The atmospheric energy constraint on precipitation change
Pendergrass, Angeline Greene
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How does rain change with global warming? This dissertation investigates the rate of global-mean precipitation increase, changes in atmospheric radiative cooling, and the changes in frequency and intensity of rain events. We examine changes in global-mean precipitation and atmospheric radiative cooling in comprehensive climate model simulations. In a realistic forcing scenario including both greenhouse gases and aerosols, clear-sky absorption of shortwave radiation is correlated with the rate of global-mean precipitation increase. In a scenario forced by increasing carbon dioxide alone, we make radiative transfer calculations to separate the responses of clear-sky atmospheric radiative cooling due to warming, moistening, and the carbon dioxide itself. Clear-sky atmospheric radiative cooling increases in response to vertically uniform warming and constant relative humidity moistening of the atmosphere. These increases are partially offset by decreases due to the direct radiative effects of carbon dioxide and black carbon. Global-mean precipitation increases by the same rate as the change in clear-sky atmospheric radiative cooling. Changes in the frequency of rain and the rate at which it falls constitute the increase in global-mean precipitation. We develop a methodology for characterizing the frequency and amount of rainfall as functions of the rain rate. We define two modes of response, one in which the distribution of rainfall increases in equal fraction at all rain rates (the increase mode) and one in which the rainfall shifts to higher or lower rain rates without a change in mean rainfall (the shift mode). We apply this description of change to simulations of global warming in climate models. We also calculate the response of the tropical rainfall distribution to ENSO phases in models and observations and apply the increase and shift modes. In addition to the increase and shift modes of change, some models show a substantial increase in rainfall at the highest rain rates. In some models this extreme mode can be shown to be associated with increases in grid-scale precipitation.
- Atmospheric sciences