Cloud-rain interactions: As complex as it gets

Abstract

The effect of aerosol on clouds and precipitation poses one of the largest uncertainties in the estimation of the anthropogenic contribution to climate change. Often the local effect of aerosol on the radiation field both directly and indirectly by changing cloud properties can be more than an order of magnitude larger than the effect of greenhouse gases. Recent works also suggest that some of the local aerosol effects such as the heating of the atmosphere from aerosol absorption can have a large-scale impact. However, due to the inhomogeneous distribution and short lifetime of aerosols, the inherent complexity of cloud microphysics and dynamics, and the strong coupling of aerosol-related processes with meteorology, it is still challenging to estimate the overall effect that aerosols exert on the radiation field and climate. The climatic effect of aerosol-cloud interaction is not limited to the radiation field. By changing the cloud microphysical and radiative properties, aerosols may affect precipitation amount and patterns. Precipitation processes are located at the end of the 'food chain' of aerosol-cloud processes. Rain rates and patterns are the final result of many cloud processes and feedbacks, some of which are affected by aerosols. Changes in precipitation patterns could lead to serious hydrological consequences. For example if the same amount of rain precipitates in shorter time, e.g. heavier rain rates, the probability of floods increases, a larger portion of the water is drained by rapid surface run-off and the amount of water penetrating the subsurface and available as an underground reservoir of drinking water is reduced. Moreover, the subsurface water distribution depends heavily on topography and geomorphology. Therefore small changes in the timing or location of precipitation may dramatically alter the surface and subsurface water distribution and affect the water reservoir. In studying aerosol-cloud-precipitation interactions, the largest challenge is determining how to separate the effect of meteorological processes from aerosol effects. Simple statistical correlations between observed or retrieved aerosol and cloud properties do not imply causality. With the help of sophisticated cloud numerical models where certain variables or processes can be controlled in sensitivity simulations, aerosol-precipitation casual relationships could be examined in specific conditions. However, it is not always clear whether such aerosol-precipitation relationships seen in the model could be applied to more general meteorological scenarios. Aerosol-cloud-precipitation interaction is a highly complex problem involving processes and feedbacks that span the size range from an aerosol particle (10-7m) to a cloud (103m), and all the way to synoptic scale systems (106m). These feedbacks determine whether a droplet, initiated at a size of few microns, could grow within the time scale of a cloud's lifetime to reach a raindrop size of a few mm, and whether this raindrop will fall all the way to the surface and be available as fresh water. These feedbacks include dynamic and thermodynamic processes of precipitating particles, which in conjunction with other processes determine the micro and macrophysical properties of the cloud and hence determine the cloud's effect on the regional radiation field and local climate. Thus, aerosol, cloud, precipitation and radiation interactions are inherently linked, and need to be addressed as a single problem when attempting to better understand human-induced changes in the climate system. Focus on Aerosol Precipitation Contents Drizzle rates versus cloud depths for marine stratocumuli A B Kostinski Characteristics of vertical velocity in marine stratocumulus: comparison of large eddy simulations with observations Huan Guo, Yangang Liu, Peter H Daum, Gunnar I Senum and Wei-Kuo Tao Dispersion bias, dispersion effect, and the aerosol-cloud conundrum Yangang Liu, Peter H Daum, Huan Guo and Yiran Peng The impact of smoke from forest fires on the spectral dispersion of cloud droplet size distributions in the Amazonian region J A Martins and M A F Silva Dias A conceptual model for the link between Central American biomass burning aerosols and severe weather over the south central United States Jun Wang, Susan C van den Heever and Jeffrey S Reid © 2008 IOP Publishing Ltd.