Impacts of Aerosol−Radiation Interactions on the Wintertime Particulate Pollution under Different Synoptic Patterns in the Guanzhong Basin,China

The effects of aerosol-radiation interactions(ARI)are not only important for regional and global climate,but they can also drive particulate matter(PM)pollution.In this study,the ARI contribution to the near-surface fine PM(PM_(2.5))concentrations in the Guanzhong Basin(GZB)is evaluated under four unfavorable synoptic patterns,including“northlow”,“transition”,“southeast-trough”,and“inland-high”,based on WRF-Chem model simulations of a persistent heavy PM pollution episode in January 2019.Simulations show that ARI consistently decreases both solar radiation reaching down to the surface(SWDOWN)and surface temperature(TSFC),which then reduces wind speed,induces sinking motion,and influences cloud formation in the GZB.However,large differences under the four synoptic patterns still exist.The average reductions of SWDOWN and daytime TSFC in the GZB range from 15.2%and 1.04°C in the case of the“transition”pattern to 26.7%and 1.69°C in the case of the“north-low”pattern,respectively.Furthermore,ARI suppresses the development of the planetary boundary layer(PBL),with the decrease of PBL height(PBLH)varying from 18.7%in the case of the“transition”pattern to 32.0%in the case of the“north-low”pattern.The increase of daytime near-surface PM_(2.5)in the GZB due to ARI is 12.0%,8.1%,9.5%,and 9.7%under the four synoptic patterns,respectively.Ensemble analyses also reveal that when near-surface PM_(2.5)concentrations are low,ARI tends to lower PM_(2.5)concentrations with decreased PBLH,which is caused by enhanced divergence or a transition from divergence to convergence in an area.ARI contributes 15%-25%toward the near-surface PM_(2.5)concentrations during the severe PM pollution period under the four synoptic patterns.

Identification of synoptic patterns for extreme rainfall events associated with landfalling typhoons in China during 1960–2020

Extreme rainfall associated with landfalling typhoon(ERLTC)can cause severe disasters and economic impacts throughout China.Improving the accuracy of ERLTC forecasts is therefore crucial in disaster prevention and mitigation.The top 26 ERLTC events in China during 1960–2020 are investigated based on multi-source datasets.These ERLTC events are categorized into five main types according to the geographical location of the extreme precipitation and its position relative to the tropical cyclone(TC)center,namely:the typhoon inner-core rainfall in Taiwan(TWIC),typhoon inverted trough rainfall in Taiwan(TWIT),weak typhoon rainfall in Hainan(HNWK),strong typhoon rainfall in Zhejiang(ZJST)and inland typhoon remnant rainfall(ILRM).All the ERLTC events occurred in the weakening stage of TC after reaching its lifetime maximum intensity in convective cloud(TBB≤−32℃)regions over complex local terrain.The translational speeds of 20 TCs(76.9%of the total)were smaller than the climatological average(20.6 km h^(−1))during the extreme precipitation events.The differences are as follows:the TWIC and TWIT types are featured with different season,track and water vapor channel although both occurred in Taiwan.The other three types are distinguished by spinning track and strong convective cloud for HNWK type,strong TC intensity and binary TC interactions for ZJST type;and stagnation and strong westerly trough activity for ILRM type,respectively.These results are expected to provide useful clues for an in-depth understanding of ERLTC events over China.

The spring Yellow Sea fog：synoptic and air–sea characteristics associated with different airflow paths

The fog occurs frequently over the Yellow Sea in spring（April–May）, a climatical period of Asian monsoon transition. A comprehensive survey of the characteristic weather pattern and the air-sea condition is provided associated with the fog for the period of 1960–2006. The sea fog is categorized by airflow pathways of backward trajectory cluster analysis with the surface observations derived from international comprehensive oceanatmosphere dataset（I_COADS） I_COADS datasets and contemporaneous wind fields from the National Centers for Environmental Prediction（NCEP）/National Center for Atmospheric Research（NCAR） reanalysis. On the basis of the airflow paths, the large-scale lower-tropospheric circulation patterns and the associated surface divergence,the distribution of a vertical humidity, the horizontal water vapor transportation and the air-sea temperature difference are investigated and the major findings are summarized as follows.（1） Four primary clusters of the airflow paths that lead to spring sea fog formation are identified. They are originated from the northwest, east,southeast and southwest of the Yellow Sea, respectively.（2） Springtime Yellow Sea fog occurs under two typical weather patterns： the Yellow Sea high（YSH） and cyclone and anticyclone couplet（CAC）. Each pattern appears by about equal chance in April but the YSH occurrence drops to around one third and the CAC rises to around two third of chance in May.（3） The common feature in the two types of synoptic conditions is that surface divergence center is located over the Yellow Sea.（4） For the YSH type of fog, water vapor comes mainly from local evaporation with a well-defined dry layer present in the lower atmosphere; for the CAC type of fog, however, water vapor comes mainly from areas outside the Yellow Sea with a thick surface layer of high humidity.（5） With the differences in weather patterns and its associated vertical distribution of the humidity and the transportation of water vapor, there are two types of sea fogs. Most fogs of the CAC types are ＂warm＂ fog, while fogs of YSH type have nearly equal chance to be ＂warm＂ and ＂cold＂ fog.

