Cloud condensation nuclei (CCN) play an important role in the processes leading to precipitation formation and cloud radiative forcing on climate. Increase in CCN concentrations due to anthropogenic activities alters microphysical properties of clouds by increasing the number concentrations and reducing the sizes of cloud droplets. The changes in cloud microphysics are also closely related to the changes in precipitation efficiency, cloud water content, fractional cloudiness and cloud lifetime. In recent years the sensitivity of the cloud droplet spectral shape and the rate of raindrop formation to the concentration and size distribution of CCN have increasingly been investigated both in observational and numerical studies.
The differences of cloud properties and precipitation formation mechanism of the clouds forming over the continents and over the ocean are attributed not only to different concentrations of atmospheric aerosols serving as CCN but also to different thermal conditions (lapse rates and humidity). However, most previous studies were focused on only one specific thermodynamic condition and the main goal of most numerical simulations of CCN effects on cloud microphysics was the reproduction of a cloud that matched in cloud height with the specific observed clouds in comparison, thus the numerical simulations do not cover the wide variability of cloud dimensions observed under similar thermodynamic conditions. Therefore there is little room to discuss the effect of thermodynamic conditions on precipitation mechanism.
The goal of this paper is to address the issue of aerosol-cloud-precipitation interaction in different thermodynamic conditions using a cloud model and give some observational support. A two dimensional cloud model with detailed (using hydrometeor size bins) cloud microphysics was employed for numerical simulations. Two different CCN concentrations, representing maritime and continental environments, are used as input CCN data and the model was run on eight thermodynamic conditions, represented by the convective available potential energy (CAPE), that are selected from observational soundings.
Model results showed that small CAPE conditions developed a shallow cloud with negligible precipitation when continental CCN were used while maritime CCN produced somewhat deeper clouds with significant precipitation. In case of strong convection (large CAPE), the sensitivity of the precipitation rate to the CCN concentrations was relatively low. In general both maritime and continental clouds produced significant precipitation but continental, rather than maritime, clouds were deeper and eventually produced stronger updrafts. Another important factor is found to be the altitude difference of lifting condensation level (LCL) and freezing level (FL). Some thermodynamic soundings with a large CAPE but a deep FL-LCL did not produce significant precipitation, because the cloud could not penetrate the FL and thus ice process was prohibited. This demonstrates that the presence of ice phase precipitating particle in the clouds is critical to initiating heavy precipitation.
As an effort to find some observational support for these model results, 6-hourly sounding data at one Korean weather station and the hourly precipitation data from a nearby (20 km apart) Automatic Meteorological Observing Station (AMOS) were analyzed for the year 2002. Among the 1460 rawinsonde soundings, only 30 cases were classified as temporally and spatially isolated convective cases. Average CAPE for the 30 isolated convective cases was about 700 J kg-1 and these 30 cases were divided into two groups-below and above 700 J kg-1. Analysis of these cases showed that there was less precipitation for deeper FL-LCL than thinner FL-LCL for similar CAPE, probably because of high FL and thus prohibition of ice process. In below 700 J kg-1 CAPE group, deeper FL-LCL exhibited less precipitation amount: r2 of FL-LCL vs. precipitation amount was 0.44. This is consistent with the model result. In above 700 J kg-1 CAPE group, the tendency was weak.