Measurement and Meteorological Analysis of 7Be and 210pb in Aerosol at Waliguan Observatory

Beryllium-7 （^7Be） and lead-210 （^210pb） activities were measured from October 2002 to January 2004 at Waliguan Observatory （WO： 36.287°N, 100.898°E, 3816 m a.s.l （above sea level） in northwest China. ^7Be and ^210pb activities are high with overall averages of 14.74-3.5 mBq m^-3 and 1.8±0.8 mBq m^-3 respectively. For both 7Be and ^210pb, there are significant short-term and seasonal variations with a commonly low value in summer （May-September） and a monthly maximum in April （for ^7Be） and in December （for ^210pb）. The ratio of ^7Be/^210pb showed a broad maximum extending from April to July, coinciding with a seasonal peak in surface ozone （O3）. The seasonal cycles of ^7Be and ^210pb activities were greatly influenced by precipitation and thermal dynamical conditions over the boundary layer, especially for ^210Pb. The vertical mixing process between the boundary layer and the aloft air modulates the variations of ^7Be and ^210pb at WO in summer. It is indicated that air mass had longer residence time and originated from higher altitudes at WO in the spring-summer time and the winter in 2003. During an event with extremely high weeklyaveraged ^7Be concentration （24.8 mBq m^-3） together with high O3 levels and low water mixing ratio, we found that air masses had been convectively transported a long distance to WO from high latitude source regions in central Asia, where significant subsiding motions were observed. In another case with the extreme ^210pb activity of 5.7 mBq m^-3 high CO2 level and specific humidity （in winter）, air masses had come from south China and north Indian regions where 222Rn activities were high. This study, using ^7Be and ^210pb as atmospheric tracers, has revealed that complex interactions of convective mixing from the upper troposphere and long-range transports exist at WO.

Effect of Different Microphysical Parameterizations on the Simulations of a South China Heavy Rainfall

A heavy rainfall event in south China was simulated by the Weather Research and Forecasting(WRF) model with three microphysics schemes, including the Morrison scheme, Thompson scheme, and Milbrandt and Yau scheme(MY), which aim to evaluate the capability to reproduce the precipitation and radar echo reflectivity features, and to evaluate evaluate their differences in microphysics and the associated thermodynamical and dynamical feedback. Results show that all simulations reproduce the main features crucial for rainfall formation. Compared with the observation, the MY scheme performed better than the other two schemes in terms of intensity and spatial distribution of rainfall. Due to abundant water vapor, the accretion of cloud droplets by raindrops was the dominant process in the growth of raindrops while the contribution of melting was a secondary effect. Riming processes, in which frozen hydrometeors collect cloud droplets mainly, contributed more to the growth of frozen hydrometeors than the Bergeron process. Extremely abundant snow and ice were produced in the Thompson and MY schemes respectively by a deposition process. The MY scheme has the highest condensation and evaporation, but the lowest deposition. As a result, in the MY scheme, the enhanced vertical gradient of condensation heating and evaporation cooling at low levels produces strong positive and weak negative potential vorticity in Guangdong, and may favor the formation of the enhanced rainfall center over there.

Effect of the Initial Vortex Structure on Intensity Change During Eyewall Replacement Cycle of Tropical Cyclones:A Numerical Study

This study investigates the effect of the initial tropical cyclone(TC)vortex structure on the intensity change during the eyewall replacement cycle(ERC)of TCs based on two idealized simulations using the Weather Research and Forecasting(WRF)model.Results show that an initially smaller TC with weaker outer winds experienced a much more drastic intensity change during the ERC than an initially larger TC with stronger outer winds.It is found that an initially larger TC vortex with stronger outer winds favored the development of more active spiral rainbands outside the outer eyewall,which slowed down the contraction and intensification of the outer eyewall and thus prolonged the duration of the concentric eyewall and slow intensity evolution.In contrast,the initially smaller TC with weaker outer winds corresponded to higher inertial stability in the inner core and weaker inertial stability but stronger filamentation outside the outer eyewall.These led to stronger boundary layer inflow,stronger updraft and convection in the outer eyewall,and suppressed convective activity outside the outer eyewall.These resulted in the rapid weakening during the formation of the outer eyewall,followed by a rapid re-intensification of the TC during the ERC.Our study demonstrates that accurate initialization of the TC structure in numerical models is crucial for predicting changes in TC intensity during the ERC.Additionally,monitoring the activity of spiral rainbands outside the outer eyewall can help to improve short-term intensity forecasts for TCs experiencing ERCs.