Low-Level Temperature Inversions and Their Effect on Aerosol Condensation Nuclei Concentrations under Different Large-Scale Synoptic Circulations

Knowledge of the statistical characteristics of inversions and their effects on aerosols under different large-scale synoptic circulations is important for studying and modeling the diffusion of pollutants in the boundary layer. Based on results gen- erated using the self-organizing map （SOM） weather classification method, this study compares the statistical characteristics of surface-based inversions （SBIs） and elevated inversions （EIs）, and quantitatively evaluates the effect of SBIs on aerosol condensation nuclei （CN） concentrations and the relationship between temperature gradients and aerosols for six prevailing synoptic patterns over the the Southern Great Plains （SGP） site during 2001-10. Large-scale synoptic patterns strongly influ- ence the statistical characteristics of inversions and the accumulation of aerosols in the low-level atmosphere. The activity, frequency, intensity, and vertical distribution of inversions are significantly different among these synoptic patterns. The verti- cal distribution of inversions varies diurnally and is significantly different among the different synoptic patterns. Anticyclonic patterns affect the accumulation of aerosols near the ground more strongly than cyclonic patterns. Mean aerosol CN con- centrations increase during SBIs compared to no inversion cases by 16.1%, 22.6%, 24.5%, 58.7%, 29.8% and 23.7% for the six synoptic patterns. This study confirms that there is a positive correlation between temperature gradients and aerosol CN concentrations near the ground at night under similar large-scale synoptic patterns. The relationship is different for different synoptic patterns and can be described by linear functions. These findings suggest that large-scale synoptic patterns change the static stability of the atmosphere and inversions in the lower atmosphere, thereby influencing the diffusion of aerosols near the ground.

Evaluation of meteorological predictions by the WRF model at Barrow, Alaska and Summit, Greenland in the Arctic in April 2019

Accurate meteorological predictions in the Arctic are important in response to the rapid climate change and insufficient meteorological observations in the Arctic.In this study,we adopted a high-resolution Weather Research and Forecasting(WRF)model to simulate the meteorology at two Arctic stations(Barrow and Summit)in April 2019.Simulation results were also evaluated by using surface measurements and statistical parameters.In addition,weather charts during the studied time period were also used to assess the model performance.The results demonstrate that the WRF model is able to accurately capture the meteorological parameters for the two Arctic stations and the weather systems such as cyclones and anticyclones in the Arctic.Moreover,we found the model performance in predicting the surface pressure the best while the performance in predicting the wind the worst among these meteorological predictions.However,the wind predictions at these Arctic stations were found to be more accurate than those at urban stations in mid-latitude regions,due to the differences in land features and anthropogentic heat sources between these regions.In addition,a comparison of the simulation results showed that the prediction of meteorological conditions at Summit is superior to that at Barrow.Possible reasons for the deviations in temperature predictions between these two Arctic stations are uncertainties in the treatments of the sea ice and the cloud in the model.With respect to the wind,the deviations may source from the overestimation of the wind over the sea and at coastal stations.

Circulation-Type Classification over Mexico by the Era-Interim Database and Cost733

In this paper an approach of a synoptic classification by cluster k-means (CKM) and the European Gro&#223wettertypes (GWT) was performed and executed in COST733 package. The methodology used a CKM with nine clusters and GWT with 16. The COST733 evaluated a dataset of 30-years since 1986 to 2015. The variables selected were mean sea level pressure (mslp), geopotential height (z500 and z850), wind speed and direction (u10, v10 and u, v at 850 hPa) and relative vorticity (vo) with a 0.75° × 0.75° resolution of the data grid at 00:00, 06:00, 12:00 and 18:00 UTC and 0 steps. These results were evaluated using COST733 to find the quality of measurements by the explained variance (EV) or reduction ratio in error and pseudo F value (PF) to determine the certainty of the results. GWT-16 showed better yearly values in the evaluation with 32.7 (EV) and 354.3 (PF) against the CKM-9 of 54.2 (EV) and 1621.8 (PF). Finally, it was concluded that GWT-16 could be used for classification of synoptic systems over Mexico and the analysis of meteorological phenomena triggers on increases or decreases of atmospheric pollution in areas over Mexico.

