TRMM data show a super-Clausius Clapeyron sensitivity in heavy rain in a warmer climate

September 1, 2011 — Click on image to enlarge.

The water holding capacity of the Earth’s atmosphere is governed by the Clausius-Clapeyron (CC) Law which states that there is approximately a 7% increase in atmospheric water vapor per degree rise in temperature. It has been an ongoing debate in the scientific community whether or not global rainfall should be increasing at the CC rate as a result of global warming.

In a recent paper, Lau and Wu (2011) address the above question by examining oceanic rainfall distribution ranked by rain rates, using TMI (microwave) and PR (radar) rainfall data from the Tropical Rainfall Measuring Mission (TRMM) for the period 1998-2009. The geographic locations of light, moderate, and heavy rains (Fig.1), defined by the bottom 5% (B5), the inter-quartile range (I25) and the top 10% (T10), indicate that moderate and heavy rains are confined to SST warmer than 27o C. The joint probability distributions with cloud top temperature and radar-echo-top height (Fig. 2) show that B5 rain is primarily associated with warm, low cloud, I25 with mixed-phase precipitation, middle to high clouds, and T10 with high clouds and ice-phase precipitation. Both TMI and PR (Fig. 3) show that heavy rains (>50-70 percentile) have a super-CC sensitivity with extreme heavy rains (top 5%) exhibiting 80-90% increase per degree rise in SST. In contrast, light and moderate rains which account for 30-50% of the tropical rainfall amount show a reverse CC sensitivity, with 5-10% reduction per degree rise. The differences are related to changes in large-scale dynamics and tropical convection.

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