Abstract:Using observed rainfall data from 26 stations in Southwest China for the period 1951–2017, in this study, we investigated the climatic characteristics of rainfall over Southwest China during spring (March–April–May) and each spring month. The rainfall in May was found to account for 55.3% of the total spring rainfall over Southwest China and to have the most significant interannual variations of the spring months. The spring rainfall over Southwest China also shows clear interdecadal variations, which are dominated by the interdecadal variations of the May rainfall. Significant abrupt interdecadal changes (AIDCs) also occur in the spring rainfall, with increased AIDCs having occurred in the middle and the later 1990s after a period of decreased IDACs in the later 1970s. Although the time series of spring rainfall over Southwest China exhibits an insignificant linear trend, the eastern part of Southwest China shows a clear decreasing trend in terms of area, and the western part of Southwest China shows a significant increasing trend. Lastly, the spring rainfall over Southwest China shows significant interannual and interdecadal periodicities, which are also present in the April and May rainfalls. We note that no interdecadal periodicity is evident for March rainfall. In general, the interannual and interdecadal periodicities of spring rainfall are dominated by the variations of May rainfall.

Abstract:On the basis of the MTCSWA (Multiplatform Tropical Cyclone Surface Winds Analysis) data, the climatic statistical characteristics of tropical cyclones (TCs) at different sizes, the variation of the averaged radial extent of different significant wind speed thresholds (i.e., 34, 50, and 65 kt, 1 kt≈0.514 m/s) in different quadrants, and the axisymmetricity of the radial extent of different wind speed thresholds and their correlation with TC intensity over the western North Pacific from 2007 to 2016 are investigated. In this study, the size and intensity of TCs are defined as the azimuthally averaged radius at which wind speed decreases to 34 kt (≈17.5 m/s) from the center and the maximum sustained winds near the TC center, respectively. The mean size of TCs over the western North Pacific Ocean is 221.9 km, with small TCs of 96.4 km and large TCs of 346.4 km. The activity location of large TCs is more concentrated, with a more northward distribution, than that of small TCs. The peak value of the mean size of all TCs occurs in August and October. The mean values of the azimuthally averaged radial extents of the 34, 50, and 65 kt wind speed thresholds are 221.9, 121.0, and 77.4 km, respectively. According to the definition of axisymmetricity, the radial extent of the 34 kt wind speed threshold has the lowest axisymmetricity and the 65 kt wind speed threshold has the highest axisymmetricity. Positive correlations exist between the radial extents of the 34, 50, and 65 kt wind speed thresholds and the TC intensity during the TC’s lifetime, among which the correlation between the radial extent of the 65 kt wind speed threshold and the TC intensity is the lowest. For a certain wind speed threshold, the correlation between its radial extent and TC intensity varies in different quadrants at different stages of the TC’s lifetime. The higher the wind speed is, the weaker the correlation between the axisymmetricity of its radial extent and TC intensity. Only the axisymmetricity of the radial extent of the 34 kt wind speed threshold shows a weak positive correlation with TC intensity during the TC’s lifetime.

Abstract:A persistent cold event affected China through late January to early February 2018, causing severe damage to the life and property of the country. The event exhibits a resemblance to the well-known long-lasting snowstorms of January 2008 (“0801” event in brief), because both the events occurred during the La Niña phase along with reduced sea ice in the Arctic during the preceding autumn. In this study, the observational features of the 2018 event and the possible factors for its occurrence are investigated through a comparison with the “0801” event. The 2018 cold event persisted for a relatively shorter period than the “0801” event, with cold air outbreak along with an eastern pathway rather than a northwestern or northern pathway, as was seen in “0801”. Large-scale atmospheric circulation exhibited a substantial difference between the two cases. The Ural blocking and the East Asian grand trough, the two key systems consisting of East Asian winter monsoon (EAWM), which formed a “west-high-east-low” pattern, were significantly stronger in the 2018 event. In comparison, the East Asian grand trough in the “0801” event did not deepen as evidently, despite the similarly enhanced Ural blocking. Instead, a substantially deepened southern branch trough, another important composition of EAWM, developed in “0801” and resulted in a “north-high-south-low” pattern. Also, the Siberian high persisted for a shorter period in the 2018 event. Besides, tropical convection activities exhibited difference between the two cases. Enhanced convection occurred in the Maritime Continent and equatorial western Pacific in the 2018 case, but mainly over the equatorial Indian Ocean during the “0801” event. The latter contributed to the strengthening of the southern branch trough and the western Pacific subtropical high leading to intensified water vapor transportation from the Bay of Bengal and the western Pacific, and intensified precipitation. In comparison, the convection in 2018 was substantially weaker and not so favorable for transporting moisture to South China.

