28: Clouds and Precipitation GEORGE A ISAAC1 AND JOHN HALLETT2 1Cloud Physics and Severe Weather Research Division, Meteorological Service of Canada, Toronto, ON, Canada 2Division of Atmospheric Sciences, Desert Research Institute, Reno, NV, US Clouds and the precipitation that comes from them are important elements of the hydrological cycle. Clouds provide a blanket for our Earth, both shielding it from radiation from the Sun and trapping heat escaping from the surface. They also generate precipitation through the condensation-coalescence mechanism, which involves only liquid cloud drops, or through ice initiation leading to large ice particles and eventually snow or rain. Mean annual precipitation amounts reach a maximum near the equator, near 8 mm day−1 , and decrease poleward to about 1 mm day−1 . A good understanding of both cloud and precipitation processes is very important for climate and weather predictions. This paper outlines some of the most important processes and provides reference material where more detailed information can be obtained. INTRODUCTION the earth in the tropical regions is much greater than that at the poles, which also accounts for a greater amount of Earth is covered with clouds as any satellite photo of our precipitation in that region. At the poles, the temperatures planet will show. Figure 1 shows a cloud climatology from are low and the stratiform clouds that exist barely produce the GEWEX Surface Radiation Budget data set (Whitlock precipitation, less than 1 mm per day. et al., 1995). Almost everywhere, the annual mean cloud There are many uncertainties in our knowledge about cover is greater than 50% and in some large areas it is clouds and precipitation. However, much progress has been greater than 80%. These clouds literally provide a blanket made and this article will briefly summarize our current for our Earth, both shielding it from radiation from the Sun knowledge, and point to more comprehensive articles where and trapping heat escaping from the surface. Understanding additional information can be found. Earlier textbooks on clouds is extremely important for both climate and weather cloud physics which provide useful information include predictions because their presence or absence can strongly those written by Fletcher (1962) and Mason (1971). More affect surface temperatures. Figure 2 shows that for a recently, a general textbook on cloud physics has been station in northern Canada, the presence of cloud in the written by Rogers and Yau (1989) and a more detailed wintertime makes the surface warmer, while it provides a book on the microphysics of clouds and precipitation was cooling effect in the summertime. published by Pruppacher and Klett (1997). Life cannot exist without water and in most cases we get our water from precipitation. Figure 3 shows the annual mean precipitation rate as compiled by Xie and Arkin CLOUD FORMATION AND TYPES (1996, 1997) using gauge observations, satellite estimates and numerical model outputs. Precipitation amounts reach a Clouds can exist in many forms in the atmosphere. See maximum near the equator, near 8 mm day−1, and decrease the World Meteorological Organization International Cloud poleward to about 1 mm day−1. The higher temperatures in Atlas (WMO, 1975, 1987), the AMS Glossary (Glickman, the tropical regions produce strong convection and more 2000) and Scorer (1972) for a full description of cloud precipitation, with cloud base temperatures being greater types. Cirrus clouds form at temperatures below −40 ◦C, than 20 ◦C. It should also be mentioned that the area of generally occur above 5 km, and they cover wide areas in a Encyclopedia of Hydrological Sciences. Edited by M G Anderson. 2005 Canadian Crown Copyright 2 METEOROLOGY AND CLIMATOLOGY Annual mean cloud cover 60N 30N EQ 30S 60S 0 60E 120E 180 120W 60W 0 0.2 0.4 0.6 0.8 Figure 1 Mean annual cloud cover as seen from satellite as a function of latitude and longitude. From the GEWEX surface radiation budget data set (Whitlock et al., 1995. 1995 American Meteorological Society). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs Inuvik, January, 1961−1990 Inuvik, July, 1961−1990 −15 25 C) C) Scattered ° ° −20 Broken 20 Overcast All −25 15 −30 Scattered 10 −35 Broken Overcast Mean monthly temperature ( All Mean monthly temperature ( −40 5 N NEE SE SE SW W NWCalm All NNEE SESESWWNWCalm All (a)Surface wind direction (b) Surface wind direction Figure 2 January and July mean monthly temperature at Inuvik, Northwest Territories, Canada, as a function of cloud cover and surface wind direction. ‘‘Scattered’’ indicates 0–1 tenth sky, ‘‘broken’’ 2–8 tenths, and ‘‘overcast’’ 9–10 tenths sky coverage (Isaac and Stuart, 1996. 