Aerosol indirect radiative forcing of climate change (enhancement of cloud drop number concentration and by anthropogenic aerosols and resultant change in cloud albedo, commonly denoted the aerosol first indirect effect or "Twomey effect") is considered the most uncertain forcing of climate change over the industrial period, despite numerous studies demonstrating such modification of cloud properties and several studies quantifying resulting changes in shortwave radiative fluxes. We have previously used ground-based remote sensing of cloud optical depth by narrowband radiometry and liquid water path (LWP) by microwave radiometry to demonstrate substantial (factor of 2) day-to-day variation in cloud drop effective radius (re) at the ARM Southern Great Plains site that is weakly associated with variation in aerosol loading as characterized by light-scattering coefficient at the surface (Kim et al., JGR, 2003), but the substantial scatter suggests the importance of meteorological influences on cloud drop size as well. Here we extend that study by examining key meteorological variables that may contribute to variation in re. The mixing height is determined by the vertical potential temperature gradient. The Brunt-Vaisala frequency, the (angular) oscillation frequency of a parcel of air following vertical displacement in a system initially at rest, is determined as the square root of the product of gravity and potential temperature gradient, the potential temperature gradient being determined by radiosonde. The analysis indicates a correlation of smaller droplets with higher Brunt-Vaisala frequency at height above the top of the mixed layer but just beneath the cloud top, such that the more stable the inversion, the smaller the droplets. Such a correlation would result in enhancement of the aerosol first indirect effect would be enhanced in situations of highly stable cloud tops.
This page was last updated 2005-02-05.
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