Abstract:In order to improve model performance, the impact of the new generation WRF-CMAQ (Weather Research and Forecasting model-Community Multi-scale Air Quality model) air quality model system performance of different resolutions for the main district of Beijing was evaluated in 2018. Based on the data set, with PM2.5 as the primary pollutant, forecast grade accuracy of BJ01 (resolution of 1 km) and BJ03 (resolution of 3 km) domains were found to be better compared to that of the official forecast. More than 50% accuracy rate was achieved with BJ01 and BJ03 domains. Compared with the accuracy rate on the first day of official forecast (59%), accuracy rate using the proposed system reached over 60%. A comprehensive scoring method based on the IAQI (Individual Air Quality Index) accuracy and the grade accuracy is adopted. Results show that BJ03 domain achieved the highest score (75.0 points) followed by BJ01 domain. The official forecast scored 70.6 points while BJ09 (resolution of 9 km) domain achieved the lowest score of 69.1 points. Based on the analysis of the prediction results of 2018 long time series of the model system, the model’s predicted PM2.5 concentration is observed to be consistent with that of the observation trend. In addition, the analysis reveals that the correlation coefficient between the model result of BJ03 domain and that of the observation is 0.76. Good peak value simulation performances are achieved in BJ03 and BJ09 domains where there are large area coverages. Similar error trends in peak value simulation of the three model domains are observed. It is evident that the results from the model with coarse resolution are higher than that of the model with fine resolution, which covers a smaller area. Consistent with the forecast comprehensive score, the statistical analysis results reveal that BJ03 domain prediction has the best performance with an average deviation of 0.83 μg/m³. Compared with the observation forecast, BJ09 domain forecast is generally higher whereas BJ01 domain forecast is observed to be lower. Spatial difference analysis of different resolutions from the same site yields inconsistent results. This study shows that best performance is achieved by BJ01, BJ03, and BJ09 areas for the Nongzhanguan, Wanliu, and Dongsi stations, respectively.

Abstract:Formation and variation dynamics of western Pacific subtropical high (WPSH) have always been an important subject of interest for Chinese meteorologists. However, geopotential height field indexes commonly used in current studies and operational services cannot accurately describe the evolution characteristics of the subtropical high with a global warming background. It is known that the stream function of horizontal circulation defined by the three-pattern decomposition of global atmospheric circulation can equivalently describe the three-dimensional spatial geometric images of the subtropical high. Based on this information, this study proposes the use of the zero contour of the 500 hPa stream function of horizontal circulation as the objective standard to redefine the area, intensity, ridge line, and westward ridge point indexes of the subtropical high. The objectivity and rationality of the newly defined indexes of subtropical high are verified by taking short-term structural evolution characteristics of the subtropical high during summer in 2018 and its relationship with precipitation in eastern China. Results reveal accurate depiction of the large-scale eddy circulation characteristics of the subtropical high with the use of the newly defined indexes of based on the stream function of the horizontal circulation. Additionally, the dynamic and thermodynamic equations concerning the stream function of three-pattern decomposition of global atmospheric circulation provide a new theoretical tool for studying the mechanism of the evolution of the subtropical high anomaly by using the newly defined indexes.

Abstract:The characteristics of wind fields and transport pathways in the Guanzhong Basin were revealed based on data from 151 surface meteorological stations in 2017. Wind speed and wind direction data from five representative stations in the basin were analyzed by calculating windy days, daily average wind speed and wind direction frequency in four seasons, and hourly wind field and transport trajectories were calculated by CALMET and trajectory model. The results show that daily average wind speed in the Guanzhong Basin is about 1-3 m s^-1, which is higher in summer but lower in autumn and winter. The dominant wind direction in the center of the basin mainly includes northeast and southwest winds along the terrain, while the dominant wind direction around the basin shows a converging trend along the terrain to the center of the basin. Seasonal changes in dominant wind direction at each station are not significant. The wind fields in the basin can be classified into three types, namely, systematic wind field, feeblish systematic wind field, and local atmospheric circulation. The local atmospheric circulation occurs most frequently in the year (74.7%), followed by feeblish systematic wind field (17.3%), and systematic wind field (8%). Airflows of systematic wind field are mainly transported northeasterly, while airflows of feeblish systematic wind field or local atmospheric circulation are transported northeasterly, westerly (including southwesterly), and southeasterly. Trajectories of the local atmospheric circulation are limited to a small area, concentrated between the central basin and southern mountains, which indicates that this wind field is not conducive to air pollution dispersion.

