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With the development of refined numerical forecasts, problems such as score distortion due to the division of precipitation thresholds in both traditional and improved scoring methods for precipitation forecasts and the increasing subjective risk arising from the scale setting of the neighborhood spatial verification method have become increasingly prominent. To address these issues, a general comprehensive evaluation method (GCEM) is developed for cross-scale precipitation forecasts by directly analyzing the proximity of precipitation forecasts and observations in this study. In addition to the core indicator of the precipitation accuracy score (PAS), the GCEM system also includes score indices for insufficient precipitation forecasts, excessive precipitation forecasts, precipitation forecast biases, and clear/rainy forecasts. The PAS does not distinguish the magnitude of precipitation and does not delimit the area of influence; it constitutes a fair scoring formula with objective performance and can be suitable for evaluating rainfall events such as general and extreme precipitation. The PAS can be used to calculate the accuracy of numerical models or quantitative precipitation forecasts, enabling the quantitative evaluation of the comprehensive capability of various refined precipitation forecasting products. Based on the GCEM, comparative experiments between the PAS and threat score (TS) are conducted for two typical precipitation weather processes. The results show that relative to the TS, the PAS better aligns with subjective expectations, indicating that the PAS is more reasonable than the TS. In the case of an extreme-precipitation event in Henan, China, two high-resolution models were evaluated using the PAS, TS, and fraction skill score (FSS), verifying the evaluation ability of PAS scoring for predicting extreme-precipitation events. In addition, other indices of the GCEM are utilized to analyze the range and extent of both insufficient and excessive forecasts of precipitation, as well as the precipitation forecasting ability for different weather processes. These indices not only provide overall scores similar to those of the TS for individual cases but also support two-dimensional score distribution plots which can comprehensively reflect the performance and characteristics of precipitation forecasts. Both theoretical and practical applications demonstrate that the GCEM exhibits distinct advantages and potential promotion and application value compared to the various mainstream precipitation forecast verification methods. [ABSTRACT FROM AUTHOR]

The roles of the Tibetan Plateau (TP) and Yunnan‐Guizhou Plateau (YGP) in regional precipitation extremes over the Sichuan Basin (SCB) in summer under specific circulation background remain unclear. This study quantifies the impact of TP or YGP on the regional extreme precipitation event (REPE) which occurred during 04:00 Beijing time (BJT) on 11 August to 03:00 BJT on 13 August 2020 with a rainfall center located in the western SCB based on numerical experiments. Results show that the accumulated precipitation regionally averaged over the western SCB during the REPE can be reduced by 84% (51%) when the TP (YGP) is absent, and the decreased precipitation is mainly caused by the reduced stratus precipitation. Mechanism analysis indicates that relative to the control experiment including the TP and YGP, the absence of TP or YGP leads to reduced temperature over the SCB induced by anomalous cold advection from the upstream regions where the terrains are removed, and further results in much more stable stratification and thereafter weakens the ascending motions and reduces the stratus precipitation over the western SCB. On the other hand, the absence of TP or YGP induces an anomalous anti‐cyclone at 850 hPa over SCB, which further weakens both the uplift effect of terrain on airflow along the windward slope and water vapor transport over the western SCB and thereafter reduces the precipitation over this region. This study deepens the understanding of the topographic effect on regional extreme precipitation over SCB in summer from the thermodynamic and dynamic processes. Plain Language Summary: Tibetan Plateau (TP) and Yunnan‐Guizhou Plateau (YGP) play important roles in the formation of summer regional extreme precipitation event (REPE) over Sichuan Basin (SCB) under specific circulation background. This study addresses how the TP or YGP affects the REPE over SCB during 04:00 Beijing time (BJT) on 11 August to 03:00 BJT on 13 August 2020 based on numerical experiments. Results show the absence of TP (YGP) can reduce the accumulated precipitation during the REPE regionally averaged over the western SCB by 84% (51%). The absence of TP or YGP leads to reduced temperature over SCB induced by anomalous cold advection from the upstream regions where the terrains are removed and much more stable stratification, and thereafter weakens the large‐scale ascending motions and reduces the stratus precipitation over western SCB. Meanwhile, an anomalous anti‐cyclone at 850 hPa over the SCB is induced by the absence of TP or YGP. This can further weaken the uplift effect of terrain on airflow along the windward slope and water vapor transport over western SCB and result in decreased accumulated precipitation over this region. Findings of this study can provide a base for the refined simulation and prediction of REPE over regions with complex terrain. Key Points: The regional precipitation extreme over Sichuan Basin (SCB) on 11–13 August 2020 cannot be reproduced without Tibetan Plateau (TP) or YGPThe absence of TP (YGP) can reduce the accumulated precipitation regionally averaged over western SCB by 84% (51%)The absence of TP or YGP weakens the topographic lifting effect on airflow along the windward slope and reduces precipitation in western SCB [ABSTRACT FROM AUTHOR]

