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Based on the quality-controlled observational spectral width data of the Beijing mesosphere–stratosphere–troposphere (MST) radar in the altitudinal range of 3–19.8 km from 2012 to 2014, this paper analyses the relationship between the proportion of negative turbulent kinetic energy (N-TKE) and the horizontal wind speed and the vertical shear of horizontal wind domain and gives the distributional characteristics of atmospheric turbulence parameters obtained by using different calculation models. Three calculation models of the spectral width method were used in this study – namely the H model (Hocking, 1985), N-2D model (Nastrom, 1997) and D–H model (Dehghan and Hocking, 2011). The results showed that the proportion of N-TKE in the H model, N-2D model and D–H model increases with the horizontal wind speed u and/or the vertical shear of horizontal wind speed ∂u∂z, and the maximum values are 60 %, 45 % and 35 %, respectively. When the∂u∂z is greater than 0.006 s−1, the N-TKE of the H model increases sharply with ∂u∂z; the increase rate is about 20%0.002s-1. For these three models, the results are similar except that the vertical shear of the horizontal wind speed is greater than 0.006 s−1. When ∂u∂z>0.006 s−1, the proportion of N-TKE in the N-2D and H models increases with ∂u∂z, while the proportion in the D–H model is less than 10 % and has slight variation. However, it is still necessary to consider the applicability of the N-2D model and D–H model in some weather processes with strong winds. The distributional characteristics with height of the turbulent kinetic energy dissipation rate ε and the vertical eddy diffusion coefficient Kz derived by the three models are consistent with previous studies. Still, there are differences in the values of turbulence parameters. Also, the range resolution of the radar has little effect on the differences in the range of turbulence parameters' values. The median values of ε in the H model, N-2D model and D–H model is 10−3.2–10−2.7, 10−3.0–10−2.6 and 10−3.3–10−2.8 m2 s−3, respectively. The median values of Kz in these three models are 100.3–100.7, 100.4–100.7 and 100.1–100.5 m2 s−1.

The I3RC Community Monte Carlo model of three-dimensional (3D) radiative transfer (I3RC-CM) was improved by specifying surfaces with a widely used bidirectional reflectance model known as the RPV (Rahman-Pinty-Verstraete). The surface-cloud damping effect on top-of-atmosphere (TOA) reflectivity (hereafter, the damping effect) is evaluated as the atmospheric attenuation of incoming shortwave radiation on its way down to the surface and shortwave radiation reflected at the surface that is transmitted back to the TOA. A series of simulations were performed using the improved I3RC-CM to investigate this effect over different land covers, in which the land covers are characterized by the 3 RPV parameters: the intensity of surface reflectance, the anisotropy of the surface, and the asymmetry parameter. The damping effect is greatly affected by the land cover, especially the intensity of surface reflectance. The intensity of surface reflectance can lead to an approximate deviation of 0.40 in the damping effect, while the influence of the asymmetry parameter and the surface anisotropy on the damping effect is less than 0.10. The damping effect strengthens as the surface reflectance intensity and asymmetry parameter increase, but slightly weakens as the surface anisotropy increases. The land cover change has a stronger impact on the damping effect at the absorbing wavelengths than at the non-absorbing wavelengths due to the cloud absorption, particularly at 2.13 µm, but the cloud absorption offers only a partial explanation for the differences in damping effect changes with values less 0.10.

To better understand the accuracy of cloud top heights (CTHs) derived from passive satellite data, ground-based Ka-band radar measurements from 2016 and 2017 in Beijing are compared with CTH data inferred from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Himawari Imager (AHI). Relative to the radar CTHs, the MODIS CTHs are found to be underestimated by−1.10 ± 2.53 km on average and 49 % of CTH differences are within 1.0 km. The AHI CTHs are underestimated by −1.10 ± 2.27 km and 42 % are within 1.0 km. Both the MODIS and AHI CTH retrieval accuracy depends strongly on the cloud depth (CD). Large differences are mainly due to the retrieval of thin clouds of CD  km, especially when the cloud base height is higher than 4 km. For clouds with CD >1 km, the mean CTH difference decreases to -0.48±1.70 km for MODIS and to -0.76±1.63 km for AHI. It is found that MODIS CTHs with higher values (i.e. >6 km) show smaller discrepancy with radar CTH than those MODIS CTHs with lower values (i.e.  km). Statistical analysis illustrate that the CTH difference between the two satellite instruments is lower than the difference between the satellite instrument and the ground-based Ka-band radar. The monthly accuracy of both CTH retrieval algorithms is investigated and it is found that summer has the smallest retrieval difference.

A cloud clustering and classification algorithm is developed for a ground-based Ka-band radar system in the vertically pointing mode. Cloud profiles are grouped based on the combination of a time-height clustering method and the k-means clustering method. The cloud classification algorithm, developed using a fuzzy logic method, uses nine physical parameters to classify clouds into nine types: cirrostratus, cirrocumulus, altocumulus, altostratus, stratus, stratocumulus, nimbostratus, cumulus or cumulonimbus. The performance of the clustering and classification algorithm is presented by comparison with all-sky images taken from January to June 2014. Overall, 92% of the cloud profiles are clustered successfully and the agreement in classification between the radar system and the all-sky imager is 87%. The distribution of cloud types in Beijing from January 2014 to December 2017 is studied based on the clustering and classification algorithm. The statistics show that cirrostratus clouds have the highest occurrence frequency (24%) among the nine cloud types. High-level clouds have the maximum occurrence frequency and low-level clouds the minimum occurrence frequency.

