Increasing Skewness of the Vertical Velocity Distribution Shifts Rainfall to Heavier Rates

Monday, 15 December 2014
Angeline G Pendergrass, National Center for Atmospheric Research, Boulder, CO, United States
How will the distributions of precipitation and vertical velocity change in response to global warming? Global-average rainfall increases more slowly than extreme rain rates with warming, implying a change in the shape of the rainfall distribution. At the same time, the tropical vertical velocity distribution weakens, particularly its asymmetric component (the Walker circulation). Recent work by the author shows that the distribution of daily rainfall shifts to heavier rain rates in response to global warming and ENSO phases in climate models. Observations confirm the shift in response to ENSO phases. 

Using an heuristic model relating the distributions of rainfall and vertical velocity, we show that an increase in skewness of the vertical velocity distribution replicates the shift of the precipitation distribution to heavier rain rates with warming. The skewness is the third moment of a statistical distribution, which measures its asymmetry. Implications of the increase in skewness of vertical velocity include and increase in dry days and a corresponding decrease in rain falling at moderate rain rates. When increased moistening that occurs with warming is also accounted for, the projected increase of extreme rainfall faster than the increase in the mean emerges. In contrast, broadening the vertical velocity distribution fails to replicate salient characteristics of the projected change in the rainfall distribution like the increase in dry day frequency. Climate models also show an increase in the skewness of daily-average vertical velocity in global warming simulations in most models.