Heavy Rain-producing Terrestrial Low-Pressure Systems Over East Asian Summer Monsoon Region: Evolution, Energetics, and Trend

Abstract:

The synoptic low-pressure systems (LPSs) formed over the downwind side of the Tibetan Plateau explain a substantial portion of summer extreme rainfall along their paths. Recent studies have found that over the East Asian landmass, the total extreme rainfall trend is, to a great extent, accounted for by the terrestrial LPSs-related component. Yet, the energy sources fueling these storms and the environmental drivers for their long-term changes remain elusive. Utilizing a probabilistic clustering method, three clusters of terrestrial LPS tracks for the period 1979-2018 are identified. Besides the differences in trajectories that distinguish the clusters into migratory and quasi-stationary types, prominent inter-cluster differences are noted in evolution, energetics, and trend in storm numbers. The Lorenz energetics suggest that the latent heat release and baroclinity are indispensable for terrestrial LPSs. While the latent heat release is essential for all three clusters, the migratory type, which has greater intensity and faster development, is more closely tied to baroclinicity. Nonetheless, the summer baroclinicity alone is not enough to sustain the LPSs as the storms dissipate after propagating out of the humid monsoon region and into the drier extratropics. Over time, the number of migratory LPSs decreases but the number of quasi-stationary LPSs increases. Using a Poisson model that links the LPS genesis to local environmental conditions, the decreasing occurrence of migratory LPSs is found to result from the weakened vertical wind shear and baroclinicity, whereas the increasing occurrence of quasi-stationary LPSs is driven by the enhanced relative humidity and reduced steering flow in the mid-to-lower troposphere.