The radiances scattered or emitted by clouds demonstrate diverse features at different wavelengths due to different cloud physical structures. This paper presents a method (the smallest-radiance-distance method, SRaDM) of revealing the physical structures of clouds. The method is based on multi-spectral radiances measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua. The principle and methodology of SRaDM is deduced and provided in this paper. Correlation analysis based on data from MODIS and Cloud Profiling Radar (onboard CloudSat), collected from January 2007 to December 2010 over an ocean area (15°N–45°N, 145°E–165°E), led to selection of radiances at 13 wavebands of MODIS that demonstrated high sensitivity to cloud physical structures; radiances at the selected wavebands were subjected to SRaDM. The Standardized Euclidean distance is introduced to quantify the degree of changes in multi-spectral radiances (termed Drd) and in physical structures (termed Dst) between cloud profiles. Statistics based on numerous cloud profiles show that Drd decreases monotonically with a decrease in Dst, which implies that small Drd always accompanies small Dst. According to the law of Drd and Dst, the new method, SRaDM, for revealing physical structures of clouds from the collocation of cloud profiles of similar multi-spectral radiances, is presented. Then, two successful experiments are presented in which cloud physical structures are captured using multi-spectral radiances. SRaDM provides a way to obtain knowledge of the physical structures of clouds over relatively larger areas, and is a new approach to obtaining 3D cloud fields.

The physical properties and radiative role of cirrus clouds remain one of the uncertainties in the Earth–atmosphere system. In this study, we present a detailed analysis of cirrus properties based on four years of surface millimetre wavelength radar measurements in Beijing, China, where summer monsoon from the ocean and winter monsoon from the continent prevails alternately, resulting in various cirrus clouds. More than 6600 cirrus clusters were studied to quantify the properties of cirrus clouds, such as the height, optical depth and horizontal extent, which can serve as a reference for parameterization and characterization in global climate models. In addition, comparison between cirrus clusters formed under summer monsoon and winter monsoon indicates the different formation and evolution mechanisms of cirrus. Statistically, the temperature of more than 90 % of cirrus bins are below −15 °C. The dependence of the radar reflectivity of cirrus particles on the height and temperature was also observed in this study, indicating that the reflectivity of cirrus bins increases (decreases) as the temperature (height) increases. In addition, it was found that there is a strong linear relationship between the mean reflectivity and the cirrus cloud depth. Due to various synoptic circumstances, the cirrus clouds in summer are warmer, higher, and thicker, with larger reflectivity than that in winter; in particular, the mean cloud-top height of cirrus clouds in summer is 2.5 km higher than that in winter. It was found that most cirrus clusters in winter are likely to be the in situ origin type cirrus but some cirrus clusters in summer are the in situ origin cirrus and others are the liquid origin type cirrus.

A high-resolution dual-band terahertz (THz) radiometer was designed to measure vertical distributions of chemical elements in the middle atmosphere of the Tibetan Plateau. A forward simulation, which always should be conducted firstly for the development of a matching retrieval algorithm, has not been done before. We use two radiative transfer models, ARTS and AM, to simulate the water vapor, ozone and carbon monoxide spectra on the plateau based on the spectral design of the THz radiometer. The emission line characteristics of the three gases in this spectral band are identified. Reasons for the differences in the spectral simulations between the two models are analyzed for individual gases. The impact of several different spectral parameter settings on the simulations are evaluated through a series of sensitivity experiments. This study suggests that the ARTS is more suitable for the development of the THz radiometer retrieval algorithm. An optimal parameter setting of the ARTS for the three elements are given.

The Atmosphere Profiling Synthetic Observation System (APSOS) is the first ground-based facility for profiling atmospheric variables and multiple constituents in the whole (neutral) atmosphere from the surface up to the lower thermosphere. It enables simultaneous observations and extensive studies of the atmospheric vertical structure and constituent transport. The program under which this new facility was developed, funded by the National Natural Science Foundation of China, was launched in 2012 for developing a cluster of state-of-the-art instruments to facilitate atmospheric studies over the Tibetan Plateau (TP). After a one-year test run in Anhui Province at the Huainan Division of the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS) (32.62°N, 116.98°E), APSOS was recently deployed at its final destination——Yangbajain (YBJ) International Cosmic Ray Observatory (30.21°N, 90.43°E; 4300 m MSL), located in YBJ valley, about 90 km northwest of the city of Lhasa, the Tibet Autonomous Region, China. Figure 1 shows an aerial view of the newly established observatory for APSOS in YBJ.

Ozonesonde launches were routinely performed in Beijing from March 2001 to February 2019 to generate a unique long-term (18 years) vertical ozone profile dataset over mainland China. This study elucidates the vertical ozone structure on various temporal scales during this 18 years period by using the entire ozonesonde data product for the first time. Moreover, the long-term variability in the integrated ozone column over the North China Plain (NCP) is also explored by comparing the retrievals from ozonesonde at the Beijing urban site and a Dobson ozone spectrometer at the Xianghe suburban site. Our results indicate that vertical ozone exhibited clear monthly variability characterized by high values of tropospheric ozone during warm seasons and high values of stratospheric ozone during cold seasons. Stratospheric intrusions frequently occurred during spring and effectively transported cold air masses with high ozone from the lower stratosphere downward into the upper troposphere. Evident interannual variability in the lower troposphere and in ozone-rich areas of the stratosphere was revealed by vertical ozone distributions. The integrated total ozone columns retrieved from ozonesonde and Dobson bear close resemblance and exhibit strong sinusoidal monthly variations. In the troposphere and boundary layer, the integrated ozone column presented a significant positive trend during 2001–2012 in Beijing; a sudden decline occurred between 2011 and 2013, which was followed by a slow and insignificant increase after the implementation of the Clean Air Action plan in 2013 on the NCP.

