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This paper proposes a ResGA-Net algorithm for cloud top height (CTH) retrieval using FY-4A satellite data. The algorithm utilizes genetic algorithms for data selection and employs a residual module-based neural network for modeling. It takes the spectral channel data from the FY-4A satellite as input features and uses CTH extracted from ground-based millimeter-wave cloud radar reflectivity as the target. By combining the large observation scale of the FY-4A satellite and the high accuracy of ground-based cloud radar observations, the model can generate satellite CTH products with higher precision. To validate the effectiveness of the algorithm, experiments were conducted using data from the Beijing area spanning from January 2020 to January 2022. The experimental results show that the metrics of the proposed ResGA-Net outperform those of various contrastive algorithms, and compared to the original FY-4A CTH product, the RMSE and MAE have decreased by 37.89% and 34.77%, while the PCC and SRCC have increased by 11.17% and 9.47%, respectively, demonstrating the superiority of the proposed method presented in this paper. [ABSTRACT FROM AUTHOR]

This paper compares the wind fields measured by the meteor radar at Mohe, Beijing, Wuhan, and Sanya stations and horizontal wind model (HWM14) predictions. HWM14 appears to successfully reproduce the height-time distribution of the monthly mean zonal winds, although large discrepancies occur in wind speed between the model and measurement, especially in the summer and winter months. For meridional wind, the consistency between model prediction and radar observation is worse than that of zonal wind. The consistency between radar measurements and model prediction at Sanya station is worse than other sites located at higher latitudes. Harmonic analysis reveals large discrepancies in diurnal, semidiurnal, and terdiurnal tides extracted from meteor radar observations and HWM14 predictions. [ABSTRACT FROM AUTHOR]

The values of ratio a of the linear relationship between specific attenuation and specific differential phase vary significantly in convective storms as a result of resonance scattering. The best-linear-fit ratio a at X band is determined using the modified attenuation correction algorithm based on differential phase and attenuation, as well as the premise that reflectivity is unattenuated in S band radar detection. Meanwhile, the systemic reflectivity bias between the X band radar and S band radar and water layer attenuation (ZW) on the wet antenna cover of the X band radar are also considered. The good performance of the modified correction algorithm is demonstrated in a moderate rainfall event. The data were collected by four X band dual-polarization (X-POL) radar sites, namely, BJXCP, BJXFS, BJXSY, and BJXTZ, and a China's New Generation Weather Radar (CINRAD/SA radar) site, BJSDX, in Beijing on 20 July 2016. Ratio a is calculated for each volume scan of the X band radar, with a mean value of 0.26 dB deg−1 varying from 0.20 to 0.31 dB deg−1. The average values of systemic reflectivity bias between the X band radar (at BJXCP, BJXFS, BJXSY, and BJXTZ) and S band radar (at BJSDX) are 0, −3, 2, and 0 dB, respectively. The experimentally determined ZW is in substantial agreement with the theoretically calculated ones, and their values are an order of magnitude smaller than rain attenuation. The comparison of the modified attenuation correction algorithm and the empirical-fixed-ratio correction algorithm is further evaluated at the X-POL radar. It is shown that the modified attenuation correction algorithm in the present paper provides higher correction accuracy for rain attenuation than the empirical-fixed-ratio correction algorithm. [ABSTRACT FROM AUTHOR]

A Chinese Ka-band solid-state transmitter cloud radar (CR) was employed to investigate clouds and precipitation over the Tibetan Plateau (TP) in summer 2014. After performing quality control and merging the output from the three work modes of CR, the Doppler spectral density data were analyzed to estimate the vertical air motion (Vair) and particle fall speed (Vfall) in clouds. The reproduced radar reflectivity was employed to retrieve the ice water content (IWC) and ice effective radius (Re). The cloud type classification algorithm was developed to classify cloud conditions into eight different categories. The vertical and daily variations of reflectivity, Vair, Vfall, IWC, and Re were then analyzed, and as a result, four conclusions were formulated. First, the clouds with reflectivity less than −10 dBZ were mainly located above 7 km, and the precipitable particles were formed below this layer (rain echo top) due to the abundant supercooled water therein. Second, the weak updraft in the range of 2–4 km caused the high occurrence of weak clouds during 04:00–12:00 Beijing local time (BT), and the rain echo top did not exceed 7 km due to the existing downdraft in 4–6 km. After 14:00 BT, convective clouds quickly developed, and the averaged updraft, reflectivity, and the echo top notably increased. Third, particular cloud types commonly exhibited weak reflectivity, low IWC, and less vertical variations, while the others more obvious vertical variations with larger IWC and Re. Last, compared with the radar sensitivity and range from radar, Vair biases that were introduced by the Doppler spectra broadening due to turbulence, wind shear, and radar beam width, could be neglected. [ABSTRACT FROM AUTHOR]

