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'Plum rains' pummel southern China

Heavy rains have set in across southern China and bring the risk of flooding and mudslides. These rains are known as the plum rains since they tend to occur at the time when plums ripen around late spring and early summer. They also affect Japan and Korea during nearby months of the year.

Image: DTN estimate of rainfall in the 24 hours to 2:35am China Standard Time Sunday 8 June.

The plum rains are due to the collision of humid air coming from the South China Sea and dry air coming from inland China.

Image: 850hPa wet bulb potential temperature (GFS model), a parameter that is incorporates both temperature and humidity. This shows the boundary of different airmasses over southern China.

This collision of airmasses sets up a nearly stationary boundary running from west to east. The boundary is known as the mei-yu front when it sets up across China (mei-yu means plum rains). Large clusters of thunderstorms develop and move eastwards along the mei-yu front. These thunderstorms pose the greatest risk of heavy rainfall along the boundary.

Image: Large clusters of thunderstorms moving eastwards along the mei-yu front around sunrise on Saturday 7 June.

Image: DTN lightning strikes in the 24 hours to 7am China Standard Time on Sunday 8 June.

Let’s zoom in on one of the thunderstorm systems to highlight some features. The large cloud mass is nearly circular and has a diameter of about 300km. The shadow to the west indicates the vertical height of the thunderstorms. The bubbly texture on the left of the system indicates where the cloud is shooting beyond the tropopause above 14km. This kind of thunderstorm system has the potential to produce heavy long-lived rainfall.

Image: A large thunderstorm system near Wuhan around sunrise on the morning of Saturday 7 June.

As of Sunday morning, the mei-yu front has moved a little southwards and is impacting the heavily populated city of Shanghai.

Image: Large thunderstorm system impacting Shanghai on the morning of Sunday 8 June.

The mei-yu front will remain close to its current position until it breaks down by about Thursday. From now until then the plum rains will deliver widespread totals of 80-100mm and isolated totals exceeding 150mm, leading to the risk of flash and riverine flooding and mudslides.

Image: Accumulating rainfall totals from Sunday to Thursday according to the ECMWF model.