Sandstorm is a serious natural disaster in north China. It is of great significance to carry out relevant research to improve the forecast level of this kind of catastrophic weather. Based on the RegCM-dust model, an extended period numerical prediction analysis of a typical severe sandstorm process in north China is conducted, and the results are compared with NCEP reanalysis data and other analysis results. The results show that the regions with high sediment discharge simulated by the model are mainly located in southern Xinjiang, Mongolia and western Inner Mongolia. The model has a certain forecasting ability for 10 m wind speed, but the simulated wind speed is smaller than the reanalysis data. The changes of dust column content and total sedimentation simulated by the model can reflect the characteristics of the dust storm weather process. The simulated sand-dust mixing ratio has a certain correspondence with the urban pollution index, which indicates that the model has certain forecasting ability for the pollution weather caused by sand-dust.

The ECMWF-HR cloud forecast products are verified by using the total cloud cover inversion products of FY-2G satellite from October 2019 to September 2020 and the diurnal variation characteristics of ECMWF-HR total cloud cover products in the central and eastern parts of Northwest China are diagnosed in selected key areas to provide references for the application of cloud forecasting. The results show that the total cloud forecasted by the ECMWF-HR is relatively stable and has obvious diurnal characteristics in the study area. Forecast deviation is small in the daytime and at night it increases by 10%-20%. Meanwhile, there are obviously seasonal characteristics of cloud forecast product, and it has positive deviation in summer half year and the spatial distribution of the deviation is even. It shows regional distribution characteristics in winter half year with negative deviations in the western Qilian Mountains and positive deviation in Gansu and the south part of Shanxi, and the deviation is lower in winter half year than in summer half year in other areas. In general, the cloud forecast product of ECMWF-HR is relatively reliable in the study area, but in two regions, there are significant anomalies. Total cloud forecast needs to be increased by about 10%-30% in the western Qilian Mountains and decreased by about 20%-30% in Gansu and the south part of Shanxi on the base of ECMWF-HR product. The model correction results are relatively close to the satellite inversion results, with an average absolute deviation of 4.5% and similar diurnal variation characteristics.

The northwest region of China is located in the hinterland of Eurasia, in which the source of water vapor is scarce, and drought is its main climatic feature. In recent years, with the continuous increase of regional precipitation, the warming and wetting in Northwest China has attracted great attention from all walks of life. In order to scientifically respond to social concerns, the team used multi-source data to conduct in-depth research on the phenomenon of warming and wetting in Northwest China from multi-scale and multi-dimensional perspectives, and found that the trend of wetting in Northwest China had significant and nonlinear enhancement characteristics. It is recognized that the wetting in Northwest China is expanding eastward, and the land surface evapotranspiration there has a special negative feedback mechanism on climate warming. It is estimated that the warming and wetting trend will still maintain in Northwest China in this century, and the wetting trend is driven by multi-factor comprehensive driving mechanism. The multi-aspect impacts of the warming and wetting in Northwest China are evaluated, and the technical countermeasures to deal with the warming and wetting there are put forward, and the research results of “the enhancement and eastward expansion of climate warming and humidification, formation mechanism and important environmental impacts in Northwest China” are formed. The major consultation report based on the research results has played an important decision-making support for the national strategies such as the development of the western region in the new era and the ecological protection and high-quality development of the Yellow River Basin. The research results were selected as “China's Top Ten Scientific and Technological Progress in Ecological Environment” in 2022, and have also received extensive attention from the international academic communities.

The weather research and forecasting (WRF) model along with its three-dimensional data assimilation (3D-Var) system was used to improve the initial field by assimilating radar reflectivity and radial velocity from Doppler radars. A rare strong squall line occurring on 4 March 2018 in Jiangxi Province was simulated and studied by using the adjusted initial field. It is found that only assimilation of rainwater, snow, graupel particles retrieved from radar reflectivity and water vapor derived from radar reflectivity could not make a stable improvement in forecasting the composite reflectivity, especially got an opposite effect on forecasting surface gale and precipitation. The assimilation of reflectivity data could significantly improve forecast skill when radar radial velocity was assimilated jointly. The reason is that assimilation of radar reflectivity can efficiently adjust initial hydrometeors and thermal field, but it has little effect on initial dynamic field. As the simulation time went by, the adjustment of dynamic field was unreasonable, a false divergence wind field appeared in the upper troposphere, thus a stratiform cloud area appeared in front of the squall line, which was not captured in reality. So the model could not improve the simulation of vertical wind shear and cold pool as well as rear inflow, and then there was a large gap between forecasted results and observations. Only assimilation of radar radial velocity could improve simulation results, and assimilation of both could efficiently adjust initial hydrometeors and thermal fields as well as dynamic field, and make the physical configuration more compatible with reality.The vertical wind shear and wind field structure were more favorable to occurrence of the strong squall line, and then formed a strong cold pool close to reality, further the forecasting results of radar composite reflectivity, surface wind and precipitation of the squall line agreed much better with observations compared with only assimilation of radar radial velocity.

