Factors Determining the Impact of Aerosols on Surface Precipitation from Clouds: An Attempt at Classification

Abstract

The simulation of the dynamics and the microphysics of clouds observed during the Large-Scale Biosphere–Atmosphere Experiment in Amazonia—Smoke, Aerosols, Clouds, Rainfall, and Climate (LBA–SMOCC) campaign, as well as extremely continental and extremely maritime clouds, is performed using an updated version of the Hebrew University spectral microphysics cloud model (HUCM). A new scheme of diffusional growth allows the reproduction of in situ–measured droplet size distributions including those formed in extremely polluted air. It was shown that pyroclouds forming over the forest fires can precipitate. Several mechanisms leading to formation of precipitation from pyroclouds are considered. The mechanisms by which aerosols affect the microphysics and precipitation of warm cloud-base clouds have been investigated by analyzing the mass, heat, and moisture budgets. The increase in aerosol concentration increases both the generation and the loss of the condensate mass. In the clouds developing in dry air, the increase in the loss is dominant, which suggests a decrease in the accumulated precipitation with the aerosol concentration increase. On the contrary, an increase in aerosol concentration in deep maritime clouds leads to an increase in precipitation. The precipitation efficiency of clouds in polluted air is found to be several times lower than that of clouds forming in clean air. A classification of the results of aerosol effects on precipitation from clouds of different types developing in the atmosphere with high freezing level (about 4 km) is proposed. The role of air humidity and other factors in precipitation’s response to aerosols is discussed. The analysis shows that many discrepancies between the results reported in different observational and numerical studies can be attributed to the different atmospheric conditions and cloud types analyzed.

Anthropogenic aerosols may have increased upper tropospheric humidity in the 20th century

“…We note that such an effect has been seen in a simulation of radiative-convective equilibrium of an aquaplanet with a constant sea surface temperature, using a two-dimensional cloud-resolving convection parameterization (Grabowski, 2003). With prescribed radiation, up- Khain et al (2005Khain et al ( , 2008 and are supported by observational studies of Zhang et al (2007) and Koren et al (2005), who found increased upper level cloudiness associated with an increased amount of pollution (see Sect. 4).…”

Online coupled regional meteorology chemistry models in Europe: current status and prospects

“…In models with a resolution of the order of magnitude of 10 km, deep convection impacts aerosol concentrations through microphysics and (Schroeder vection (Lupkes andSchlunzen, 1996) et al, 2006) Schlunzen, 1996) advection complex storm dynamics. However, the processes lack a complete understanding (Khain et al, 2008;Khain, 2009). Few attempts have been made to implement these processes in convection parametrizations (Grell and Freitas, 2013).…”

“…Khain et al, 2008). However, they are computationally more expensive and are usually only used for research applications.…”

“…The supersaturation needed for activating a CN is determined by the modified Köhler theory that takes the effects of surfactants and slightly soluble species into account. Another advantage of coupling two moment cloud schemes with prognostic aerosols is a more physically detailed approach to autoconversion (Khairoutdinov and Kogan, 2000;Liu et al, 2005). The most widely used approach is to assume specific size distributions for cloud droplets and rain drops (Seifert and Beheng, 2001).…”

Predicting global atmospheric ice nuclei distributions and their impacts on climate

“…Increased IN are thereby depicted to reduce the net cooling impact of such clouds on the planet and vice versa (8). Nevertheless, we must note that the sign and magnitude of the impact remains in question, because cloud modeling studies find that IN increases may lead as well to precipitation decreases, and their impact cannot necessarily be isolated from impacts of increases in all types of aerosols on the warm phase (>0°C) regions of clouds (9), nor the dynamical context in which clouds form (10). The sign and magnitude of the effects of increased IN on high (cirrus) clouds is similarly uncertain, but the impact is likely to be a decrease in the net planetary warming of this cloud type.…”