Abstract:In order to help weather forecasters understand the physical mechanisms concealed among observational phenomena and scientifically improve their warning skills, from the operational point of view on weather forecasting and warning, this paper emphatically discusses the scientific relationships between the mechanisms triggering linear convective gales and the structure evolution, thermodynamic and dynamic processes of convective storms and the cloud-water macrophysical processes, and also explains their “dominant characters” in the contemporary operational observation system. Besides, some scientific problems with different views or interpretations are discussed as well. The major results are as follows. The surface linear gales incurred by convective storms are directly derived from the internal vertical movement of storms, while major compositions of the vertical velocity are contributed by the cold pool forcing and the downward vertical gradient of the disturbed pressure for thermodynamic effect. The thermodynamic effect and cold pool effect are directly related to cloud-water macrophysical processes within storms, such as evaporation (condensation)， melting (sublimation), and their evolutions can be demonstrated by a series of observed phenomena, such as weak echo slot, descending rear inflow jet, MARC (mid-altitude radial convergence), mesocyclone, gust front and the acute variations of meteorological elements on surface. The pattern variations of linear convective storms are induced by the inner dynamic processes of storm systems or by the interaction between storm systems and ambient atmosphere. It can not be definitely considered that all linear convective storms should develop into squall lines with bow echoes. The RKW theory can essentially interpret the interaction between the integrated squall line and the vertical shear of ambient wind. Virtually, the ambient wind performs the principal function of leading the movement and propagation of squall line, and the development or maintenance of squall line is possibly controlled by its inner thermodynamic and dynamic processes.

Abstract:In order to explore the impacts of urban agglomeration in the Guangdong-Hong Kong-Macao Greater Bay Area on localized extreme heavy rainfall, this paper conducts 3DVar assimilation of Doppler radar data to simulate an extreme heavy rainfall event that occurred in the Greater Bay Area on 22 May 2020, based on the WRF-ARW mesoscale numerical model, GSI-3DVar assimilation system and ERA5 reanalysis data provided by ECMWF. The influence process and mechanism of urban area on local extreme rainfall are studied. The results show that, compared with no assimilation of any observational data, adding the assimilation of radar reflectivity and radial wind data can improve the simulation ability of precipitation, especially for the heavy rainstorm magnitude of more than 250 mm. Observation and control experiment jointly show that the urban agglomeration in the Greater Bay Area acts as a local “heat source”, increasing the temperature of the boundary layer through sensible and latent heat, resulting in significant heat island effect, which then strengthens the convective instability within lower atmosphere. On the other hand, strong friction dissipation reduces the wind speed in boundary layer so that it catches more warm and moist air within the urban area, forming stronger thermal instability and moisture convergence, and then leading to the rainfall center located at the inner part of the edge of urban area. The sensitivity experiment (i.e., removal of urban land use) further shows that friction dissipation caused by urban underlying surface affects dynamic thermodynamic environments in boundary layer below 800 hPa, resulting in stronger southwesterly and unstable atmospheric condition over downstream of the urban area in urban removal experiment. Also, the convection is lifted by the local topography, enhancing vertical upward movement, which finally results in stronger rainfall intensity than the control experiment and the location of the rainfall area more inclined to the downstream of the urban area.

Abstract:By using weather radar, upper-level and ground observations and NCEP 1°×1° reanalysis data, this paper analyzes the characteristics of mixed-type convective weather during the formation and development of Jianghuai cyclone in spring, and the reasons why different types of convective weather occur and intensify. The results show that there are differences in spatio-temporal distribution and convective characteristics of different types of severe convective weather. The localized hail is mainly produced by discrete convective line in the formation stage of cyclone, the banded short-time severe precipitation is produced by TL/AS MCS which is located on herringbone convective line in the formation stage of cyclone and behind the S-shaped convective line in the development stage of cyclone, and the large-scale gale is produced by TS MCS on the S-shaped convective line in the development stage of Jianghuai cyclone. Jianghuai cyclone is the result of the baroclinic development of the large-scale weather systems, convective activities enhance the convergence of the low-level〖JP2〗 front, strengthening the formation and development of cyclone. The generation of severe convective weather is closely related to the dynamic and thermodynamic field of Jianghuai cyclone. In the cyclone formation stage, the southwest vortex combined with the mountainous terrain provides the environmental field conducive to the formation of hail in the southwest of Hubei Province. The warm shear line in the cyclone formation stage and the cold shear line affected by the south branch trough in the cyclone development stage provide the environmental field conducive to the formation of storm train effect which produces 〖JP2〗short-time severe precipitation. In the cyclone development stage, the cold shear line provides the environmental field conducive to the formation of the rear inflow jet, which is the main factor of gale formation.

