The Changing Character of Precipitation

SEPTEMBER 2003 AMERICAN METEOROLOGICAL SOCIETY | W hy does it rain? If a parcel of air rises, it expands in the lower pressure, cools, and therefore condenses moisture in the parcel, producing cloud and, ultimately, rainfall—or perhaps snowfall. So a key ingredient is certainly the many and varied mechanisms for causing air to rise. These range from orographic uplifting as air flows over mountain ranges, to a host of instabilities in the atmosphere that arise from unequal heating of the atmosphere, to potential vorticity dynamics. The instabilities include those that result directly in vertical mixing, such as convective instabilities, to those associated with the meridional heating disparities that give rise to baroclinic instabilities and the ubiquitous fronts and low and high pressure weather systems. Thus cold air pushing underneath warmer air (advancing cold front) or warm air gliding over colder air (advancing warm front), and so on, can all provide opportunities for air to rise. Henceforth we use the term “storm” as shorthand for all the potential disturbances that create upward motion in the atmosphere, since many will in fact be thunderstorms, or extratropical cyclones manifested as rainor snowstorms. Although the distribution of these events around the globe vary with climate, the basic mechanisms for causing air to rise presumably do not in and of themselves change. They need to be better understood and modeled, but from a societal standpoint, the fact that their relative importance can change with time is a significant but often overlooked consequence. Similarly, mechanisms involved in the actual condensation process are important. After a cloud forms, if the cloud droplets grow large enough (> 4-μm diameter) or the air cools sufficiently for the cloud droplets to freeze, then rain and snow form, respectively. The microphysics of cloud droplets matters. Human interference, such as putting various kinds of pollutants and aerosols into the atmosphere, can make important differences in the number and size of cloud droplets, precipitation formation, within-cloud heating, and the cloud’s lifetime (e.g., Rosenfeld 2000; Ramanathan et al. 2001; Kaufman et al. 2002). Lightabsorbing aerosols can short-circuit the hydrological THE CHANGING CHARACTER OF PRECIPITATION