Wednesday, 5 June 2002
Effects of aerosols on rain formation as seen from experiments with a 2000-bin cloud microphysical model
A 2000-bin cloud microphysical parcel model is used to investigate the effect of cloud condensational nuclei (CCN) concentration and size distribution on warm rain formation both in maritime and continental environmental thermodynamic conditions. As concerns to sensitivity of precipitation to aerosol characteristics, clouds are separated into three groups: 1) in clouds belonging to the first cloud group collisions do not actually occur, and, therefore, the clouds of this type do not precipitate; 2) in clouds of the second group collisions are triggered in the vicinity of cloud top, so that only some fraction of the cloud water content (CWC) is converted into rain, and 3) in clouds belonging to the third group the most of the CWC is converted into rain well below cloud top. Clouds belonging to different groups have different depth. The heights separating different cloud groups depend on thermodynamic conditions in the environment. According to the results of our experiments, under maritime conditions clouds belonging to the first group have a depth below 2.5 km, the depth of clouds of the second group ranges from 2.5 to 5-6 km, and clouds belonging to the third group are deep clouds with depth exceeding 5-6 km. Clouds developing under unstable continental conditions belong either to the first or to the second group depending of the air humidity and other thermodynamic conditions. Clouds belonging to the second group turn out to be the most sensitive to changes in CCN spectra. The increase in the concentration of CCN of the medium size (second CCN mode) slows down cloud droplet growth rate and increases the time needed to initiate collision. At the same time, an increase in the concentration of large CCN reduces the height of collision initiation and increases the fraction of CWC converted into rain, resulting in significant rain enhancement. Even a comparatively small amount of large CCN can lead to increase in precipitation from these clouds several times. Sensitivity of clouds belonging to the first and the third group to concentration of large CCN is much lower that that of clouds of the second group. Clouds of the first group are too small to produce rain even in case of significant concentration of large CCN. Clouds belonging to the third group actually realize all their precipitation potential, so that an additional amount of large CCN cannot increase precipitation. Moreover, an “injection” of a significant amount of large CCN leads to a decrease in the precipitation amount in this case. It is shown that precipitation from clouds developed under maritime conditions is not sensitive to concentration of very small (less than about 0.01 micron in radius) CCN. Since vertical velocities in maritime cloud are comparatively small, small CCN remain largely non-activated. Application of the results is discussed in relation to the problems of cloud droplet spectra and rain formation over oceans and continents in cases of penetration of aerosols of different size, as well as in relation to the problem of artificial rain enhancement by cloud seeding.