With the development of refined numerical forecasts, the problems such as the score distortion due to the division of precipitation thresholds in both traditional and improved scoring methods for precipitation forecast and the increasing subjective risk arisen from the scale setting of the neighbourhood spatial verification method have become increasingly prominent. To solve this issue, a general comprehensive evaluation method (GCEM) has been developed for cross-scale precipitation forecasts by directly analysing the proximity of precipitation forecasts and observations in this study. In addition to the core element of the precipitation forecast accuracy score (PAS) index, the GCEM system also includes score indices for insufficient precipitation forecasts, excessive precipitation forecasts, precipitation forecast biases and clear/rainy forecasts. The PAS does not distinguish the magnitude of precipitation and delimit the area of influence, it constitutes a fair scoring formula with objective performance and can be suitable for evaluating the rainfall events such as general and extreme precipitation. The PAS can be used to calculate the accuracy of numerical models or quantitative precipitation forecasts, enabling the quantitative evaluation of the comprehensive capability of various refined precipitation forecasting products. Based on the GCEM, comparative experiments between the PAS and TS are conducted for two typical precipitation weather processes. The results show that relative to TS, the PAS aligns with subjective expectations much more, indicating that the PAS is more reasonable than the TS. In addition, other indices of the GCEM are utilized to analyse the range and extent of both insufficient and excessive forecasts of precipitation, as well as the precipitation forecast ability in two weather processes. These indices not only provide overall scores for individual cases similar to the TS but also offer two-dimensional score distribution plots, which can comprehensively reflect the performance and characteristics of precipitation forecasts. Both theoretical and practical applications demonstrate that the GCEM exhibits distinct advantages and potential promotion and application value compared to the various mainstream precipitation forecast verification methods. [ABSTRACT FROM AUTHOR]

Based on the hourly rainfall gauge data and ERA5 reanalysis for the period 1980–2020, typical synoptic patterns responsible for summer regional hourly extreme precipitation events (RHEPE) over the middle and lower Yangtze River basin have been objectively identified using a circulation clustering method. It is found that the Meiyu front with different locations and intensities imbedded in the East Asian summer monsoon, and landfalling typhoons are the leading contributors. As the dominant synoptic pattern, the Meiyu front pattern is associated with ∼92% of the total RHEPE occurrence and can be categorized into a southerly strong‐Meiyu type and a northerly weak‐Meiyu type. The RHEPE occurrence shows a predominant morning peak associated with the southerly strong‐Meiyu type and a secondary late afternoon peak related to the northerly weak‐Meiyu type, in which the Meiyu front is pushed northward by the strengthened western North Pacific subtropical high accompanied by accelerated low‐level southwesterly flow. Plain Language Summary: Using ERA5 reanalysis and hourly gauge rainfall measurements in summers of 1980–2020, this study investigates the driving mechanisms and temporal variation of summer regional hourly extreme rainfall events over the middle and lower Yangtze River basin. Typical synoptic patterns responsible for the summer regional hourly rainfall extremes are objectively identified using spectral clustering analysis. The Meiyu front with different locations and intensities imbedded in East Asian summer monsoon and the landfalling typhoons are the synoptic patterns leading to the regional hourly rainfall extremes. The diurnal twin peaks (morning and late afternoon) in the occurrence of regional hourly rainfall extremes are related to the Meiyu front with different locations and intensities. The results from this investigation may help improve the prediction and climate risk assessment of regional extreme rainfall events. Key Points: The Meiyu front imbedded in the East Asian summer monsoon and landfalling typhoons are the leading contributors of summer regional hourly extreme precipitation events (RHEPE) over middle and lower Yangtze River basinThe Meiyu front pattern can be sorted into a southerly type with strong Meiyu front and a northerly type with weak Meiyu frontThe northerly weak Meiyu front pattern with active convection contributes the most to the afternoon diurnal peak of RHEPE occurrence [ABSTRACT FROM AUTHOR]