Monitoring atmospheric carbon dioxide (CO2) from space-borne state-of-the-art hyperspectral instruments can provide a high precision global dataset to improve carbon flux estimation and reduce the uncertainty of climate projection. Here, we introduce a carbon flux inversion system for estimating carbon flux with satellite measurements under the support of “The Strategic Priority Research Program of the Chinese Academy of Sciences—Climate Change: Carbon Budget and Relevant Issues”. The carbon flux inversion system is composed of two separate parts: the Institute of Atmospheric Physics Carbon Dioxide Retrieval Algorithm for Satellite Remote Sensing (IAPCAS), and CarbonTracker-China (CT-China), developed at the Chinese Academy of Sciences. The Greenhouse gases Observing SATellite (GOSAT) measurements are used in the carbon flux inversion experiment. To improve the quality of the IAPCAS-GOSAT retrieval, we have developed a post-screening and bias correction method, resulting in 25%–30% of the data remaining after quality control. Based on these data, the seasonal variation of XCO2 (column-averaged CO2 dry-air mole fraction) is studied, and a strong relation with vegetation cover and population is identified. Then, the IAPCAS-GOSAT XCO2 product is used in carbon flux estimation by CT-China. The net ecosystem CO2 exchange is −0.34 Pg C yr−1 (±0.08 Pg C yr−1), with a large error reduction of 84%, which is a significant improvement on the error reduction when compared with in situ-only inversion.

To determine the performance and data accuracy of the 50 MHz Beijing Mesosphere–Stratosphere–Troposphere (MST) radar, comparisons of radar measured horizontal winds in the height range 3–25 km with radiosonde observations were made during 2012. A total of 427 profiles and 15 210 data pairs were compared. There was very good agreement between the two types of measurement. Standard deviations of difference (mean difference) for wind direction, wind speed, zonal wind and meridional wind were 24.86◦ (0.77◦), 3.37 (−0.44), 3.33 (−0.32) and 3.58 (−0.25) m s−1, respectively. The annual standard deviations of differences for wind speed were within 2.5–3 m s−1 at all heights apart from 10–15 km, the area of strong winds, where the values were 3–4 m s−1. The relatively larger differences were mainly due to wind field variations in height regions with larger wind speeds, stronger wind shear and the quasi-zero wind layer. A lower MST radar SNR and a lower percentage of data pairs compared will also result in larger inconsistencies. Importantly, this study found that differences between the MST radar and radiosonde observations did not simply increase when balloon drift resulted in an increase in the real-time distance between the two instruments, but also depended on spatiotemporal structures and their respective positions in the contemporary synoptic systems. In this sense, the MST radar was shown to be a unique observation facility for atmospheric dynamics studies, as well as an operational meteorological observation system with a high temporal and vertical resolution.

In this paper, interannual variations of convective and stratiform precipitations in summer East Asian monsoon (SEAM) dominion are analysed using 16-year (1998–2013) Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) rain products (2A25 and 3A25). It is found that the climatological mean patterns of convective and stratiform rain rates (RRs) are generally similar to each other, but the area fraction (AF) of stratiform precipitation can always reach about 80% and dominates in summer East Asia. Nonetheless, convective precipitation produces similar rainfall quantity with stratiform rainfall. For the RR, AF and contribution fraction (CF), their interannual standard deviation and coefficient of variation (CV) spatial patterns are similar between summer convective and stratiform precipitations in East Asia, because of the fact that they are all maybe influenced by SEAM. Differently, only the intensity of interannual variability (i.e. CV) for convective precipitation AF is much greater than that for stratiform precipitation, while CV for both convective and stratiform precipitations RR/CF are similar in most areas. Moreover, the interannual variability of SEAM is more highly correlated with that of RR for convective precipitation than that for stratiform precipitation in most land regions. While the opposite conclusion is found in ocean regions. Generally, the SEAM intensity in the lower troposphere mainly controls interannual variability of the atmospheric stability and both stratiform and convective precipitations RR. Then, atmospheric stability variation further modulates the AF and CF variations of both stratiform and convective precipitations on the interannual scale. This work mainly provides possible evidence of linkage between large-scale circulation and convective and stratiform precipitations on the interannual scale. It suggests that the possible dominant mechanism should be explored using more observations and together with model simulations in the future work.

In this study, the authors focus on the cut-off low pressure systems (COLs) lingering over East Asia in late spring and early summer and quantify the two-way stratosphere–troposphere exchange (STE)...

The 10-year climatological features related to midlatitude double tropopause events (DTs) are examined using ERAInterim data from 2003 to 2012. The analysis is based on tropopauses defined by lapse rate. Results show that DTs are permanent or semi-permanent in the midlatitudes, and high DT frequency bands move poleward in winter and equatorward in summer, which is consistent with the seasonal movement of the subtropical jet. Based on our statistics, the second tropopause is found at about 100 hPa in the subtropics and at slightly lower altitudes in sub-polar regions. The thickness between the first and second tropopause is smaller in the subtropics and increases with latitude. Next, the origin of air sandwiched between the first and second tropopause of DTs is studied with a revised version of the UK Universities Global Atmospheric Modelling Programme Offline Trajectory Code (Version 3) diabatic trajectory model. The results show that, in the lower or middle troposphere, air is transported into the DTs from lower latitudes, mainly in the tropics. The dominant source regions are mainly areas of deep convection and steep orography, e.g., the western Pacific and Himalayan Mountains, and they show strong seasonality following the seasonal shift of these strong upwelling regions.

