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In this paper, the spatial and temporal characteristics of convective precipitation (CP) and large-scale precipitation (LSP) in southern China during 1980–2020 are analyzed using monthly mean precipitation data from MERRA-2. In addition, the possible effects of relative humidity on CP and LSP are explored. The results indicate the following. (1) The LSP dominates the proportion of total precipitation (TP). Both LSP and CP are more prevalent in the south and less prevalent in the north, but there is a difference in the regions of their maximum centers. (2) Significant interannual and seasonal variations are observed in precipitation. TP and LSP tended to be higher than average after the 1990s, while for the CP, a significant negative trend has dominated the past ten years. There are significant increasing trends for TP and LSP, with area-averaged linear trends of 78.1 mm/decade and 85.9 mm/decade, respectively, while that of CP is − 17.1 mm/decade. The increasing trends of LSP are mainly contributed by the precipitation of summer and autumn. (3) The variations of LSP are affected by relative humidity in the troposphere, while CP is only influenced by the changes in relative humidity due to air temperature or specific humidity. The trend of relative humidity is − 0.33%/decade, mainly due to rising temperature in the troposphere. (4) Changes in relative humidity caused by temperature or specific humidity alone act on large-scale precipitation through both interannual and interdecadal processes, causing large-scale precipitation to increase, and the convective precipitation is mainly affected by the interdecadal processes. [ABSTRACT FROM AUTHOR]

The Yangtze River Delta (YRD) region, located over East China, is an international urban agglomeration radiating Asia Pacific with convenient transportation, advanced manufacturing industry, and modern service industry. The intensive human activities have exerted substantial effects on the weather and climate over the YRD region during the past decades, thus bringing more challenges to accurate weather and climate forecasting. However, the weekly cycles of surface air temperature and precipitation linked closely to human activities and their corresponding physical processes remain insufficiently understood. Here, we investigate the weekly cycles of summertime surface air temperature and precipitation and the weekend effect over the YRD region for the period of 2008–2019 using high-resolution observational data and reanalysis data. The results demonstrate that lower surface air temperature and higher precipitation generally manifest during weekends compared to weekdays over the YRD region particularly over the areas with intensive human activities. Further analysis of the underlying physical processes points to the aerosol-cloud-radiation interaction which largely explains the weekend effect in summertime temperature and precipitation over the YRD region. [ABSTRACT FROM AUTHOR]

The deep blue sky is an indicator of a lower concentration of aerosols and a cloudless sky. With increasing human emissions, a trend towards days with fewer deep blue skies might indicate a decline in a good living environment for humans. This study investigates the long-term changes of the deep blue sky in China from 1980 to 2018. Due to a lack of direct measurements, we use atmospheric visibility and low cloud cover to classify blue sky days into three grades: light blue day, medium blue day, and deep blue day. Climatologically, annual deep blue days increase from southeast China to northwest China, with the maximum number in Xinjiang and eastern Inner Mongolia and the minimum number in western Qinghai and southern Hebei. From 1980 to 2018, annual deep blue days show a prominent decreasing trend in most of China, with area-mean annual deep blue days decreasing by −0.48 days per year (d/y) in China, and the variation becomes more obvious after 2013. The maximum decreasing trend is observed in eastern China. The most prominent decreases of deep blue days are seen in winter. Both air pollution and the change in meteorological conditions contribute to the decrease of wintertime deep blue days in China. Specifically, the decrease in surface wind speed hinders the cleaning of air by winds, the increase in surface air temperature, and decrease in relative humidity is favorable for low cloud increase, and the increasing emission of pollution reduces atmospheric visibility. [ABSTRACT FROM AUTHOR]

As an intrinsic feature of daily surface air temperature (SAT) variability found in station measurements, temporal asymmetry (TA) can be taken as an evaluation metric to access the quality of SAT re-analysis product. In this study, TA calculated from four SAT variables, i.e., daily mean SAT (Tmean), daily maximum SAT (Tmax), daily minimum SAT (Tmin) and diurnal temperature range (TDTR = Tmax − Tmin), is applied to evaluate synoptic-scale performance of four reanalysis products (NCEP-2, JRA-55, ERA-I, and ERA-5) over China. The results show that four re-analyses overall overestimate the TA of daily Tmax and Tmin variability over China, but with a comparatively consistent estimated TA for Tmean. Moreover, the TA of Tmean variability for these four re-analyses shares high spatial consistency with those from the observation. However, four re-analyses own the similar region-dependent spatial patterns of overestimated TA for Tmax and Tmin variability, especially for Tmax. Since high TA is an indicator for strong nonlinear feature, only Tmean reanalysis is the most suitable to explore synoptic-scale extreme events, such as heat waves and cold waves, which are highly related to the strong nonlinear processes. [ABSTRACT FROM AUTHOR]

