Abstract:The investigation of climate change in arid and semiarid regions has long been a prominent scientific issue of extensive interest,particularly concerning the patterns and future trends of dry and wet climatic variations.Previous research has made significant advancements in unraveling the facts and mechanisms underlying these climatic changes across diverse arid and semiarid regions worldwide.Although the 6^th IPCC evaluation report indicates a projected exacerbation of global aridity trends in the future,several contentious issues persist.This paper provides a comprehensive review of studies focusing on interdecadal changes in dry and wet conditions,particularly interdecadal drought,in arid and semiarid regions within the context of global warming.The current state of related research is systematically analyzed,and key scientific challenges faced by arid and semiarid region studies are identified.

Abstract:Short-lived Climate Forcers (SLCFs) have a significant impact on air pollution and climate change.The Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) pays a special attention on SLCFs,which includes an assessment of natural emissions of SLCFs in addition to the anthropogenic SLCFs.The understanding of natural emissions of SLCFs and their climate feedbacks would be more important in the future with the reduction of anthropogenic SLCFs under climate warming.In this paper,the latest findings from IPCC AR6 are introduced in terms of natural emissions of SLCFs,changes in emissions under historical and future climate scenarios,and their climate feedbacks.Under climate warming,future emissions of lighting NO_x,biogenic VOCs,and biomass burning will likely increase,and the sensitivity in emissions of soil NO_x,dust,sea salt,and dimethyl sulfide (DMS) to climate change is uncertain.Meanwhile,due to changes in emissions,atmospheric burden,or lifetime of SLCFs in response to climate warming,the overall effect will likely result in a negative feedback of -0.20 W/m²/℃ (-0.41～+0.01 W/m²/℃),which may slightly mitigate climate warming.

Abstract:Based on the observation and a set of downscaling bias corrected model (NEX-GDDP-CMIP6) data,this study has investigated the model performance in simulating precipitation extremes over Southwest China using Taylor diagram and GEV fitting methods.Furthermore,the future changes of precipitation extremes that with different return intervals and their associated population exposure are also explored.Results show that NEX-GDDP-CMIP6 models and the multi-model ensemble (N-CMIP6-MME) can reasonably capture the spatial-temporal characteristics of changes in precipitation extremes over Southwest China,and N-CMIP6-MME out performs most of individual models.In the future,the precipitation extremes are expected to significant increases over most regions of Southwest China,including the extreme events that occurring once in 10 or 20 years,which will further increase the risk of population exposure to these extremes.Additionally,for the more extreme events (once in 20-year),the increasing magnitude of its occurring probability would be much greater and it would also exert a larger increase of its associated population exposure,when compared to the events of once in 10-year.Around the year of 2050,the occurring probabilities of the RX1day (RX5day) extremes that occurring once in 20-year are expected to increase by 175.2％(148.9％),216.0％(162.4％),210.9％(156.8％),and 274.3％(207.1％) under SSP1-2.6,SSP2-4.5,SSP3-7.0and SSP5-8.5 scenario,respectively.Correspondingly,thepopulation exposuresare projected to respective increase by 129.1％(118.8％),177.7％ (135.1％),182.4％(143.2％),and 237.5％(161.5％).Further analyses indicate that the increase of population exposure to the precipitation extreme over Southwest China is mainly due to the significant increase of precipitation extremes,and the changes in populations and their interaction generally present a negative contribution.

Abstract:Based on reanalysis datasets of land precipitation,circulation,radiation,and indices representing atmospheric waves,anomalies of the East-Asian circulation and the summer precipitation in North China of the years in which El Niño decayed fast in spring are compared with those of El Niño slow decaying years between 1951—2020;And the reason why El Niño decayed fast in spring in some years was discussed from the perspective of atmospheric wave intensity.Results show that,compared with the other years without El Niño decaying,the precipitation in North China in July and August of the fast decaying years are significantly greater,especially in August.In contrast,the summer precipitation in North China are not significantly greater in the slow decaying years.There was an anomalous anticyclone at 850 hPa over the Philippines to the South China Sea from June to August in the fast decaying years,which was much stronger than that in the slow decaying years.The western Pacific subtropical high (WPSH) at 500 hPa was significantly northward-displaced in August in the fast decaying years,while the WPSH in the slow decaying years was more westward rather than northward.The 200 hPa subtropical westerly jet in August of the fast decaying years was significantly northward,while the corresponding westerly jet in the slow decaying years was slightly southward.And the Walker Circulation from June to August in the fast decaying years was much stronger than that in the slow decaying years.These circulation anomalies in the fast decaying years provided favorable conditions for more precipitation in North China from July to August.According to the composite analysis of near-surface wind,the outbreak of anomalous equatorial easterly wind occurred around the month in which El Niño decayed fast over the Western and Central Pacific.However,in the slow decaying years,the anomalous equatorial easterly wind was much weaker and unsignificant.The convection over the Indian Ocean remained active and spread to the Maritime continent during the fast decay of El Niño.Such continuous active convection very likely induced the outbreak of anomalous near-surface easterly wind by triggering atmospheric waves,and consequently resulted in the fast decay of El Niño.