Based on the hourly precipitation data from April to September during 1960-2019 at 9 national meteorological stations and 400 regional meteorological stations built year by year in Longnan of Gansu Province from 2008 to 2019, NCEP FNL 1°×1° reanalysis data and MICAPS data, the spatial-temporal distribution and mesoscale characteristics of short-time heavy precipitation in Longnan of Gansu Province were analyzed. The results are as follows: (1) The occurrence frequency of short-time heavy precipitation in Longnan became more from the northwest to the southeast, with two relatively concentrated areas. The short-time heavy precipitation with rainfall intensity greater than 50 mm·h^-1 occurred in Cheng county, Hui county and Kang county in the east of Longnan. The occurrence frequencies of short-time heavy precipitation and rainstorm were more in the southeast of Longnan, and for short-time heavy precipitation it was also relatively high in the mountainous areas in the northwest. (2) Since 1960, the stations occurring short-time heavy precipitation in Longnan increased slowly.The monthly variation showed a single-peak type, with the maximum in August and accounting for 37.5% of the total stations occurring short-time heavy precipitation. The ten-day variation presented a double-peak type, with two peaks in early July and early August, respectively. The stations occurring short-time heavy precipitation was the most from late July to mid-August, accounting for 47.2% of the total stations. Diurnal variation showed that there was more short-time heavy precipitation at night than in the day, there were multiple peaks. The stations occurring short-time heavy precipitation increased significantly since 15:00 BST. The diurnal peaks of short-time heavy precipitation occurred at 23:00 BST, accounting for 9.4% of the total stations. (3) The short-time heavy precipitation in Longnan was closely linked to rainstorms. The mesoscale concept modes of short-time heavy precipitation in Longnan mainly showed three types, including low vortex shear, northwest air flow following the trough moving eastward and the southwest air flow beside the sub-tropical high.

Based on the daily minimum temperature observation data in southeast of Gansu Province during 1969-2018, the variation of extreme low temperature events was analyzed, then 74 circulation characteristic quantities from National Climate Center were used to research the circulation system affecting the extreme low temperature events. The results are as follows: (1) The frequency of annual extreme low temperature events in southeast of Gansu Province reduced obviously with a rate of 2.3 d·(10 a)-1 in the last 50 years, and the reduction was most significant in summer and slowest in winter. There was an abrupt change of extreme low temperature days in 1987, after the abrupt change, the frequency of extreme low temperature events was relatively lower. (2) Compared with the climatic mean, the intensity of extreme low temperature events tended to increase, before 1987, the low temperature intensity anomaly increased with a rate of 0.2 ℃·(10 a)-1, while after 1987, the extrem low temperature intensity anomaly increased with a rate of 1.2 ℃·(10 a)-1. (3) The intensity of extreme low temperature in spring, summer, autumn and winter mainly ranged from -5.0～5.0 ℃, 10.0～15.0 ℃, -5.0～10.0 ℃, -20.0～-10.0 ℃, and the occurrence frequencies were 61.9%, 90.1%, 73.4% and 73.1%, respectively. (4) There was a positive correlation between extreme low temperature events and Eurasian meridional circulation in southeast of Gansu Province. The extreme low temperature events was related with cold air, the area index of western Pacific subtropical high and the intensity of polar vorticity center in the northern hemisphere in winter, while it was related with subtropical high northern boundary of South China sea, subtropical high northern boundary of the western Pacific and polar vorticity intensity of the Pacific in spring. The composited analysis of geopotential height fields indicated that Mongolia was a cold high pressure center on ground, and polar vorticity center was located in the eastern hemisphere from 500 hPa to 100 hPa, the westerly belt prevailed meridional circulation, and east Asian large trough in the westerly belt was deep and westward, the region of southeast of Gansu was controlled by strong northwest airflow after the trough, which was favourable to forming extreme low temperature events.

