Abstract
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 dln Nddln Na 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-Nd 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 Nd 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 location is small (though positive), suggesting that the MAX-MIN 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.
Highlights
The subtropical cloud-capped marine boundary layer has a strong climate impact through the decks’ high solar reflectivity whereas the emitted long-wave radiation remains close to that of surface emission under clear skies (Hartmann et al, 1992)
We focused on October months only, to reduce influences from the seasonal cycle
Episodes with high Nd over the Arica Bight are associated with a weaker anticyclone, weaker surface and freetropospheric winds, and thinner clouds, shown by Wood et al (2008) and George and Wood (2010)
Summary
The subtropical cloud-capped marine boundary layer has a strong climate impact through the decks’ high solar reflectivity whereas the emitted long-wave radiation remains close to that of surface emission under clear skies (Hartmann et al, 1992). A deeper understanding of the processes affecting the radiative properties of maritime warm clouds, both large-scale and microphysical, is necessary for developing confidence in future climate predictions This is becoming important as more aerosol indirect effects become incorporated into climate models. Lwp,met or the change in cloud albedo A with cloud droplet number Nd , all else held constant, Twomey, 1977), is considered in the Intergovernmental Panel on Climate Change (IPCC) 2007 assessment Other effects, such as influences on the cloud albedo through changes in liquid water path (LWP) induced by changes in. Cloud properties (cloud top height, fraction, Pacific where the Andes dictate along-shore winds that can liquid water path/thickness and droplet number) are derived transport aerosol from the more developed southern regions primarily from MODIS data.
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