Abstract:This study investigates the characteristics and underlying processes of the month-to-month reversal of air temperature anomalies in Northeast Asia during early summer (May to June) on the interannual timescales,focusing on the influences of soil moisture anomalies in eastern Europe.Utilizing observational and reanalysis data spanning from 1979 to 2020,the results reveal that the dominant mode of intermonthly variation in air temperature anomalies exhibits a reversal pattern,with warmer (colder) conditions in May and colder (warmer) conditions in June.This reversal mode is directly linked to circulation anomalies over Northeast Asia.Further analysis demonstrates that reduced soil moisture in eastern Europe during May contributes to a warmer May and a colder June in Northeast Asia.The potential physical processes driving these effects are explored.Lower soil moisture in May leads to local soil warming and subsequent warming of the lower troposphere.This,in turn,weakens (enhances) the lower tropospheric meridional temperature gradient and baroclinicity in the Mediterranean region (northern Europe).Consequently,high-frequency transient wave activity weakens (enhances),facilitating the formation of an anomalous high-pressure system over central and eastern Europe,thereby generating Rossby wave sources through transient vorticity forcing.These associated Rossby waves propagates eastward along the polar front jet,resulting in the formation of a barotropic anomalous high-pressure system over Northeast Asia and inducing surface warming.While the soil moisture anomalies persist until June,their intensity weakens.Similarly,this favors the formation of an anomalous high-pressure system and Rossby wave source;however,their centers shift westward towards western Europe.The associated Rossby wave activity leads to the formation of a barotropic anomalous low-pressure system,causing cooling over Northeast Asia.Notably,the activity characteristics of Rossby waves (wave source,activity centers,and propagation pathway) associated with the soil moisture anomalies in eastern Europe exhibit distinct differences between May and June,closely tied to the atmospheric climatology over northern Eurasia.When soil moisture is high in May,the aforementioned physical processes exhibit reverse tendencies.

Abstract:The Tibetan Plateau exhibits complex train,resulting in significant variability in soil conditions across different regions.Gravel and soil organic matter have a substantial influence on soil thermal and hydraulic properties.In this study,we employ the regional climate model RegCM4.7 coupled with CLM4.5 to modify the surface data and implement a corresponding soil thermal and hydraulic parameterization scheme.We conduct two tests:one considering the influence of gravel (test2)and another considering the combined influence of gravel and organic matter (test3).Our results indicate that test2 significantly improves the simulation accuracy for the western part of the Plateau by incorporating gravel effects but exhibits poor performance for the eastern part.Test3 further enhances the simulation of shallow soil properties in the central and eastern regions based on test2.The regional average root mean square error of shallow soil temperature decreases from 2.11 ℃ to 0.47 ℃,and the regional average root mean square error of shallow soil moisture decreases from 0.05 mm³·mm^-3 to 0.01 mm³·mm^-3 when transitioning from test1 to test3.Furthermore,all three schemes successfully simulate the surface temperature of the Tibetan Plateau,with test3 exhibiting the smallest error.The root mean square error of the regional average decreases from 2.18 ℃ to 0.74 ℃,bringing it closer to the reanalysis data.

Abstract:This study analyzes the spatiotemporal variation of heat discomfort days characterized by Humidex and cold discomfort days represented by Wind Chill Index in different latitudes (high latitudes,middle latitudes,and low latitudes) of the northern hemisphere during 1961—2014,using NCEP/NCAR reanalysis data and simulations from six CMIP6 models.The investigation focuses on the attribution of these discomfort days.The results show a significant increase in the frequency of heat discomfort in the middle and low latitudes,and a significant decrease in the frequency of cold discomfort in the middle and high latitudes of the northern hemisphere.Furthermore,using the optimal fingerprint method,we find that the increased frequency of heat discomfort in middle latitude and the decreased frequency of cold discomfort in high latitude can be attributed to ALL,ANT,and GHG forcings,with GHG forcing dominating changes in the frequency of heat discomfort in mid-latitude areas and the frequency of cold discomfort in high-latitude areas.In the low latitudes,the significant increase in the frequency of heat discomfort can be attributed to anthropogenic greenhouse gas forcing,while AER has the opposite effect.The frequency of cold discomfort may be attributed to the internal variability of the climate system.In the future,the frequency of heat (cold) discomfort in the middle and low latitudes (middle and high latitudes) is expected to increase (decrease) under the uncoordinated and rapid development with higher radiative forcing.However,low radiative forcing and sustainable development can ensure a stable level of human comfort.