Large contributions of emission reductions and meteorological conditions to the abatement of PM_(2.5)in Beijing during the 24th Winter Olympic Games in 2022

To guarantee the blue skies for the 2022 Winter Olympics held in Beijing and Zhangjiakou from February 4 to 20,Beijing and its surrounding areas adopted a series of emission control measures.This provides an opportunity to determine the impacts of large-scale temporary control measures on the air quality in Beijing during this special period.Here,we applied the WRF-CMAQ model to quantify the contributions of emission reduction measures and meteorological conditions.Results show that meteorological conditions in 2022 decreased PM_(2.5)in Beijing by 6.9 and 11.8μg/m^(3)relative to 2021 under the scenarios with and without emission reductions,respectively.Strict emission reduction measures implemented in Beijing and seven neighboring provinces resulted in an average decrease of 13.0μg/m^(3)(-41.2%)in PM_(2.5)in Beijing.Over the entire period,local emission reductions contributed more to good air quality in Beijing than nonlocal emission reductions.Under the emission reduction scenario,local,controlled regions,other regions,and boundary conditions contributed 47.7%,42.0%,5.3%,and 5.0%to the PM_(2.5)concentrations in Beijing,respectively.The results indicate that during the cleaning period with the air masses from the northwest,the abatements of PM_(2.5)were mainly caused by local emission reductions.However,during the potential pollution period with the air masses from the east-northeast and west-southwest,the abatements of PM_(2.5)were caused by both local and nonlocal emission reductions almost equally.This implies that regional coordinated prevention and control strategies need to be arranged scientifically and rationally when heavy pollution events are forecasted.

Errors and Correction of Precipitation Measurements in China

In order to discover the range of various errors in Chinese precipitation measurements and seek a correction method, 30 precipitation evaluation stations were set up countrywide before 1993. All the stations are reference stations in China. To seek a correction method for wind-induced error, a precipitation correction instrument called the ＂horizontal precipitation gauge＂ was devised beforehand. Field intercomparison observations regarding 29,000 precipitation events have been conducted using one pit gauge, two elevated operational gauges and one horizontal gauge at the above 30 stations. The range of precipitation measurement errors in China is obtained by analysis of intercomparison measurement results. The distribution of random errors and systematic errors in precipitation measurements are studied in this paper. A correction method, especially for wind-induced errors, is developed. The results prove that a correlation of power function exists between the precipitation amount caught by the horizontal gauge and the absolute difference of observations implemented by the operational gauge and pit gauge. The correlation coefficient is 0.99. For operational observations, precipitation correction can be carried out only by parallel observation with a horizontal precipitation gauge. The precipitation accuracy after correction approaches that of the pit gauge. The correction method developed is simple and feasible.

Seasonal and spatial distributions of phytoplankton biomass associated with monsoon and oceanic environments in the South China Sea

Seasonal variations of phytoplankton/chlorophyll-a （Chl-a） distribution, sea surface wind, sea height anomaly, sea surface temperature and other oceanic environments for long periods are analyzed in the South China Sea （SCS）, especially in the two typical regions off the east coast of Vietnam and off the northwest coast of Luzon, using remote sensing data and other oceanographic data. The results show that seasonal and spatial distributions of phytoplankton biomass in the SCS are primarily influenced by the monsoon winds and oceanic environments. Off the east coast of Vietnam, Chl-a concentration is a peak in August, a jet shape extending into the interior SCS, which is associated with strong southwesterly monsoon winds, the coastal upwetling induced by offshore Ekman transport and the strong offshore current in the western SCS. In December, high Chl-a concentration appears in the upwelling region off the northwest coast of Luzon and spreads southwestward. Strong mixing by the strong northeasterly monsoon winds, the cyclonic circulation, southwestward coastal currents and river discharge have impacts on distribution of phytoplankton, so that the high phytoplankton biomass extends from the coastal areas over the northern SCS to the entire SCS in winter. These research activities could be important for revealing spatial and temporal patterns of phytoplankton and their interactions with physical environments in the SCS.