The complexity of inhomogeneous surface-atmosphere radiation transfer is one of the foremost problems in the field of atmospheric physics and atmospheric radiation. To date, the influence of surface properties on shortwave radiation has not been well studied. The daily downward surface shortwave radiation of the latest FLASHFlux/CERES (Fast Longwave And Shortwave Fluxes_Time Interpolated and Spatially Averaged/Clouds and the Earth’s Radiant Energy System) satellite data was evaluated against in situ data. The comparison indicated that the differences between the two data sets are unstable and large over rugged terrain compared with relatively flat terrain, and the mean absolute error of the satellite products reaches 31.4 W m−2 (12.3%) over rugged terrain. Based on the SSF (single satellite footprint)/CERES product, the influence of surface properties on the distribution of downward surface shortwave radiation (DSSR) was analyzed. The influence of surface properties on DSSR over the Tibetan Plateau is about twice as large as that in two other regions located at the same latitude (eastern China-western Pacific and subtropical North Pacific). A simulation was carried out with the help of the I3RC (International Intercomparision of Three-Dimensional Radiation Code) Monte Carlo 3D radiative transfer community model. The results showed that DSSR increases as surface albedo increases. Moreover, the impact of surface albedo on DSSR is larger if the spatial distribution of clouds is more non-uniform. It is hoped that these results will contribute to the development of 3D radiative transfer models and the improvement of satellite inversion algorithms.

The spectral characteristics of anvils in tropical areas (25°S–25°N) have been investigated on the basis of data from the tropical rainfall measuring mission’s (TRMM) precipitation radar (PR) and a visible and infrared scanner (VIRS), from 1998 to 2007. The anvils’ vertical structures were captured by TRMM PR and categorized into two subtypes: ice anvils with an echo base of ≥6 km and mixed anvils with an echo base between 3 and 6 km. Visible and infrared signals for the anvils, which are from reflectance at 0.63 and 1.6 μm (hereafter referred to as RF1 and RF2, respectively) and the equivalent brightness temperatures of a black body at 3.7, 10.8, and 12.0 μm (hereafter referred to as TB3, TB4, and TB5, respectively), were derived simultaneously by use of TRMM VIRS. The findings reveal that the normalized frequency distribution (i.e., probability density functions, PDF) of anvil tops and bases have a bimodal distribution whereas that of anvil thickness has a single-peak curve. For visible signals, the PDF distribution of RF1 (RF2) for anvils, mixed anvils, and ice anvils has an approximately symmetric distribution with a tropics-wide averages of 0.74, 072, and 0.80 (0.21, 0.21, and 0.20), respectively. It can be concluded that ice anvils are optically thicker and contain many more ice-cloud droplets at the cloud top than mixed anvils. RF1 of anvils is usually lower over land than over ocean, by ~0.1, whereas RF2 of anvils is usually higher over land than over ocean, by ~0.3. This implies that anvil clouds have thinner optical depth and their cloud tops consist of many more small ice droplets over land than over ocean. For infrared signals, TB4 is regarded as a representative channel. The PDF distribution of TB4 for anvils and mixed anvils is broad, with tropics-wide averages of 229.2 and 232 K, respectively. They contain two peaks and the secondary peak lies at a much lower value. For ice anvils, the PDF distribution of TB4 is a single-peak curve with a tropics-wide average of 219.5 K. Mean TB4 of anvils, mixed anvils, and ice anvils is usually lower over land than over ocean, by ~3.6 K, ~1.6 K, and 5 K, respectively. In addition, land–ocean differences between RF1-to-RF2 reflectance ratios are more obvious than those between TB4 and TB5 brightness temperatures. Furthermore, these spectral signals have been proved to be a potentially useful way of revealing the presence of anvils over land and ocean, and especially for separating anvils into the mixed and ice subtypes. Finally, long-term mean values reveal that the spectral signals of anvils and of stratiform and convective precipitating clouds are indeed different, and that the difference varies with different geographical location. In real time, however, uncertainty still exists when only spectral signals are used to discriminate anvils from stratiform and convective precipitating clouds, which means active satellite observations are indispensable.

Unlike other cloud types, high-level clouds play an important role, often imposing a warming effect, in the earth–atmosphere radiative energy budget. In this paper, macro- and microphysical characteristics of cirrus clouds, such as their occurrence frequency, geometric scale, water content, and particle size, over northern China (land area, herein called the L area) and the Pacific Ocean (ocean area, herein the O area) are analyzed and compared based on CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) products from 1 January 2007 to 31 December 2010. Over both areas, statistical analysis shows that cirrus occurrence approached 33% in summer whereas it was only ~10% in winter, >50% of cirrus cloud thicknesses were in the range of ~(0.25–1.5) km, there were >98% ice particles in high-level clouds, and temperature had a closer linear relationship with ice effective radius (IER) than height. Also, the seasonal difference of this linear relationship is minor over both land and ocean. Comparisons reveal that the mean occurrence frequency, mean cloud thickness, range of cloud-base and cloud-top height, IER, and ice water content of cirrus in summer were generally greater in winter, and greater over the O area than over the L area. However, the relationship between IER and temperature over land is close to that over ocean.

A historical severe rainfall case in the metropolitan area of Beijing was studied using a C-band dual-polarization radar system. The QPE results from the evaluation was then validated using 5-min resolution gauge observation from an operational national weather station where is in the severe rainfall area. Results show that the rainfall estimation is more consistent with the surface rain gauge observation compared with the S-band single polarization observation and the algorithm of R(KDP): Brandes (2002) is the best for this case.

