June July August Research Articles

The relationship between the intensified heat waves and deteriorated summertime ozone pollution in the Beijing–Tianjin–Hebei region, China, during 2013–2017

Summertime ozone (O3) pollution has frequently occurred in the Beijing–Tianjin–Hebei (BTH) region, China, since 2013, resulting in detrimental impacts on human health and ecosystems. The contribution of weather shifts to O3 concentration variability owing to climate change remains elusive. By combining regional air chemistry model simulations with near-surface observations, we found that anthropogenic emission changes contributed to approximately 23% of the increase in maximum daily 8-h average O3 concentrations in the BTH region in June–July–August (JJA) 2017 (compared with that in 2013). With respect to the weather shift influence, the frequencies, durations, and magnitudes of O3 exceedance were consistent with those of the heat wave events in the BTH region during JJA in 2013–2017. Intensified heat waves are a significant driver for worsening O3 pollution. In particular, the prolonged duration of heat waves creates consecutive adverse weather conditions that cause O3 accumulation and severe O3 pollution. Our results suggest that the variability in extreme summer heat is closely related to the occurrence of high O3 concentrations, which is a significant driver of deteriorating O3 pollution.

Regional Reconstruction of Po River Basin (Italy) Streamflow

The Po River Basin (PRB) is Italy’s largest river system and provides a vital water supply source for varying demands, including agriculture, energy (hydropower), and water supply. The current (2022) drought has been associated with low winter–early spring (2021–2022) snow accumulation in higher elevations (European Alps) and a lack of late spring–early summer (2022) precipitation, resulting in deficit PRB streamflow. Many local scientists are now estimating a 50- to 100-year (return period) drought for 2022. Given the importance of this river system, information about past (paleo) drought and pluvial periods would provide important information to water managers and planners. Annual streamflow data were obtained for thirteen gauges that were spatially located across the PRB. The Old World Drought Atlas (OWDA) provides annual June–July–August (JJA) self-calibrating Palmer Drought Severity Index (scPDSI) data for 5414 grid points across Europe from 0 to 2012 AD. In lieu of tree-ring chronologies, this dataset was used as a proxy to reconstruct PRB regional streamflow. Singular value decomposition (SVD) was applied to PRB streamflow gauges and gridded scPDSI data for two periods of record, referred to as the short period of record (SPOR), 1980 to 2012 (33 years), and the long period of record (LPOR), 1967 to 2012 (46 years). SVD serves as both a data reduction technique, identifying significant scPDSI grid points within the selected 450 km search radius, and develops a single vector that represents the regional PRB streamflow variability. Due to the high intercorrelations of PRB streamflow gauges, the SVD-generated PRB regional streamflow vector was used as the dependent variable in regression models for both the SPOR and LPOR, while the significant scPDSI grid points (cells) identified by SVD were used as the independent variables. This resulted in two highly skillful regional reconstructions of PRB streamflow from 0 to 2012. Multiple drought and pluvial periods were identified in the paleo record that exceed those observed in the recent historical record, and several of these droughts aligned with paleo streamflow reconstructions of neighboring European watersheds. Future research will utilize the PRB reconstructions to quantify the current (2022) drought, providing a first-time paleo-perspective of drought frequency in the watershed.

Global Distribution of Clouds over Six Years: A Review Using Multiple Sensors and Reanalysis Data

A six-year global study of cloud distribution and cloud properties obtained from observations of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), the Atmospheric Infrared Sounder (AIRS), and the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) data is presented in this study. From the CALIPSO observations, the highest clouds for both daytime and night-time were found in the Inter Tropical Convergence Zone (ITCZ) region. The lowest cloud heights were found towards the poles due to the decrease in the tropopause height. Seasonal studies also revealed a high dominance of clouds in the 70 °S–80 °S (Antarctic) region in the June–July–August (JJA) season and a high dominance of Arctic clouds in the December–January–February (DJF) and September–October–November (SON) seasons. The coldest cloud top temperatures (CTT) were mostly observed over land in the ITCZ and the polar regions, while the warmest CTTs were mostly observed in the mid-latitudes and over the oceans. Regions with CTTs greater than 0 °C experienced less precipitation than regions with CTTs less than 0 °C.

