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

by D. Painemal, P. Zuidema
Atmospheric Chemistry and Physics ()
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Synoptic and satellite-derived cloud property vari-ations for the southeast Pacific stratocumulus region associ-ated 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 re-gion with the largest mean oceanic N d , for the five Octo-ber months of 2001, 2005, 2006, 2007 and 2008. The abil-ity of the satellite retrievals to capture composite differences is assessed with ship-based data. N d and ship-based accu-mulation mode aerosol concentrations (N a) correlate well (r =0.65), with a best-fit aerosol activation value d lnN d d lnN a of 0.56 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 character-ized 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 albe-dos, 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 signif-icantly 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 loca-Correspondence to: D. Painemal ( tion is small (though positive), suggesting that the MAX-MIN N d composite differences in radiative properties pri-marily 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 in-creased cloud fraction, despite accompanying weaker free tropospheric subsidence.

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