Abstract:The observational data of meteorological stations during 1980～2019 in China was used in this study, including daily precipitation, daily maximum temperature, humidity, cloud cover and wind speed. Then holiday climate index (HCI) is constructed by these meteorological elements to show holiday climatic suitability over China. The annual mean holiday comfort day (HCD) is 131.1 days over China, with a bimodal distribution throughout the year, which is more in spring and autumn while less in winter and summer. Affected by the effective temperature and wind speed, the annual mean HCD show a significant increasing trend. The bimodal distribution of HCD is more prominent caused by effective temperature, cloud cover and precipitation. The spatial distribution of annual mean HCD is not even in China, which is greater in North China, Huanghuai, Jianghuai, Xinjiang and Yunnan province than those in other regions. HCD in Tibetan is the least in China. HCD in most areas over China has an increasing trend, except for central South China and eastern Hebei province. The spatial distributions of HCD in winter, spring and autumn show increasing trends in most part of China, while decreasing trends in summer. Closely attention on the influence of climate change on holiday comfort over China is conducive to the rational development and utilization of climate resources, providing protection for the sustainable development of tourism industry.

Abstract:Based on datasets of three drought indices (i.e., the Standardized Precipitation Index, SPI-12, Standardized Precipitation Evapotranspiration Index, SPEI-12, and self-calibrated Palmer Drought Severity Index, scPDSI) for the period 1901~2020, this study investigates the long-term characteristics of drought in China over the past 120 years and then explores reasons for their inconsistency. Results indicate a significant drying trend in southwestern China, the Loess Plateau, southern Northeast China, and southern Xinjiang, while regions with significant wetting trends are located in North China, the east part of Northwest China, and the north part of Northeast. As far as drought events are concerned, both Northeast China and Northwest China are characterized by prolonged duration and higher intensity, but the drought tends to decrease in general, especially in eastern Northwest China and northern Northeast China. On the contrary, the duration and intensity of drought events increased in southern Northeast China. There are approximately 2820000km2 of land area each year that has experienced drought, and among them about 30500 km2 of land areas are threatened by extreme drought. Drought severity increases with time, with rapid growth after the mid-1990s. During the past 120 years, drought area exhibits significant interannual and decadal variabilities, with the main periods of 2~3 years and 18~22 years, respectively. In humid regions, three drought indices show good consistency, while in semi-arid and arid regions, their consistency is relatively low. In humid areas, the wet-dry variation is dominated by precipitation. In semi-arid and arid areas, besides precipitation, both temperature anomaly and soil characteristics also play an important role in drought. Therefore, more attention should be paid to the drought index selection over arid and semi-arid areas in the context of global warming.

Abstract:The performances of CMIP5 and CMIP6 Earth System models in simulating China’s terrestrial vegetation biomass were assessed based on two sets of the observation biomass carbon density data, the first to ninth (1950-2018) China Forest Resource Inventory data and a set of remote sensing land cover data. The impacts of different Land-Use Harmonization (LUH) datasets on vegetation biomass simulations of CMIP5 and CMIP6 models were further explored. Our results show that both CMIP5 and CMIP6 models overestimate China"s total vegetation carbon storage during 1995-2004 (28.0 ± 6.0 Pg C for CMIP5, 25.3 ± 7.7 Pg C for CMIP6, and the two sets of reference data are 18.1 Pg C and 18.7 Pg C respectively). The spatial distributions of vegetation carbon density as simulated in CMIP6 models are better than those in CMIP5, with improved Taylor Skill Score(TSS) values and reduced model uncertainties. During 1950-1990, China’s vegetation in CMIP5 and CMIP6 are carbon sources of ?89.4 Tg C yr-1 and ?58.2 Tg C yr-1, respectively. These carbon sources significantly increase to ?256.6 Tg C yr-1 (CMIP5) and ?171.0 Tg C yr-1(CMIP6) in the 1980s. During 1990-2014, in CMIP5 the vegetation carbon sources are much weaker (?48.1 Tg C yr-1) , while in CMIP6 those sources are changed to weak sinks (42.8 Tg C yr-1, P<0.05). The differences of China"s vegetation carbon between CMIP5 and CMIP6 are closely related to their different land change conditions. Compared to Land-Use Harmonization (LUH1) in CMIP5, the forest covers and their changes in LUH2(in CMIP6) are more consistent with China Forest Inventory data and are closer to that since 1980s. This study implies that LUH datasets used in CMIPs have large bias from China’s forest and crop trajectories over the past 65 years. More accurate land cover datasets are essential for improving the simulation of vegetation carbon in the future CMIP models.