Multitime scale variations of sea surface temperature in the China seas based on the HadISST dataset

The variability of the sea surface temperature（SST） in the China seas has been studied in seasonal,interannual and interdecadal scales based on the monthly data of HadISST spanning from 1870 to 2007. The main results obtained are SST in the China offshore changes most actively at the seasonal scale with the intensity diminishing from north to south,as the temperature differences between summer and winter reaching 17 and 4 C in the northern and southern areas,respectively. Moreover,seasonal variation near the coastal regions seems relatively stronger than that far from the coastline;significant interannual variations are detected,with the largest positive anomaly occurring in 1998 in the overall area. But as far as different domains are concerned,there exists great diversity,and the difference is also found between winter and summer. Differed from the seasonal variations,where the strongest interannual variability takes place,resides to the south of that of the seasonal ones in the northern section,nevertheless in the South China Sea,the most significant interannual variability is found in the deep basin;interdecadal changes of summer,winter and annual mean SST in different domains likewise present various features. In addition,a common dominant warming in recent 20 a are found in the overall China offshore with the strongest center located in the vicinity of the Changjiang Estuary in the East China Sea,which intensifies as high as 1.3 C during the past 130 a.

Research on the Relationship of ENSO and the Frequency of Extreme Precipitation Events in China

Based on a daily precipitation observation dataset of 743 stations in China from 1951 2004, the F distribution function is used to calculate the probability distribution of daily precipitation and to define extreme precipitation events. Based on this, the relationship of ENSO and the frequency of extreme precipitation events is studied. Results reveal that ENSO events have impact on extreme precipitation events, with different magnitudes at different regions and seasons. In general, during winter and spring, extreme precipitation events occur more often during E1 Nino events than during La Nina events. While during summer and autumn, the opposite is found. The relationship of a two season-lag ENSO and extreme precipitation frequency shows different pattern. Extreme precipitation events occur more often in several regions if an ENSO warm phase happened in the central-eastern tropical Pacific two seasons before. No similar impacts of El Nino and La Nina on the frequency of extreme precipitation events are found.

Changes in the Diurnal Cycles of Precipitation over Eastern China in the Past 40 Years

This study analyzed the interdecadal changes in the diurnal variability of summer （June-August） precipitation over eastern China during the period 1966 2005 using hourly station rain gauge data. The results revealed that rainfall diurnal variations experienced significant interdecadal changes. Over the area to the south of the Yangtze River, as well as the area between the Yangtze and Yellow Rivers, the percentages of morning rainfall （0000 1200 LST） to total rainfall in terms of amount, frequency and intensity, all exhibited increasing interdecadal trends. On the contrary, over North China, decreasing trends were found. As a result, diurnal rainfall peaks also presented pronounced interdecadal variations. Over the area between the Yangtze and Yellow Rivers, there were 16 out of 46 stations with afternoon （1200-0000 LST） frequency peaks in the first 20 years of the 40-year period of study, while only eight remained in the latter 20 years. In North China, seven stations experienced the opposite changes, which accounted for about 21% of the total number of stations. The possible causes for the interdecadal changes in diurnal features were discussed. As the rainfall in the active monsoon period presents morning diurnal peaks, with afternoon peaks in the break period, the decrease （increase） of rainfall in the active monsoon period over North China （the area south of the Yangtze River and the area between the Yangtze and Yellow Rivers） may contribute to interdecadal changes in diurnal rainfall variability.