In this study, the temperature and relative humidity profiles retrieved from five ground-based microwave radiometers in Beijing were assimilated into the rapid-refresh multi-scale analysis and prediction system-short term (RMAPS-ST). The precipitation bifurcation prediction that occurred in Beijing on 4 May 2019 was selected as a case to evaluate the impact of their assimilation. For this purpose, two experiments were set. The Control experiment only assimilated conventional observations and radar data, while the microwave radiometers profilers (MWRPS) experiment assimilated conventional observations, the ground-based microwave radiometer profiles and radar data into the RMAPS-ST model. The results show that in comparison with the Control test, the MWRPS test made reasonable adjustments for the thermal conditions in time, better reproducing the weak heat island phenomenon in the observation prior to the rainfall. Thus, assimilating MWRPS improved the skills of the precipitation forecast in both the distribution and the intensity of rainfall precipitation, capable of predicting the process of belt-shaped radar echo splitting and the precipitation bifurcation in the urban area of Beijing. The assimilation of the ground-based microwave radiometer profiles improved the skills of the quantitative precipitation forecast to a certain extent. Among multiple cycle experiments, the onset of 0600 UTC cycle closest to the beginning of rainfall performed best by assimilating the ground-based microwave radiometer profiles. [ABSTRACT FROM AUTHOR]

Using ground-based remote sensing equipment (wind profile radar and microwave radiometer) data and ground automatic station data, a ground-based remote sensing sounding profile system (FAS) is constructed, which aims to make use of its advantages of high resolution, high accuracy, and low cost to make up for the lack of space–time density in existing conventional sounding layer information. The retrieval results of remote sensing sounding profiles in Beijing from May 2021 to September 2022 were tested and evaluated. The results show that the correlation coefficient between FAS and conventional sounding specific humidity is 0.89, the root–mean–square deviation is 1.53 g/kg, and the evolution trends of different data sources of convective available potential energy (CAPE) and vertical wind shear are synchronous. A case study was conducted to evaluate the effectiveness of 40 severe convective processes in the Beijing Plain area. The results show that, due to the minute-level time resolution of FAS, the retrieved convective parameters could track the evolution trend of the atmospheric state with high timeliness, dynamically describe the configuration of thermodynamic parameters, and indicate the time-varying local convective potential and instability level. Therefore, it has certain short-term forecasting significance for the occurrence time, intensity, and convection type. [ABSTRACT FROM AUTHOR]

This study demonstrates the capability of a cloud model in simulating a real-world tornado using observed radiosonde data that define a homogeneous background. A reasonable simulation of a tornado event in Beijing, China, on 21 July 2012 is obtained. The simulation reveals the evolution of a descending reflectivity core (DRC) that has commonalities with radar observations, which retracts upward right before tornadogenesis. Tornadogenesis can be divided into three steps: the downward development of mesocyclone vortex, the upward development of tornado vortex, and the eventual downward development of condensation funnel cloud. This bottom-up development provides a numerical evidence for the growing support for a bottom-up, rapid tornadogenesis process as revealed by the state-of-the-art mobile X-band phase-array radar observations. The evolution of the simulated tornado features two replacement processes of three near-surface vortices coupled with the same midlevel updraft. The first replacement occurs during the intensification of the tornado before its maturity. The second replacement occurs during the tornado's demise, when the connection between the midlevel mesocyclone and the near-surface vortex is cut off by a strong downdraft. This work shows the potential of idealized tornado simulations and three-dimensional illustrations in investigating the spiral nature and evolution of tornadoes. [ABSTRACT FROM AUTHOR]