Abstract:Urban ventilation corridor was designed to improve urban air circulation, alleviate urban thermal environment and improve human comfort, has been widely used in cities all over the world. Based on the observation data of meteorological stations, satellite remote sensing data, basic geographic information and urban planning of Pingdingshan city, with the method of mathematical statistics, land surface temperature retrieval, geographic information systems and remote sensing techniques , we analyzed wind environment, urban heat island (UHI), green source and ventilation potential of the city, meanwhile constructed the structure of urban ventilation corridor preliminarily. The results showed that: (1) The dominate wind in Pingdingshan was mainly from northwest, northeast and south. (2) The spatial distribution of UHI was multicenter, and the intensity of UHI decreased from the central city to surrounding area. Urban green source was mainly distributed around the city, which can introduce fresh air into urban center. (3) Based on the comprehensive assessment of urban ventilation potential, urban surface roughness, sky view factor etc., the corridor system of “six main and ten secondary corridors” in Pingdingshan city was constructed.

Abstract:Fog and haze can transform from/to each other under such a typical evolution pattern as “heavy haze (HH) – extremely dense fog (EDF) – heavy haze”. To explore the key meteorological factors for the transformation from HH to EDF, the near-surface circulation patterns of 247 EDF and 96 HH cases in the central area of the Huaihe River Basin (HRB) at 08:00 (high-intensity period) for 40 years were objectively classified respectively, using the multi-element oblique rotation principal component analysis method, together with the ERA5 reanalysis data. Based on the results of classification, the formation mechanisms for EDF under various circulation patterns were analyzed, and the differences of meteorological conditions in the surface and boundary layer of HH and EDF with similar large-scale circulation patterns were compared. The results show that: (1) The circulation of EDF can be divided into five types, with a cold high in northwest or northeast China for each type, and a sub-synoptic system in the HRB, such as weak high, frontal or inverted trough; The studied area is located in the north (33%) or inside (19%) of the weak high, in front of the front (29%) or the invert trough (11%) or at the bottom of the cold high (7%), prevailing with southwest, south or east wind with average wind speed lower than 1.6 m/s at ground level. At 925 hPa, the study area is located in the warm ridge and low humidity area, with an average wind speed of 2 m/s. In the vertical, the relative humidity (RH) decreases rapidly with height, and to below 60% at 925hPa or 850hPa, which is conducive to the surface radiation cooling, with an average temperature decrease range larger than 3℃, forming a deep near-surface inversion, with a temperature difference of 2-4 ℃ between 975 hPa and the ground. (2) The near-surface circulation for HH can be divided into three types, which are all related to high pressure system. The study area is located in the front (56%), bottom (19%) or rear (26%) of the surface high, but there is no sub-synoptic scale system or the system is weak in the HRB. (3) The reasons for HH cannot develop into EDF include: ① insufficient water vapor source (due to northerly wind on the ground); ② low cooling range at ground level (the average lower than 3.1 ℃) with relative high wind speed (the average over 2.2 m/s). (4)The sub-synoptic scale system near the study area is the key reason for whether HH can develop into EDF. This system determines local meteorological conditions, such as whether there are light winds, significant cooling, and sufficient sources of water vapor. The inter-monthly variation and generation and disappearance time of EDF under various circulation patterns are also analyzed.

Abstract:Time series analysis methods have been developed before to identify dust weather based on pollutant concentrations, but the criteria used are subject to considerable uncertainty. Therefore, in this study, we propose an objective identification method for dust weather based on the K-means clustering method by using the hourly concentration of PM2.5 and PM10 from the environmental monitoring stations. The core idea of this method is as follows: first select the optimal number of classifications K for cluster analysis; then classify the cluster groups that show large scattering in the distribution of PM2.5 and PM10 concentrations until no further classification is needed. This method is applied to identify dust weather in Xi"an from February to April 2018. The results show that this method can effectively identify the main dust weather events. Based on this method, typical characteristics of dust weather can be obtained: the ratio of PM2.5 to PM10 concentration is less than 43.5%, and the PM10 concentration is greater than 228μg/m3, which is consistent with our knowledge that the PM10 concentration is high and mainly consists of coarse particles during the dust event. Overall, this method has a clear physical basis, and it is easy to operate, suitable for massive data processing, and promising for applications in relevant areas.

Abstract:Based on coherent Doppler wind lidar and ground-based conventional observational data, a typical dust weather process occurring in Hulunbuir was analyzed using machine learning and the HYSPLIT model. The study revealed that the dust event started with a sudden increase in southerly wind. Subsequently, the wind direction shifted to south-southwest, resulting in a reduction in wind speed and a weakening of dust transport. However, when the wind shifted to westerlies, the dust transport intensified again. The transport of dust ceased after a decrease in westerly wind speeds. During the dust transport period, turbulence was relatively weak, and the mixing layer height remained limited. Machine learning particle size calculations indicated PM10 dominating the early transport and both PM10 and PM2.5 showing substantial growth in the later phase. This divergence in particle sizes across different transport periods suggests a potential change in dust sources. HYSPLIT revealed that in the early phase of dust transport originated from northwestern Mongolia, passing through Xilingol League in China before reaching Hulunbuir. In the later phase, dust transport directly entered Hulunbuir from the southern regions of Russia, resulting in an escalation of dust pollution. Finally, using total mass flux analysis, it was determined that the response to dust occurred earlier in the period from the pre-dust period until the beginning, compared to ground-level particulate concentration. The total mass flux threshold for this dust event was established using box-and-whisker plot. Variations in total mass flux and the establishment of thresholds could serve as novel indicators for dust event early warning systems.

