Abstract:The seasonal transition of the convective (STC) in the Maritime Continent (MC) during the boreal spring can largely influence the establishment of the Asian summer monsoon, but the interannual variability of STC in MC and its causes are still not well understood. In this paper, based on the climatic characteristics of STC in MC, we define some characteristic indicators of convection and circulation, further analyze the interannual variation characteristics of STC in MC, and explore the possible effects of sea surface temperature anomalies (SSTA) in the tropics and their mechanisms. The results show that the interannual variabilities of STC in MC and the evolution of atmospheric circulation over the surrounding areas have an overall consistency, and are mainly regulated by the El Ni?o ? South Oscillation (ENSO) events. In some special years, the distribution of the SSTA in the eastern tropical Indian ocean could stimulate an anomalous local atmospheric circulation, which will ultimately lead to some changes on the convective activity in MC.

Abstract:The summer precipitation in China is mainly affected by the East Asian summer monsoon, and the main factor affecting the monsoon is the land-sea thermal contrast. Based on the ECMWF / ERA5 reanalysis data and CN05.1 gridded precipitation data set from 1979 to 2022, this paper defines the European-Atlantic land-sea thermal contrast and two types of East Asian-Pacific land-sea thermal contrast: East Asia-North Pacific and East Asia-Subtropical High land-sea thermal contrast. The characteristics of three land-sea thermal contrast and the spatial and temporal characteristics of summer precipitation in China are studied, and the effects of land-sea thermal contrast on the mean and variability of summer precipitation in China and the influence of variability on extreme precipitation are analyzed. The results show that: (1) The precipitation in Northeast China, the lower reaches of the Yangtze River and the Tibet Plateau is significantly affected by the land-sea thermal contrast. The mean and variability of summer precipitation in these areas are increasing, and the increase of variability increases the probability of extreme precipitation in these areas. (2) The land-sea thermal contrast between East Asia-Pacific and Europe-Atlantic has maintained an increasing trend over the past 40 years and has shown an increasing trend under the medium emission scenario, and the land surface warming is higher than that of the ocean. (3) The main contribution of precipitation generation and change in Northeast China is the land-sea thermal contrast between East Asia-Pacific and East Asia-Subtropical High. The main contribution to the precipitation in the lower reaches of the Yangtze River is the land-sea thermal contrast between East Asia and the Pacific. The main impact on precipitation in the plateau area is the thermal contrast between East Asia-subtropical high and Europe-Atlantic sea and land. Finally, the results of LBM simulation further confirm that the Euro-Atlantic thermal contrast increases the precipitation in the western plateau and decreases the precipitation in the eastern plateau. These findings provide an important basis for us to understand the temporal and spatial variation of summer precipitation in China and the physical mechanism behind it.

Abstract:This paper used the ERA5 reanalysis data to investigate the atmospheric circulation configurations and their evolution processes for three types of extreme low temperature events in the arid and semi-arid regions of China (dryland for brevity) during the winter half-year. This study examined the activity characteristics of transient eddies under different circulation configurations and their feedback forcing to the abnormal circulation. The results showed that the continuous amplification of tilted ridge and trough over the Eurasian continent was the key reason for extreme low temperature events in the dryland. This pair of ridge and trough was primarily maintained by the incoming low-frequency Rossby wave energy. Under its influence, transient eddies were abnormally active on the southern and northern sides of the titled ridge and trough, in which transient eddies in the south of trough guided part of cold air masses southward and further intensified the extreme low temperature events. In addition, the convergence and divergence of transient vorticity fluxes favored the continuous maintenance of tilt ridge and trough, which was conducive to amplification and eastward expansion of the low-level Siberian cold high pressure, and thus led to extreme low temperatures affecting the entire dryland. When the tilted ridge and trough were elongated longitudinally, the ridge over the Ural Mountain weakened and moved eastward as Rossby wave energy emitting downward. Correspondingly, the low-level Siberian cold high pressure also exhibited an eastward displacement, and the activity of transient waves was suppressed. The extreme low temperatures are mainly confined to the eastern dryland.

