Microphysical variability in southeast Pacific Stratocumulus clouds: synoptic conditions and radiative response Rosenstiel School of Marine and Atmospheric Sciences University of Miami, Florida, USA
Received: 04 November 2009 – Published in Atmos. Chem. Phys. Discuss.: 27 November 2009 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.
Revised: 20 April 2010 – Accepted: 26 June 2010 – Published: 09 July 2010
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.
Citation: Painemal, D. and Zuidema, P.: Microphysical variability in southeast Pacific Stratocumulus clouds: synoptic conditions and radiative response, Atmos. Chem. Phys., 10, 6255-6269, doi:10.5194/acp-10-6255-2010, 2010.