Developing a Climate Prediction System over Southwest China Using the 8-km Weather Research and Forecasting (WRF) Model: System Design, Model Calibration, and Performance Evaluation

Abstract A high-resolution, short-term climate prediction system for summer (June–July–August) climate over Southwest China has been developed using the Weather Research and Forecasting (WRF) Model nested with a global climate prediction system (PCCSM4). The system includes 12 ensemble members generated by PCCSM4 with different initial conditions, and the finest horizontal resolution of WRF is 8 km. This study evaluates the ability of the WRF Model to predict summer climate over Southwest China, focusing on the system design, model tuning, and evaluation of baseline model performance. Sensitivity simulations are first conducted to provide the optimal model configuration, and the model performance is evaluated against available observational data using reforecast simulations for 1981–2020. When compared to PCCSM4, the WRF Model shows major improvements in predicting the spatial distribution of major variables such as 2-m temperature, 10-m wind speed, and precipitation. WRF also shows better skill in predicting interannual temperature variability and extreme temperature events, with higher anomaly correlation coefficients. However, large model biases remain in seasonal precipitation anomaly predictions. Overall, this study highlights the potential advantages of using the high-resolution WRF Model to predict summer climate conditions over Southwest China.

Sea Surface Temperature and Salinity in Lombok Strait Reconstructed From Coral Sr/Ca and δ18O, 1962–2012

Coral geochemical tracers have been used in studies of the paleoclimatology and paleoceanography of the tropics and subtropics. We measured Sr/Ca and oxygen isotope ratios (δ18O) in a coral sample collected from the southern part of Lombok Strait, a significant outlet of the Indonesian Throughflow (ITF) to the Indian Ocean, to reconstruct the historical record of sea surface temperature (SST) and seawater δ18O. Seawater δ18O can be used to approximate sea surface salinity (SSS) because it reflects the balance of evaporation and precipitation. The resulting time series reconstructed SST and SSS, covering the period 1962–2012, shows no clear trend of global warming, although the record includes a large cooling event (~4°C) during 1996–1997. Although neither SST nor SSS shows a systematic relationship with El Niño–Southern Oscillation and Indian Ocean Dipole (IOD), weak but significant correlations are found partly. In addition, the coral data show signals of major IOD and El Niño events in 1994 and 1997, respectively, although climatic trends recorded in the coral are not consistent with those found along the Java-Sumatra coast. To evaluate other influences on the ITF in Lombok Strait, we compared our coral record with coral records from sites in the Java Sea, the southern part of Makassar Strait, and Ombai Strait. During the northwest monsoon (December–January–February), variations in SST and SSS at Lombok Strait site are similar to those at the Java Sea and southern Makassar sites for the period 1962–1995, which suggests that low-salinity water from the Java Sea is carried at least to the southern part of Makassar Strait where it suppresses the ITF upstream from Lombok Strait. However, the SST and SSS records differ at the three sites during the southeast monsoon (June–July–August), indicating that surface conditions in Lombok Strait vary separately from those in the Java Sea. In the longer term, although global warming has been widely identified in the Indonesian Seas, the coral record shows no clear warming trend in the southern part of Lombok Strait, where fluctuations in the ITF may be modulating the distribution of heat in the surface waters of the western Pacific and eastern Indian Ocean.

The substantial role of May soil temperature over Central Asia for summer surface air temperature variation and prediction over Northeastern China

The slowly varying soil temperature can exert local and nonlocal influences on regional climate system, and may thus provide a critical source of subseasonal-to-seasonal climate prediction. In this study, we identify that soil temperature in May over the key region of Central Asia (42 °N–50 °N, 62 °E–80°E, KRCA) from Noah, Mosaic, CLM and ERA-interim datasets is closely linked to variations of the surface air temperature, daily maximum temperature and hot days over Northeastern China in summer (June–July–August), with correlation coefficients of regional average detrended time series ranging from 0.42 to 0.54, and all significant at the 99% confidence level for the period of 1979–2018. The possible physical mechanism behind the substantial downstream impacts of soil temperature over Central Asia are explored via diagnostical analysis combined with regional climate model experiments. Warmer soil temperature in May over the KRCA can keep to the summer and cause positive anomalies of geopotential height in summer over Northeastern China through the Rossby wave propagation, and associated stronger subsidence warming, less cloud cover, more solar radiation reaching the surface, higher planetary boundary layer, and stronger thermal advection at 850 hPa, which provide favorable conditions for warmer surface air temperature particularly in the daytime as well as more hot days. This study further reveals that soil temperature over Central Asia in May makes important contribution to prediction of summer surface air temperature, daily maximum temperature and hot days over Northeastern China in terms of regional average time series and spatial patterns. Our findings highlight the previously-unknown substantial role of antecedent soil temperature condition over Central Asia for summer surface air temperature variation and prediction over Northeastern China.