Probability Distribution and Projected Trends of Daily Precipitation in China

Based on observed daily precipitation data of 540 stations and 3,839 gridded data from the high-resolution regional climate model COSMO-Climate Limited-area Modeling(CCLM)for 1961–2000,the simulation ability of CCLM on daily precipitation in China is examined,and the variation of daily precipitation distribution pattern is revealed.By applying the probability distribution and extreme value theory to the projected daily precipitation(2011–2050)under SRES A1B scenario with CCLM,trends of daily precipitation series and daily precipitation extremes are analyzed.Results show that except for the western Qinghai-Tibetan Plateau and South China,distribution patterns of the kurtosis and skewness calculated from the simulated and observed series are consistent with each other;their spatial correlation coefcients are above 0.75.The CCLM can well capture the distribution characteristics of daily precipitation over China.It is projected that in some parts of the Jianghuai region,central-eastern Northeast China and Inner Mongolia,the kurtosis and skewness will increase significantly,and precipitation extremes will increase during 2011–2050.The projected increase of maximum daily rainfall and longest non-precipitation period during flood season in the aforementioned regions,also show increasing trends of droughts and floods in the next 40 years.

A three-dimensional coupled physical-biological model study in the spring of 1993 in the Bohai Sea of China

A three-dimensional （3-D） coupled physical and biological model was used to investigate the physical processes and their influence on the ecosystem dynamics of the Bohai Sea of China. The physical processes include M2 tide, time - varying wind forcing and river discharge. Wind records from 1 to 31 May in 1993 were selected to force the model. The biological model is based on a simple, nitrate and phosphate limited, lower trophic food web system. The simulated results showed that variation of residual currents forced by M2 tide, fiver discharge and time-varying wind had great impact on the distribution of phytoplankton biomass in the Laizhou Bay. High phytoplankton biomass appeared in the upwelling region. Numerical experiments based on the barotropic model and baroclinic model with no wind and water discharge were also conducted. Differences in the results by the baroclinic model and the barotropic model were significant： more patches appeared in the baroclinic model comparing with the barotropic model. And in the baroclinic model, the subsurface maximum phytoplankton biomass patches formed in the stratified water.

Projection of PM_(2.5) and Ozone Concentration Changes over the Jing-Jin-Ji Region in China

Based on GISS-E2-R model simulations, the changes in PM2.5 and ozone concentrations during 2016– 35 are analyzed over the Jing-Jin-Ji region under different future emissions scenarios: 2.6, 4.5, 6.0, 8.5 Representative Concentration Pathways scenarios(RCP2.6, RCP4.5, RCP6.0, and RCP8.5), compared to the baseline periods of 1851–70(pre-industrial) and 1986–2005(present day). The results show that PM2.5 increases under all emissions scenarios, with the maximum value occurring in the southeastern part of the region under most scenarios. As for ozone, its concentration is projected to increase during 2016–35 under all emissions scenarios, compared to the baseline periods. The temporal evolutions of PM2.5 and ozone show PM2.5 reaching a peak during 2020–40, while ozone will likely increase steadily in the future.

Bias Analysis in the Simulation of the Western North Pacific Tropical Cyclone Characteristics by Two High-Resolution Global Atmospheric Models

This study compares the atmosphere-only HighResMIP simulations from FGOALS-f3-H(FGOALS)and MRIAGCM3-2-S(MRI)with respect to tropical cyclone(TC)characteristics over the Western North Pacific(WNP)for the July-October months of 1985-2014.The focus is on investigating the role of the tropical easterly jet over the Western Pacific(WP_TEJ)in modulating the simulation biases in terms of their climatological distribution and interannual variability of WNP TC genesis frequency(TCGF)based on the analysis of the genesis potential index(GPI).Results show that the two models reasonably capture the main TC genesis location,the maximum center of frequency,and track density;however,their biases mainly lie in simulating the intense TCs and TCGF distributions.The MRI better simulates the windpressure relationship(WPR)but overestimates the proportion of super typhoons(SSTYs).At the same time,FGOALS underestimates the WPR and the proportion of SSTYs but better simulates the total WNP TC precipitation.In particular,FGOALS overestimates the TCGF in the northeastern WNP,which is strongly tied to an overestimated WP_TEJ and the enhanced vertical circulation to the north of its entrance region.In contrast,the MRI simulates a weaker WP_TEJ and vertical circulation,leading to a negative TCGF bias in most of the WNP.Both models exhibit comparable capability in simulating the interannual variability of WP_TEJ intensity,but the composite difference of large-scale atmospheric factors between strong and weak WP_TEJ years is overestimated,resulting in larger interannual anomalies of WNP TCGF,especially for FGOALS.Therefore,accurate simulations of the WP_TEJ and the associated oceanic and atmospheric factors are crucial to further improving WNP TC simulations for both models.

