Ji Niea, Adam H. Sobelb,c, Daniel A. Shaevitzc, and Shuguang Wangc
a. Department of Atmospheric and Oceanic Sciences, Peking University, Beijing, China
b. Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA
c. Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, USA
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A useful starting hypothesis for predictions of changes in precipitation extremes with climate is that those extremes increase at the same rate as atmospheric moisture does, which is ∼7%K−1 following the Clausius–Clapeyron (CC) relation. This hypothesis, however, neglects potential changes in the strengths of atmospheric circulations associated with precipitation extremes. As increased moisture leads to increased precipitation, the increased latent heating may lead to stronger large-scale ascent and thus, additional increase in precipitation, leading to a super-CC scaling. This study investigates this possibility in the context of the 2015 Texas extreme precipitation event using the Column Quasi-Geostrophic (CQG) method. Analogs to this event are simulated in different climatic conditions with varying surface temperature (Ts) given the same adiabatic quasigeostrophic forcing. Precipitation in these events exhibits super-CC scaling due to the dynamic contribution associated with increasing ascent due to increased latent heating, an increase with importance that increases with Ts. The thermodynamic contribution (attributable to increasing water vapor; assuming no change in vertical motion) approximately follows CC as expected, while vertical structure changes of moisture and diabatic heating lead to negative but secondary contributions to the sensitivity, reducing the rate of increase.
Fig. A schematic of the scaling of precipitation extremes with temperature in a CQG system. A is under the current climate. Upper shows an upper level synoptic wave, lower level fronts, and a low-pressure center. Lower shows the side view of the convecting and precipitating region over the low-pressure center. B is in a warmer climate: the fractional changes of vertical motion and precipitation if the large-scale adiabatic QG forcing F (and Advq ) is unchanged. The darker color of the cloud cartoon indicates that the convective system is stronger in a warmer climate.
Full Text: https://doi.org/10.1073/pnas.1800357115