Abstract:In order to investigate the non-uniform distribution characteristics of the flood season precipitation and trend, this paper analyzes the temporal distribution characteristics and evolution trend of the flood season precipitation in Jinhua from 1971 to 2020 by using the dayily precipitation data combined with the ECMWF reanalysis data, and by applying the improved index of precipitation concentration and concentration period. The results show that the initial day of the precipitation concentration period during is on June 13, which coincides with the initial day of the monsoon season; Jinhua region generally has a relatively uniform temporal distribution of precipitation, but with large inter-annual variability; and There is a quasi 18-year cycle of precipitation concentration and period of concentration. In the SST field, the anomalously high SST in the east-central Pacific Ocean in the winter before the year of precipitation anomaly concentration, and then the SST difference in the east-central Pacific Ocean at the equator gradually turns into a significant negative anomaly, which suggests that the anomalously concentrated year of precipitation anomaly may occur in the decaying stage of El Ni?o. The most notable anomalies in the situation field are the strong and westerly subtropical high pressure in the western Pacific and the anomalous anticyclone in the Philippine region. Anomalous anticyclones in the Philippine region allow for strong moisture transport from the southwesterly flow along the eastern coast of China during the early summer season, an the same time causing the subtropical high pressure to strengthen and extend westward. The potential gradient between the stronger subtropical high pressure and the westerly trough can increase the transport of warm and humid air from the southwest and slow down the movement of the westerly trough ridge, in order to form a more stable circulation configuration, resulting in the emergence of sustained precipitation. And after the strong subtropical high pressure continues to extend west and lift north, the westerly wind belt system southward is blocked, the large warm high pressure of the sinking air flow will lead to persistent high temperature and drought heat weather, which will lead to an abnormal concentration of precipitation.

Abstract:Abstract: Soil heat flux (G) is one of key factors in affecting the surface energy balance over the Qinghai-Tibetan Plateau. The estimation and spatiotemporal distribution analysis can provide a reference for the surface energy balance in the region. In this paper, the applicability and accuracy of the model was evaluated by combining SEBAL model inversion data with observation data over the Qinghai-Tibetan Plateau from 2003 to 2018 based on the MODIS, the China regional surface meteorological element driven data set, and the 1 km all-weather surface temperature data set in Western China. We found that SEBAL model had high accuracy for inverting soil heat flux (G) over the Qinghai-Tibetan Plateau. On this basis, soil heat flux (G) value were inverted by remote-sensing data, and analyzed the spatiotemporal distribution characteristics of G in the region from 2003 to 2018. The results showed that: (1)The mean value of G showed a fluctuating downward trend in multiyear, with the maximum valley value appearing in 2011, and the maximum peak value in 2016. The mean value of G showed a fluctuating downward trend in every season, except in winter. The mean value ranges of G in different seasons showed a trend of summer > spring > autumn > winter , and the order of magnitude of fluctuation changes was consistent with former. (2)The mean value of G showed spatial distribution characteristics that the highest area in the northern Qaidam Basin and its surrounding areas, the southwestern region such as Ngari region and circumjacent areas followed, the rest of the area was mostly low in general, and with obvious spatial heterogeneity. The mean value of G in different seasons showed spatial distribution characteristics were basically consistent with former. (3)The central and southeastern areas showed an increasing trend, while the northern, western and southwestern areas showed a decreasing trend. The proportion of areas with an increasing trend was highest in winter and lowest in summer, the proportion of areas with a decreasing trend was highest in summer and lowest in winter. The results of this study demonstrated the applicability of the SEBAL model for inverting G, and enriched the studies of surface energy balance over the Qinghai-Tibetan Plateau.

Abstract:In recent years, reanalysis data are more and more used in wind energy resource assessment. In this paper, the observation data from 40 ground stations in China are used to evaluate the ground wind speed error of ERA5 and ERA5-Land reanalysis data with different resolutions. The results show that the mean Bias Error (BE) of multi-year mean wind speed of ERA5 and ERA5-Land data is 0.08 m s-1 and -0.06 m s-1 respectively, the maximum BE is 0.46 m s-1 and -0.19 m s-1 respectively, the Relative Bias Error (RBE) is 4.4% and -2.0%, and the maximum RBE is 33.0% and -10.1% respectively; The slope of the linear fitting equation of monthly mean wind speed is 0.93 and 0.97 respectively, the intercept is 0.29 m s-1 and 0.02 m s-1 respectively, and the correlation coefficient is 0.98 and 0.99 respectively; The mean Root Mean Square Error (RMSE) of monthly mean wind speed is 0.17 m s-1 and 0.14 m s-1 respectively, the maximum RMSE is 0.49 m s-1 and 0.22 m s-1 respectively, the Relative Root Mean Square Error (RRMSE) is 7.4% and 5.7%, and the maximum RRMSE is 35.2% and 13.3% respectively. The error of ground wind speed of ERA5-Land high-resolution data is relatively low, which is conducive to improving the accuracy of wind energy resource assessment.