Analysis of lower-boundary climate factors contributing to the summer heatwave frequency over eastern Europe using a machine-learning model

A machine-learning (ML) model, the light gradient boosting machine (LightGBM), was constructed to simulate the variation in the summer (June–July–August) heatwave frequency (HWF) over eastern Europe (HWF_EUR) and to analyze the contributions of various lower-boundary climate factors to the HWF_EUR variation. The examined lower-boundary climate factors were those that may contribute to the HWF_EUR variation—namely, the sea surface temperature, soil moisture, snow-cover extent, and sea-ice concentration from the simultaneous summer, preceding spring, and winter. These selected climate factors were significantly correlated to the summer HWF_EUR variation and were used to construct the ML model. Both the hindcast simulation of HWF_EUR for the period 1981–2020 and its real-time simulation for the period 2011–2020, which used the constructed ML model, were investigated. To evaluate the contributions of the climate factors, various model experiments using different combinations of the climate factors were examined and compared. The results indicated that the LightGBM model had comparatively good performance in simulating the HWF_EUR variation. The sea surface temperature made more contributions to the ML model simulation than the other climate factors. Further examination showed that the best ML simulation was that which used the climate factors in the preceding winter, suggesting that the lower-boundary conditions in the preceding winter may be critical in forecasting the summer HWF_EUR variation.摘要本文使用LightGBM机器学习模型模拟了欧洲东部夏季热浪频率的变化, 并分析了多个底边界层气候因子的贡献. 所选取的气候因子包括前期冬季, 前期春季以及同期夏季的下垫面海温, 土壤湿度, 积雪以及海冰. 分析结果说明LightGBM模型能够较好的模拟出欧洲东部夏季热浪频率的变化, 其中海温因子对模拟的贡献最大. 进一步的分析研究显示, 使用前期冬季的气候因子进行的模拟可以获得最佳模拟结果, 意味着前期冬季的下垫面气候因子可能对夏季欧洲东部热浪频率变化的预报能起到关键作用.

Seasonal climate drivers of peak NDVI in a series of Arctic peatlands

Changes in plant cover and productivity are important in driving Arctic soil carbon dynamics and sequestration, especially in peatlands. Warming trends in the Arctic are known to have resulted in changes in plant productivity, extent and community composition, but more data are still needed to improve understanding of the complex controls and processes involved. Here we assess plant productivity response to climate variability between 1985 and 2020 by comparing peak growing season NDVI (Normalised Difference Vegetation Index data from Landsat 5 and 7), to seasonal-average weather data (temperature, precipitation and snow-melt timing) in nine locations containing peatlands in high- and low-Arctic regions in Europe and Canada. We find that spring (correlation 0.36 for peat dominant and 0.39 for mosaic; MLR coefficient 0.20 for peat, 0.29 for mosaic), summer (0.47, 0.42; 0.18, 0.17) and preceding-autumn (0.35, 0.25; 0.33, 0.27) temperature are linked to peak growing season NDVI at our sites between 1985 and 2020, whilst spring snow melt timing (0.42, 0.45; 0.25, 0.32) is also important, and growing season water availability is likely site-specific. According to regression trees, a warm preceding autumn (September–October–November) is more important than a warm summer (June–July–August) in predicting the highest peak season productivity in the peat-dominated areas. Mechanisms linked to soil processes may explain the importance of previous-Autumn conditions on productivity. We further find that peak productivity increases in these Arctic peatlands are comparable to those in the surrounding non-peatland-dominant vegetation. Increased productivity in and around Arctic peatlands suggests a potential to increased soil carbon sequestration with future warming, but further work is needed to test whether this is evident in observations of recent peat accumulation and extent.