Abstract:Two moderate La Niña and El Niño events occurred in early 2008 and early 2016,respectively.During these two distinct El Niño/Southern Oscillation (ENSO) events,Yunnan experienced low temperatures and snowstorms.This study investigates the causes of these two extreme cold events during different ENSO events using statistical methods based on atmospheric circulation data,sea surface temperature (SST),and monthly temperature records from 124 observation stations in Yunnan.The findings are as follows:1) The extremely cold events in Yunnan were more pronounced in February for both 2008 and 2016.2) During the two different ENSO events in February,the variations in atmospheric circulation and temperature exhibited notable differences.In La Niña (El Niño) years,the Siberian high strengthened (weakened),with high geopotential height in the north (west) and low geopotential height in the south (east).The western Pacific subtropical high weakened (strengthened),and anomalous northerly (southerly) winds intensified on the northwest side of the Philippine anomalous cyclone (anticyclone).Additionally,the East Asian winter Monsoon was strong (weak),resulting in low (high) temperatures in the eastern region of Yunnan.3) The atmospheric circulation over the northeast Pacific responded to SST anomalies in the central and eastern equatorial Pacific in 2008 and 2016.Furthermore,the northerly current in the west of the Philippine cyclone triggered by the cold SST in the central and east of the equatorial Middle East Pacific significantly impacted the strengthening and southward activity of the East Asian winter monsoon in 2008,while the warm SST had no significant impact on the atmospheric circulation over the Philippines region in 2006.4) The effects of the Arctic Oscillation (AO) and Arctic Sea ice on the strengthening of the Siberian high in February 2008 and 2016 exhibited different characteristics.The positive phase of AO had a greater impact on the strengthening of the Siberian high in February 2008,whereas the reduction in Arctic Sea ice had a significant impact on the strengthening of the Siberian high in February 2016.

Abstract:To enhance our understanding of the causes behind interannual summer precipitation anomalies in North China and improve climate monitoring and prediction technologies,this study examines the relationship between the East Asian subtropical summer monsoon index (EAMI),the North China atmospheric dynamic rise index (HBDRI),and the summer precipitation in North China.Correlation analysis,synthesis,and circulation anomaly regression reconstruction methods are employed using summer precipitation data in North China and NCEP/NCAR reanalysis circulation data.The main findings are as follows:1) The EAMI and HBDRI exhibit a significant correspondence with summer precipitation in North China.Stronger values of both indices are associated with above-average summer precipitation in North China,while weaker values are associated with below-average summer precipitation.In cases where the strength of the two indices is inconsistent,regional precipitation in North China may be above average,but the overall precipitation in the entire region remains relatively normal.2) Anomalies in summer precipitation in North China result from the combined effects of the East Asian subtropical summer monsoon and the upward motion of atmospheric dynamics in North China.In years with stronger EAMI and HDBRI values,the Baikal Lake trough deepens at the 500 hPa level during summer,the Northwest Pacific subtropical high shifts northward,and North China falls under influence of a circulation pattern characterized by a high-pressure system in the east and a low-pressure system in the west.Consequently,the movement of the western low trough eastward is hindered,leading to a sustained upward motion of the atmosphere over North China.This pattern is accompanied by an increased intensity of the Indian summer monsoon and the East Asian subtropical summer monsoon at the 850 hPa level.During this period,the westerly wind water vapor transport from the tropical Indian Ocean,the southerly wind water vapor transport from the East Asian subtropical region,or the southeast wind water vapor transport strengthens,ensuring an ample supply of water vapor to North China.Such a configuration of high-and low-level circulations is highly conducive to generating increased summer precipitation in North China.Conversely,unusually below-average summer precipitation occurs when the aforementioned conditions are not met.3) The stronger values of the EAMI and HBDRI during the early April-May period can serve as climate monitoring and prediction indicators for abnormally high summer precipitation in North China.

