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TRACER STUDIES OF TRANSPORT AND TRANSFORMATION IN CUMULI
Citation:
Stith, J., A. Alekzweeny, AND D. Burrows. TRACER STUDIES OF TRANSPORT AND TRANSFORMATION IN CUMULI. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-95/117 (NTIS PB95255717), 1995.
Description:
The results from a study of the transport and transformation of pollutants are reported. irborne measurements near Champaign, llinois and Milwaukee, Wisconsin were made during the summers of 1990 and 1992. easurements of the aerosol size distribution, wind, turbulence, cloud microphysics and trace gas measurements were collected from 31 research flights. he main objective was to document transport and transformation by cumuli, but data on out of cloud transformation processes were also collected. uring the 1990 study SF6 tracer gas was used to tag regions of the cloud bass and the air just above the rising cumulus. he first tracer experiment was used to follow the behavior of the cloud base region and the second was used to follow the entrainment of air into the rising cloud. n the largest cloud that was studied, low level air from below the cloud base was transported without dilution through the midlevels of the cloud. n smaller clouds studied, a more uniform dilution was observed between cloud base air and air from near the entrainment altitude. he dilutions in the lower levels of the small clouds could be explained by a simple buoyancy sorting model. he model offered possible explanations for the behavior of the tracer that was released at the cloud base of two non-precipitating cumuli. n one case the tracer was carried to the top of the cloud and in the second, it was not found past cloud midlevels. he buoyancy model suggested that in the first came air with 60% or greater percentages of cloud base air could attain the cloud top. he second case required 80% or greater percentages of cloud base air. n increase in the relative sizes of aerosol in the accumulation mode was observed in an area that was likely affected by the venting of cumuli in the area. imilar increases in size were not observed in evaporating regions of stratiform clouds. ypothesis is proposed to explain the measurements. he cumuli activate much smiler aerosol which, after aqueous phase reactions and evaporation. have a much greater relative increase in size than the larger aerosol activated by stratum clouds. he results from the entrainment experiments suggested that, during the early stages of entrainment the tracer remained mostly out of the cloud and was carried alongside the upper cloud regions by the circulation present there. n each experiment, concentrated SF was first found on the edges of tho cloud turrets. ater, the tracer mixed into the main portion of the turrets and rapidly diluted. he observations are consistent with the hypothesis that entrainment occurs through a vortex-like circulation that brings air from just the rising cloud top into the central region of the cloud. he eddy correlation method was used to determine the transfer velocity of gases and aerosols over lake Michigan downwind of Chicago. he results show downward transfer velocities (deposition) of 0.15 and 0.86 cm s-1 for O3 and aerosols in the size range of 0.1 to 3.0 um in diameter and upward transfer velocities of 0.04 and 0.54 cm s-1 for CO2 and water vapor about 7.5 km from the shoreline. t mid-lake much lower transfer velocities were measured. he turbulence intensity, in the subrange, was found to decrease as the air traveled over the cooler water.