Lightning climatology across the Chinese continent from 2010 to 2020

Understanding the lightning climatological distribution is of great significance but remains challenging which is in part due to the lack of continuous and uniform lightning observations. In this paper, for the first time, an 11-year ground-based cloud-to-cloud lightning dataset of China National Lightning Detection Network from 2010 to 2020 is used to investigate the lightning climatology over China's land area in terms of spatial distribution, seasonal, monthly and diurnal variations. Lightning activity in China's land area generally decreases from south to north and from east to west. The mean cloud-to-ground lightning density of China's land area is 0.9 fl km−2 yr−1, with the average maximums (exceeding 10 fl km−2 yr−1) are found in the south and east coastal provinces, such as Guangdong, Hainan, Fujian and Zhejiang Province. The lightning activity in spring, summer and autumn seems consistent throughout the daytime cycle, with the lightning activity is rather weak in the morning and most active in the afternoon, while winter lightning appears to be more active in the nighttime and in the very early morning produced from nocturnal thunderstorms. The cloud-to-ground lightning activity in the tropical and subtropical areas within 15–25°N is mainly concentrated in the late spring and early summer, with two weak peaks in May and August, while lightning in mid latitudes within 25–55°N mainly occurs in the summer (June–July-August). The percentage of positive cloud-to-ground lightning flashes with peak current higher than 75 kA is more than 3 times than that of negative cloud-to-ground lightning flashes. Meanwhile, the mean peak current of cloud-to-ground lightning flashes shows an inverse diurnal pattern to the cloud-to-ground lightning activity.

Multi-predictor ensembles improving seasonal prediction of summer rainfall over the Bohai Sea Rim based on statistical downscaling of BCC_CSM1.1 m

A model-circulation-based statistical downscaling (MCSD) method and its multi-predictor ensemble (MPE) schemes are proposed in this study to improve summer (June–July-August) rainfall prediction over the Bohai Sea Rim (BSR) area in the northern China where quite low prediction skills were found in Beijing Climate Center Climate System Model version 1.1 m (BCC_CSM1.1 m). This MCSD method, using both the empirical orthogonal function and singular value decomposition, can largely eliminate noise within variables and extract leading coupled patterns between the observed BSR summer rainfall and simultaneously predicted large-scale atmospheric circulation to significantly add prediction skill and spatial resolution. Three circulation anomaly predictors, the 500-hPa geopotential height (H500), 200-hPa zonal wind (U200) and 850-hPa meridional wind, are selected to implement statistical downscaling, and their MPE means are compared for verifications. Cross validations show considerable improvements of the MCSD with the MPE schemes, especially H500 and its ensemble mean with U200 (H5U2), where almost all temporal correlation coefficients between the predictions and observation are significantly positive over most BSR area, and particularly, spatial correlation coefficients reach 0.24 and 0.19 from −0.05 of the original model and the anomaly sign consistency rates reach 58.2% and 58.4% from 48.4%. In MCSD, to deal with the deficiency of underestimated rainfall anomaly, an intensity inflation for the downscaled rainfall prediction is carried out, which makes temporal correlation coefficients for the area-averaged H500 and H5U2 predictions improved from about 0.54 to 0.7. Overall, not only the tendency and spatial distribution but also magnitude accuracy of summer rainfall prediction over BSR can be significantly improved by the MCSD and its MPEs, providing a potentially effective tool for the mid-latitude seasonal prediction of summer rainfall usually showing low skills in dynamical models.

Investigating sources of surface ozone in central Europe during the hot summer in 2018: High temperatures, but not so high ozone

The changing emissions and climate have largely influenced the contribution of different sources on surface ozone, while the relations between the emissions, meteorological conditions and ozone sources are highly uncertain. In this study we modeled the sources of surface ozone in central Europe in the summer (June–July–August) of 2018, one of the warmest summers in Europe, with the regional chemical transport model CAMx and its source apportionment tool OSAT. Additional simulations with the anthropogenic emissions in 2000 were performed as high-emission scenarios to investigate the effects of reduced emissions on ozone sources, and a specific analysis on the coupled effects from meteorology was conducted based on measurements and backward trajectory calculations at three Swiss sites. The model results indicate that the highest contribution to summer afternoon surface ozone in Europe in 2018 comes from the boundary import (65%), followed by off-road traffic (11%) and road traffic (8%). Compared to the case using 2000 emissions, the lower emissions in 2018 lead to lower ozone in most of the areas in Europe except for some urban areas and shipping routes, and the highest decrease is from the road traffic for both the whole domain and the Swiss sites. The backward trajectory analysis at Swiss sites in 2000 (base year), 2003 (the warmest year) and 2018 (the 3rd warmest year) show that 2018 is the year with the highest fraction of air mass from the high ozone areas (from east in southern Switzerland and from north in northern Switzerland). The results indicate that the reduced anthropogenic emissions are the main reason responsible for the reduced high level ozone in the hot summer in central Europe.