Abstract:Model evaluation is an indispensable part of model development. In this study, the authors evaluated the Common Land Model (CoLM) using the latest benchmark software called International Land Model Benchmarking (ILAMB) and compared this model with the Community Land Model version 5 (CLM5) from NCAR. As a land surface model benchmark software, ILAMB can automatically generate graphical diagnostics for model variables and score model performance. Results show that CoLM generally performs well, and its simulation result is close to the benchmark data. Compared with CLM5, CoLM is slightly inferior for gross primary productivity and hydrologic processes. For radiation, CoLM performs as well as CLM5, and is even better for variables such as surface upward long-wave radiation and surface net radiation. Comparing the forcing data of CRUNCEPv7 and GSWP3v1, the authors found differences in the climate average state and observed that the model performance under GSWP3v1 forcing is better. The latent heat flux simulated by CoLM and CLM5 under CRUCNEPv7 forcing has a significant positive bias in the Amazon plains, eastern and southern Asia, and eastern North America, but the bias decreases under GSWP3v1 forcing. The two models overestimate the sensible heat flux in north Africa and central Asia with either of the two forcing datasets. Regarding the radiative process, the surface upward shortwave radiation simulated by CoLM was mainly on the high side in the world, which to a certain extent caused the low level of surface net radiation. The surface upward long-wave radiation scores of four experiments are similar, but a difference exists in the specific spatial distribution.

Abstract:A multiscale examination of temperature and precipitation in Mohe from 1958 to 2019 was performed using the Ensemble Empirical Mode Decomposition (EEMD) method and the sliding average t-test method. The results showed that the annual average temperature in Mohe increased by 0.43℃ (10 a)^－1 with the mid and late 1980s as the abrupt change points. The daily maximum temperature increased by 0.61℃ (10 a)^－1 with the mid and late 1980s as the abrupt change points, while the daily minimum temperature increased by 0.21℃ (10 a)^－1 with the early to mid 1980s and late 1990s as the multiple abrupt change points. Furthermore, the rainfall, nighttime rainfall, and daytime rainfall showed a similar change pattern from 1958 to 2019. The trend in the three types of rainfall obtained by EEMD was 14.05 mm (10 a)^－1, 7.71 mm (10 a)^－1, and 5.12 mm (10 a)^－1, respectively. No abrupt change points were detected for all types of precipitation. Both the EEMD and the linear trend analysis found an increase in temperature and precipitation, while the trends obtained by EEMD were greater than that by the linear method, which implies that the traditional linear analysis method underestimates the trends in temperature and precipitation in Mohe.

Abstract:The Precipitation-surface air Temperature Relationship (PTR) constitutes an important indicator of the ecological environment and is closely related to the occurrence of extreme drought/heat wave (or flood/coldness) events. Research has been taking place in several caves of the Qingjiang River basin of the Three Gorges Area. The cave sediments are characterized by long carbon and oxygen isotope sequences which may lead to the construction of a long sequence PTR. Based on modern meteorological observation and reanalysis data, the evolution characteristics and forming reason of the PTR in the Qingjiang River basin can be analyzed, which can provide some physical basis for understanding the relationship between the carbon–oxygen coupling coefficient of the stalagmites and PTR. Based on daily surface air temperature and precipitation observations from six stations, namely Badong (31°2'N, 110°22'E), Jianshi (31°36'N, 109°43'E), Lichuan (30°17'N, 108°56'E), Enshi (30°17'N, 109°28'E), Wufeng (30°12'N, 110°40'E), and Yichang (30°42'N, 111°18'E), and three sets of reanalyses, namely the NCEP Climate Forecast System Reanalysis (CFSR), Japan Meteorological Agency 55-year reanalysis (JRA55), and European Centre for Medium-range Weather Forecasts Interim Reanalysis (ERA-interim), this study investigated the seasonal, interannual, and decadal variability of the PTR and its connection with the East Asian summer monsoon. The following results were obtained: 1) PTR exhibits an obvious seasonality while a significant negative correlation is observed only in summer. 2) Summer PTR exhibits a decadal variability with a periodicity of 20–25 years, close to IPO/PDO. Significant shifts occurred in 1974, 1986, and 1992, with PTR altering from “insignificant” to “significant” and back to “insignificant” again. Specifically, PTR between 1977 and 1992 was significantly negative. 3) There is an overall correspondence between the decadal variability of summer PTR and the strength of the East Asian Summer Monsoon (EASM). When PTR was significantly negative (1977–1992), EASM was weak. However, when PTR was insignificant (1965–1976 and 1993–2011), EASM was strong. 4) Physically, when the summer monsoon weakens, the north and south air flow converges to the south. When the water vapor and ascending motion increases in Qingjiang River basin, it causes an increase in the cloud coverage and precipitation but it reduces the surface heat flux and decreases the surface temperature, leading subsequently to a negative PTR. When the summer monsoon strengthens, the north and south air flow converges to the north, which is not beneficial for the increase of water vapor. On the contrary, it is beneficial for the descending motion. In addition, cloud coverage reduces and temperature increases, resulting in the increase of the adiabatic rate and the positive contribution of precipitation efficiency to precipitation. It is important that, on one hand, the increase of temperature can increase precipitation, and on the other hand, the impact of cloud coverage/precipitation on the temperature weakens but does not disappear. Both conditions make PTR insignificant. 5) The decadal variability of PTR may be linked to the IPO/PDO.