Abstract:The Western Pacific-South China Sea-East Indian Ocean (two oceans and one sea) region has an important impact on the weather and climate, national security, as well as social economy of China. However, due to the limitation of data conditions, the existing research on high sea winds are mainly focused on the offshore, resulting in the insufficient understanding of the temporal and spatial distribution, the characteristics of changes and their mechanisms of high sea winds in the two oceans and one sea region. So, it is urgent to use new high-resolution data for in-depth research, but at present, there are relatively few comparative studies between ERA5 reanalysis surface winds and the observation data. Therefore, this paper compares the ICOADS moored buoy observation data with the ERA5 reanalysis data in the two oceans and one sea region, and the results show that the ERA5 reanalysis 10 m winds can well reflect the distribution and variation characteristics of sea surface wind field. In addition, the ERA5 reanalysis data has high temporal and spatial resolution, long time-series and complete data records. Using the ERA5 reanalysis data is feasible and has certain advantages for climate analysis of high sea winds. However, it should be noted that the ERA5 reanalysis surface wind speed generally has a systematic bias of underestimating the observed wind speed. In particular, the greater the observed wind speed, the greater the deviation of ERA5 from the observed wind speed.

Abstract:The return sounding observation is a new high-altitude observation technique developed by China. In addition to the observation of vertical profile in the ascending stage, at the same time, the atmospheric sounding in the period of floating and falling is added to this technique and the space-time densification of the sounding profile is realized automatically. In this paper, with the ERA5 reanalysis data as the “true value”, the return sounding simulation system is used to construct the return sounding simulation observation. Besides, Observing System Simulation Experiments (OSSEs) are conducted based on CMA-MESO regional model and 3D-Var assimilation system. The results of numerical experiments show that compared with the traditional single-rise sounding observation, the simulated sounding observation of the descending section of the return sounding can effectively improve the precipitation forecasting skills of CMA-MESO in the circumstances with national network, and the ETs scores for different precipitation grades can be improved by about 2%-5%. At the same time, the forecast of factor field (temperature, humidity and wind field) is improved, with improvement rate about 2%-5%. In addition, the analysis results of typical weather cases suggest that the increase of return sounding observations can improve the initial model deviation, thus more accurate simulation of precipitation distribution. Generally, the research results of this paper would provide a theoretical support for the future scientific layout and application of the return sounding.

Abstract:Beijing Daxing International Airport, as a super large international hub with huge throughput, has a large amount of aircraft exhaust emissions， which can impact the atmospheric environment around the airport greatly, and should not be ignored. In order to study the impact of aircraft exhaust emissions on the atmospheric environment under adverse meteorological conditions, the adverse meteorological conditions period in 2020 is selected based on the Beijing air quality data. According to the projected basic data of the 2025 planning of Daxing International Airport, the Emissions and Dispersion Modeling System (EDMS) is used to establish the list of exhaust emission sources of the airport and simulate the diffusion of air pollution. The results of pollution analysis show that there are meteorological characteristics such as light calm wind, high humidity and strong grounding temperature inversion with thin thickness during adverse meteorological conditions. Beijing has experienced heavy pollution processes, accompanied by the low visibility. Under the influence of adverse meteorological conditions, the parameters such as friction velocity and mixing layer height are at a lower level, indicating that the diffusion conditions of air pollution are poor. The model prediction results show that aircraft exhaust would have a great impact on the surrounding environment, and the ground concentration of pollutants might have the time distribution law of low in the daytime but high at night. Such characteristics of pollution meteorology and aircraft emission concentration distribution under adverse meteorological conditions could provide some references for the research on airport air pollution control measures.

