Sensitivity of a Cloud Microphysical Model to an Urban Environment

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Abstract
Observations showed increased concentrations of cloud condensation nuclei (CCN) in air samples collected over and downwind of St. Louis when compared to upwind samples. Aircraft observations of urban clouds showed corresponding increased concentrations of cloud base droplets. In addition, observations indicated higher cloud bases and decreased elevations of average first echo base heights in the St. Louis/East St. Louis area as compared with similar clouds over rural areas. The purpose of this paper is to examine the possible role of CCN chemical composition and number concentration in producing the observed phenomena. A closed parcel model of condensation and collection was employed for this purpose. The results suggest that the observed differences of depth from cloud base to first echo height between urban and rural clouds do not result from concentration differences in any CCN size range. Results of model calculations also suggest that variations in chemical composition of the largest CCN (≥1.0 μm radius) were not responsible for the observed urban/rural differences. A hypothesis based on observations and model results is presented for explaining the observed differences in cloud base to first echo depth in terms of differences between the evolution and strength of updrafts in urban and rural clouds.

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