Diurnal and seasonal variation, intensity, and structure of deep convective systems (DCSs; with 20-dBZ echo tops exceeding 14 km) over the Tibetan Plateau–South Asian monsoon region from the Tibetan Plateau (TP) to the ocean are investigated using 14 yr of Tropical Rainfall Measuring Mission (TRMM) data. Four unique regions characterized by different orography are selected for comparison, including the TP, the southern Himalayan front (SHF), the South Asian subcontinent (SAS), and the ocean. DCSs and intense DCSs (IDCSs; with 40-dBZ echo tops exceeding 10 km) occur more frequently over the continent than over the ocean. About 23% of total DCSs develop into IDCSs in the SHF, followed by the TP (21%) and the SAS (15%), with the least over the ocean (2%). The average 20-dBZ echo-top height of IDCSs exceeds 16 km and 9% of them even exceed 18 km. DCSs and IDCSs are the most frequent over the SHF, especially in the westernmost SHF, where the intensity—in terms of strong radar echo-top (viz., 40 dBZ) height, ice-particle content, and lightning flash rate—is the strongest. DCSs over the TP are relatively weak in convective intensity and small in size but occur frequently. Oceanic DCSs possess the tallest cloud top (which mainly reflects small ice particles) and the largest size, but their convective intensity is markedly weaker. DCSs and IDCSs show a similar diurnal variation, mainly occurring in the afternoon with a peak at 1600 local time over land. Although most of both DCSs and IDCSs occur between April and October, DCSs have a peak in August, whereas IDCSs have a peak in May.

A method is proposed to simultaneously calibrate shortwave (0.3–4 μm) global, direct, and scattering solar irradiance (GSI, DSI, and SSI, respectively) measurements. The method uses the World Radiation Reference (WRR) as a calibration standard and on-site radiation measurements as inputs. Two simple but effective techniques are used in the calibration. The first is to scale SSI and GSI detection sensitivities under overcast skies, which is based on the assumption that SSI should be equal to GSI if DSI is completely scattered and absorbed. The second is a new method to retrieve aerosol optical thickness (AOT), using the ratio of horizontal DSI (HDSI) to GSI measurements under clear and clean conditions. Thereafter, retrieved AOTs are used to drive a radiative transfer model to calculate atmospheric transmittance and then a ratio of GSI to the transmittance. Deviation of this ratio to the WRR is regarded as an indicator of GSI uncertainty, and the calibration transfer coefficient is derived as the WRR ratio. The method is applied to calibrate radiation measurements at Xianghe, China, during 2005. It is estimated from the derived transfer coefficients on 36 clear and clean days that uncertainties of DSI, GSI, and SSI measurements are within −4.0% to 2.9%, −5.9% to 2.4%, and −6.1% to 4.9%, respectively. The calibration is further validated based on comparisons of AOT at 750 nm retrieved from HDSI/GSI to Aerosol Robotic Network (AERONET) AOT products. The maximum deviation between two AOT products is 0.026. The unique advantage of this method lies in its potential applications in correcting historic radiation measurements and monitoring radiometer performance.

The energetics of the third stage of a snowstorm over China was analyzed using ECWMF data. The analysis of the energy budget for the Middle East trough and the western Pacific trough that developed toward China on 25–28 January 2008 showed the advection of the geopotential by the ageostrophic wind to be both a crucial source and the primary sink of the eddy kinetic energy centers associated with the troughs. The magnitudes of the energy conversion terms, interaction kinetic energy conversion and baroclinic conversion, were too small to explain the development of the energy centers and the jet streaks. The energy centers gained energy at their entrance regions via the convergence of the ageostrophic geopotential fluxes, and then lost energy at their exit regions by the same fluxes. At the entrance regions, the fluxes converged, increasing the geopotential gradient, which generated a stronger geostrophic wind and higher kinetic energy, resulting in an ascending motion in this area. When the troughs moved to China, the ascending motion caused by the convergence of the fluxes at entrance region intensified the snowstorms over central and southern China.

A mesoscale weather research and forecasting (WRF) model was used to simulate a cold vortex that developed over Northeast China during June 19-23, 2010. The simulation used high vertical resolution to reproduce the key features of the cold vortex development. Characteristics of the associated stratosphere-troposphere exchange (STE), specifically the spatiotemporal distribution of the cross-tropopause mass flux (CTF), were investigated using the Wei formula. The simulation results showed that the net mass exchange induced by the cold vortex was controlled by stratosphere-to-troposphere transport (STT) processes. In the pre-formation stage of the cold vortex (i.e., the development of the trough and ridge), active exchange was evident. Over the lifecycle of the cold vortex, STT processes prevailed at the rear of the trough and moving vortex, whereas troposphere-to-stratosphere transport (TST) processes prevailed at the front end. This spatial pattern was caused by temporal fluctuations of the tropopause. However, because of the cancellation of the upward flux by the downward flux, the contribution of the tropopause fluctuation term to the net mass exchange was only minor. In this case, horizontal motion dominated the net mass exchange. The time evolution of the CTF exhibited three characteristics: (1) the predominance of the STT during the pre-formation stage; (2) the formation and development of the cold vortex, in which the CTF varied in a fluctuating pattern from TST to STT to TST; and (3) the prevalence of the STT during the decay stage.

The third precipitation episode of China’s great snowstorms of 2008 was analyzed using station observations and ECMWF six-hourly data. The variation of the shape of the upper-level subtropical jet played an important role in the rainfall over southern China. With the eastward movement of the trough, the jet shape changed from two straight jets to a tilting jet over China and then it moved southward. With these variations, the south-north movement of ascending flow and precipitation area over southern China occurred.