We present the mapping at fine spatial scale of aerosol optical properties using a mobile laboratory equipped with LIDAR (Light Detection And Ranging), sun photometer and in situ instruments for performing on-road measurements. The mobile campaign was conducted from 9 May to 19 May 2017 and had the main objective of mapping the distribution of pollutants in the Beijing and North China Plain (NCP) region. The highest AOD (Aerosol Optical Depth) at 440 nm of 1.34 and 1.9 were recorded during two heavy pollution episodes on 18 May and 19 May 2017, respectively. The lowest Planetary Boundary Layer (PBL) heights (0.5–1.5 km) were recorded during the heavy pollution events, correlating with the highest AOD and southern winds. The transport of desert dust from the Gobi Desert was captured during the mobile measurements, impacting Beijing during 9–13 May 2017. Exploring the NCP outside Beijing provided datasets for regions with scarce ground measurements and allowed the mapping of high aerosol concentrations when passing polluted cities in the NCP (Baoding, Tianjin and Tangshan) and along the Binhai New Area. For the first time, we provide mass concentration profiles from the synergy of LIDAR, sun photometer and in situ measurements. The case study along the Binhai New Area revealed mean extinction coefficients of 0.14 ± 0.10 km−1 at 532 nm and a mass concentration of 80 ± 62 μg/m3 in the PBL (<2 km). The highest extinction (0.56 km−1) and mass concentrations (404 μg/m3) were found in the industrial Binhai New Area. The PM10 and PM2.5 fractions of the total mass concentration profiles were separated using the columnar size distribution, derived from the sun photometer measurements. This study offers unique mobile datasets of the aerosol optical properties in the NCP for future applications, such as satellite validation and air quality studies. [ABSTRACT FROM AUTHOR]

Based on application of the atomic filter technology in a signal detection system of lidar, the diurnal observation of sodium lidar were obtained using the system at the National Space Science Center of the Chinese Academy of Sciences at Beijing Yanqing station (40.5° N, 116° E) in April 2014. During the lidar observation period, among the 103 cases of continuous daytime observations, the longest time was 181 h. In the case of a continuous observation period of 5 days (13–18 October 2014), the signal-to-noise ratio reached to 19:1 at 12:00–13:00 Local Time of the daytime, when the spatial and time resolutions were respectively set to 96 m of 167 s. The improvements resulted in the highest detection level of any existing diurnal lidars in China. Some interesting phenomena such as the sporadic sodium layer have also been observed during the daytime. The daytime capability extended the observing time range of the earlier systems that were limited to only nighttime observations. This innovation provides a useful method for the studies of diurnal tides, photochemistry, gravity waves, and correlative modeling studies. [ABSTRACT FROM AUTHOR]

Ground-based microwave radiometers (MWRPS) can provide continuous atmospheric temperature and relative humidity profiles for a weather prediction model. We investigated the impact of assimilation of ground-based microwave radiometers based on the rapid-refresh multiscale analysis and prediction system-short term (RMAPS-ST). In this study, five MWRP-retrieved profiles were assimilated for the precipitation enhancement that occurred in Beijing on 21 May 2020. To evaluate the influence of their assimilation, two experiments with and without the MWRPS assimilation were set. Compared to the control experiment, which only assimilated conventional observations and radar data, the MWRPS experiment, which assimilated conventional observations, the ground-based microwave radiometer profiles and the radar data, had a positive impact on the forecasts of the RMAPS-ST. The results show that in comparison with the control test, the MWRPS experiment reproduced the heat island phenomenon in the observation better. The MWRPS assimilation reduced the bias and RMSE of two-meter temperature and two-meter specific humidity forecasting in the 0–12 h of the forecast range. Furthermore, assimilating the MWRPS improved both the distribution and the intensity of the hourly rainfall forecast, as compared with that of the control experiment, with observations that predicted the process of the precipitation enhancement in the urban area of Beijing. [ABSTRACT FROM AUTHOR]