Abstract:Based on the particulate matter and meteorological measurements at the Dongtan Atmospheric Composition Observing Station (shortened Dongtan Station) from 2008 to 2015, the arriving air mass is classified, the PM2.5 concentration level and its annual variation are examined, and the potential source of higher level of PM2.5 is also identified in this article. The long-term variation of PM2.5 concentration presents an in-significant trend from 2008 to 2015, but the percentage of fine particles (PM2.5) keeps increasing. The ratio of PM2.5/PM10 increases from 0.84 to 0.92, indicating more and more data of secondary aerosols are observed at Dongtan Station. The air mass of 8-year big sample data at Dongtan Station can be aggregated into 3 types of back trajectories, that is, land, ocean, and land/ocean mixing types, accounting for 32.0%, 38.8% and 29.3% respectively. Among them, the PM2.5 background mass concentration ranges stably within 11-15 μg·m-3 in ocean air mass, 〖JP2〗but 29-56 μg·m-3 in land air mass, showing largely seasonal variability. The potential source of relatively higher PM2.5 observed at Dongtan Station presents clear seasonal transition from areas north to Shanghai including North China and, Huanghuai regions as well as Jiangsu and Anhui provinces in autumn and winter to southern Yangtze River Delta region including northern Zhejiang Province and its seaboard extending to Fujian Province in summer by PSCF analysis. In general, higher PM2.5 loading at Dongtan Station is mostly contributed by air mass from Shanghai and its neighboring city clusters including Suzhou, Wuxi, Changzhou, Hangzhou, Jiaxing and Huzhou etc. It is noted that air mass recycled from Yellow Sea and Bohai Sea is also an important source area for the elevated PM2.5 observed at Dongtan Station in spring, autumn and winter.

Abstract:Using the data of CHAP PM2.5 and MODIS MCD19A2, the ERA5 reanalysis data and the SNPP/VIIRS satellite monitoring fire point data as well as the least squares method and other analytical methods, this paper explores the spatio-temporal distribution pattern of PM2.5 concentrations in southwest Yunnan and the causes of seasonal bursts from the aspect of backward trajectory of pollutants, circulation situation, high and low air dynamic structure configuration, etc.. The results show that the spatial distribution of PM2.5 concentration and aerosol optical depth (AOD) in the study area are in the pattern of high values in the south and west but low values in the north and east. The PM2.5 concentration is the lowest in July but is the highest in March in a year. The stable transport of pollution sources from February to April causes the spring PM2.5 concentration value in the study area to be higher and the fluctuation to be smaller than in the other three seasons. However, the spatial distribution of the percentage change is more obvious. The percentage reduction in PM2.5 concentration in the past 20 years was mostly by from -30% to -20%, and the percentage change in summer, autumn and winter was mainly less than -30%. More than 90% of the fire points in eastern Myanmar and northern Laos, which are adjacent to the study area, occur from February to April. Under the guidance of the westerly air flow, the westward (southwest-oriented) pollutants are transported over a short distance through the high-fire point area, causing subsidence under the dynamic action of low-layer radiation dispersion in the upper air, and resulting in a seasonal increase in PM2.5 concentration in the study area. A large range of meteorological conditions conducive to the spread of pollutants and the cleaning of more precipitation can reduce the contribution of overseas air pollutants to PM2.5 pollution in southwest Yunnan. The effects of changes in fire points outside the key areas on PM2.5 concentration and AOD show a significant positive correlation. Their effects on PM2.5 lagged behind the change in fire points by about 2 days, and the correlation between the two gradually weakens from south to north.