Revealing the synoptic patterns related to hourly extreme precipitation (EP) is very important to deepen our recognition and understanding of EP formation. The predominant synoptic patterns associated with the summer regional hourly EP events (RHEPE) over the central‐eastern Tibetan Plateau (CETP) and Sichuan Basin (SCB) have been systematically identified. Results show the summer RHEPE over CETP and SCB are dominated by the background large‐scale circulations featured by the configuration of eastward‐extended South Asia high (SAH) and westward‐extended Western North Pacific Subtropical High (WNPSH) and their northward advance, except for that an obvious low‐level vortex imbedded in the background large‐scale circulations is mainly responsible for the summer RHEPE over SCB. The frequency and intensity of the total summer RHEPE over CETP and SCB all show an obvious increasing trend during 2000–2020, which is largely contributed by the synoptic pattern characterized by the configuration of eastward extended SAH and westward extended WNPSH. Plain Language Summary: To completely indicate the synoptic patterns leading to the summer regional hourly extreme precipitation events (RHEPE) over the central‐eastern Tibetan Plateau (CETP) and Sichuan Basin (SCB), this study has identified the dominant synoptic patterns of summer RHEPE over the two regions. Results show that the configuration of eastward extended South Asia high (SAH) and westward extended Western North Pacific Subtropical High (WNPSH) and their northward advance dominates the prime circulation patterns of summer RHEPE over CETP and SCB. However, in addition to the opposing motion of the SAH and WNPSH, a low‐level vortex imbedded in the background large‐scale circulations is mainly responsible for the summer RHEPE over SCB. The findings of this study may help us to deepen our understanding of the RHEPE formation over the two regions with complex terrain and provide a base to further improve the prediction of extreme precipitation. Key Points: Summer rainfall extremes over both central‐eastern Tibetan Plateau (CETP) and Sichuan Basin (SCB) are dominated by the combination of eastward extended SAH and westward extended Western North Pacific Subtropical HighThe summer rainfall extremes over SCB are mainly induced by a low‐level vortex imbedded in the background of large‐scale circulationsThe dominant synoptic patterns over CETP and SCB all contribute to the increasing trends of frequency and intensity of the summer regional hourly EP events [ABSTRACT FROM AUTHOR]

This study examines the impacts of sea surface temperature (SST) configuration on the monthly prediction of summer monsoon over the western North Pacific (WNP) by conducting several sets of hindcast experiments using the Beijing Climate Center Climate System Model and its atmospheric component model. The results show that the atmosphere-only model exhibits limited skill in predicting the WNP monsoon rainfall and circulation, and this skill can hardly be improved by simply increasing the frequency of prescribed SST observation. Compared to the atmosphere-only model, the coupled model shows much better performance in predicting the WNP monsoon rainfall and circulation, which can be further improved by adopting the observed SST with relatively higher frequency in the model initialization. This indicates that the high frequency of observed SST used is much more important in the coupled model than in the uncoupled model. In addition, the uncoupled model forced by the SST predicted by coupled model tends to produce better prediction of WNP monsoon rainfall and circulation than that forced by the observed SST. Both the coupled model and the atmosphere-only model forced by the coupled model predicted SST can well reproduce the surface latent heat flux and shortwave radiation flux over the WNP, leading to a reasonable SST-monsoon relationship and thus skillful predictions of WNP monsoon. Therefore, although the Tier-1 approach based on coupled model is increasingly popular, the Tier-2 approach based on atmosphere-only model is still feasible for the monthly prediction of WNP summer monsoon despite the lack of air-sea interaction. To obtain more skillful Tier-2 prediction, we recommend seeking for SST forcing that is unrealistic but consistent with the atmospheric model rather than SST forcing with very high accuracy. [ABSTRACT FROM AUTHOR]

The northern mountainous area of the Pearl River Delta (PRD) is a center of convection and heavy rainfall of coastal southern China during the summer monsoon season. Although there are frequent nighttime convective activities and rainfall over the region, the key dynamical and physical processes responsible for the nocturnal convection initiation (CI) are complex and not yet well documented. In this study, high‐resolution reanalyses from the Variational Doppler Radar Assimilation System (VDRAS) were used to explore the triggering mechanisms of a representative nocturnal convection event that occurred over the northern PRD on August 14, 2014. Results showed that the enhanced prevailing low‐level southerly winds, deflected easterly winds caused by the blocking of multi‐scale orography, and northern downslope winds associated with an orographic cooling effect were crucial for generating low‐level convergence and strong updrafts before CI. The low‐level convergence and moisture flux in the boundary layer started to increase at 00:20 local solar time (LST), and by the CI time (01:00 LST), the convergence intensity more than doubled. The intensification of low‐level convergence and moisture transport was strongest before 00:40 LST and was dominated by a zonal component that was closely related to the deflected easterly winds. Using Doppler radar data assimilation, this is the first study to reveal how the deflected flows and the thermally induced downslope winds combine with prevailing monsoonal winds to affect a nocturnal CI over the complex mountainous region of the South China coast. Key Points: The combination of complex orography and monsoonal flows is crucial for the nocturnal convection initiation along the South China coastEnhanced low‐level monsoonal flows, terrain‐induced easterly winds, and downslope winds converge and generate strong updraftsThe terrain‐deflected easterly winds play an important role in low‐level convergence and moistening [ABSTRACT FROM AUTHOR]