Abstract:Significant uncertainties exist in the ability of numerical models to reproduce heavy rainfall events over warm sector during the pre-summer rainy season in southern China. One such event occurred with heavy rainfall along the coastal line of Guangdong Province during the period of 29–30 May 2020, and all operational numerical models failed to predict this event at that time. Consequently, eight experiments were conducted to assess the influence of nudging surface-intensive observations into the numerical simulations. The findings reveal that nudging all surface meteorological elements, in what is termed the control experiment (EXP1), reproduces well the development of linear convection and the spatial and temporal evolution of heavy precipitation along the coastline of Guangdong Province. Sensitivity experiments focusing on nudging different surface elements indicate that nudging surface water vapor plays a pivotal role in convection initiation, primarily attributed to the swift escalation of relative humidity from 80% to near saturation (99%) within 3 hours in the low levels, accompanied by a marked reduction in convective inhibition (CIN), lifting condensation level (LCL), and level of free convection (LFC). Nudging surface temperature accentuates thermal buoyancy by amplifying potential temperature perturbations, thereby influencing the initiation and organization of convection. The occurrence and development of convection are considerably delayed in the absence of the nudging surface temperature, and its organization is less structured. Nudging surface wind helps to correct the near-surface southwesterly wind direction, aligning the convection evolution and the rainfall more closely with the observations. Additional six sensitivity experiments were carried out to further explore the impact of nudging duration on simulations. The results suggest that nudging all surface meteorological elements for 6 hours mirrors the outcomes of the control run, effectively reproducing well the heavy rainfall along the coastline. Despite a swift rise in water vapor within the initial 3 hours, the maintenance of water vapor for a certain time (another 3 hours) promotes rapid convection development. Thus, the numerical prediction performance of heavy rainfall in the warm sector over southern China can be improved to a certain extent by nudging surface intensive observations using the surface-grid nudging technique during the initial several (6) hours of model integration.

Abstract:The widely use of automatic weather station has greatly improved the accuracy of meteorological monitoring, providing important support for forecasting and warning, meteorological services, climate analysis and scientific research. Based on the daily precipitation data of the national and automatic weather station in 2015-2021, the multi-scale spatial and temporal distribution of precipitation, the spatial category and intensity characteristics of heavy rainfall, and a brief weather background were analyzed. The main conclusions are as follows:(1) Based on the area-density weights of the rainstorm stations in each district, the spatial classification index of the heavy rainfall is developed with 4 categories of local, regional, wide-range and territory-coverage considering the independence of the assessment factors and the non-uniform distribution of the stations. Based on the area weight of the number of rainstorm stations and the disaster-causing effects of different magnitude rainfall, the assessment index of heavy rainfall intensity is developed. By using the percentile method to determine the intensity grade of heavy rainfall, the spatial range and intensity of heavy rainfall are closely connected.(2) The precipitation in Xiamen was characterized by significant regional heterogeneity and local characteristics due to the topographic and geomorphological characteristics of mountains, sea and bays. The mean annual precipitation and heavy rainfall frequency increased gradually from the coast to inland, and the frequency of rainstorm was closely related to the distribution of topography. (3) In 2015-2021, there were 106, 37, 16 and 5 rainstorms of the aforementioned category in Xiamen, accounting for 65%, 22%, 10% and 3%, respectively.Heavy rainfall occurred in every month, and concentrated in the main flood season from May to September, with peaking in June and August.There were 8 extremely heavy, 24 heavy, 33 relatively and 99 general cases, and the total intensity and average intensity ranked firstly in 2016 and the weakest in 2020. The spatial category of the 8 extremely heavy events was territory-wide scale, while the 99 general heavy rainfall were all local-range events.(4) The main influencing systems of the regional torrential rain processes included warm-cold air activities, tropical cyclones, southerly stream, northward uplift of convergence zone, low-level wind shear and strong convection, etc. The wide-range especially the territory-wide heavy rainfall caused mainly by warm-cold air activities and tropical cyclones. A rare territory-coverage winter heavy rainfall event occurred on 9_th^{Dec 2015 due to the impacting of} weak cold air and the strong warm moist airflow.The results can be used as a scientific reference for monitoring, forecasting, assessment and service of heavy rainfall.