Here we investigate the key features of early and late South China Sea summer monsoon (SCSSM) onset. To carry out the investigation, we select five extreme early and nine extreme late onset years (14 days or more, before and after the normal onset date are considered "late" and "early" onset years, respectively). Since the mean SCSSM onset occurs in May, we analyze the changes in composite mean meteorological fields during May. Next, we examine the differences in daily evolution (from 10 to 2 days before the onset) of various vital variables between the early and late SCSSM onset years. We demonstrate that during the early SCSSM onset years, surface air temperature (SAT) is warmer over the North Pacific Ocean, South China Sea (SCS), and Northern China, which gradually extend towards East Asia. These SAT anomalies over Northern China, East Asia, and SCS are due to the advection of warm anomalies from the North Pacific Ocean. Consequently, a dipole-like sea level pressure (SLP) pattern had developed, with lower SLP over East Asia, Northern China, SCS, and North Pacific Ocean, and higher SLP over the Indian Ocean. It subsequently brought moisture-laden wind from the Indian Ocean towards SCS and East Asian landmass and is responsible for early SCSSM onset. [ABSTRACT FROM AUTHOR]

The uncertainty in trend estimates caused by temporal scale effect has rarely been studied. In response to this challenge, this study aims to explore temporal scale effects on trend detection based on in situ measurements throughout China given that they provide full temporal details of surface elements. Three common temporal scales (daily, monthly, and yearly) were used to present findings. Two different trend detection methods (i.e., simple linear regression and breaks for additive seasonal and trend) were employed to check the dependence of temporal scale effects on methods. It was found that temporal scale effects were dependent on the type of elements, the time span of datasets, and trend detection methods. They are more significant for elements with fast changes (e.g., snow depth) compared to those with gradual changes (e.g., albedo, air temperature). And temporal scale effects from daily to monthly scale are generally larger than those from monthly to yearly scale. The former is almost independent of trend detection methods, while the latter shows a clear dependence (i.e., dealing with seasonality or not). Trend estimates of surface albedo and longwave net radiation are mainly affected by temporal scale effects, while those of snow depth, shortwave net radiation, and air temperature are affected by both temporal scale effects and the choice of trend detection methods. The feedbacks between these elements show a clear dependence on temporal scale, which are generally stronger on monthly scale but weaker on yearly scale. [ABSTRACT FROM AUTHOR]

The variability of air temperature extremes exerts a great influence on agricultural production and the global hydrologic cycle. It has been the focus of attention for the past several decades. Using observed surface air temperature from 192 meteorological stations in South China maintained by the China Meteorological Administration, this study computed and analyzed 10 extreme temperature indices and the mean temperature at multiple spatiotemporal scales for the period from 1961 to 2018. These indices were analyzed with particular reference to the growing season of rice. Results showed that the variation trends of all annual indices exhibited different north–south patterns across decades, and the most recent 20-year period experienced greater warming than previous periods. The regional averaged rates of the annual mean maximum temperature, the annual mean minimum temperature, summer days, and tropical nights were 0.163 °C decade−1, 0.197 °C decade−1, 1.2 days decade−1, and 5.4 days decade−1, respectively. Except for the month of April, the southern region mostly experienced stronger warming than the northern region, especially in summer and autumn. Nighttime warming was usually greater than daytime warming, especially in June and October. Most temperature indices showed very weak correlations with large-scale atmospheric oscillations. [ABSTRACT FROM AUTHOR]

The Arctic polar vortex (APV) system plays an important role in controlling winter and spring atmospheric circulation pattern in China. Here, we evaluate the anomalous APV-induced spring vegetation variability and lagged productivity responses in China. We found that both strong and weak APV conditions have almost equally negative impacts on spring vegetation growth in China, e.g., negative NDVI anomalies occurred in 48.6 and 53.2% of China's vegetated areas under strong and weak APV conditions, respectively. However, large seasonal compensation effects were associated with weak APV conditions, and beneficial lagged vegetation productivity responses occurred in 67.2% of China's vegetated areas, whereas adverse responses occurred in only 32.8% of vegetated areas. Under a strong APV, adverse lagged vegetation productivity responses occurred in 54.5% of China's vegetated areas, whereas beneficial responses occurred in only 45.5% of vegetated areas. The temperature, precipitation, and solar radiation changes caused by anomalous APV-induced changes in circulation patterns are the main reasons for spring vegetation variability. The lagged vegetation productivity responses were attributed to anomalous APV-related precipitation and air temperature anomalies in the following summer and autumn. This improved understanding of the strong links between APV anomalies and vegetation dynamics in China should facilitate early warning of vegetation productivity reductions under anomalous APV conditions. [ABSTRACT FROM AUTHOR]