Abstract:This study analyzes the frontogenesis characteristics of rainstorms during the Meiyu period in 2020 using ERA5 reanalysis data and the precipitation data from automatic stations in Jiangsu Province.The main results are as follows:1) The characteristics of frontogenesis during the Meiyu period in 2020 are significant.Heavy rain is associated with middle-low frontogenesis,with deformation frontogenesis being the main contributor.2) Heavy rainfall during the Meiyu period can be categorized into two types.Severe convective rainstorms exhibit a larger frontogenesis range,higher development,greater intensity,and more overlap between total frontogenesis and decomposition terms.However,stable rainfall shows opposite characteristics.3) Two typical cases,namely the strong convective rainfall event on “6.28” and the stable rainfall event on “7.11” are selected.Frontogenesis occurs along shear lines and in regions of significant pseudo-equivalent potential temperature gradient.Strong convective rainfall is observed near the main frontal zone and in high-energy zones of pseudo-equivalent potential temperature,indicating the presence of multiple secondary frontogenesis centers.Stable heavy precipitation,on the other hand,is concentrated near the main frontal zone.These different rainfall distributions arise from variations in triggering mechanism and precipitation properties.4) The vertical front zone in both processes tilts northward from low to high.In the “6.28” process,a notable transport of warm and moist airflow is observed on the south side of the frontal zone,with strong upward motion within the frontal zone.In contrast,the “7.11” process exhibits less pronounced features and weaker convection.The magnitude and impact of frontogenesis differ across various precipitation processes and stages.Rainstorms are primarily generated by divergence and deformation frontogenesis,while the tilting term frontolysis in convective precipitation and frontogenesis in stable precipitation.Quantitative analysis of each decomposition term reveals that low-level divergence and deformation frontogenesis initially increase and then decrease in both processes.Deformation and divergence frontogenesis below 700 hPa are the main contributors,while the middle-high atmosphere experiences frontolysis.

Abstract:This study evaluates the performance of several existing tropical cyclone (TC) genesis potential indices (GPIs) in reproducing the observed TC genesis frequency (TCGF) in the North Atlantic and western North Pacific (WNP) from 1979 to 2017.The results demonstrate that the existing GPIs can reasonably replicate the climatological distribution,annual cycle,and interannual variability of TCGF in the North Atlantic.However,they fail to reproduce the interannual variability of TCGF in the WNP.To address this limitation,a modified GPI is developed for the WNP,utilizing best-track TC data and ERA-Interim reanalysis data spanning 1979—2017.The modified GPI incorporates earth vorticity (the Coriolis parameter) and relative vorticity separately instead of absolute vorticity and excludes relative humidity,which is commonly used in existing GPIs.Compared with the existing GPIs,the modified GPI can reproduce not only the climatological distribution and annual cycle but also the interannual variability of TCGF in the WNP,particularly in the southeastern region.Additionally,the modified GPI is evaluated using NCEP/NCAR,JRA-55,and ERA5 reanalysis datasets,demonstrating consistent and improved performance in reproducing the interannual variability of TCGF,especially when employing JRA-55 or NCEP/NCAR reanalysis datasets.

Abstract:This study analyzes the spatial-temporal characteristics and influencing factors of sensible heat flux based on data observed by the Large Aperture Scintillometer (LAS) from April 2018 to April 2019 in Pancheng town.The results indicate the following:1) In the town,the sensible heat flux exhibits a unimodal diurnal variation pattern,significantly larger during the daytime compared to the nighttime.The sensible heat flux is significantly higher on clear days than on cloudy days and slightly lower at night compared to cloudy days.The average hourly sensible heat flux ranges between 2.25—200.53 W·m^-2 and 13.10—132.52 W·m^-2 for clear and cloudy days,respectively.2) The sensible heat flux is significantly higher in summer than in winter.In August,the values during the daytime and at night are 112.19 and 23.54 W·m^-2,respectively.In February,the values during the daytime and at night are 35.57 and 11.57 W·m^-2,respectively.3) On clear days with inflows from different directions,notable differences in sensible heat fluxes are observed in the town.As the proportion of impervious layers increases,the proportion of net radiation allocated to sensible heat fluxes also increases significantly.When the proportion exceeds 60％,the upward trend becomes less evident.4) Using the Monin-Obukhov length as a criterion for judging atmospheric stability,the C²_n method exhibits a low misjudgment rate among the five methods when using LAS data,making it a more suitable method for determining atmospheric stability at night in the town.5) Among the surface parameters affecting sensible heat flux,changes in effective height have the greatest impact.Changes in wind speed have a larger effect,especially in autumn and winter.The change in Bowen's ratio has a minor effect,while the effects of changes in temperature,roughness length,and displacement height are negligible.