Warm Convective Clouds Research Articles

The scavenging of two different types of marine aerosol particles calculated using a two-dimensional detailed cloud model

Our 2-D dynamic model including spectral microphysics and scavenging has been evaluated for a warm precipitating convective cloud at Day 261 (18 September 1974) of the GATE campaign. Two different chemical species ((NH 4 ) 2 SO 4 and NaCl) of aerosol particles were followed in the air, inside the drops in the cloud, and inside the drops reaching the ground. Concerning the dynamics and microphysics, as well as the scavenging and wet deposition, the model results agree quite well with available observations. The cloud rained after 19 min of cloud life time. For the considered aerosol loading of the atmosphere, rough estimates are derived for the total material processed by such a warm convective cloud as input for larger scale models. In particular, the following conclusions could be drawn for the situation considered. (1) If a drop spectrum forms on an aerosol spectrum where the small particles consist of (NH 4 ) 2 SO 4 and the large ones of NaCl, the resulting small drops also mainly consist of (NH 4 ) 2 SO 4 and the larger drops of NaCl. Collision and coalescence causes a redistribution of the chemical species such that the precipitation sized drops consist of NaCl to about 70%. (2) The mixing ratio of aerosol material in the drops is a function of the age of the drops and their history and therefore the variation of the mixing ratio with drop size depends on the entrainment and evoluion of the relative humidity. The mixing ratio decreased with increasing drop radius at almost all grid points due to continuous activation of fresh particles. (3) Assuming that the sulfate aerosol would not consist of (NH 4 ) 2 SO 4 particles but instead consist of NH 4 HSO 4 particles the acidic cloud water has a pH of 4.7 which agrees with observations of marine precipitation. (4) The scavenging efficiency of the cloud considered is closely related to its precipitation efficiency (both near 40%). About 90% of the total amount of aerosol material scavenged is incorporated into the cloud water through nucleation scavenging. About 30% of the aerosol material in the rain at the ground is due to scavenging below cloud base. (5) The scavenging coefficient of the considered storm was on the order of 10 -4 s -1 . Differences between two commonly used formulations for calculating the scavenging coefficient Λ can be explained in terms of humidity changes of the atmosphere. DOI: 10.1034/j.1600-0889.1991.t01-2-00004.x

Workshop on Warm Convective Clouds, 9–10 February 1987, Paris, France

1 Centre de Recherches en Physique de L'Atmosphere, Magny-les-Hameaux, France.2 LAMP, University of Clermont-Ferrand, France.3 DFVLR, West Germany.4 Meteorological Office, Bracknell, UK.5 University of Bonn, West Germany.

A theoretical study of progressive developments in rain- drop size distribution and other characteristics in rain showers from 'warm' convective type clouds

A study, on a theoretical basis, of the expected size distribution of raindrops and rainfall intensity at various phases of rain showers from an overhead large cumulus cloud of 'warm' type has been made, assuming that each raindrop is the result of growth on a 'giant' sea-salt nucleus, and that droplet growth beyond a certain sizes is due mainly to coalescence following collisions between cloud droplets. The study, based on assumptions of plausible physical conditions in a cloud in regard to (a) concentration and size distribution of' giant sea-salt nuclei in air, (b) mean cloud liquid water content and (c) updraft rate and its variations with time, brings out features which are in general conformity with what are observed in rain from warm convective type clouds, and provides some corroboration of the present accepted theory of coalescence growth of raindrops, and of the salt-nucleus hypothesis of rain formation in such clouds.