Abstract:The spatial distribution of summer rainfall anomalies over eastern China often characterized by meridionally banded structure. The possible change of it in response to global warming is of great significance to water resource management and disaster prevention. Previous studies show critical role of climate variability on modulating these rainfall modes while seldom studies considered model’s internal variability on investigating their responses to increased greenhouse gas. Based on model simulations with different forcings from the fifth Coupled Model Inter-comparison Project (CMIP5), this paper analyzes the response of the leading modes of eastern China summer rainfall to increased CO2 concentration with consideration of model internal variability. The results show that increased CO2 would not change the leading modes of eastern China summer rainfall. The tripole and dipole mode during the recent decades would still be the leading modes in the abrupt quadruple CO2 experiment (4×CO2) and 1% per year increased until quadrupled CO2 experiment (1%CO2) with the dipole mode plays a more dominant role. However, the frequency, intensity and trend of these modes will change. Compared to pre-industrial control simulation (piControl), the temporal variability of the tripole and dipople mode both decrease under 4×CO2 forcing. The variability of dipole mode intensifies while tripole mode weakens under 1%CO2 forcing. With the gradually accumulation of CO2, the occurrence of the positive and negative phase of these modes will change. The phase of tripole mode which featured with drought over the Yangtze River Basin while flood over North and South China would happen more frequently. So does the “Southern flood and northern drought” phase of the dipole mode. Compared to piControl simulation, the sea surface temperature and atmospheric circulation anomalies associated with the dipole and tripole mode both weaken under 4×CO2 forcing. Whereas under 1%CO2 forcing, their differences are regional dependent. The tripole mode is associated with weakened anomalies over the Indian and western North Pacific Ocean while the dipole mode is associated with stronger central and eastern tropical Pacific and North Pacific sea surface temperature anomalies.

Abstract:In this study, rapidly intensifying (RI) tropical cyclones (TCs) with ‘Atypical’ convection in the Northwest Pacific were screened by using reanalysis data and infrared cloud images. A group of slow-intensifying (noRI) typhoons with ‘Typical’ convection were gathered for comparison. It is found that the RI group has a stronger maximum region of surface latent heat flux on the left side of the vertical wind shear (VWS) direction. To further explore possible mechanisms, the ‘Atypical’ convection case ‘Lekima’ (2019) was selected for sensitivity numerical experiments. The diagnostic results indicate that the surface latent heat flux on the left side of the VWS direction helps establish convective-instability in the region. Consequently, the effect of the "Ventilation effect" was weaken through the "Boundary layer entropy recovery" mechanism caused by the surface latent heat flux. Under easterly VWS, convection activity can be enhanced in high surface latent heat flux regions during the transition from the down-shear to the up-shear, thereby stimulating strong upper outflow channels. The establishment of outflow channels (Outflow blocking mechanism) can resist the environmental easterly flow, which weakening the strength of VWS, and allowing the typhoon to develop vertically.

Abstract:The raindrop spectrum, as a high-frequency observation data that directly describes the characteristics of raindrop size and quantity, has been widely used in the fine evaluation and analysis of raindrop changes in different weather conditions and types. The study utilized raindrop spectrum observation data from 2019 to 2022 in the Shaanxi Province. We have analyzed the overall characteristics of precipitation under the influence of five weather circulations, the drop spectra characteristics of rainstorm and non-rainstorm processes, and the drop spectra characteristics of convective precipitation and stratiform precipitation. The results indicate that the contribution of precipitation with rainfall rate>5mm h-1 to the total rainfall of the process varies significantly among different circulation systems. The main reason for this situation is the difference in the number concentration and contribution of raindrops in each diameter intervals. For example, the southwest airflow type is often characterized by stable precipitation formed by long-term high concentration small raindrops, while the northwest airflow type is characterized by strong precipitation formed by short-term high concentration large raindrops. The characteristics difference of drop spectrum between different circulation rainstorm and non-rainstorm further increase, and heavy rainfall have a significant promoting effect on raindrop spectral parameters. From the variation of number concentration with diameter, the number proportion and diameter spectral width of convective precipitation raindrops with rainfall rate>5mm h-1 in the rainstorm are generally higher than those of non-rainstorm. The distribution range and magnitude of Dm-Nw between convective precipitation and stratiform precipitation are nearly similar, with differences in the location of mean and high value areas. Compared with similar regions such as Zhaosu, Zhuhai, and Palau Island, most of convective precipitation in Shaanxi tends to have oceanic convective precipitation characteristics. A few continental convective precipitation occur in flat straight airflow weather or northwest airflow weather. Based on the above analysis, this study differs from the traditional Z-R empirical relationship and optimizes the fitting of Z-R precipitation estimation equations for various circulation precipitation based on raindrop spectra, which helps to improve the accuracy of radar precipitation estimation in different weather scenarios in the local area in Shaanxi.

