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Based on the precipitation data of automatic weather stations, CMA-STI and ERA5 data from 2000 to 2020, this paper found that, less than half of the tropical cyclone activities were accompanied by heavy rain that occurred in the Jianghuai region, and only 30% of the tropical cyclones were in the company of daily precipitation in the Jianghuai area reaching the average daily amount in the Meiyu period of that year. Except for a few "path-turning type" tropical cyclones and "northwest-traveling type" tropical cyclones that continued to move eastward or northward after landfall, most of the other tropical cyclones were accompanied by the weakening of Meiyu. The WRF model and the built-in TC bogus method were used to simulate the 2017 typhoon Merbok and the 2019 typhoon Danas to verify the influence mechanism of TC on Meiyu. The results show that the model well simulated the path, precipitation area and intensity of the two typhoons. By comparing the tests before and after the removal of the typhoons, it was found that the "northwest-traveling type" typhoon Merbok strengthened a low-level jet with the westward airflow at the southern edge at the early stage, a Yangtze-Huaihe valley shear strengthened, a low pressure headed northward after landing, and a subtropical high was stable, forming a circulation situation favorable for precipitation. After removing Merbok, the low-level jet broke, the subtropical high retreated southward, the water vapor in the South China Sea could not be transported to the Yangtze-Huaihe valley, and the precipitation in the middle and lower reaches of the Yangtze River decreased. With the northward movement of both the path-turning type typhoon Danas and the the subtropical high, convective instability decreased and vertical ascending motion weakened. A large amount of water vapor from the western Pacific was transported to the vicinity of the typhoon eye. The water vapor transport and precipitation in the Yangtze-Huaihe valley decreased and the Meiyu was interrupted. After removing Danas, the precipitation in the east increased, the subtropical high was stable, and the Meiyu was maintained. [ABSTRACT FROM AUTHOR]

Based on CMPAS precipitation products and ERA5 reanalysis datasets, the performance of the ECMWF global high-resolution products (ECMWF) have been analyzed during the typical Meiyu (from 10th June to 20th July) period in 2020. Using the method of object-based diagnostic evaluation (MODE), the performance of the ECMWF in simulating the spatial characteristics of heavy precipitiation, such as the centroid latitude, the centroid longitude, the rainfall area, the major axis length, width and axis angle of precipitation, have been comprehensively revealed. The results show that the ECMWF tends to have a northward and westward bias in reproducing the Meiyu rain belt, in terms of the daily precipitation. Regarding the spatial morphology characteristics of the rain belt, the ECMWF has a larger rainfall area and has a steeper slope than that in the observation. The diurnal variation of precipitation within the rain belt peaks around early-morning in the observation, and the ECMWF can well forecast the diurnal variation of precipitation. In addition, the results also show that the frequency of the northward bias in forecasting the rain belt peaks in the evening to midnight and afternoon, while the bias of the southward mainly occurs in the morning. The low-level jet exhibits similar diurnal variations but with a 3 hours earlier peak than the observed precipitation. The peak time of low-level jet in the ECMWF forecast is 3 hours earlier than that in observed. Further analysis reveals that the zonal location of the rain belt is highly correlated with the intensity of low-level jet. The rain belt moves further northward when the low-level jet is strengthened, and visa-versa. Finally, two heavy rainfall events were chosen to represent the typical northward bias and relatively accurate forecast of the zonal position of the rain belt. The results show that the northward bias of the rain belt results from the stronger low-level jet 3 hours ahead in the ECMWF forecast.

Revisiting the defined Meiyu onset of Central Weather Bureau (CWB), this study applied a newer reanalysis dataset and added multiple timing and duration criteria to improve the Meiyu onset index. The previous Meiyu onset index was based on horizontal and vertical wind shears using older-generation reanalysis data. The horizontal shear captures the cyclonic vorticity while the vertical shear depicts overturning. However, this older index tends to predict the onset date too early from the actual maximum precipitation. After applying the modification that is described in this paper, the newer Meiyu onset index consistently leads the maximum precipitation in Taiwan only by a few days, except for two years over the 30-year analysis period. The implication of this modified and improved Meiyu onset index is that it can substitute model precipitation that tends to be problematic, as well as studying climate change impacts. [ABSTRACT FROM AUTHOR]

Revisiting the defined Meiyu onset of Central Weather Bureau (CWB), this study applied a newer reanalysis dataset and added multiple timing and duration criteria to improve the Meiyu onset index. The previous Meiyu onset index was based on horizontal and vertical wind shears using older-generation reanalysis data. The horizontal shear captures the cyclonic vorticity while the vertical shear depicts overturning. However, this older index tends to predict the onset date too early from the actual maximum precipitation. After applying the modification that is described in this paper, the newer Meiyu onset index consistently leads the maximum precipitation in Taiwan only by a few days, except for two years over the 30-year analysis period. The implication of this modified and improved Meiyu onset index is that it can substitute model precipitation that tends to be problematic, as well as studying climate change impacts.