Clouds are significant in the global radiation budget, atmospheric circulation, and hydrological cycle. However, knowledge regarding the observed climatology of the cloud vertical structure (CVS) over Beijing is still poor. Based on high-resolution radiosonde observations at Beijing Nanjiao Weather Observatory (BNWO) during the period 2010–2017, the method for identifying CVS depending on height-resolved relative humidity thresholds is improved, and CVS estimation by radiosonde is compared with observations by millimeter-wave cloud radar and ceilometer at the same site. Good consistency is shown between the three instruments. Then, the CVS climatology, including the frequency distribution and seasonal variation, is investigated. Overall, the occurrence frequency (OF) of cloudy cases in Beijing is slightly higher than that of clear-sky cases, and the cloud OF is highest in summer and lowest in winter. Single-layer clouds and middle-level clouds are dominant in Beijing. In addition, the average cloud top height (CTH), cloud base height (CBH), and cloud thickness in Beijing are 6.2 km, 4.0 km, and 2.2 km, respectively, and show the trend of reaching peaks in spring and minimums in winter. In terms of frequency distribution, the CTH basically resides below an altitude of 16 km, and approximately 43% of the CBHs are located at altitudes of 0.5–1.5 km. The cloud OF has only one peak located at altitudes of 4–8 km in spring, whereas it shows a trimodal distribution in other seasons. The height at which the cloud OF reaches its peak is highest in summer and lowest in winter. To the best of our knowledge, the cloud properties analyzed here are the first to elucidate the distribution and temporal variation of the CVS in Beijing from a long-term sounding perspective, and these results will provide a scientific observation basis for improving the atmospheric circulation model, as well as comparisons and verifications for measurements by ground-based remote sensing equipment. [ABSTRACT FROM AUTHOR]

Lidar observations of metal layers play a significant role in research on the chemistry and dynamics of the mesosphere and lower thermosphere. This work reports on Fe lidar observations conducted in Beijing and Mohe. Utilizing the same laser emission system, a 1064 nm seed laser was injected into an Nd: YAG laser to generate a single longitudinal-mode pulse 532 nm laser, which pumped a dye laser to produce a 572 nm laser. The 572 nm laser and the remaining 1064 nm fundamental frequency laser passed through a sum–frequency module to generate a 372 nm laser to detect the Fe layer. According to a total of 52.6 h of observations for 10 nights in Beijing, the Fe layer has an average column density of 1.24 × 1010 cm−2, an RMS width of 4.4 km and a centroid altitude of 89.4 km. In Mohe, observed for 16 nights and a total of 91.5 h, the Fe layer has an average column density of 1.08 × 1010 cm−2, an RMS width of 4.6 km and a centroid altitude of 89.5 km. The probability of the occurrence of sporadic Fe layers was 42.4% in Beijing and 29.4% in Mohe. Compared to simultaneously observed Na layers, the occurrence probabilities of sporadic Fe layers were higher than those of sporadic Na layers in both stations. Based on the two cases observed in Beijing, it is conjectured that the formation mechanism of sporadic metal layers above approximately 100 km has a more significant influence on sporadic Fe layers than on sporadic Na layers. The lower thermospheric Fe layers with densities significantly larger than those of the main layer were observed during two nights in Mohe. This work contributes to the refinement of the global distribution of Fe layers and provides abundant observational data for the modeling and study of the metal layers. [ABSTRACT FROM AUTHOR]

The mechanism of aerosol pollution transport remains highly elusive owing to the myriad of influential factors. In this study, ground station data, satellite data, ground-based LiDAR remote sensing data, sounding data, ERA5 reanalysis and a backward trajectory model were combined to investigate the formation process and optical properties of winter aerosol pollution in Beijing and surrounding areas. The analysis of ground station data shows that compared to 2019 and 2021, the pandemic lockdown policy resulted in a decrease in the total number of pollution days and a decrease in the average concentration of particulate matter in the Beijing area in 2020. The terrain characteristics of the Beijing-Tianjin-Hebei (BTH) made it prone to northeast and southwest winds. The highest incidence of aerosol pollution in Beijing occurs in February and March during the spring and winter seasons. Analysis of a typical heavy aerosol pollution process in the Beijing area from 28 February to 5 March 2019 shows that dust and fine particulate matter contributed to the primary pollution; surface air temperature inversion and an average wind speed of less than 3 m/s were conducive to the continuous accumulation of pollutants, which was accompanied by the oxidation reaction of NO2 and O3, forming photochemical pollution. The heavy aerosol pollution was transmitted and diffused towards the southeast, gradually eliminating the pollution. Our results provide relevant research support for the prevention and control of aerosol pollution. [ABSTRACT FROM AUTHOR]

