ATMOSPHERIC MOISTURE RESIDENCE TIMES AND CYCLING - IMPLICATIONS FOR RAINFALL RATES AND CLIMATE-CHANGE

New estimates of the moistening of the atmosphere through evaporation at the surface and of the drying through precipitation are computed. O verall, the e-folding residence time of atmospheric moisture is just o ver 8 days. New estimates are also made of how much moisture that prec ipitates out comes from horizontal transport versus local evaporation, referred to as 'recycling'. The results depend greatly on the scale o f the domain under consideration and global maps of the recycling for annual means are produced for 500 km scales for which global recycling is 9.6%, consisting of 8.9% over land and 9.9% over the oceans. Even for 1000 km scales, less than 20% of the annual precipitation typicall y comes from evaporation within the domain. While average overall atmo spheric moisture depletion and restoration must balance, precipitation falls only a small fraction of the time. Thus precipitation rates are also examined. Over the United States, one hour intervals with 0.1 mm or more are used to show that the frequency of precipitation ranges f rom over 30% in the Northwest, to about 20% in the Southeast and less than 4% just east of the continental divide in winter, and from less t han 2% in California to over 20% in the Southeast in summer. In midlat itudes precipitation typically falls about 10% of the time, and so rai nfall rates, conditional on when rain is falling, are much larger than evaporation rates. The mismatches in the rates of rainfall versus eva poration imply that precipitating systems of all kinds feed mostly on the moisture already in the atmosphere. Over North America, much of th e precipitation originates from moisture advected from the Gulf of Mex ico and subtropical Atlantic or Pacific a day or so earlier. Increases in greenhouse gases in the atmosphere produce global warming through an increase in downwelling infrared radiation, and thus not only incre ase surface temperatures but also enhance the hydrological cycle, as m uch of the heating at the surface goes into evaporating surface moistu re. Global temperature increases signify that the water-holding capaci ty of the atmosphere increases and, together with enhanced evaporation , this means that the actual atmospheric moisture should increase. It follows that naturally-occurring droughts are likely to be exacerbated by enhanced potential evapotranspiration. Further, globally there mus t be an increase in precipitation to balance the enhanced evaporation but the processes by which precipitation is altered locally are not we ll understood. Observations confirm that atmospheric moisture is incre asing in many places, for example at a rate of about 5% per decade ove r the United States. Based on the above results, we argue that increas ed moisture content of the atmosphere therefore favors stronger rainfa ll or snowfall events, thus increasing risk of flooding, which is a pa ttern observed to be happening in many parts of the world. Moreover, b ecause there is a disparity between the rates of increase of atmospher ic moisture and precipitation, there are implied changes in the freque ncy of precipitation and/or efficiency of precipitation (related to ho w much moisture is left behind in a storm). However, an analysis of li near trends in the frequency of precipitation events for the United St ates corresponding to thresholds of 0.1 and 1 mm/h shows that the most notable statistically significant trends are for increases in the sou thern United States in winter and decreases in the Pacific Northwest f rom November through January, which may be related to changes in atmos pheric circulation and storm tracks associated with El Nino-Southern O scillation trends. It is suggested that as the physical constraints on precipitation apply only globally, more attention should be paid to r ates in both observations and models as well as the frequency of occur rence.