Abstract: Based on the ground meteorological observation data of Hohhot station from 1981 to 2018, air quality data from 2014 to 2018 and nighttime light data from 1992 to 2013, the atmospheric self-cleaning ability index (ASI) was calculated, and the variation characteristics of atmosphere self-cleaning ability in Hohhot city in the past 38 years and its influencing factors were discussed.The results show that ASI was negatively correlated with air quality index (AQI), and the relationship between them could be expressed by a power function. The smaller the ASI was, the greater the risk of air pollution would be. The annual average ASI in Hohhot  declined significantly, especially after the 21st century. Compared to precipitation, wind speed and mixed layer height were more important to ASI. The positive correlation between the cold air active days and ASI was significant. After the 21st century, the cold air activity was obviously weak, which had a certain influence on ASI. The nighttime light index and the nighttime light area had a significantly negative correlation with ventilation. To some extent, the urbanization process played a role in the decline of ASI, especially after the 21st century.

Based on  daily rainfall  from 34 national  meteorological observation stations in the Hetao area , Inner Mongolia during 1961-2018, extreme precipitation events and extreme precipitation processes were analyzed in the past 58 years. Atmospheric circulation features of typical extreme precipitation processes were aslo analyzed. The main results are as follows: (1) Average annual precipitation decreased from southeast to  northwest, and precipitation was rich from July to August. The precipitation decreased significantly in August and increased significantly in May, June and December. (2) The thresholds of extreme precipitation became lower from southeast to  northwest, the historical maximum extremum occurred in Wushenzhao  of Ordos,  the historical minimum extremum occurred in Hailisu of Bayannur. The frequency of extreme precipitation events was more than five times in most areas, the intensity became weaker from southeast to northwest. (3) The extreme precipitation events was most from July to August, and its intensity was strongest in August. The frequency of extreme precipitation events and the intensity increased significantly in September. (4) The annual average extreme precipitation process intensity decreased significantly, the process precipitation was concentrated in Tumote Zuo Banner, Ejin Horo Banner and the city of Huhhot. (5) The extreme precipitation processes were influenced by the southwest flow at the front of trough, and the plentiful vapor and exceptionally strong updraft flow could easily cause extreme precipitation events.

Based on 2-meter temperature data from the second generation monthly dynamic extended range forecast (DERF2.0) model, observational temperature data at 69 weather stations in Gansu Province, reanalysis data of NCEP/DOE and sea surface temperature data of NOAA，the forecast errors of January temperature in Gansu Province by DERF2.0 model from 1992 to 2013 and their relationship with external forcing were analyzed. The results are as follows: (1) The simulated effects of January temperature by DERF2.0 model in eastern Yellow River of Gansu (known as Hedong for short) were better than that in most regions of western Yellow River of Gansu (known as Hexi for short), especially  in Gannan, Linxia, Lanzhou, Dingxi, Pingliang and Qingyang, the average errors between forecast and observation were small and stable, and the linear tendency rates of forecasted temperature in January were consistent with observation from 1992 to 2013, while the average errors were bigger and unstable in most regions of Hexi, and the change trends of forecasted temperature were contrary to actual observation. (2) Although the model could well reflect the inter-annual variation and spatial distribution pattern of January temperature in Gansu, the abnormal centers and values of temperature change were significantly different from the observation. (3) The EOF1 of error field reflected consistent overestimate or underestimate to  January temperature, the EOF2 presented an opposite distribution pattern in Hedong and Hexi, while the EOF3 appeared a reverse phase distribution pattern in Gannan Plateau and other parts of Gansu Province. (4) The main modes of forecast error field were significantly correlated with circulation and sea surface temperature (SST) in key areas, which indicated that the response of model to circulation and SST anomalies was deficient. Therefore, it was partially possible to reduce forecast errors of January temperature in Gansu by adjusting the response ability of DERF2.0 model to circulation and SST in key areas.

The predictionmodel for the highestand lowest temperature is established by the CMSVM regressionmethod based on ECMWF data and the extreme temperature observations from 8 automaticweather stations inQingyang ofGansu Province from 2003 to2007.The effect test in 2008 operation forecast indicated that the rate ofaverage forecastaccuracy is64% and 71% for the highestand lowest temperature in 5 days, which has the good instruction for the real-timeweather service. The forecast accuracy rate of the highest and lowest temperature decreased with the forecast time increase, and the accuracy rate for the lowest temperature is higher than thatof the highest temperature. The forecast effect for the lowest temperature is better in spring and summer than that in autume and winter, but it is opposite for the highest temperature. The absolute and the average forecasterrors for the highestand lowest temperature are added with the forecast time increase, and the absolute forecasterror for the highest temperature is bigger than thatof the lowestbut the average error is close and positive. The forecast effect test in Septemper of 2008 indicated that the forecast values presented the similar trend to real time observations for the highest and lowest temperature.