A torrential rainfall that occurred in Beijing during the period of 21–22 July 2012 is simulated by the Weather Research and Forecasting Model in order to investigate the probable mechanisms for the initiation and organization of warm-sector mesoscale convective systems (MCSs). The simulated results show that the cyclone, which formed in Hetao area, Inner Mongolia and moved eastward slowly, played a key role in the formation and development of warm-sector precipitation, although the favorable atmospheric environment and the configuration of weather systems are also important, which caused the trigger and organization of convective cells along Taihang Mountains. It is the interaction of the local terrain convergence line and the southerly airflows of Hetao cyclone that cause the continuous trigger of convective cells along Taihang Mountains. While, the triggers of convective cells in the plains are caused by the gravity waves, which is related to the development and eastward movement of Hetao cyclone. It must be pointed out that the merging and coupling between the cells that triggered in Taihang Mountains and moved southwesterly and the cells that triggered in plains and moved northeasterly are the key factors for the formation and development of MCSs during the warm-sector precipitation. In addition, the back-building processes and the cold pool forcing are also important for the formation and development of MCSs in this study. [ABSTRACT FROM AUTHOR]

In this study, based on the National Centers for Environmental Prediction (NCEP) Final Analysis (FNL) data, the reliability and performances of their application on clean days and polluted days (based on the PM2.5 mass concentrations) in Beijing were assessed. Conventional meteorological factors and diagnostic physical quantities from the NCEP/FNL data were compared with the L-band radar observations in Beijing in the autumns and winters of 2017–2019. The results indicate that the prediction reliability of the temperature was the best compared with those of the relative humidity and wind speed. It is worth noting that the relative humidity was lower and the near-surface wind speed was higher on polluted days from the NCEP/FNL data than from the observations. As far as diagnostic physical quantity is concerned, it was revealed that the temperature inversion intensity depicted by the NCEP/FNL data was significantly lower than that from the observations, especially on polluted days. For example, the difference in the temperature inversion intensity between the NCEP/FNL data and the observation ranged from −0.56 to −0.77 °C on polluted days. In addition, the difference in the wind shears between the NCEP/FNL reanalysis data and the observations increased to 0.40 m/s in the lower boundary layer on polluted days compared with that on clean days. Therefore, it is suggested that the underestimation of the relative humidity and temperature inversion intensity, and the overestimation of the near-surface wind speed should be seriously considered in simulating the air quality in the model, particularly on polluted days, which should be focused on more in future model developments. [ABSTRACT FROM AUTHOR]

As one of the key technologies of HF communication, the maximum usable frequency (MUF) prediction method has been widely discussed. To experimentally confirm the reliability of commonly used MUFs prediction models for high-frequency communication, we have compared maximum observed frequencies (MOFs) and predicted MUFs to assess the accuracy of two typical prediction models. The root-mean-square error (RMSE) and relative RMSE (RRMSE) between oblique sounding MOFs and the predicted MUFs were used to assess the model's accuracy. The oblique sounding path was from Changchun to Jinyang, and the vertical-sounding ionosonde was located in Beijing, which was approximately the midpoint of the oblique sounding circuit. The statistical analysis results show that: (a) the trend of prediction results from the Lockwood and the Istituto Nazionale di Geofisica e Vulcanologia (INGV) model are in good agreement with the observations: the mean RMSE and RRMSE of the INGV model are less than those of the Lockwood model; (b) in the four different periods (sunrise, daytime, sunset, and nighttime) of the whole day, the maximum difference of RMSE between the Lockwood and INGV model is 0.14 MHz (the INGV performs better than the LWM), with the corresponding differences of RRMSE being 0.31% at sunrise and 0.68% at daytime; (c) in the four seasons of spring, summer, autumn, and winter, the minimum RMSE values of the Lockwood and INGV models are 1.51 MHz and 1.37 MHz, respectively, which are obtained in winter, and the corresponding RRMSEs are 11.47% and 11.79%, respectively; (d) in the high and low solar activity epochs, the mean RMSEs of the Lockwood and INGV models are 1.63 MHz, and 1.54 MHz, with corresponding mean RRMSE values of 11.47% and 11.55%. In conclusion, the INGV model is more suitable for MUF prediction over Beijing and its adjacent mid-latitude regions from the RMSE comparison of the two models. [ABSTRACT FROM AUTHOR]

The distribution and diurnal variation of short-duration heavy rainfall (SDHR) and the influence of a complex underlying surface were studied by using fine-scale hourly precipitation data in the Beijing–Tianjin–Hebei (BTH) region during the summers of 2014–2020. Areas prone to SDHR are located mainly in the southern foothills of the Yanshan Mountains, the foothills area, and the trumpet-shaped topographic entrance area north of Beijing, areas inland of the west coast of Bohai Bay, and the northern Beijing urban area. Owing to the influence of topography and the geographical location, the distribution and diurnal variation SDHR is significantly different in the western and northern mountainous areas, the foothills, and the plains. Compared to the underlying urban surface, the topography and the land–sea interface have considerable effects on the distribution of SDHR. A key finding is that the foothills of northern of Beijing, eastern slope and piedmont area of the Taihang Mountains, and the land–sea interface of Bohai Bay play important roles in the formation and propagation of SDHR. [ABSTRACT FROM AUTHOR]