The spectral sampling rate and range of CO2 absorption bands are critical for the optimal design of hyperspectral instrument for CO2 observation satellite. Undersampling of spectra in space-based spectrometer significantly contaminates signals measured in the CO2 1.61 μm-band. The CO2 dry-air column (XCO2) error due to spectral undersampling can be up to ~1 ppm, which is the target precision of the Chinese Carbon Satellite (TanSat) for a single sounding. Undersampling error depends on surface albedo, solar zenith angle, and scattering properties in the atmosphere. The spectral sampling rate is recommended to be greater than 2.0 pixels per full width at half maximum to avoid undersampling. Reduction of spectral resolution and the use of narrower spectral regions can improve spectral sampling with little changes in CO2 retrieval sensitivity without losing much information. The full-band approach provides direct constraints on the wavelength-dependent surface albedo and particle scattering from the measurements. To keep a broader band, we recommend reduction of the spectral resolution by a factor of two.

An algorithm for retrieving the surface pressure from oxygen A-band measurements in the future Chinese CO2 satellite (CarbonSpec/TanSat) was developed. The full physical radiative transfer model, vector radiative transfer model based on successive order of scattering, which is based on the successive order of scattering approach, was used to simulate the measurements of CarbonSpec/TanSat, as well as the kernel matrix in the inversion algorithm, and then the surface pressure and other related atmospheric parameters such as aerosol optical depth (AOD), surface albedo, and temperature were derived through optimal estimation theory. Sensitivities of the algorithm to surface albedo, solar zenith angle (SZA), viewing zenith angle (VZA), aerosol type, and AOD were investigated, and the results showed that the absolute error of retrieved surface pressure increases with decreasing surface albedo or increasing SZA and VZA. An accuracy of

The mechanism of the CO2 diurnal cycle is a basis for investigating the carbon budget and its impacts on climate and environment change. Regional diurnal variations in CO2 concentration based on observations and modeling have been studied widely. However, few studies have focused on the pattern of the CO2 diurnal cycle in China. In this study, a three-dimensional global chemical transport model, Goddard Earth Observing System (GEOS)-Chem, was applied to simulate the CO2 concentration and its variation over China from 2004 to 2012. Meanwhile, we also analyzed the CO2 concentration as observed by two eddy covariance flux observation towers, one located in Beijing (39°58′N, 116°22′E) and one in Hefei (31°55′N, 117°10′E), using LI-COR 7500A infrared gas analyzers. Observations showed the amplitude of the CO2 diurnal cycle at Hefei to be larger than at Beijing, due to stronger ecological activities. GEOS-Chem successfully captured the main aspects of the diurnal cycle of the CO2 concentration in the boundary layer observed at both Beijing and Hefei. However, some discrepancies between the model and observations did exist; specifically, the model tended to underestimate the amplitude of the CO2 diurnal cycle. The data also showed that traffic emissions significantly enhanced the CO2 concentration in the boundary layer.

The ground-based Atmospheric Emitted Radiance Interferometer (AERI) was deployed in Shouxian, China in 2008 to measure down-welling infrared radiances with high resolution. Based on AERI observations, we propose a new method for retrieving vertical temperature and water vapor profiles in the planetary boundary layer (PBL). The method exclusively uses NCEP-2, a global reanalysis data as a first-guess profile in an iterative recursive algorithm. The PBL profiles of temperature and moisture under clear sky conditions in Shouxian have been retrieved using this new method. Compared with coincident radiosonde measurements, we find that AERI is able to obtain more accurate temperature and water vapor profiles in the PBL. The retrieval results with high temporal resolution can be used to monitor the PBL stability and evolution.

In the present reported study, the vertical distributions of local atmospheric refractivity were retrieved from groundbased GPS observations at low elevation angles. An improved optimization method was implemented at altitudes of 0–10 km to search for a best-fit refractivity profile that resulted in atmospheric delays most similar to the delays calculated from the observations. A ray-tracing model was used to simulate neutral atmospheric delays corresponding to a given refractivity profile. We initially performed a “theoretical retrieval”, in which no observation data were involved, to verify the optimization method. A statistical relative error of this “theoretical retrieval” (−2% to 2%) indicated that such a retrieval is effective. In a practical retrieval, observations were obtained using a dual-frequency GPS receiver, and its initial value was provided by CIRA86aQ_UoG data. The statistical relative errors of the practical retrieval range from −3% to 5% were compared with co-located radiosonde measurements. Results clearly revealed diurnal variations in local refractivity profiles. The results also suggest that the general vertical distribution of refractivity can be derived with a high temporal resolution. However, further study is needed to describe the vertical refractivity gradient clearly.

It has been several years since the Greenhouse Gases Observing Satellite (GOSAT) began to observe the distribution of CO2 and CH4 over the globe from space. Results from Thermal and Near-infrared Sensor for Carbon Observation-Cloud and Aerosol Imager (TANSO-CAI) cloud screening are necessary for the retrieval of CO2 and CH4 gas concentrations for GOSAT TANSO-Fourier Transform Spectrometer (FTS) observations. In this study, TANSO-CAI cloud flag data were compared with ground-based cloud data collected by an all-sky imager (ASI) over Beijing from June 2009 to May 2012 to examine the data quality. The results showed that the CAI has an obvious cloudy tendency bias over Beijing, especially in winter. The main reason might be that heavy aerosols in the sky are incorrectly determined as cloudy pixels by the CAI algorithm. Results also showed that the CAI algorithm sometimes neglects some high thin cirrus cloud over this area.