Abstract:Using the 1998-2018 NCEP/NCAR global analysis data, atmospheric observation data, and the Tibetan Plateau Vortex (TPV) and shear line yearbooks, a comparative analysis was conducted on the structural characteristics of high-impact eastward moving plateau vortices with quasi-straight long (QSLTPVs ) and short paths (QSSTPVs ) adopting synthetic method. The relationship between the intensity and structure of long and short path vortices was further discussed, thereby indicating that the evolution of long and short path vortices is to some extent determined by the structure of the low vortex itself. The results show that the long and short path vortices have the same structural characteristics, namely, the off-center vortex circulation vertical structure, the coincidence of the center of the low vortex and the positive vorticity center before its departure from the Tibetan Plateau, and the consistent trend of the vortex variation of the low vortex in different activity stages, and the upward motion mainly occurs in the south wind area east of the TPV vertical axis. The differences in the structural characteristics of long and short path vortices are clearly manifested during strengthening stage. The positive vorticity column and upward motion column associated with the long path vortex are stronger than those of the short path vortex. The positive vorticity column associated with the long path vortex tilts northward and has a symmetrical distribution compared to the short path vortex. The convective intensity of positive vorticity coming into the vortex area of the long path vortex is stronger than that of the short path vortex. The transfer of high-altitude westerly momentum of the long path vortex is more evident than that of the short path vortex. The center of south wind associated with the long path vortex is further to the east than that of the short path vortex. The intersection of east and west winds of the long path vortex is further to the south and stronger than that of the short path vortex. The positive vorticity advection center overlying the vortex area of the long path vortex is lower when it departure from the Tibetan Plateau and lasts longer, deviating to the east of the TPV, while the short path vortex is on the opposite side. These differences indicate that the long path vortex has a dynamic mechanism that is conducive to the long-term eastward movement of the TPV.

Abstract:Atmospheric low frequency oscillations are closely related to summer precipitation and related catastrophic weather in China, and it is of great significance to study the effects of 10-30-day low frequency oscillations on weather and climate. In this paper, the effects and mechanisms of the 10-30-day oscillations at mid-high latitudes and low latitudes on the average summer precipitation in China during the period 1991-2020 are investigated by using station observations data and ERA5 reanalysis data, etc. According to the singular value decomposition(SVD) analysis, the 10-30-day oscillations at mid-high latitude are closely related to the spatial distribution of inverse-phase precipitation over Yangtze River and South China Sea, When the 10-30-day oscillations in north of Lake Baikal are weak and the oscillations in south of Lake Baikal are strong, the precipitation exhibits a significant decrease in the Yangtze River basin while showing a notable increase from the South China Sea to South China; The key region of the low latitudes 10-30-day oscillations is located in the northwestern Pacific, where precipitation in southern China exhibits a significant positive anomaly during periods of strong oscillation within this region. The 10-30-day oscillations at mid-high latitudes mainly affect the mean position of the summer subtropical westerly jet stream through wave-current interactions, which makes the position of the secondary circulation near the jet stream to be anomalous, thus indirectly affecting the precipitation in the Yangtze River Basin and the South China Sea to South China, and making it show anomalous inverse-phase distributions; Distinguishing it from the mid-high latitudes, the 10-30-day oscillations at low latitudes over northwest Pacific propagate northwestward to southern China, directly inducing precipitation anomalies.