THE CHANGING CHARACTER OF PRECIPITATION BY KEVIN E. TRENBERTH, AIGUO DAI, ROY M. RASMUSSEN, AND DAVID B. PARSONS As climate changes, the main changes in precipitation will likely be in the intensity, frequency, and duration of events, but these characteristics are seldom analyzed in observations or models. hy does it rain? If a parcel of air rises, it expands pushing underneath warmer air (advancing cold in the lower pressure, cools, and therefore front) or warm air gliding over colder air (advancing W condenses moisture in the parcel, producing warm front), and so on, can all provide opportuni- cloud and, ultimately, rainfall—or perhaps snowfall. ties for air to rise. Henceforth we use the term “storm” So a key ingredient is certainly the many and varied as shorthand for all the potential disturbances that mechanisms for causing air to rise. These range from create upward motion in the atmosphere, since many orographic uplifting as air flows over mountain will in fact be thunderstorms, or extratropical cy- ranges, to a host of instabilities in the atmosphere that clones manifested as rain- or snowstorms. Although arise from unequal heating of the atmosphere, to po- the distribution of these events around the globe vary tential vorticity dynamics. The instabilities include with climate, the basic mechanisms for causing air to those that result directly in vertical mixing, such as rise presumably do not in and of themselves change. convective instabilities, to those associated with the They need to be better understood and modeled, but meridional heating disparities that give rise to from a societal standpoint, the fact that their relative baroclinic instabilities and the ubiquitous fronts and importance can change with time is a significant but low and high pressure weather systems. Thus cold air often overlooked consequence. Similarly, mechanisms involved in the actual con- densation process are important. After a cloud forms, AFFILIATIONS: TRENBERTH, DAI, RASMUSSEN, AND PARSONS—The if the cloud droplets grow large enough (> 4-µm di- National Center for Atmospheric Research,* Boulder, Colorado ameter) or the air cools sufficiently for the cloud drop- *The National Center for Atmospheric Research is sponsored by lets to freeze, then rain and snow form, respectively. the National Science Foundation The microphysics of cloud droplets matters. CORRESPONDING AUTHOR: Dr. Kevin E. Trenberth, National Human interference, such as putting various kinds of Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307 pollutants and aerosols into the atmosphere, can make E-mail: [email protected] important differences in the number and size of cloud DOI: 10.1175/BAMS-84-9-1205 droplets, precipitation formation, within-cloud heat- In final form 27 March 2003 ing, and the cloud’s lifetime (e.g., Rosenfeld 2000; ©2003 American Meteorological Society Ramanathan et al. 2001; Kaufman et al. 2002). Light- absorbing aerosols can short-circuit the hydrological AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER 2003 | 1205 cycle by depositing heat directly into a layer that include the correct combination of frequency and would otherwise be heated by the latent heating in intensity of precipitation—it is unlikely that useful precipitation originating from heat absorbed at the forecasts or simulations will result. surface and lost through evaporation (see also Menon Moreover, we argue later that it is the characteris- et al. 2002). tics of rain that are more apt to change as climate These aspects have been the focus of much meteo- changes. This means that prospects are greater for rological and climate research but are not the focus changes in the extremes of floods and droughts than of this article. The other main ingredient in the open- in total precipitation amount. These aspects are also ing statement about why it rains is the assumption that very important for agriculture, hydrology, and water there is moisture present. Where exactly does the resources, yet have not been adequately appreciated moisture come from? It is argued that this aspect of or addressed in studies of impacts of climate change. precipitation is one that has been underappreciated Hence, a central purpose of this article is to highlight and is worthy of more attention. After all, it will not the need for more attention to this topic. rain at all unless there is a supply of moisture. Yet time In this paper, we discuss issues with the changes after time in major droughts, such as the one experi- to be expected in precipitation, problems in model- enced in the western United States in the summer of ing it, and the need to look at the problems in new 2002, this aspect seems to be overlooked and fruitless ways; ways that advance understanding and improve calls are made for