Estimation of Drought Change Trends in Northeast China in 2020-2050 and Its Impact on Maize Yield

Based on the results of nine Coupled Model Intercomparison Project Phase 5 (CMIP5) coupling models, the temperature and precipitation data of 114 stations in Northeast China were compared and analyzed. The simulation effect of CMIP5 model on precipitation and temperature in Northeast China was evaluated. The research shows that the Geophysical Fluid Dynamics Laboratory Earth System Models (GGFDL-ESM2G) have the best simulation effect on precipitation and temperature in Northeast China. Based on the SPEI index, the relationship between the drought trend of maize growing season and the yield change rate of maize in Northeast China was analyzed, and the future drought (2020-2050) and corn yield in Northeast China were estimated. The cumulative Standardized Precipitation Evapotranspiration Index (SPEI) analysis of the northeast maize growing season (May-September) shows that the drought in the northeastern region showed an intensifying trend from 1980 to 2010, especially in the first ten years of the 21st century. The cumulative SPEI index has a significant positive correlation with the yield of maize in Northeast China, and has a certain indicator effect on the yield of maize in Northeast China. The three scenarios of GFDL-ESM2G model show that under the three scenarios of Representative Concentration Pathways (RCP), the warming in Northeast China is significant;under the RCP4.5 scenario, the precipitation in Northeast China is increasing;in the RCP2.6 and RCP8.5 climate scenarios, precipitation is presented and reduces the trend of drought. Estimates of drought trends in Northeast China show that under the RCP4.5 climate scenario, the drought in Northeast China showed a slowing trend from 2020 to 2050. Under the RCP2.6 and RCP8.5 climate scenarios, the drought in Northeast China showed an increasing trend. Under the RCP2.6 and RCP8.5 climate scenarios, the yield change rate of maize in Northeast China showed a downward trend, indicating that climate warming caused the drought in Northeast China to increase, which had a negative impact on corn yield increase. In severe drought years, drought may cause northeast corn production seriously reduced. However, under the RCP4.5 scenario, drought has little effect on corn yield.

Spatial Inhomogeneity of Atmospheric CO_(2) Concentration and Its Uncertainty in CMIP6 Earth System Models

This paper provides a systematic evaluation of the ability of 12 Earth System Models(ESMs)participating in the Coupled Model Intercomparison Project Phase 6(CMIP6)to simulate the spatial inhomogeneity of the atmospheric carbon dioxide(CO_(2))concentration.The multi-model ensemble mean(MME)can reasonably simulate the increasing trend of CO_(2) concentration from 1850 to 2014,compared with the observation data from the Scripps CO_(2) Program and CMIP6 prescribed data,and improves upon the CMIP5 MME CO_(2) concentration(which is overestimated after 1950).The growth rate of CO_(2) concentration in the northern hemisphere(NH)is higher than that in the southern hemisphere(SH),with the highest growth rate in the mid-latitudes of the NH.The MME can also reasonably simulate the seasonal amplitude of CO_(2) concentration,which is larger in the NH than in the SH and grows in amplitude after the 1950s(especially in the NH).Although the results of the MME are reasonable,there is a large spread among ESMs,and the difference between the ESMs increases with time.The MME results show that regions with relatively large CO_(2) concentrations(such as northern Russia,eastern China,Southeast Asia,the eastern United States,northern South America,and southern Africa)have greater seasonal variability and also exhibit a larger inter-model spread.Compared with CMIP5,the CMIP6 MME simulates an average spatial distribution of CO_(2) concentration that is much closer to the site observations,but the CMIP6-inter-model spread is larger.The inter-model differences of the annual means and seasonal cycles of atmospheric CO_(2) concentration are both attributed to the differences in natural sources and sinks of CO_(2) between the simulations.

