Abstract
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. 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.
Highlights
The first aerosol indirect effect (Twomey, 1974) describes how, in the absence of changes in cloud macrophysical properties, increased aerosol concentrations lead to increased cloud albedo by increasing the droplet concentration Nd and average droplet surface area
This is because (a) MODIS Terra overestimates the daytime mean fc and Lp because there are significant afternoon decreases in cloud cover and liquid water path in this region (Rozendaal et al, 1995; Wood et al, 2002); (b) we neglect the albedo bias associated with the sub-1◦ variability in cloud properties, which is estimated to be less than 7.5% for marine low clouds
We explore contributions to the subseasonal temporal and spatial variability of albedo over the southeast Pacific and find that cloud microphysics does not contribute more than 10% to this albedo variance
Summary
The first aerosol indirect effect (Twomey, 1974) describes how, in the absence of changes in cloud macrophysical properties, increased aerosol concentrations lead to increased cloud albedo by increasing the droplet concentration Nd and average droplet surface area. It is not possible to determine this with observations alone, it is useful to examine further how patterns of subseasonal meteorological variability relate to the cloud variability and albedo. This study investigates such variability in the southeast Pacific (SEP), a subtropical marine stratocumulus region. We investigate how changes in large scale meteorology, macrophysical and microphysical cloud properties are associated with changes in albedo, with a view toward providing useful constraints for regional and global models based on variability and to examine how cloud properties covary in a relatively simple regional system.
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