cloud seeding to produce rain and models. We focus on conceptual aspects to illustrate snow.1 Cloud seeding cannot create moisture, nor can the main points, as definitive data often do not exist. it make the large-scale environment more favorable These aspects are some of the drivers behind a new for convection onset. It may harvest more moisture initiative at the National Center for Atmospheric Re- locally, but this is likely at the expense of precipita- search (NCAR) on the water cycle, which will be tion downstream. briefly introduced later. The character of precipitation depends not only on the nature of the storm, but also on the available FACTORS INVOLVED IN THE CHARAC- moisture. We speak glibly about rainfall, or more gen- TERISTICS OF PRECIPITATION. Moisture erally, precipitation, and often place it on a par with sources. So where does rain come from? The precipi- other atmospheric variables such as temperature, table water (column-integrated water vapor amount) pressure, wind, etc. Yet most of the time it does not in midlatitudes is typically 25 mm (an inch or so; e.g., rain. And when it does, the rain rate varies. At the very Trenberth and Guillemot 1994, 1998) and this is also least we could always consider how frequent and how close to the global mean value. So how can we possi- intense the rain is when it does fall, in addition to the bly get more than an inch of rainfall? The efficiency total amounts.2 Steady moderate rains soak into the of rainfall mechanisms is not that great, perhaps 30% soil and benefit plants, while the same rainfall (Fankhauser 1988; Ferrier et al. 1996), as not all amounts in a short period of time may cause local storm-ingested water vapor is converted to precipi- flooding and runoff, leaving soils much drier at the tation and it is not possible to dry out the air com- end of the day. This example highlights the fact that pletely. Instead, the relative humidity of the air left the characteristics of precipitation are just as vital as behind is typically about 70% overall as there are dry the amount. In models it may be possible to “tune” downdrafts but also moist cloud debris that remain. parameters to improve amounts, but unless the So perhaps only about 7.5 mm of the precipitable amounts are right for the right reasons—and these water is really available for precipitation. However, it can and often does rain this amount in an hour and, as we argue below, the average rain rate globally is 1 Denver Water along with the Upper Arkansas Water Conser- −1 vancy District and the Southeastern Colorado Water Conser- probably about 45 mm day when it is raining, and vancy District signed a $700,000 contract for cloud seeding so about 6 times the locally available moisture can fall during the 2002/03 winter and spring snow season with a goal in 1 day. of enhancing precipitation. The method uses ground-based The global average precipitation rate from global silver iodide generators when the time is judged right for precipitation estimates (Huffman et al. 1997; Xie and −1 seeding. Arkin 1997) is about 2.8 mm day . Hence, this is also 2 Rain rate refers to the average amount of rain per unit time, the global evaporation rate, as the imbalance is tiny. regardless of whether or not it rains only part of the time. We Moreover, evaporation is continuous, subject to use “intensity” to refer to the rain rate conditional on rain ac- availability of moisture from the surface (which is not tually falling. a limitation over the oceans), and increases to 1206 | SEPTEMBER 2003 about 5 mm day −1 in summer (Trenberth and ing (and thus the 2.8 versus 45 mm day −1 for the glo- Guillemot 1998). So the moisture supply for moder- bal mean). This fact alone highlights the importance ate or heavy precipitation locally does not come di- of not only how much moisture there is in the atmo- rectly from evaporation. Instead it has to come from sphere but also its location. The former is tied to tem- transport, and thus from convergence of low-level perature, which determines the moisture-holding moisture elsewhere in the atmosphere. These facts capacity of the atmosphere, along with the relative are reconciled when we recognize that it only rains humidity. about 5%-10% of the time on average. The frequency Given this factor of 16 or so, we can take the square of rainfall locally varies wildly from 0% to over 50% root to convert it from an areal measure to a linear (e.g., Trenberth 1998; Dai 2001a) but depends on the measure of scales of systems. What this means is that area and threshold used to detect precipitation; val- on average, rainfall-producing weather systems reach ues in Trenberth (1998) are for 2° latitude by 2.5° out to distances about 3–5 times the radius of the pre- longitude grid squares, not point measurements.