Typhoon is considered to play a key role in the dynamical exchange processes taking place between the troposphere and stratosphere. In this paper, the impact of typhoon on the ozone distribution in the upper troposphere and middle stratosphere is investigated using ozone profiles measured by Aura’s Ozone Monitoring Instrument and meteorological fields from reanalysis data. During the passage of Typhoon Hai-Tang in 2005 over the western North Pacific, low values of ozone column above 200 hPa and ozone mixing ratio between the upper troposphere and the middle stratosphere (from 200 to 50 hPa) are observed right above the typhoon’s track, a result due to the strong upward propagation of air associated with overshooting convection in typhoon. A comparative analysis of different stages of Hai-Tang suggests that in the region where typhoon has higher intensity the troposphere-to-stratosphere transport is enhanced. These results indicated that the typhoon has a significant impact on the ozone variation in the upper troposphere and the middle stratosphere.

The propagation of wave packets and its relationship with the subtropical jet was investigated for the period 26–29 January 2008 over southern China using ECMWF Interim re-analysis data. Wave packets propagated from the north to the south side of an upper front with eastward development along the upper front during this period. Due to the eastward development of propagation, the acceleration of geostrophic westerly winds shifted eastward along the front. There were two primary sources of the propagation of wave packets at around 30°N. The first was the temperature inversion layer below 500 hPa, and the second was baroclinic zones located along the polarward flank of the subtropical jet in the middle and upper troposphere. Most wave packets propagated horizontally from the baroclinic zones and then converged on the zero meridional gradients of zonal winds.

A mobile Rayleigh temperature lidar was deployed in Golmud (36.25°N 94.54°E), Qinghai in China for making measurements of mesosphere temperature from 55 up to 90 km. The mesospheric thermal structure was obtained during the winter seasons of Year 2013 and Year 2014. At the altitude of 57~79 km, lidar temperatures were much colder than the MSIS-00 model predictions. However, in the lower mesosphere region, lidar measurements were in reasonable agreement compared with the TIMED-SABER satellite data.

The local features of transient kinetic energy and available potential energy were investigated using ECMWF (European Centre for Medium-Range Weather Forecasts) Interim Reanalysis data for the stratospheric sudden warming (SSW) event of January 2009. The Western Europe high plays important roles in the propagation of the energy from North America to Eurasian. When the Western Europe high appeared and shifted eastward, energy conversions increased and energy propagated from North America to Eurasian as a form of interaction energy flow. The baroclinic conversion between transient-eddy kinetic energy (K e) and transient-eddy available potential energy (A e) and the horizontal advection of geopotential height were approximately one order of magnitude less than K e and A e generation terms. So, these terms were less important to this SSW event.

Naked-eye observation of cloud cover has widely resisted automation. Replacement of human observation by instruments is an inexorable trend for the development of ground-based macroscopic cloud observation. In this paper, cloud covers from an all-sky imager (ASI) are compared with those from a meteorological observer (MO) through field experiments performed at three sites in China. The correlation coefficient between ASI and MO is 0.77 for all cases. The ASI cloud fractions have great agreement with MO for clear sky, overcast sky, and sky loaded with low- and middle-level clouds. About 78% of the ASI cases had deviations between ±1 tenth compared to MO cloud cover. High-level cloud (or aerosol) is the main reason causing this difference. It is partially due to MO, who takes aerosol as high, thin cloud. Another reason might be that ASI made a wrong estimation for high-level cloud (or aerosol) because of its detector and the cloud-determination algorithm. Distinguishing high, thin cloud from aerosol is a challenge, and is the main problem that needs to be resolved for future developments of ASI. A new, improved method is discussed at the end of this paper.

In the paper, we propose a new method of identifying the clear sky based on the Atmospheric Emitted Radiance Interferometer (AERI). Using the Atmospheric Radiation Measurement (ARM) Mobile Facility (AFM) dataset in Shouxian in 2008, we simulate the downwelling radiances on the surface in the 8–12 μm window region using Line-By-Line Radiative Transfer Model (LBLRTM), and compare the results with the AERI radiances. The differences larger (smaller) than 3 mW (cm2 sr cm−1)−1 suggest a cloudy (clear) sky. Meanwhile, we develop the new algorithms for retrieving the zenith equivalent cloud base height (CBHe) and the equivalent emissivity (ɛe), respectively. The retrieval methods are described as follows. (1) An infinitely thin and isothermal blackbody cloud is simulated by the LBLRTM. The cloud base height (H) is adjusted iteratively to satisfy the situation that the contribution of the blackbody to the downwelling radiance is equal to that of realistic cloud. The final H is considered as CBHe. The retrieval results indicate that the differences between the CBHe and observational cloud base height (CBH) are much smaller for thick low cloud, and increase with the increasing CBH. (2) An infinitely thin and isothermal gray body cloud is simulated by the LBLRTM, with the CBH specified as the observed value. The cloud base emissivity (ɛc) is adjusted iteratively until the contribution of the gray body to the downwelling radiance is the same as that of realistic cloud. The corresponding ɛc is ɛe. The average ɛe for the low, middle, and high cloud is 0.967, 0.781, and 0.616 for the 50 cases, respectively. It decreases with the increasing CBH. The retrieval results will be useful for studying the role of cloud in the radiation budget in the window region and cloud parameterizations in the climate model.