Characteristics of Pre-summer Daytime Cloud Regimes over Coastal South China from the Himawari-8 Satellite

Using the high spatiotemporal resolution(2 km-and-10 min)data from the Advanced Himawari Imager onboard the Himawari-8 satellite,this study documents the fine-scale characteristics of daytime cloud regimes(CRs)over coastal South China during the pre-summer rainy season(April–June).Six CRs(CR1–CR6)are identified based on the joint frequency distribution of cloud top brightness temperature and cloud optical thickness,namely,the optically thin-to-moderate cloud mixture,optically thin warm clouds with cirrus,optically thick warm clouds,weak convective cloud mixture,strong convective clouds,and extreme,deep convective clouds.The optically thick warm clouds are the major CR during April and May,with higher frequencies over land,especially along the urban agglomeration,rather than the offshore which may be an indicator of the higher aerosol concentrations being a contributing factor over the cities.The CRs with weak convective cloud mixtures and strong convective clouds appear more frequently over the land,while the two CRs with optically thinner clouds occur mainly offshore.Synoptic flow patterns(SPs)are objectively identified and examined focusing on those favoring the two major rainproducing CRs(CR5 and CR6)and the highly reflective CR with optically thick warm clouds(CR3).The two SPs favoring CR5 and CR6 are characterized by abundant moisture with low-level jets after monsoon onset,and a northwest highsoutheast low pattern with strong dynamic convergence along the coastline,respectively.The non-convective CR3 with high reflectance is related to a SP that features the western North Pacific subtropical high extending more westward,leading to a moderate moisture supply and a wide range of convective available potential energy,but also,large convective inhibition.

Meteorological mechanism for a large-scale persistent severe ozone pollution event over eastern China in 2017

An intensive and persistent regional ozone pollution event occurred over eastern China from 25 June to 5 July 2017.73 out of 96 selected cities,most located in the Beijing-TianjinHebei and the surrounding area(BTHS),suffered severe ozone pollution.A north-south contrast ozone distribution,with higher ozone(199±33μg/m3)in the BTHS and lower ozone(118±25μg/m^3)in the Yangtze River Delta(YRD),was found to be dominated by the position of the West Pacific Subtropical High(WPSH)and mid-high latitude wave activities.In the BTHS,the positive anomalies of geopotential height at 500 hPa and temperature at the surface indicated favorable meteorological conditions for local ozone formation.Prevailing northwesterly winds in the mid-high troposphere and warm advection induced by weak southerly winds in the low troposphere resulted in low-moderate relative humidity(RH),less total cloud cover(TGC),strong solar radiation and high temperatures.Moreover,southerly winds prevailing over the BTHS aggravated the pollution due to regional transportation of O3 and its precursors.On one hand,the deep sinking motion and inversion layer suppressed the dispersion of pollutants.On the other hand,O3-rich air in the upper layer was maintained at night due to temperature inversion,which facilitated O3 vertical transport to the surface in the next-day morning due to elevated convection.Generally,temperature,UV radiation,and RH showed good correlations with O3 in the BTHS,with rates of 8.51(μg/m^3)/℃(within the temperature range of 20-38℃),59.54(μg/m^3)/(MJ/m^2)and-1.93(μg/m^3)/%,respectively.

Differences in Meteorological Conditions between Days with Persistent and Non-Persistent Pollution in Beijing, China

We compared the regional synoptic patterns and local meteorological conditions during persistent and non-persistent pollution events in Beijing using US NCEP–Department of Energy reanalysis outputs and observations from meteorological stations. The analysis focused on the impacts of high-frequency（period 〈 90 days） variations in meteorological conditions on persistent pollution events（those lasting for at least 3 days）. Persistent pollution events tended to occur in association with slow-moving weather systems producing stagnant weather conditions, whereas rapidly moving weather systems caused a dramatic change in the local weather conditions so that the pollution event was short-lived. Although Beijing was under the influence of anomalous southerly winds in all four seasons during pollution events, notable differences were identified in the regional patterns of sea-level pressure and local anomalies in relative humidity among persistent pollution events in different seasons. A region of lower pressure was present to the north of Beijing in spring, fall, and winter, whereas regions of lower and higher pressures were observed northwest and southeast of Beijing, respectively, in summer. The relative humidity near Beijing was higher in fall and winter, but lower in spring and summer. These differences may explain the seasonal dependence of the relationship between air pollution and the local meteorological variables. Our analysis showed that the temperature inversion in the lower troposphere played an important part in the occurrence of air pollution under stagnant weather conditions.Some results from this study are based on a limited number of events and thus require validation using more data.