarily reflects synoptic changes. A one-point spatial 25 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 25524 Abstract …