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Detecting Clues from Tropopause for Persistent Rainfall

A team from the State Key Laboratory of Numerical Modelling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences identify early signs of potential strong rainfall.

As the flood season comes to an end in China, consistent strong precipitation events in many parts of China has resulted in severe flooding over the summer of this year. Meteorological researchers are now looking to uncover potential signals that point towards early signs of persistent strong rains, in order to better prepare for any impending flooding.

A team led by Professor Xiao Ziniu together with their collaborators set out to identify this signal that could potentially show precursor signs of strong rainfall. The researchers used an analysis tool known as the intra-seasonal oscillation (ISO) of isentropic potential vorticity (PV) for tracking these early signals near the tropopause after persistent strong precipitation in the South of China. The tropopause is the boundary in the earth’s atmosphere between the troposphere and the stratosphere. This layer is a thermodynamic gradient stratification layer which marks the end of the troposphere.

The researchers discovered that precursor signals that results in persistent heavy precipitation in South China come from two regions; the Arctic region and the tropical monsoon region.

“20 days before the peak rainfall, the western Tibetan Plateau and the northern side of the East Asian westerly jet near the tropopause are two transit points for the anomalous potential vorticity to strengthen and change propagating direction,” said Zhao Liang, the first author of the study and a senior engineer with LASG.

Results of the study was published in Climate Dynamics in early September 2020.

Identifying predictive early configuration of synoptic systems for the occurrence of persistent heavy rainfall events is important in preparing for potential flood disasters.

Based on this study, 10 days before the peak rainfall, the joint action of the South Asia High and the Okhotsk Sea pattern developing a blocking high compresses the anomaly cold air between the two highs, and forms a narrow and steady cold air transport channel on the inclined isentropic surface. It enables the cold air travel to a lower latitude and continually meet with the warm and moist air. The persistent heavy precipitation in South China is then brought by the anomaly strong subtropical high.

“We found that when the anomaly low pressure is surrounded by three anomaly high pressure systems, including South Asia high, Okhotsk Sea blocking high and the western Pacific subtropical high, this special situation is often responsible for the following persistent heavy rainfall in South China,” said Professor Xiao.

Professor Xiao also shared that their findings will provide hope in predicting future regional persistent heavy rainfall. [APBN]