Subseasonal variability of low cloud radiative properties over the southeast Pacific Ocean University of Washington, Seattle, Washington, USA
Received: 01 October 2009 – Published in Atmos. Chem. Phys. Discuss.: 25 November 2009 Abstract. Subseasonal variability of cloud radiative properties in the persistent
southeast Pacific stratocumulus deck is investigated using MODIS satellite
observations and NCEP reanalysis data. A once-daily albedo proxy is derived
based on the fractional coverage of low cloud (a macrophysical field) and
the cloud albedo, with the latter broken down into contributions from
microphysics (cloud droplet concentration) and macrophysics (liquid water
path). Subseasonal albedo variability is dominated by the contribution of
low cloud fraction variability, except within 10–15° of the South
American coast, where cloud albedo variability contributes significantly.
Covariance between cloud fraction and cloud albedo also contributes
significantly and positively to the variance in albedo, which highlights how
complex and inseparable the factors controlling albedo are. Droplet
concentration variability contributes only weakly to the subseasonal
variability of albedo, which emphasizes that attributing albedo variability
to the indirect effects of aerosols against the backdrop of natural
meteorological variability is extremely challenging.
Revised: 29 March 2010 – Accepted: 20 April 2010 – Published: 29 April 2010
The dominant large scale meteorological variability is associated with the
subtropical high pressure system. Two indices representing changes in the
subtropical high strength and extent explain 80–90% of this variability,
and significantly modulate the cloud microphysical, macrophysical, and
radiative cloud properties. Variations in droplet concentration of up to
50% of the mean are associated with the meteorological driving. We
hypothesize that these fluctuations in droplet concentration are a result of
the large scale meteorology and their correlation with cloud macrophysical
properties should not be used as evidence of aerosol effects. Mechanisms by
which large scale meteorology affects cloud properties are explored. Our
results support existing hypotheses linking cloud cover variability to
changes in cold advection, subsidence, and lower tropospheric stability.
Within 10° of the coast interactions between variability in the surface
high pressure system and the orography appear to modulate both cloud
macrophysical properties and aerosol transport through suppression of the
marine boundary layer depth near the coast. This suggests one possible way
in which cloud macrophysical properties and droplet concentration may be
correlated independently of the second aerosol indirect effect. The results
provide variability constraints for models that strive to represent both
meteorological and aerosol impacts on stratocumulus clouds.
Citation: George, R. C. and Wood, R.: Subseasonal variability of low cloud radiative properties over the southeast Pacific Ocean, Atmos. Chem. Phys., 10, 4047-4063, doi:10.5194/acp-10-4047-2010, 2010.