In Beijing, the probability of snowfall is decreasing as a result of global warming. At the same time, Beijing has suffered severe air pollution. In this paper, the concentration and particle size characteristics of BC (Black Carbon) in snow during the winter of 2015 in Beijing were analyzed by the SP2 method. The average concentration of BC in snow meltwater in Beijing is 82 ng/mL, with a minimum value of 62.9 ng/mL and a maximum of 210.6 ng/mL. The BC particle size in snow and ice in the Beijing area is mostly concentrated in the range of 70–400 nm. After log-normal, the BC particle size above 600 nm is still small, which should be closely related to the nature of the local BC emission source. The concentration of BC in snow is highly susceptible to meteorological conditions and local pollution levels. When Beijing is under the control of the east wind or the southeast wind, aerosols in the urban areas can easily accumulate in the northwestern mountains and then settle or participate in the snowfall process, resulting in an increase in BC aerosol accumulation in the snow, thus further changing the optical properties of snow in the Beijing area. [ABSTRACT FROM AUTHOR]

Studies on urban extreme precipitation and its influencing factors are significant for prevention and reduction of meteorological disasters; however, few studies focus on hourly extreme precipitation (HEP) events due to the limitation of observation. By using the summer hourly precipitation data in Beijing from 1980 to 2020, the spatial distribution and temporal variation of HEP as well as its local environmental influencing factors are investigated. It is found that both summer precipitation amount and frequency of HEP are affected by topography, with high values in windward slope area. The summer precipitation amount of HEP is 160–200 mm, accounting for 42–47% of the annual summer precipitation amount, while the frequency proportion of HEP is only 5.5–6.0%. Although the summer precipitation amount and frequency in Beijing both decrease in the past 41 years, those for HEP present an opposite trend mainly due to the increasing HEP since 2003 and this is a phenomenon worthy of attention. A similar bimodal pattern in diurnal variation is found for the summer precipitation amount and frequency of HEP, with two peaks in 19–22 LT and 01–05 LT, respectively, indicating that HEPs are more concentrated in the evening and early morning especially in urban area. Moreover, the urbanization process of Beijing is consistent with the change trend of HEP, implying that the stronger the urban heat island intensity (UHI), the higher the probability of HEP. Furthermore, the convergence lines of terrain are also conducive to local heavy rainfall, and lower tropospheric stability (LTS) and convective available potential energy (CAPE) as well as aerosols may also affect HEP in Beijing. [ABSTRACT FROM AUTHOR]

Transported pollution plays an important role in the atmospheric environment of eastern China. This study analyzed the characteristics of surface winds at different air quality levels using meteorological station observations of both wind and mass concentrations of particulate matter with aerodynamic diameters <2.5 μm (PM2.5) over Hong Kong and Beijing. In recent decades, wind directions at the surface exhibit a similar pattern for both good and poor air quality levels at all three stations, indicating a weak relationship between surface winds and air quality in Hong Kong. However, winds at a height of 1–2 km govern pollution accumulation. This dominant role is illustrated by a sudden change in wind direction within this layer and a simultaneous pollution accumulation stage on 8 January 2014. The controlling influence of winds at 1–2 km on both the deterioration and improvement of air quality is also supported by a distinct vertical wind distribution for all 21 monotonic increasing stages and 17 decreasing stages of PM2.5. In contrast, air pollution is transported to Beijing throughout the atmospheric layer that extends from the surface to a height of more than 3 km. This key difference may be due to variations in meteorology, topography, and emission sources between Hong Kong and Beijing. The results that layer of 1–2 km in Hong Kong and of surface to 3 km in Beijing is the height where pollution transport is most likely to occur are critical for forecasting severe haze episodes in eastern China. [ABSTRACT FROM AUTHOR]