Meteorological influences on daily variation and trend of summertime surface ozone over years of 2015–2020: Quantification for cities in the Yangtze River Delta

We quantify the meteorological influences on daily variations and trends of maximum daily 8-h average ozone (MDA8 O3) concentrations by using multiple linear regression (MLR) and Lindeman, Merenda, and Gold (LMG) approaches. Different from previous region-based studies, we pay special attention to meteorological influences at city scale. Over 2015–2019, daily changes in key meteorological parameters could explain 47%–74% of the observed daily variations in summertime MDA8 O3 concentrations in Yangtze River Delta (YRD) and four cities (Shanghai, Nanjing, Hangzhou, and Hefei), with RH being the top driver. Over years of 2015–2020, daily concentrations of MDA8 O3 obtained from MLR equations (MDA8O3_MLR) of the local cities always had better performance than those of YRD. Compared with the observed daily MDA8 O3 in June–July–August (JJA) over the studied period, daily MDA8O3_MLR of the local cities (of YRD) had correlation coefficients of 0.73 (0.63), 0.75 (0.74), 0.79 (0.78), and 0.76 (0.73) in Shanghai, Nanjing, Hangzhou, and Hefei, respectively, and the MDA8O3_MLR of the local cities (of YRD) captured 54% (17%), 63% (51%), 52% (27%) of the observed O3-polluted days (days with MDA8 O3 concentration exceeding 160 μg m−3) in Shanghai, Nanjing, and Hangzhou, respectively. The meteorologically driven trends (Trend_Met) in MDA8 O3 were calculated using the established MLR equations. Over 2015–2019, the observed trends (Trend_Obs) and Trend_Met in MDA8 O3 were mostly positive in YRD, Nanjing, Hangzhou, and Hefei. In Shanghai, Trend_Obs, Trend_Met, and anthropogenically driven trend (estimated as Trend_Obs minus Trend_Met) of MDA8 O3 in JJA over 2015–2019 were −1.3, +1.0, and −2.3 μg m−3 y−1, respectively, indicating that the emission control measures alleviated O3 pollution in this city. Our results suggest that it is necessary to establish MLR equations at city scale to account for the role of meteorology in the actions of O3 pollution control.

Differentiating lightning in winter and summer with characteristics of the wind field and mass field

Abstract. Lightning in winter (December–January–February, DJF) is rare compared to lightning in summer (June–July–August, JJA) in central Europe north of the Alps. The conventional explanation attributes the scarcity of lightning in winter to seasonally low values of variables that create favorable conditions in summer. Here we systematically examine whether different meteorological processes are at play in winter. We use cluster analysis and principal component analysis and find physically meaningful groups in ERA5 atmospheric reanalysis data and lightning data for northern Germany. Two thunderstorm types emerged: wind-field thunderstorms and CAPE (convective available potential energy) thunderstorms. Wind-field thunderstorms are characterized by increased wind speeds, high cloud shear, large dissipation of kinetic energy in the boundary layer, and moderate temperatures. Clouds are close to the ground, and a relatively large fraction of the clouds are warmer than −10 ∘C. CAPE thunderstorms are characterized by increased convective available potential energy (CAPE), the presence of convective inhibition (CIN), high temperatures, and accompanying large amounts of water vapor. Large amounts of cloud-physics variables related to charge separation such as ice particles or cloud base height further differentiate both wind-field thunderstorms and CAPE thunderstorms. Lightning in winter originates in wind-field thunderstorms, whereas lightning in summer originates mostly in CAPE thunderstorms and only a small fraction in wind-field thunderstorms. Consequently, typical weather situations of wind-field thunderstorms in the study area in northern Germany are strong westerlies with embedded cyclones. For CAPE thunderstorms, the area is typically on the anticyclonic side of a southwesterly jet.