A 10-yr Rainfall and Cloud-to-Ground Lightning Climatology over Coastal and Inland Regions of Guangdong,China during the Pre-Summer Rainy Season

A comparative analysis of the spatiotemporal distribution characteristics of rainfall and lightning in coastal and inland areas of Guangdong Province of China during the pre-summer rainy season(PSRS)from 2008 to 2017 reveals distinct patterns.In the inland target region(ITR),rainfall is concentrated in the central and eastern mountainous areas.It exhibits a bimodal diurnal variation,with peaks in the afternoon and morning.The afternoon peak becomes more pronounced during the post-monsoon-onset period because of the increased rainfall frequency.Similarly,in the coastal target region(CTR),rainfall concentrates around mountainous peripheries.However,CTR’s rainfall is weaker than ITR’s during the pre-monsoon-onset period,primarily associated with the lower-level moisture outflow in CTR,but it strengthens significantly during the post-monsoon-onset period owing to enhanced moisture inflow.CTR’s diurnal rainfall variation transitions from bimodal to a single broad peak during the post-monsoon-onset period,influenced by changes in both rainfall frequency and intensity.In contrast to rainfall,the spatiotemporal distribution of lightning centers remains relatively stable during the PSRS.The strongest center is located over ITR’s plains west of the rainfall center,with a secondary center in the western plains of CTR.Lightning activity significantly increases during the post-monsoon-onset period,particularly in ITR,primarily because of the increased lightning hours.The diurnal lightning flash density and lightning hours show a single afternoon peak in the two target regions,and the timing of the peak in ITR is approximately two hours later than in CTR.Composite circulation analysis indicates that during early morning,the lower atmosphere is nearly neutral in stratification.The advected warm,moist,unstable airflow,combined with topography,favors convection initiation.In the afternoon,solar radiation increases thermal instability,further enhancing the convection frequency and intensity.Improved moisture and thermal conditions contribute to an increase in rainfall and lightning during the post-monsoon-onset period.Moreover,the occurrence of lightning is found to be closely linked to the most unstable convective available potential energy,low-level vertical wind shear,and updraft intensity.

Anomalies of Precipitation over China on Days with Tropical Cyclone Activity over the Bay of Bengal: Role of Moisture Transport

Tropical cyclones over the Bay of Bengal(BoBTC)affect the precipitation over China,with distinct seasonal and daily variabilities.This study quantitatively examines the daily standardized precipitation anomalies(SPAs)over China on the days with BoBTC activities(storm-days)and related circulations,based on rainfall measurements at surface meteorological stations and ECMWF reanalysis data on a 0.25°×0.25°resolution during 1979-2019.Significant positive SPA is found over the stations in the two adjacent regions around BoB(Southwest China in May/November and southern Tibetan Plateau in October)and three distant regions(Southeast China and the northeastern boundary of the Qinghai-Tibet Plateau in May,and central North China in October).The SPA distributions are remarkably consistent with the integrated water vapor transport(IVT)anomalies.Enhanced IVT is found associated with the interaction between southwesterly(southerly)of the BoBTC circulation and low-level monsoonal flow in May(midlevel westerly in winter months).The probabilities of extreme precipitation(EP)occurrences over the above regions all increase on storm-days.For adjacent regions,EP is significantly correlated with the northward IVT anomalies to the east of BoBTC circulation,which strengthen the water vapor input through the southern border.Such IVT anomalies are stronger in May,benefited by the deep monsoonal southwesterlies than those in November.For distant regions,EP is more closely related to the IVT anomaly extending back from BoB.Enhanced moisture from BoB concentrates along a local low-level convergence line over Southeast China,being further facilitated by coexistence of the BoBTC depression and midlevel westerly trough in midlatitudes.Our results highlight the interactions between BoBTCs and local weather systems that influence the general precipitation anomalies and occurrence of EP over China,especially over distant regions.

