Forecasts at a 4 km convection‐permitting resolution over China during the summer season have been produced with the Weather Research and Forecasting model at Nanjing University since 2013. Precipitation forecasts from 2013 to 2014 are evaluated with dense rain gauge observations and compared with operational global model forecasts. Overall, the 4 km forecasts show very good agreement with observations over most parts of China, outperforming global forecasts in terms of spatial distribution, intensity, and diurnal variation. Quantitative evaluations with the Gilbert skill score further confirm the better performance of the 4 km forecasts over global forecasts for heavy precipitation, especially for the thresholds of 100 and 150 mm d−1. Besides bulk characteristics, the representations of some unique features of summer precipitation in China under the influence of the East Asian summer monsoon are further evaluated. These include the northward progression and southward retreat of the main rainband through the summer season, the diurnal variations of precipitation, and the meridional and zonal propagation of precipitation episodes associated with background synoptic flow and the embedded mesoscale convective systems. The 4 km forecast is able to faithfully reproduce most of the features while overprediction of afternoon convection near the southern China coast is found to be a main deficiency that requires further investigations.

This paper is the first to document the development of a type of bow echo, termed merger‐formation bow echo (MFBE), in southeast China evolving from a subtropical squall line (SL) merging with a preline convective cell (CC). Although this MFBE did not produce damaging surface winds, its intense rain rate resulted in local flooding. The evolution of the kinematic, thermodynamic, and microphysical structures is investigated using the variational Doppler radar analysis system (VDRAS) analysis and polarimetric radar observations. Key factors of this MFBE event including the rear‐inflow jet (RIJ) and cold pool exhibited different characteristics from those in classical bow echoes and limited MFBE cases. As the SL propagated towards the coast, a CC was triggered along a sea breeze front. The SL did possess a RIJ, but a bow‐shaped reflectivity did not appear until the SL‐CC merger. The RIJ weakened and became elevated during the merger, while the leading edge of SL cold pool accompanied by a weak diverging outflow did not advance with the reflectivity field. The updraft was strengthened due to the merger, resulting in enhanced precipitation falling ahead of the original SL. The subcloud evaporation locally cooled the air ahead of the SL and merged with the original SL cold pool. This combined cold pool advanced forward rapidly and caught up with the bowing radar reflectivity to form this MFBE. This study illustrates the processes of a SL‐CC merger leading to the formation of a different type of MFBE that did not produce damaging surface winds.

No abstract

On 4 October 2015, a miniature supercell embedded in an outer rainband of Typhoon Mujigae produced a major tornado in Guangdong province of China, leading to 4 deaths and up to 80 injuries. This study documents the structure and evolution of the tornadic miniature supercell using coastal Doppler radars, a sounding, videos, and a damage survey. This tornado is rated at least EF3 on the enhanced Fujita scale. It is by far the strongest typhoon rainband tornado yet documented in China, and possessed double funnels near its peak intensity. Radar analysis indicates that this tornadic miniature supercell exhibited characteristics similar to those found in United States landfalling hurricanes, including a hook echo, low-level inf low notches, an echo top below 10 km, a small and shallow mesocyclone, and a long lifespan (3 h). The environmental conditions—which consisted of moderate convective available potential energy (CAPE), a low lifting condensation level, a small surface dewpoint depression, a large veering low-level vertical wind shear, and a large cell-relative helicity—are favorable for producing miniature supercells. The mesocyclone, with its maximum intensity at 2 km above ground level (AGL), formed an hour before tornadogenesis. A tornado vortex signature (TVS) was identified between 1 and 3 km AGL, when the parent mesocyclone reached its peak radar-indicated intensity of 30 m s−1. The TVS was located between the updraft and forward-flank downdraft, near the center of the mesocyclone. Dual-Doppler wind analysis reveals that tilting of the low-level vorticity into the vertical direction and subsequent stretching by a strong updraft were the main contributors to the mesocyclone intensification.

The average annual number of tornadoes in China is not more than 100 from 1961 to 2010 (Fan & Yu, 2015). However, recent significant tornado events such as the EF4 Funing tornado, the EF4 Caihong tornado, and the EF3 Kaiyuan tornado have drawn great attention of social media in China (Xue et al., 2016). Despite of the advances in tornado analysis and large eddy simulation over the past decades (Mashiko, 2016;Mashiko et al., 2009;Sun et al., 2019), the prediction of tornadoes remains difficult. One of the reasons is the lack of instruments that can capture the internal structures of tornadic storms, especially the low-level atmosphere conditions (McLaughlin et al., 2009).