Typhoon, mountain terrain key drivers of heavy rains

The heavy rainfall currently drenching northern China, especially the Beijing-Tianjin-Hebei region, is being driven by a combination of atmospheric circulation and terrain effects, forecasters said.

Chen Tao, chief forecaster at the National Meteorological Center, said the latest bout of heavy precipitation is closely linked to evolving atmospheric conditions. A stronger-than-usual western Pacific subtropical high, positioned farther north than normal, has steered warm, moist air into North China, providing abundant water vapor to fuel the rains.

Atmospheric instability along the high's periphery has triggered intense convective rainfall in some areas, resulting in heavy downpours and prolonged durations, with extreme levels of accumulated rainfall reported.

The terrain of the Yinshan, Taihang and Yanshan mountains has also amplified the rainfall. The process of orographic lifting, when air moves over an elevated terrain, in areas just ahead of these ranges has led to localized deluges, with some sites recording precipitation at levels classified as extremely heavy.

While Typhoon Co-may — the eighth of the year — has mainly affected eastern China, it has had an indirect link to the rainfall in the north. Typhoon activity can influence atmospheric circulation, including the position and strength of the subtropical high, thereby altering the northward transport of moisture, Chen said.

He Na, chief forecaster at the Beijing Meteorological Observatory, told reporters that Co-may has contributed to rainfall in Beijing, albeit indirectly.

"Beijing already had sufficient water vapor for rainfall over the past few days," He said. "With the additional moisture transported from the distant typhoon, the overall humidity level increased even further. The typhoon did have an impact on the amount of precipitation in Beijing."

Water vapor from Co-may was carried northward via the outer edge of the subtropical high. One branch of moisture sustained the typhoon's circulation, while the other extended toward Beijing, according to forecasters.

From July 23 through Tuesday, parts of Beijing, Tianjin and Hebei province recorded accumulated rainfall ranging from 100 to 450 millimeters. In some areas, including Beijing's Miyun district and the cities of Baoding and Chengde in Hebei, totals exceeded 500 mm.

Heavy rainfall and thunderstorms are forecast to continue across North China through Friday. The National Meteorological Center on Tuesday renewed its orange alert for heavy rain — the second-highest level in its four-tier warning system — and maintained a yellow alert for severe convective weather, the third-highest.

Significant precipitation is expected across Shaanxi and Shanxi provinces, the Beijing-Tianjin-Hebei cluster, parts of Guangdong and Fujian provinces, and areas along the Yangtze River. Some locations may see torrential rain, short-term downpours, thunderstorms, strong winds and hail.

Typhoon Co-may, currently moving northwest and gaining strength, is forecast to make landfall on Wednesday along the coasts of Zhejiang and Jiangsu provinces. A blue typhoon alert, the lowest of the four levels, was renewed on Tuesday morning.