To assess the quality of the retrieved products from ground-based microwave radiometers, the "clear-sky" Level-2 data (LV2) products (profiles of atmospheric temperature and humidity) filtered through a radiometer in Beijing during the 24 months from January 2010 to December 2011 were compared with radiosonde data. Evident differences were revealed. Therefore, this paper investigated an approach to calibrate the observed brightness temperatures by using the model-simulated brightness temperatures as a reference under clear-sky conditions. The simulation was completed with a radiative transfer model and National Centers for Environmental Prediction final analysis (NCEP FNL) data that are independent of the radiometer system. Then, the least-squares method was used to invert the calibrated brightness temperatures to the atmospheric temperature and humidity profiles. A comparison between the retrievals and radiosonde data showed that the calibration of the brightness temperature observations is necessary, and can improve the inversion of temperature and humidity profiles compared with the original LV2 products. Specifically, the consistency with radiosonde was clearly improved: the correlation coefficients are increased, especially, the correlation coefficient for water vapor density increased from 0.2 to 0.9 around the 3 km height; the bias decreased to nearly zero at each height; the RMSE (root of mean squared error) for temperature profile was decreased by more than 1 degree at most heights; the RMSE for water vapor density was decreased from greater than 4 g/m3 to less than 1.5 g/m3 at 1 km height; and the decrease at all other heights were also noticeable. In this paper, the evolution of a temperature inversion process is given as an example, using the high-temporal-resolution brightness temperature after quality control to obtain a temperature and humidity profile every two minutes. Therefore, the characteristics of temperature inversion that cannot be seen by conventional radiosonde data (twice daily) were obtained by radiometer. This greatly compensates for the limited temporal coverage of radiosonde data. The approach presented by this paper is a valuable reference for the reprocessing of the historical observations, which have been accumulated for years by less-calibrated radiometers. [ABSTRACT FROM AUTHOR]

In this study, an extreme rainstorm that occurred in the Beijing–Tianjin–Hebei (BTH) region in China on 19–20 July 2016 is simulated and analyzed using the Weather Research and Forecasting model, coupled with a multilayer urban canopy scheme, to reveal the impact of urbanization on the extreme precipitation process in the region. The results show that the urban heat island effect (that is, surface warming and an increased near-ground sensible heat flux, which leads to increased vertical motion and atmospheric instability layer strengthening) plays a dominant role in the urban modification of rainfall during the early stages of urbanization, resulting in an increase of 6–10 mm in average hourly precipitation in urban and downwind areas. With the further development of urbanization in the BTH region, particularly in the big cities of Beijing and Tianjin, the large-scale expansion of the urban surface reduces the surface moisture, the evaporation of surface water from the ground, and the height of the atmospheric boundary layer, leading to an urban dry island effect brought about by the lack of near-surface water vapor, which inhibits an increase in precipitation. The positive effect of the urban heat island on precipitation was offset by the urban dry island effect, so the increase in precipitation in the urban areas was not obvious, but an increased range of 8–10 mm was noted. The existence of large cities changes the position of the strong upward movement of air, and convective upward movement is more likely to occur between the suburbs. With the further expansion of the underlying surface of the adjacent cities of Beijing and Tianjin, the upward movement between the two cities coincides, leading to an obvious increase in precipitation between the two cities. [ABSTRACT FROM AUTHOR]

A heavy rainstorm occurred in Beijing on 19–20 July 2016. The Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) was used to investigate the effects of anthropogenic aerosols on precipitation and microphysical processes. Three conditions were simulated by altering the anthropogenic emissions. When the anthropogenic emissions were increased by 10 times, the area-average accumulated rainfall amount and maximum accumulated rainfall amount both decreased. The cloud water mixing ratio increased and the rain mixing ratio decreased. The radii of cloud droplets decreased, and the collision efficiency of cloud water by rain and the autoconversion rate of cloud water into rain were both low. When the anthropogenic emission was 10% of the original emission, the area of accumulated rainfall amounts greater than 25 mm in the Beijing area was 10% larger than those of the other two tests. The collision efficiency and autoconversion rate of cloud water into rain were high for large contact areas and large cloud droplets. The graupel mixing ratio was the largest. Thus, the process of melting of graupel into rain was the largest. In the WRF-Chem model, the aerosols did not participate as ice nuclei (IN) in the ice-phase microphysical processes, and therefore the aerosols could influence only the warm rain processes and mix-phased processes near the freezing level line. For no influence on ice-phase microphysical processes, the snow and ice mixing ratios did not show many differences among the different tests. [ABSTRACT FROM AUTHOR]