A Long-Time-Step-Permitting Tracer Transport Model on the Regular Latitude–Longitude Grid

If an explicit time scheme is used in a numerical model, the size of the integration time step is typically limited by the spatial resolution. This study develops a regular latitude–longitude grid-based global three-dimensional tracer transport model that is computationally stable at large time-step sizes. The tracer model employs a finite-volume flux-form semiLagrangian transport scheme in the horizontal and an adaptively implicit algorithm in the vertical. The horizontal and vertical solvers are coupled via a straightforward operator-splitting technique. Both the finite-volume scheme's onedimensional slope-limiter and the adaptively implicit vertical solver's first-order upwind scheme enforce monotonicity. The tracer model permits a large time-step size and is inherently conservative and monotonic. Idealized advection test cases demonstrate that the three-dimensional transport model performs very well in terms of accuracy, stability, and efficiency. It is possible to use this robust transport model in a global atmospheric dynamical core.

A Deep-Learning and Transfer-Learning Hybrid Aerosol Retrieval Algorithm for FY4-AGRI:Development and Verification over Asia

The Advanced Geosynchronous Radiation Imager(AGRI)is a mission-critical instrument for the Fengyun series of satellites.AGRI acquires full-disk images every 15 min and views East Asia every 5 min through 14 spectral bands,enabling the detection of highly variable aerosol optical depth(AOD).Quantitative retrieval of AOD has hitherto been challenging,especially over land.In this study,an AOD retrieval algorithm is proposed that combines deep learning and transfer learning.The algorithm uses core concepts from both the Dark Target(DT)and Deep Blue(DB)algorithms to select features for the machinelearning(ML)algorithm,allowing for AOD retrieval at 550 nm over both dark and bright surfaces.The algorithm consists of two steps:①A baseline deep neural network(DNN)with skip connections is developed using 10 min Advanced Himawari Imager(AHI)AODs as the target variable,and②sunphotometer AODs from 89 ground-based stations are used to fine-tune the DNN parameters.Out-of-station validation shows that the retrieved AOD attains high accuracy,characterized by a coefficient of determination(R2)of 0.70,a mean bias error(MBE)of 0.03,and a percentage of data within the expected error(EE)of 70.7%.A sensitivity study reveals that the top-of-atmosphere reflectance at 650 and 470 nm,as well as the surface reflectance at 650 nm,are the two largest sources of uncertainty impacting the retrieval.In a case study of monitoring an extreme aerosol event,the AGRI AOD is found to be able to capture the detailed temporal evolution of the event.This work demonstrates the superiority of the transfer-learning technique in satellite AOD retrievals and the applicability of the retrieved AGRI AOD in monitoring extreme pollution events.

Analysis of Boundary Layer Structure,Turbulence,and Flux Variations before and after the Passage of a Sea Breeze Front Using Meteorological Tower Data

A detailed analysis of a sea breeze front(SBF)that penetrated inland in the Beijing–Tianjin–Hebei urban agglomeration of China was conducted.We focused on the boundary layer structure,turbulence intensity,and fluxes before and after the SBF passed through two meteorological towers in the urban areas of Tianjin and Beijing,respectively.Significant changes in temperature,humidity,winds,CO_(2),and aerosol concentrations were observed as the SBF passed.Differences in these changes at the two towers mainly resulted from their distances from the ocean,boundary layer conditions,and background turbulences.As the SBF approached,a strong updraft appeared in the boundary layer,carrying near-surface aerosols aloft and forming the SBF head.This was followed by a broad downdraft,which destroyed the near-surface inversion layer and temporarily increased the surface air temperature at night.The feeder flow after the thermodynamic front was characterized by low-level jets horizontally,and downdrafts and occasional updrafts vertically.Turbulence increased significantly during the SBF’s passage,causing an increase in the standard deviation of wind components in speed.The increase in turbulence was more pronounced in a stable boundary layer compared to that in a convective boundary layer.The passage of the SBF generated more mechanical turbulences,as indicated by increased friction velocity and turbulent kinetic energy(TKE).The shear term in the TKE budget equation increased more significantly than the buoyancy term.The atmosphere shifted to a forced convective state after the SBF’s passage,with near isotropic turbulences and uniform mixing and diffusion of aerosols.Sensible heat fluxes(latent heat and CO_(2)fluxes)showed positive(negative)peaks after the SBF’s passage,primarily caused by horizontal and vertical transport of heat(water vapor and CO_(2))during its passage.This study enhances understanding of boundary layer changes,turbulences,and fluxes during the passage of SBFs over urban areas.