UC Berkeley

Spring and summer rainfall over East Asia is characterized by unique organized bands of precipitation, stretching from China to Korea and Japan in climatology, on top of the typical local convective systems. This banded precipitation has moderate to high intensity and can persist for several days to a few weeks. It undergoes a northward migration from May to August, starting from south China in Guangdong and Guangxi provinces to the Yangtze River Basin and eventually to north China and the Korean Peninsula. The persistent precipitation during this period is referred to as Meiyu in China, Baiu or Tsuyu in Japan, and Changma in Korea; these names capture the timing and the long-lasting nature of this precipitation. Meiyu and Baiu mean “plum rain”, denoting the season when the plum ripens; Changma means “long period of rain”. This rainy season is sometimes referred to as the East Asian summer monsoon, denoting the seasonally reversing flow that carries warm moist air from the Pacific and the Indian Ocean into continental East Asia.

Previously studies of banded precipitation in East Asia, or the East Asian rainbands, have focused on the total precipitation within specific regions, and have yielded little to no information on individual events or the exact position, shape, and size of the individual rainbands. In this work, I have developed a novel method to identify individual East Asian rainbands in the 6-hourly precipitation from ERA-interim reanalysis product from 1979-2018, by using criteria on precipitation threshold and longitudinal connectedness. This method employs a combination of image processing techniques, manual labelling, and deep neural networks to identify and catalog the long and narrow bands of precipitation. We find that the northward migration of the East Asian rainbands is continuous, and not step-wise, as suggested by previous studies that relied on rainfall data averaged over pentads or longer periods of time.

With our individually identified rainbands, we examine the atmospheric circulation associated with the rainbands to understand their dynamics and seasonal migration. We find that the mid-tropospheric northerly flow north of the East Asian rainbands to be the most important circulation feature that determines the latitudinal position of the rainbands. The seasonal climatology shows that rainbands are located at the convergence of northerly and southerly flows at the latitude of the greatest meridional gradient in moist static energy. On the days with rainbands identified, however, the northerly is much stronger than the seasonal mean while the southerly remains the same. We hypothesize that the southerly flow continually supplies warm moist air for the front, while the sporadic strong northerly triggers the frontogenesis. We have found no strong direct correlation between the latitude or strength of the zonal wind and jet stream to the latitudes of the identified East Asian rainbands. Our finding does not contradict previous studies that have associated the seasonal migrations of the rainbands with those of the jet stream, as they do happen simultaneously.

Using a simple one-layer barotropic quasi-geostrophic model of potential vorticity to guide our analysis of the ERA-interim data, we show that northerly winds crucial for the formation of the rainbands are part of the orographic Rossby wave excited by the Tibetan Plateau. The location and the speed of this northerly wind are determined by the mid-tropospheric westerlies upstream(west) of the Tibetan Plateau. In early spring, strong westerlies impinging on the Tibetan Plateau are likely to generate Rossby waves with long wavelengths and strong northerlies in East Asia, thus limiting the northward migration of the rainbands. In late spring and summer, the impinging westerlies weaken, the Rossby wavelengths shorten, and the northerlies over East Asia also weaken, permitting rainband migration northwards. On the interannual scale, the East Asian rainbands could covary with the northerly wind and also with the upstream zonal wind in some years. In other years, the correlation cannot be established, as other factors such as the subtropical high over North Pacific and the southerly moisture supply might play a role.

The westerly wind interacting with the Tibetan Plateau is part of the jet streams over East Asia, and we present a detailed characterization of the jet streams over this region. We have developed a method to identify in the 6-hourly reanalysis data the jet streams both on a 2-D surface and in the 3-D domain and to capture the subtropical jet and the eddy-driven jet. This method uses a threshold of 20 m/s for the wind speed and the connected-component algorithm. It can also distinguish the subtropical jet from the eddy-driven jet. Most importantly, the subtropical jet over Asia has a core at 200 hPa, and stays around 25°N in winter until April, when it starts to move northward smoothly to around 45°N in mid-summer. The eddy-driven jet has its core between 200 and 300 hPa. Its latitudinal position has great variability even within a month; it can be between 45°N and 60°N in winter, and between 50°N and 70°N in summer. Both the East Asian rainbands and subtropical jet migrate northward from spring to summer, but they are not perfectly collocated in latitude. The role of jet streams in the season of the rainbands is more on interacting with the Tibetan Plateau to generate Rossby waves downstream than on directly controlling the frontogenesis.