Effects of Land Cover Changes on Compound Extremes over West Africa Using the Regional Climate Model RegCM4

This study aims to characterize the impacts of the Sahel–Sahara interface reforestation on compound extremes in the Sahel region during the West African monsoon season (June–July–August–September, JJAS). For this purpose, we performed a simulation with the standard version of the RegCM4 model, and another simulation with the altered version of the same model, taking into account the incorporated forest. Results show that reforestation may strongly influence the frequency of individual extreme events (dry and warm days) by decreasing them over and off the reforested zone. The reduction in these extreme dry and warm days may be due partly to the strengthening of the atmospheric moisture content over most parts of the West African domain and the weakening of the sensible heat flux south of 16° N. The analysis also shows an increase in extreme wet days over and off the reforested zone, which could be associated partly with the strengthening of evapotranspiration over most parts of the West African domain, including the reforested area. The analysis of compound extremes shows a strong occurrence of the compound dry/warm mode over the northern Sahel for both runs, probably due to the weak precipitation recorded in this zone. Both experiments also simulated a strong compound wet/warm mode occurrence over the Sahel due to a high rainfall occurrence over this region. When comparing both runs, the impact of the reforestation was to decrease (increase) the compound extreme dry/warm (wet/warm) mode over the reforested zone. The dry/warm mode decrease is consistent with that of individual extreme dry and warm days, while the compound wet/warm mode increase may be driven by that of the extreme wet days. Finally, when considering the seasonal cycle, the dry/warm mode exhibits a more substantial decrease in the beginning (June–July, JJ) than during the peak of the West African summer monsoon season (August–September, AS). Moreover, reforestation similarly affects the compound wet/warm mode in JJ and AS by increasing it in the reforested region and decreasing it over the Southern Sahel (south of 15° N). This work suggests that reforestation may be a good solution for West African policymakers to mitigate climate change over the region and to develop better strategies for water resource management.

Unraveling Intricacies of Monsoon Attributes in Homogenous Monsoon Regions of India

India observes the summer monsoon in June–July–August–September (JJAS) season, and the livelihood security of a huge population depends on it. The impact of the monsoon onset timing, length of monsoon season, rainfall amount, and related extreme events is huge on the Indian economy. Therefore, understanding the inherent intricacies needed a detailed investigation. In five homogenous monsoon regions of India, the trend of monsoon onset and the length of monsoon season are examined. The association between 1) monsoon onset ∼ rainfall amount, 2) length of monsoon season ∼ rainfall amount, and 3) monsoon onset ∼ length of monsoon season is investigated. Subsequently, the behavior of rainfall and extreme excess days in the ±1 standard deviation (SD) length of monsoon season is also examined in detail. The trend for monsoon onset shows late onset in all the homogenous monsoon regions except the northeast region. The length of monsoon season is found increasing significantly with high magnitude in west central and northwest regions. A significantly strong negative correlation (∼−0.6) for monsoon onset timing ∼ length of monsoon season is observed. Therefore, the change in rainfall anomaly, extreme excess days, and rainy days is done concerning the length of the monsoon season. In the cases of the −1 SD (+1 SD) length of monsoon season, rainfall anomaly and extreme excess days are low (high) in most parts of the homogenous monsoon regions. Extreme excess days showed a significant association with rainy days, which indicates a high possibility of rainy days converting into extreme excess days. However, the increase in extreme excess days in the +1 SD length of monsoon season is limited to a great extent in JJAS and June only. Morlet wavelet power spectrum shows the delay (advance) of power in −1SD (+1 SD) length of monsoon season.