The South China coast suffers frequent heavy rainfall every warm season. Based on the objective classification method of principal components analysis, the key role of the synoptic pattern in determining the heavy rainfall processes that occurred over the South China coast in the warm season during 2008–18 is examined in this study. We found that heavy rainfall occurs most frequently under three typical synoptic patterns (P1–P3 hereafter) characterized by strong low-level onshore winds. P1 and P3 feature a prevailing southwesterly monsoonal flow in the lower troposphere, with heavy rainfall frequently occurring over the inland windward region in the afternoon associated with the orographic lifting and solar heating. The onshore wind of P3 is stronger than P1 as the western Pacific subtropical high extends more westward to 122°E, which induces stronger low-level convergence along the coastline than P1 when the ageostrophic wind veers from the offshore to onshore direction in the early morning. Hence, a secondary early morning rainfall peak can be found along the coastline. P2 is characterized by a low-level vortex located over the southwest portion of south China. Heavy rainfall under P2 usually initiates over the western part of the coastal region in the morning and then propagates inland in the afternoon. Overall, the synoptic patterns strongly determine the spatial distribution and diurnal cycle of heavy rainfall over the South China coast. This heavy rainfall is closely related to the diurnally varying low-level onshore winds rather than the low-level jets, as well as the different interactions between the low-level onshore winds and the local orography, coastline, and land–sea breeze circulations under different synoptic patterns. [ABSTRACT FROM AUTHOR]

Precipitation diurnal cycle over Fujian in the pre-summer rainy season has been revealed based on the hourly rain gauge data during 2009-2013. The precipitation amount (PA) over northwestern Fujian and most southeast coasts shows relatively small diurnal variability. This is in contrast to large diurnal variations in the mountainous areas. Regional differences in precipitation diurnal cycles are obviously noted among the coastal, valley, hilly, and mountainous areas. The precipitation diurnal cycles are significantly affected by the terrain elevation and distance to coast, PA and precipitation frequency (PF) show much more pronounced double diurnal peaks with the terrain elevation and distance to coast increased. However, the precipitation intensity (PI) basically shows one distinct late afternoon diurnal peak for different elevations and distances to coast. Four typical patterns of precipitation diurnal cycle are further identified by cluster analysis. The four typical PF patterns show relatively apparent morning peaks over coasts, inland mountains, and hills in addition to distinct late afternoon maxima. Low PF with weak diurnal amplitude is mainly located over the coastal areas, while high PF with strong diurnal amplitude is found over the valley, hilly, and mountainous regions. The PA exhibits a weak early morning peak and a relatively strong late afternoon peak over coastal, hilly, or mountainous areas and windward slopes but only one distinct late afternoon peak over the valley regions and leeward slopes. The amplitude of PA diurnal cycle is the weakest over the coastal areas but the strongest over the valley regions or leeward slopes among the four PA patterns. The four PI diurnal cycle patterns consistently show distinct afternoon peaks. [ABSTRACT FROM AUTHOR]

The impacts of the summer plateau monsoon (PM) over the Tibetan Plateau on summer rainfall over the Tarim Basin in northwest China are investigated, based on the observed rainfall data at 34 stations and the NCEP/NCAR reanalysis data during 1961 to 2007. Results showed that the PM is well correlated to the summer rainfall over the Tarim Basin. Process analysis shows that strong PM corresponds to an anomalous cyclone over the Tibetan Plateau in the middle troposphere and an anomalous anticyclone in the upper troposphere over northwest part of Tibetan Plateau. They result in cold air moving from high latitudes into Central Asia over the western part of Tibetan Plateau. The concurrences of the cooling in the middle-upper troposphere over Central Asia leads to an anomalous cyclone over Central Asia at 500 hPa and the anomalous descending motions prevailing over the cooling region. Associated with this anomaly, there are enhanced southerly winds and corresponding ascending motion over the Tarim Basin located in the east of the cooling region. These processes lead to more summer rainfall over the Tarim Basin. [ABSTRACT FROM AUTHOR]