The generation of stratospheric gravity waves (GWs) due to typhoon is simulated by using a meso-scale model (WRF) with a typhoon case, the Matsa in 2005. An 8-day model run that covers the major stages of the Matsa’s development reproduces the key features of the typhoon. For example, good agreements in the typhoon’s track, the intensity, and the spiral clouds, as well as mean state of stratosphere, are seen between the simulation results and the observation. Simulation results clearly show that with typhoon propagates northwestward, pronounced stratospheric GWs are generated continuously in the vicinity of Matsa. The GWs exhibit the typical curve-like wave fronts away from the Typhoon Matsa, and propagate preferentially in the upstream of the background winds. These characteristics reflect that the stratospheric GWs are closely associated with the typhoon, and thus the GWs are referred to as Tropical Cyclone related Gravity Waves (TC-GWs). The results also show that these waves should have a rather large horizontal scale so that the outmost wave fronts can be seen at the distance of ∼1000 km to the typhoon center in the horizontal plane of 20 km. This is consistent with the phenomenon of stratospheric TC-GWs with ∼1000 km horizontal scale disclosed by the previous observational analysis results.

The summertime ozone valley over the Tibetan Plateau is formed by two influences, the Asian summer monsoon (ASM) and air column variations. Total ozone over the Tibetan Plateau in summer was ∼33 Dobson units (DU) lower than zonal mean values over the ocean at the same latitudes during the study period 2005–2009. Satellite observations of ozone profiles show that ozone concentrations over the ASM region have lower values in the upper troposphere and lower stratosphere (UTLS) than over the non-ASM region. This is caused by frequent convective transport of low-ozone air from the lower troposphere to the UTLS region combined with trapping by the South Asian High. This offset contributes to a ∼20-DU deficit in the ozone column over the ASM region. In addition, along the same latitude, total ozone changes identically with variations of the terrain height, showing a high correlation with terrain heights over the ASM region, which includes both the Tibetan and Iranian plateaus. This is confirmed by the fact that the Tibetan and Iranian plateaus have very similar vertical distributions of ozone in the UTLS, but they have different terrain heights and different total-column ozone levels. These two factors (lower UTLS ozone and higher terrain height) imply 40 DU in the lower-ozone column, but the Tibetan Plateau ozone column is only ∼33 DU lower than that over the non-ASM region. This fact suggests that the lower troposphere has higher ozone concentrations over the ASM region than elsewhere at the same latitude, contributing ∼7 DU of total ozone, which is consistent with ozonesonde and satellite observations.

GTS1 digital radiosonde, developed by the Shanghai Changwang Meteorological Science and Technology Company in 1998, is now widely used in operational radiosonde stations in China. A preliminary comparison of simultaneous humidity measurements by the GTS1 radiosonde, the Vaisala RS80 radiosonde, and the Cryogenic Frostpoint Hygrometer (CFH), launched at Kunming in August 2009, reveals a large dry bias produced by the GTS1 humidity sensor. The average relative dry bias is in the order of 10% below 500 hPa, increasing rapidly to 30% above 500 hPa, and up to 55% at 310 hPa. A much larger dry bias is observed in the daytime, and this daytime effect increases with altitude. The GTS1 radiosonde fails to respond to humidity changes in the upper troposphere, and sometimes even in the middle troposphere. The failure of GTS1 in the middle and upper troposphere will result in significant artificial humidity shifts in radiosonde climate records at stations in China where a transition from mechanical to digital radiosondes has occurred. A comparison of simultaneous temperature observations by the GTS1 radiosonde and the Vaisala RS80 radiosonde suggests that these two radiosondes provide highly reproducible temperature measurements in the troposphere, but produce opposite biases for daytime and nighttime measurements in the stratosphere. In the stratosphere, the GTS1 shows a warm bias (

A springtime tropopause fold event, found to be related to a cold trough intrusion from the north, was detected in the northeastern Tibetan Plateau (TP) based on various observations. A nested high-resolution mesoscale model was employed to investigate the effect of orography on the stratosphere-troposphere exchange. The model was found to be able to capture plausible tropopause fold properties. The propagation of the tropopause fold changed significantly when the terrain height in the model was altered. However, decreasing the terrain height had no significant effect on the morphology of folds. When a fold passed over an elevated surface, a leeside jet stream and a layer of humid air in the middle troposphere tended to develop. This strong leeside descent of air masses and high mid-level potential instability (PI) could give rise to deep upward motions in the leeside and inject tropospheric air into the lower stratosphere. Besides, when the flow encounters an elevated surface, forced lifting together with mid-level PI can trigger deep convective motions on the windward slope. The troposphere to stratosphere transport was found to be persistent and almost stationary over the windward slope of the TP during the evolution of the fold.

A coupled chemical/dynamical model (SOCOL-SOlar Climate Ozone Links) is applied to study the impacts of future enhanced CO and NOx emissions over eastern China on regional chemistry and climate. The result shows that the increase of CO and NOx emissions has significant effects on regional chemistry, including NOx, CO, O3, and OH concentrations. During winter, the CO concentration is uniformly increased in the northern hemisphere by about 10 ppbv. During summer, the increase of CO has a regional distribution. The change in O3 concentrations near eastern China has both strong seasonal and spatial variations. During winter, the surface O3 concentrations decrease by about 2 ppbv, while during summer they increase by about 2 ppbv in eastern China. The changes of CO, NOx, and O3 induce important impacts on OH concentrations. The changes in chemistry, especially O3, induce important effects on regional climate. The analysis suggests that during winter, the surface temperature decreases and air pressure increases in central-eastern China. The changes of temperature and pressure produce decreases in vertical velocity. We should mention that the model resolution is coarse, and the calculated concentrations are generally underestimated when they are compared to measured results. However, because this model is a coupled dynamical/chemical model, it can provide some useful insights regarding the climate impacts due to changes in air pollutant emissions.