Impact of tropical Atlantic SST anomaly on ENSO in the NUIST‐CFS1.0 Hindcasts

AbstractUsing both observations and ensemble hindcasts of Nanjing University of Information Science and Technology Climate Forecast System (NUIST‐CFS1.0) for the period of 1983–2020, the leading modes of sea surface temperature anomalies (SSTAs) over the tropical Atlantic in boreal spring and summer (March–April–May–June–July–August, MAM‐JJA) and their connections with El Niño‐Southern Oscillation (ENSO) are investigated with Empirical Orthogonal Function (EOF), partial‐correlation, and composite analyses. In both observations and hindcasts, the first EOF mode is characterized by a basin‐wide SSTA pattern, and the second mode features a meridional dipole pattern. The meridional dipole mode cannot trigger ENSO, while the basin‐wide SST warming mode contributes to a subsequent La Niña via both north‐tropical and equatorial pathways. The north‐tropical pathway involves a low‐level anomalous anticyclone over North Pacific as a Matsuno‐Gill‐type response to the basin‐wide SST warming, related to a North Pacific Meridional Mode characterized with a cold SSTA and north‐easterly wind coupling over the northeast tropical Pacific, and finally triggers La Niña via the seasonal footprinting mechanism. This pathway is less important in the hindcasts than in observations. The equatorial pathway is tightly linked to the Kelvin wave, characteristic of an eastward propagation of the coupled pattern of SST and low‐level winds and a wedge‐like 500‐hPa air temperature anomaly over the tropical Indian and western equatorial Pacific Oceans, which is more significant in the hindcasts than in observations. Further composite analyses confirm that, even without the effect of the preceding ENSO, a basin‐wide tropical Atlantic SST warming favours the development of La Niña.

Evaluation of GPM IMERG Performance Using Gauge Data over Indonesian Maritime Continent at Different Time Scales

Accurate precipitation observations are crucial for water resources management and as inputs for a gamut of hydrometeorological applications. Precipitation data from Integrated Multi-Satellite Retrievals for Global Precipitation Measurement (GPM) (IMERG) have recently been widely used to complement traditional rain gauge systems. However, the satellite precipitation data needs to be validated before being widely used in the applications and this is still missing over the Indonesian maritime continent (IMC). We conducted a validation of the IMERG product version 6 for this region. The evaluation was carried out using gauge data in the period from 2016 to 2020 for three types of IMERG: Early (E), Late (L), and Final (F) from annual, monthly, daily and hourly data. In general, the annual and monthly data from IMERG showed a good correlation with the rain gauge, with the mean correlation coefficient (CC) approximately 0.54–0.78 and 0.62–0.79, respectively. About 80% of stations in the IMC area showed a very good correlation between gauge data and IMERG-F estimates (CC = 0.7–0.9). For the daily assessment, the CC value was in the range of 0.39 to 0.44 and about 40% of stations had a correlation of 0.5–0.7. IMERG had a fairly good ability to detect daily rain in which the average probability of detection (POD) for all stations was above 0.8. However, the false alarm ratio (FAR) value is quite high (<0.5). For hourly data, IMERG’s performance was still poor with CC around 0.03–0.28. For all assessments, IMERG generally overestimated rainfall in comparison with rain gauge. The accuracy of the three types of IMERG in IMC was also influenced by season and topography. The highest and lowest CC values were observed for June–July–August and December–January–February, respectively. However, categorical statistics (POD, FAR and critical success index) did not show any clear seasonal variation. The CC value decreased with higher altitude, but with slight difference for each IMERG type. For all assessments conducted, IMERG-F generally showed the best rainfall observations in IMC, but with slightly difference from IMERG-E and IMERG-L. Thus, IMERG-E and IMERG-L data that had a faster latency than IMERG-F show potential to be used in rainfall observations in IMC.

Evaluation of Lightning Prediction by an Electrification and Discharge Model in Long-Term Forecasting Experiments

Over nearly three rainy seasons of lightning activity in North China, numerical prediction experiments were carried out using the Weather Research and Forecasting model coupled with electrification and discharge schemes (WRF-Electric). The numerical forecast results were evaluated using the neighborhood-based equitable threat score (ETS) and fraction skill score (FSS) verification methods based on nationwide observational lightning data. An algorithm was used to generate the coverage of the total flash (intracloud and cloud-to-ground flashes) by fitting the cloud-to-ground flash data. The numerical results showed that the region of lightning activity could be well predicted by the mesoscale WRF-Electric model, particularly during a 6–12-hour forecasting period. The average ETS score of the 6–12-hour forecasting period was 0.34 for a 20 km neighborhood radius. The predictive skill of the model varied not only monthly but also diurnally. The model showed better forecasting skills during the main rainy season (June–July–August) and at 14 : 00–20 : 00 local time. The predictability of the model was enhanced with increasing thunderstorm scale. On the other hand, the coverage of predicted lightning activity was relatively concentrated, and the lightning flash density was higher than the observations. The main discrepancies in the model prediction were related to the design of the discharge parameterization. Thus, in discharge parameterization, the initial threshold for lightning should be modified according to the model resolution, while the magnitude of the neutralization charge in a single discharge should be referenced to the observational results.

