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J. K. S. Ching
and
A. J. Alkezweeny

Abstract

This paper examines the exchange of material by convective cloud processes between the mixed layer and the overlying free troposphere. It describes results of a field experiment that was conducted in Lexington, Kentucky, during the period from 20 July to 24 August 1983 to study the processes associated with the vertical transport of acidic pollutants by nonprecipitating cumulus convection. An aircraft released SF6 tracer either within or above the mixed layer, and it was sampled by another aircraft at various levels within or above the mixed layer in the active cumulus convective cloud zone, as well as on the ground. The results show that mixed layer pollutants are vented above the entrainment zone of its boundary layer into the overlying cloud layer by fields of active cumulus and that such active clouds may force cloud layer air downward into the mixed layer.

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Jason K. S. Ching
and
J. A. Businger

Abstract

The equations for horizontally homogeneous planetary boundary layer flow with constant eddy viscosity are integrated in time and height. The special case for which the direction of the pressure gradient force is a periodic function of time is studied in detail. The nondimensional number F=z(4Kt)−½ is seen to be the proper scale which describes the flow response to the boundary layer.

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J. M. Godowitch
,
J. K. S. Ching
, and
J. F. Clarke

Abstract

The evolutionary cycle of the nocturnal radiation inversion layer, from formation through the time of erosion under fair weather summer conditions was investigated by time-series analyses of observations of inversion base and top heights, and inversion strength at an urban and a nonurban site in St. Louis, Missouri. The time-dependent behavior of each inversion parameter is presented from statistical and least-squares regression analyses. Differences of inversion evolution between these sites are discussed.

A surface-based inversion generally formed before sunset at the nonurban site, and the growth of the inversion top height has been described quite well by (2KTt)½, where KT is the thermal eddy coefficient. The average time of formation of an elevated inversion layer at the urban site was 2.5 h after sunset. The height of the nocturnal urban boundary layer decreased after formation under steady wind conditions.

The urban mixing height was consistently higher during the morning than at the nonurban site, although the difference diminished with time since the nonurban mixing height growth rate was greater. The slower growth rate of the urban mixing height was attributed primarily to advection of relatively cold air and lower mixing heights from the upwind nonurban environment. An overall rise of the inversion top height after sunrise was believed to be due to urban-induced upward motions, caused by low-level convergence that produced increasing mixing height growth rates. The average time for the inversion layer to erode completely was 4 h after sunrise at both sites.

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