1996 American Meteorological Society) sheet form, often with a fibrous aspect (Figure 4a). Below vapor trails caused by high-flying jet aircraft (Figure 4b). this temperature, ice crystals form even in the absence of These clouds generally do not create precipitation that insoluble nuclei in cloud drops and a few degrees lower reaches the ground and they are primarily formed of ice in diluted haze droplets. Cirrus can form from broadscale crystals. However, they can start to precipitate (Figure 4c) uplift, in the outflow of thunderstorms, or even from the and the falling ice crystals can “seed” lower layers of CLOUDS AND PRECIPITATION 3 Annual mean precipitation (mm day−1) 60 N 30 N EQ 30 S 60 S 0 60 E 120 E 180 120 W 60 W 0 Figure 3 Annual mean precipitation (mm day−1) as compiled by Xie and Arkin (1996, 1997) using gauge observations, satellite estimates and numerical model outputs (Xie and Arkin, 1996. 1996 American Meteorological Society). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs cloud and initiate precipitation. Mid-level clouds such as sublimation. The liquid to solid phase change involves altostratus or altocumulus are also formed by broadscale the freezing process, while melting occurs when solid lifting, often produced by frontal systems. They can be water changes to liquid. Phase changes occur with the composed of either ice crystals or liquid water, but they corresponding release or uptake of latent heat. For example, do not account for much of our precipitation. Lower-level in order for water to change from the liquid to vapor state, cloud types such as stratus, and stratocumulus (Figure 4d, latent heat is required to break the hydrogen bonds between e) are associated with precipitation, either snow or rain. In water molecules in the liquid. This is called the latent midlatitudes, especially in winter, these cloud types account heat of vaporization. Similarly, thelatentheatoffusionis for most of our precipitation. Cumulus clouds (Figure 4f) required to change from the solid to liquid state. The latent and especially thunderstorms (Figure 4e) create most of our heat of sublimation is released when vapor changes directly precipitation in the summer at mid- and lower-latitudes. into ice. Normally, phase changes from liquid or solid to Clouds are generally formed in circulation around low vapor occur when the air is subsaturated with respect to pressure cyclonic storm systems, with precipitation tak- liquid water or ice, and the reverse happens when the air is ing place at fronts where higher temperature, moist air if supersaturated. lifted above cooler air. Mesoscale complexes and hurri- The Clausius–Clapeyron equation, one of the most canes form towards the tropics and easterly waves form important in cloud physics, relates the saturation vapor convergence zones for precipitation in low-latitude tropical pressure with respect to water (es)orice(ei) to the latent regions. heat of either vaporization (Lv) and or sublimation (Ls) to temperature (T). For the saturation vapor pressure over water, the equation may be written: WATER PHASES, LATENT HEAT des = Lves Water in the atmosphere exists in three phases: vapor, dT RvT solid, and liquid. Cloud and precipitation formation involve −1 transforming vapor into the other two phases. A vapor where Rv is the gas constant for water vapor (461.5 J kg to liquid phase change occurs through condensation and K−1). the reverse process involves evaporation. A vapor to solid Table 1 shows the saturation vapor pressure over water phase change is called deposition, while the reverse is called and ice, and the latent heats of vaporization (condensation) 4 METEOROLOGY AND CLIMATOLOGY (a) (b) (c) (d) (e) (f) (g) (h) Figure 4 Some examples of cloud types such as: (a) cirrus cloud, (b) cirrus clouds produced by high-flying aircraft (contrails), (c) cirrus clouds with falling ice crystal streaks, (d) stratus clouds below an inversion, (e) stratocumulus deck, (f) cumulus cloud, (g) thunderstorm, and (h) mammatus. For full definitions see American Meteorological Society (AMS) Glossary (Glickman, 2000), World Meteorological Organization Cloud Atlas (WMO, 1975, 1987), and Scorer (1972). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs ( John Hallett) and sublimation as a function of temperature. The latent water is approximately 1.8, 3.8, 7.8 and 15.0 g of vapor heat of fusion, which is released when ice changes to liquid, per kg of air, respectively (List, 1968). This helps explain is the difference between Lv and Ls. The high values of why there is more precipitation in the Tropics than in the the latent heat of vaporization of ice and water, compared Arctic. with the latent heat of fusion (melting/freezing) show that water molecules are mostly bonded in both liquid and solid, ATMOSPHERIC STABILITY AND CLOUD and the bonding only changes by about 12% on melting. FORMATION It is clear from Table 1 that warm air can hold much more water than cold air. For example, at −10 ◦C, 0 ◦C, +10 ◦C In order to consider how clouds form in the atmosphere, and +20 ◦C and 1000 mb, the saturation mixing ratio over it is necessary to understand atmospheric stability and the CLOUDS AND PRECIPITATION 5 Table 1 The saturation vapor pressure over water and ice, about 1.5 km (a saturated parcel would keep rising through and the latent heats of vaporization (condensation) and buoyancy), showing the potential for convection.