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Characteristics of extreme precipitation over Yellow River Loop Valley (YRLV) and links to European blocking are investigated in this study. Spatial and temporal analysis of extreme precipitation shows that it contributes more than 30% of the total summer precipitation in the YRLV and is characterized by a strong and short period of local rainfall. Most of the extreme rains in the YRLV occur in July and August. Two typical circulation patterns were identified using a k-means clustering method. The extreme precipitation results from the combined actions of intensified high pressure over northeast China (NECH) and the westward extension of the western Pacific subtropical high (WPSH). The intensified southerly flow of the amplified NECH strengthens the water vapor transport induced by the westward extension of the WPSH from the northwest Pacific or Bay of Bengal into the YRLV. The NECH is amplified by the wave energy propagating from European blocking via the Silk Road pattern (SRP). This is the subseasonal cause of extreme precipitation over the YRLV. The composited July and August mean 500 hPa geopotential anomaly pattern for extreme precipitation years shows a high-pressure anomaly over the European continent and a negative phase of the SRP. The former provides a background for the occurrence of European blocking, and the latter explains the preexistence of the NECH and provides a linkage between the activity of European blocking and the subseasonal evolution of the NECH. Thus, the interannual variation in the extreme precipitation over the YRLV is mainly reflected by the phase of the SRP and the stationary waves over Europe. [ABSTRACT FROM AUTHOR]

Characteristics of extreme precipitation over Yellow River Loop Valley (YRLV) and links to European blocking are investigated in this study. Spatial and temporal analysis of extreme precipitation shows that it contributes more than 30% of the total summer precipitation in the YRLV and is characterized by a strong and short period of local rainfall. Most of the extreme rains in the YRLV occur in July and August. Two typical circulation patterns were identified using a k-means clustering method. The extreme precipitation results from the combined actions of intensified high pressure over northeast China (NECH) and the westward extension of the western Pacific subtropical high (WPSH). The intensified southerly flow of the amplified NECH strengthens the water vapor transport induced by the westward extension of the WPSH from the northwest Pacific or Bay of Bengal into the YRLV. The NECH is amplified by the wave energy propagating from European blocking via the Silk Road pattern (SRP). This is the subseasonal cause of extreme precipitation over the YRLV. The composited July and August mean 500 hPa geopotential anomaly pattern for extreme precipitation years shows a high-pressure anomaly over the European continent and a negative phase of the SRP. The former provides a background for the occurrence of European blocking, and the latter explains the preexistence of the NECH and provides a linkage between the activity of European blocking and the subseasonal evolution of the NECH. Thus, the interannual variation in the extreme precipitation over the YRLV is mainly reflected by the phase of the SRP and the stationary waves over Europe.

In this study, an extreme rainstorm that occurred in the Beijing–Tianjin–Hebei (BTH) region in China on 19–20 July 2016 is simulated and analyzed using the Weather Research and Forecasting model, coupled with a multilayer urban canopy scheme, to reveal the impact of urbanization on the extreme precipitation process in the region. The results show that the urban heat island effect (that is, surface warming and an increased near-ground sensible heat flux, which leads to increased vertical motion and atmospheric instability layer strengthening) plays a dominant role in the urban modification of rainfall during the early stages of urbanization, resulting in an increase of 6–10 mm in average hourly precipitation in urban and downwind areas. With the further development of urbanization in the BTH region, particularly in the big cities of Beijing and Tianjin, the large-scale expansion of the urban surface reduces the surface moisture, the evaporation of surface water from the ground, and the height of the atmospheric boundary layer, leading to an urban dry island effect brought about by the lack of near-surface water vapor, which inhibits an increase in precipitation. The positive effect of the urban heat island on precipitation was offset by the urban dry island effect, so the increase in precipitation in the urban areas was not obvious, but an increased range of 8–10 mm was noted. The existence of large cities changes the position of the strong upward movement of air, and convective upward movement is more likely to occur between the suburbs. With the further expansion of the underlying surface of the adjacent cities of Beijing and Tianjin, the upward movement between the two cities coincides, leading to an obvious increase in precipitation between the two cities.

In this study, an extreme rainstorm that occurred in the Beijing–Tianjin–Hebei (BTH) region in China on 19–20 July 2016 is simulated and analyzed using the Weather Research and Forecasting model, coupled with a multilayer urban canopy scheme, to reveal the impact of urbanization on the extreme precipitation process in the region. The results show that the urban heat island effect (that is, surface warming and an increased near-ground sensible heat flux, which leads to increased vertical motion and atmospheric instability layer strengthening) plays a dominant role in the urban modification of rainfall during the early stages of urbanization, resulting in an increase of 6–10 mm in average hourly precipitation in urban and downwind areas. With the further development of urbanization in the BTH region, particularly in the big cities of Beijing and Tianjin, the large-scale expansion of the urban surface reduces the surface moisture, the evaporation of surface water from the ground, and the height of the atmospheric boundary layer, leading to an urban dry island effect brought about by the lack of near-surface water vapor, which inhibits an increase in precipitation. The positive effect of the urban heat island on precipitation was offset by the urban dry island effect, so the increase in precipitation in the urban areas was not obvious, but an increased range of 8–10 mm was noted. The existence of large cities changes the position of the strong upward movement of air, and convective upward movement is more likely to occur between the suburbs. With the further expansion of the underlying surface of the adjacent cities of Beijing and Tianjin, the upward movement between the two cities coincides, leading to an obvious increase in precipitation between the two cities. [ABSTRACT FROM AUTHOR]