Abstract:The particle size distribution is not only affected by meteorological factors such as temperature, air humidity and wind, but also closely regulated by the boundary layer mixing such as turbulence. Based on the simultaneously observed meteorological factors and particle size distribution in the range of 14.6-660.0 nm in November 2018, the impact of different meteorological factors, especially turbulence, on the particle size distribution is investigated to further understand the roles of meteorological factors in haze development. The results show that the increased relative humidity can reduce the total particle number concentration (TPNC) in nuclear mode and Aitken mode, but raise the TPNC in accumulation mode. Warmed temperature can increase the TPNC in the nuclear mode. The increase of wind speed, turbulent kinetic energy, frictional velocity and turbulence intensity can dilute and remove the particles in the Aitken and accumulation modes, but enhance the TPNC in the nuclear mode. In contrast to the diurnal variation of turbulence, the TPNC in Aitken mode and accumulation mode is low in the daytime but higher in the nighttime. In clean days, the TPNC in the nuclear mode continues to rise in the afternoon and reaches a peak in the late afternoon. A time lag has been found in the nuclear mode TPNC increase in contrast to the turbulence development. After the turbulence develops for 3 to 5 hours, the TPNC in nuclear mode begins to increase significantly.

Abstract:The climate over China in summer 2022 was extremely abnormal, with significant regional floods and droughts and spatial difference in rainfall distribution. Based on observational and reanalysis dataset, the characteristics and possible causes of East Asia summer monsoon (EASM) and climate anomalies over China was summarized and investigated through correlation and composite analysis. It has been found that the seasonal march of EASM was ahead of climatological mean in general. The onset date of South China Sea monsoon, and the start dates of preMeiyu rainy season in South China, rainy season in Southwest China, Meiyu in Jiangnan and the middle and lower reaches of Yangtze River, rainy season in North China and Northeast China were all earlier than in normal years. The average surface temperature over China in summer 2022 ranked the highest since 1961, while the average precipitation across the whole China was the second lowest compared with the rainfall in the same period in history. The Yangtze River Basin experienced recordbreaking high temperatures and drought in midsummer. The distribution of precipitation also showed significant intraseasonal variability. In early and middle 〖JP2〗June, the main rain belt was in South China. 〖JP〗From late June through August, the western Pacific subtropical high (WPSH) shifted northward bringing the main rain belt to shift to North China, Northeast China, Huanghuai Region and eastern Northwest China, and the precipitation in the eastern part of China was “more in the north and less in the south”. The heat wave and drought persisted in the Yangtze River Valley throughout the summer. The abnormal climate in summer 2022 was closely related to the oceanic external forcing. The reinforcement of La Ni〖AKn~D〗a in spring and intensified negative SST anomalies in central Pacific, active convection over maritime continent, developing negative phase of Indian Ocean dipole mode, the Kuroshio and its warm extension area all contributed to the intensification and northwestward extension of WPSH, the northward location of the summer main rain belt and the recordbreaking heat wave in the Yangtze River Valley.

Abstract:The main characteristics of the general atmospheric circulation in October 2022 are as follows. There was a single polar vortex center in the Northern Hemisphere, which was stronger than normal. The circulation at middle-high latitudes of the Eurasian showed the “two troughs and one ridge” pattern. Compared to the normal state, the Western Pacific subtropical high was stronger, and the location was westward and northward. The monthly mean temperature was 10.8℃, higher than the normal value (10.6℃) by 0.2℃. The monthly mean precipitation was 34.4 mm, 3.4% less than normal (35.6 mm). There were two torrential rain processes in China in October: one was affected by the upper-air trough, low-level jet and subtropical high, and the other by the No.2222 Typhoon Nalgae and the high trough. In October, a total of five tropical cyclones were active in the South China Sea and the western North Pacific, with the number of the tropical cyclones more than normal by 1.2. There were no landing typhoons, fewer than normal. Among them, Typhoon Sanka (No.2219), Typhoon Nansha (No.2220) and Typhoon Nalgae (No.2222) affected the South China Sea, Hainan Island and South China. The meteorological drought to the north of the Yangtze River got relieved, but the drought to the south of the Yangtze River persisted.