The frequency of hail storms, thunderstorms, and high wind events has decreased by nearly 50 percent on average throughout China since 1960.
The findings, published in Scientific Reports, are based on one of the most comprehensive studies on trends in local severe weather patterns to date.
“Most of the data published on trends in severe weather has been incomplete or collected for a limited short period,” says Fuqing Zhang, professor of meteorology and atmospheric science at Penn State University. “The record we used is, to the best of our knowledge, the largest, both in time scale and area of land covered.”
The changes could be linked to a drop in the strength of the East Asian Summer Monsoon, scientists say.
“We believe that changes in monsoon intensity are affecting severe weather in the area because of the strong correlation we found, but we cannot say the monsoon is the exclusive cause,” says Zhang. “A monsoon is one of the major drivers of severe weather because it affects the three necessary ‘ingredients’ for severe weather, which are wind shear, instability, and triggering.”
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Wind shear is the difference between the wind speed and direction at different altitudes. Because a monsoon brings southerly winds into China, a weaker summer monsoon would decrease the overall low tropospheric wind shear. The weaker monsoons would also bring less warm, moist air from the south—one of the most common sources of instability in the atmosphere.
A common triggering mechanism for severe convective weather is lifting by the front, a high temperature gradient across the monsoon, and this would also be reduced in a weaker summer monsoon.
Climate change or pollution?
Some studies suggest that climate change may be one of the reasons that the Asian Summer Monsoon weakened. One factor in monsoon formation is the difference between the temperature above land and the temperature above adjacent ocean or sea. A warming climate would affect the difference between these two and, as a result, simulations show that this could continue decreasing the monsoon’s strength.
However, the team notes that other major changes in the area—such as an increase in industrialization and air pollution in China in the 1980s—might have played a significant role in the region’s atmospheric changes and could affect the severe weather.
While a decrease in severe weather might sound beneficial, it may not always be a good thing.
“There are many natural cycles that rely on severe weather and the precipitation it brings,” says Qinghong Zhang, professor of atmospheric and oceanic sciences at Peking University and lead author of the study, who conducted the research while on sabbatical at Penn State. “A decrease in storms could potentially lead to an increase in droughts.
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“Also, some theorize that while the frequency of severe weather decreases, their intensity could potentially increase. We cannot say if this is true yet, but it is something we will analyze in the future.”
This was the first study in its level of detail because of the amount of data collected by the Chinese National Meteorology Information Center. The study also showed that occurrences of hail remained relatively steady from 1961 through the 1980s before plummeting.
“The frequency of thunderstorms and high winds decreased gradually over the time period we studied, but not hail,” says Qinghong Zhang. “This is something we don’t fully understand at this point but plan to investigate more.”
The Chinese National Science Foundation, the National Basic Research Program of China, and the US National Science Foundation supported this research.
Source: Penn State