Genesis of Upper-Tropospheric Anticyclones over the Asian–Western Pacific Sector from Tropical–Extratropical Interaction Perspective

Abstract Upper-tropospheric anticyclones (UTACs) emerge throughout the seasons with changing location and intensity. Here, the formation mechanisms of these UTACs, especially in the Asian–Australian–western Pacific sector, were investigated based on the diagnosis of the vorticity equation as well as the contribution of the planetary waves. During June–July–August (JJA), a vigorous UTAC corresponding to the South Asian high (SAH) forms over South Asia, to the south of the Tibetan Plateau, where intense heating associated with the Asian summer monsoon rainfall and the resultant baroclinic Rossby response are the important physical processes. Meanwhile, the produced anticyclonic vorticity is farther transported by the interhemispheric divergent wind toward the Southern Hemisphere (SH), creating the SH UTAC centered over the Maritime Continent. During December–January–February (DJF), two zonally elongated UTACs reside on each side of the equator (∼10° poleward), mainly over the Maritime Continent–western Pacific sector. Upon a closer look at the NH winter, we observed that the northern parts of UTAC cannot be explained by this vorticity balance alone. Diagnosis of the wave activity flux indicated that planetary waves emanating from the cold Eurasian continent converges around the northern parts of the UTAC with its peak in the NH winter, which weakens the subtropical jet, thus generating UTAC. Configuration of the SH summer (DJF) UTAC bears resemblance with that of SAH. These results suggest that the creation of anticyclonic vorticity and its interhemispheric transportation as well as the propagation of planetary wave are the selectively important agents for the genesis of seasonally varying UTACs. Significance Statement Recent studies have provided evidence that the South Asian high (formerly Tibetan high) is not a purely thermally driven system only maintained over the elevated Tibetan Plateau. This study aims to understand the physical processes responsible for the genesis of the upper-tropospheric anticyclone, especially in the Asian–Australian–western Pacific sector, throughout the season. During summer in the Northern Hemisphere, deep heating caused by South Asian monsoon rainfall plays a crucial role in the genesis of the South Asian high. The wintertime anticyclone emerging over the subtropical western North Pacific is caused via remote influences anchored with tropical convection and the cold Eurasian continent in which atmospheric teleconnections are important. These findings provide new avenues for research on tropical–extratropical interactions with respect to the formation and variability of important climate phenomena.

Mechanisms responsible for interdecadal variation of the thermodynamic component of East Asian summer monsoon moisture transport

AbstractIn this study, the East Asian summer (June–July–August) monsoon (EASM) atmospheric moisture transport variability under global warming are examined using ERA‐Interim reanalysis data from 1979 to 2019. The total EASM atmospheric moisture transport is decomposed into the thermodynamic (δTH) and dynamic (δMCD) components, which are related to specific humidity and wind velocity, respectively. Based on empirical orthogonal function (EOF) analysis, both the second and the fourth modes of δTH are linked to δMCD, indicating that δMCD can impact the moisture distribution and consequently influence δTH indirectly. Both the first (δTH_EOF1) and the third (δTH_EOF3) modes of δTH show considerable interdecadal changes. It is noted that their principal components exhibit significant increasing trends in recent decades. The response of EASM moisture transport to global warming is probably reflected in these two modes. δTH_EOF1 is associated with negative phases of the Interdecadal Pacific Oscillation (IPO)/Pacific Decadal Oscillation (PDO). This causes the western North Pacific subtropical high (WNPSH) to extend westward and the strengthening of Pacific trade winds. As a result, positive precipitable water anomalies dominate East Asia and δTH_EOF1 is characterized by anomalous southwesterly moisture transport along the East China Sea to southern Japan. δTH_EOF3 corresponds to anomalous moisture transport over the Indochina Peninsula and Japan. The precipitable water associated with δTH_EOF3 in general represents a meridional dipole. Anomalous southward wind induced by the eastward extension of the WNPSH is the main reason for positive (negative) precipitable water anomalies in southern (northern) East Asian. The anomalous circulation associated with the anomalous east–west sea surface temperature gradient over the Indian Ocean affects atmospheric moisture in southern East Asian and can dramatically impact δTH_EOF3.