Boundary layer clouds have a larger influence on the Earth’s radiative budget than any other cloud type and their response to changing environmental conditions is a main source of uncertainty in cloud feedbacks simulated in global climate models. In particular, their response to increases in aerosol concentrations is especially unknown. Motivated by this, the Routine Atmospheric Radiation Measurement (ARM) Aerial Facility (AAF) Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) experiment was conducted over the ARM Southern Great Plains (SGP) site in Oklahoma in 2009. RACORO was unique in that a routine operation strategy was implemented, whereby a large database of cloud properties unbiased by preconceived selection criteria was obtained, with the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter collecting over 260 hours of data in a 6-month time period. Frequent in-situ observations of boundary layer clouds in a variety of meteorological conditions were made.
In this presentation, 2337 penetrations of the CIRPAS Twin Otter through shallow cumuli over an 85 hour period are used to characterize the dependence of bulk cloud properties (i.e., liquid water content, total concentration, extinction and effective radius) on aerosol concentration in the context of varying meteorological conditions. Five different meteorological categories are identified based on airmass source regions and meteorological characteristics determined from weather maps, satellite images, radar images, rawinsonde data and back trajectory analysis. Unlike for previous studies of stratus and stratocumulus that provided evidence of precipitation suppression and increased water contents under enhanced aerosol loading, there is a decrease of liquid water content with increased aerosol count. The decrease in liquid water content was correlated with a decrease in vertical velocity inside cloud. The impact of entrainment and turbulent mixing on LWC and the dispersion of cloud droplet size distributions is also examined by assessing the importance of inhomogeneous and homogeneous as a function of position in cloud. Implications of our findings for the representation of aerosol indirect effects in models are discussed.