ABSTRACT This article presents a comprehensive analysis of interannual and interdecadal variations of summer precipitation and precipitation-related extreme events in East China associated with variations of the East Asian summer monsoon ( EASM) from 1979 to 2012. A high-quality daily precipitation data set covering 2076 observational stations in China is analysed. Based on the precipitation pattern analysis using empirical orthogonal functions and the regime shift detection method, three sub-periods of 1979-1992 (period I), 1993-1999 (period II) and 2000-2012 (period III) are identified to be representative of the precipitation variability. Similar significant variability of the extreme precipitation indices is found across four sub-regions in eastern China. The spatial patterns of summer mean precipitation, the number of days with daily rainfall exceeding 95th percentile precipitation (R95p) and the maximum number of consecutive wet days ( CWD) anomalies are consistent, but opposite to that of maximum consecutive dry days ( CDD) anomalies to some extent during the three sub-periods. However, the spatial patterns of hydroclimatic intensity ( HY-INT) are notably different from that of the other three extreme indices, but highly correlated to the dry events. The changes of precipitation anomaly patterns are accompanied by the change of the EASM regime and the abrupt shift of the position of the west Pacific subtropical high around 1992/1993 and 1999/2000, respectively, which influence the moisture transport and contributes to the precipitation anomalies. In addition, the EASM intensity is linked to sea surface temperature anomaly over the tropical Indian and Pacific Ocean through its effects on convective activity over the west Pacific that induces the cyclonic or anticyclonic anomaly over the South China and northwest Pacific. [ABSTRACT FROM AUTHOR]

The changes in summer rainfall over the Tarim Basin, China, and the underlying mechanisms have been investigated using the observed rainfall data at 34 stations and the NCEP-NCAR reanalysis data during the period of 1961-2007. Results show that the summer rainfall over the Tarim Basin, which exhibits a significant increasing trend during the last half century, is closely related to the summer middle and upper tropospheric cooling over central Asia. Mechanism analysis indicates that the middle and upper tropospheric cooling over central Asia results in a location farther south of the subtropical westerly jet over western and central Asia with anomalous southerly wind at lower levels and ascending motion prevailing over the Tarim Basin. Such anomalies in the atmospheric circulations provide favorable conditions for the enhanced summer rainfall over the Tarim Basin. Further analysis suggests that the weakened South Asian summer monsoon (SASM) could be potentially responsible for the middle and upper tropospheric cooling over central Asia. This is largely through the atmospheric responses to the diabatic heating effect of the SASM. A weakened SASM can result in an anomalous cyclone in the middle and upper troposphere over central Asia. The western part of the anomalous cyclone produces more cold air advection, which leads to the cooling. This study suggests indirect but important effects of the SASM on the summer rainfall over the Tarim Basin. [ABSTRACT FROM AUTHOR]

The relation between the spatial and temporal variations of the West Asian subtropical westerly jet (WASWJ) and the summer precipitation in northern Xinjiang has been explored using the NCEP/NCAR reanalysis data and the summer precipitation data at 43 stations in northern Xinjiang during 1961 to 2007. Results show that the position of the WASWJ is more important than its strength in influencing the summer precipitation in northern Xinjiang. When the jet position is further south, the anomalous southwesterly flow crossing the Indian subcontinent along the southern foothill of the Tibetan Plateau is favorable for the southwestward warm and wet air penetrating from low latitudes into Central Asia and northern Xinjiang and more rainfall formation. Further analysis shows that the interannual variations of the jet position are well correlated with the Arctic Oscillation (AO). In the weak AO years, the middle to upper troposphere becomes colder than normal and results in an anomalous cyclonic circulation at 200 hPa over Western and Central Asia, which enhances the westerly wind over middle and low latitudes and leads to the WASWJ located further south. [ABSTRACT FROM AUTHOR]

The performance of a regional air-sea coupled model, comprising the Regional Integrated Environment Model System (RIEMS) and the Princeton Ocean Model (POM), in simulating the seasonal and intraseasonal variations of East Asian summer monsoon (EASM) rainfall was investigated. Through comparisons of the model results among the coupled model, the uncoupled RIEMS, and observations, the impact of air-sea coupling on simulating the EASM was also evaluated. Results showed that the regional air-sea coupled climate model performed better in simulating the spatial pattern of the precipitation climatology and produced more realistic variations of the EASM rainfall in terms of its amplitude and principal EOF modes. The coupled model also showed greater skill than the uncoupled RIEMS in reproducing the principal features of climatological intraseasonal oscillation (CISO) of EASM rainfall, including its dominant period, intensity, and northward propagation. Further analysis indicated that the improvements in the simulation of the EASM rainfall climatology and its seasonal variation in the coupled model were due to better simulation of the western North Pacific Subtropical High, while the improvements of CISO simulation were owing to the realistic phase relationship between the intraseasonal convection and the underlying SST resulting from the air-sea coupling. [ABSTRACT FROM AUTHOR]