Based on observation data supplied by the Chinese Meteorological Administration (CMA) and reanalysis datasets provided by the ECMWF, the multiscale causes of persistent heavy rainfall events (PHREs) that occurred from 1979 to 2018 during Meiyu periods over the middle and lower reaches of the Yangtze River (MLYR) are investigated. During Meiyu periods, precipitation shows obvious interannual variabilities. In PHRE years, the contribution rate of persistent heavy rainfall to the total precipitation is approximately 57%. Precipitation also shows significant synoptic-scale (less than 10 days) characteristics. Through the quantitative diagnosis of interactions among background-scale (greater than 30 days), quasi-biweekly-scale (10–30-days), and synoptic-scale variables, the possible causes of PHREs are explored. The results reveal that the difference in precipitation intensity between PHRE years and non-PHRE years is determined by the background water vapor, background wind and synoptic-scale wind conditions. In PHRE years, the prevailing background southwesterly winds from lower latitudes provide more background water vapor, and more mean kinetic energy is converted to perturbation energy. Moreover, the active synoptic-scale oscillations from higher latitudes and the convergence of Rossby wave disturbance energy over the MLYR could also cause the occurrence and maintenance of PHREs during Meiyu periods. The multiscale causes and corresponding circulation patterns in 2020 PHREs are similar to PHREs years.

Abstract: Convection‐permitting simulations are used to investigate the key mechanism of Meiyu precipitation diurnal cycle over China. Six days from the 2014 Meiyu season are used to produce a “north” composite rainband over the Yangtze‐Huaihe River Basin and another 6 days used to produce a “south” composite band. Both rainbands have peak rainfall in the early morning, while the south band has a secondary peak in the afternoon. Low‐level ageostrophic winds (AGWs) are found to exhibit diurnal cycles with clockwise rotations and their directions, and magnitudes depend on the background geostrophic monsoon flows. Net moisture flux into a control volume enclosing each rainband is almost purely due to AGWs. For both rainbands, net flux reaches maximum at ~04 LST, about 3–4 hr before morning precipitation peak. For the north band, a prominent minimum occurs at ~19 LST, 4 hr before the precipitation minimum. The moisture fluxes through the southern control volume boundary make the largest contributions to the net flux and its diurnal variations. The diurnal variations of the AGWs and their relationship with the background monsoon flows agree very well with the prediction of Blackadar boundary layer inertial oscillation theory, and the convergence forcing by the AGWs resulting from the inertial oscillations plays a paramount role in modulating the diurnal cycles of Meiyu front precipitation, including the creation of early morning peak and evening minimum. Feedback of latent heat release plays only a secondary role. The commonly recognized diurnal monsoon variability can be explained by the Blackadar inertial oscillation theory. [ABSTRACT FROM AUTHOR]

The Yangtze River Delta (YRD) has experienced significant urban expansion in recent years, while the Meiyu belt of China has demonstrated a decadal northward shifting trend. Thus, it is of interest to assess how urban expansion affects Meiyu precipitation and hopefully to reveal the underlying physical mechanisms involved. In this study, the urban extents over the YRD in 2001 and 2010 are derived based on land use/land cover (LULC) category data and nighttime light image data. Two parallel groups of 10-summer (2001-2010) numerical simulations are carried out with the urban extents over the YRD in 2001 and 2010, respectively. The results show that the urban expansion in the YRD tends to result in increased (decreased) Meiyu precipitation over the Huaihe River (Yangtze River) basin with intensities of 0.2-1.2 mm day. Further analysis indicates that the spatiotemporal pattern of the Meiyu precipitation change induced by the urban expansion resembles the third empirical orthogonal function (EOF) mode of the observed Meiyu precipitation. Analyses of the possible underlying physical mechanisms reveal that urban expansion in the YRD leads to changes in the surface energy balance and warming (cooling) of tropospheric (stratospheric) air temperature over eastern China. Anomalous upward (downward) motion and moisture convergence (divergence) over the Huaihe River (Yangtze River) basin occur, corresponding to the increases (decreases) of the Meiyu precipitation over the Huaihe River (Yangtze River) basin. [ABSTRACT FROM AUTHOR]

This work for the first time analyzed the vertical structures of the different stages of Meiyu precipitation systems over the Yangtze‐Huai River Valley in central China using measurements and retrievals from the Global Precipitation Measurement Mission Dual‐Frequency Precipitation Radar (GPM‐DPR) and Feng Yun satellites. GPM‐DPR‐retrieved near‐surface rain and drop size distributions were first validated against the surface disdrometer measurements and showed good agreement. Then we analyzed three cases from the Integrative Monsoon Frontal Rainfall Experiment to demonstrate the different characteristics of convective precipitation and stratiform precipitation (SP) in the developing, mature, and dissipating stages of the Meiyu precipitation systems, respectively. For statistical analysis, all Meiyu cases during the period 2016–2018 detected by GPM‐DPR were collected and classified into different types and stages. In the stratiform regions of Meiyu precipitation systems, coalescence slightly overwhelms breakup and/or evaporation processes, but it was dominant in the convective regions when raindrops fall. There were large numbers of large ice particles during the developing stage due to strong updrafts and abundant moisture, whereas there were both large ice and liquid particles in the mature stage. The vertical structures of the SP examined in this study were similar to those over the ocean regions due to high relative humidity but different to the mountainous west regions of the USA. The findings of the stage‐dependent SP vertical structures provide better understanding of the evolution of monsoon frontal precipitation, as well as the associated microphysical properties, and provide insights to improve microphysical parameterization in future models. Key Points: The near‐surface rain and drop size distributions retrieved from GPM‐DPR are reliable across the Yangtze‐Huai River Valley in central ChinaThree cases were selected during IMFRE to represent the developing, mature, and dissipating stages of Meiyu precipitation systemsCoalescence slightly overwhelms breakup and/or evaporation in stratiform region but it is dominant in convective region as raindrops fall [ABSTRACT FROM AUTHOR]