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The vertical structure of wind is a key factor in modulating air quality, from which the determination of boundary layer height (BLH) remains a major challenge. In this paper, we developed an improved threshold method to determine the BLH from radar wind profiler (RWP) measurements. The normalized signal-to-noise ratio (SNR) profiles were used instead of the original SNR profiles to avoid instrumental inconsistencies. Additionally, a peak filter was designed to indicate the BLH based on the maximum SNR by taking into account the multiple peaks in the SNR profile. This algorithm was then applied to the RWP measurements taken in the summer (June–July–August) of 2018 in Beijing to obtain the BLHs. Validation analyses suggested that the BLH retrievals from RWP exhibited high consistency with those from radiosondes, with an average correlation coefficient of 0.69 (0.66) and a root mean squared error of 0.39 (0.41) in the daytime (nighttime). Additionally, the major features of summertime BLHs in Beijing were examined. In particular, a distinct diurnal variation in BLH was observed with a peak (1630 ± 510 m) occurring at 0600 universal time coordinated (UTC) and a minimum (587 ± 343 m) at 2300 UTC. Therefore, the algorithm presented here has great potential to be applied to other regions to obtain reliable BLHs. The findings obtained here highlight the importance of vertical wind structure in air quality studies. [ABSTRACT FROM AUTHOR]

In this paper, the precipitable water vapor (PWV) data from Global Navigation Satellite System (GNSS) is introduced into ITU-R cloud attenuation model for higher temporal and spatial resolution for a tropical region. The revised model incorporates PWV value for estimation of cloud integrated liquid water content (ILWC) and then determines the cloud attenuation. In this study, ILWC along the propagation path is obtained by processing radiosonde vertical profile using the combination of a water vapor pressure cloud detection model and the Salonen and Uppala liquid water density model. From the analysis of 2 years of radiosonde data from eight sites within the tropical region, the results show that the ILWC along the path can be approximated by a power function relationship with the PWV. The estimated cloud attenuation using the improved model is compared with the values calculated using the ITU-R model and the cloud attenuation derived from a Ka-band beacon data. The results show that the proposed model is in good agreement with the ITU-R model at high percentage of time exceedance for the thin layer of stratus clouds and matches well with cloud attenuation suffered from beacon signal at low percentage of time exceedance for the thick layer of cumulonimbus clouds. [ABSTRACT FROM AUTHOR]

Based on the 30‐year (1988–2017) radiosonde observations, the climatological characteristics of the planetary boundary layer height (PBLH) over Japan, as well as the long‐term changes, are investigated. We firstly compared four different methods to determine the PBLH. Results show that the bulk Richardson number method has the best performance among all these methods. At all studied stations, monthly median PBLHs in the morning are generally higher than the ones in the evening. Bimodal structure of seasonal PBLHs are found over Japan. Daytime PBLH peaks in spring and autumn and achieves the minimum in summer and winter. Night‐time PBLH shows less seasonal variations. It is found that the PBLHs in the central and southern part of Japan could be dominant by the variations of surface temperature, while the variations of surface relative humidity (RH) may be responsible for the PBLHs in the northern part of Japan. Moreover, sea fog which is associated with high RH and temperature inversion also has an obvious influence on PBLHs in northern Japan. Trend study shows that PBLHs in most stations slightly increased during the past 30 years. [ABSTRACT FROM AUTHOR]

Water vapor (H2O) is the strongest greenhouse gas in our atmosphere. Hence, accurate measurements and a correct representation in global models of H2O in the upper troposphere/lower stratosphere (UTLS) are important for understanding and projecting climate. Here we compare balloon‐borne measurements of UTLS H2O, performed by cryogenic frostpoint hygrometers (CFH) and meteorological radiosondes (Vaisala RS41) during two intensive field campaigns in the Asian summer monsoon anticyclone region, with humidity data from three products of the European Centre for Medium‐range Weather Forecasts (ECMWF): operational analysis and forecast (termed OPERA), ERA‐Interim reanalysis, and the newly released ERA5 reanalysis. Taking CFH as a reference, we show that OPERA and ERA5 provide a more accurate representation of UTLS H2O than ERA‐Interim. In particular, OPERA and ERA5 similarly overestimate H2O mixing ratios by on average 0.7–0.8 ppmv (14–15%) and 0.7–0.9 ppmv (15–17%) at pressures 60–100 hPa, respectively, and both provide a good representation of the observed vertical distribution (including fine structures) and natural variability of UTLS H2O. In contrast, ERA‐Interim underestimates UTLS H2O by 0.6–1.7 ppmv (14–30%), and it fails to capture relevant features of the vertical distribution of UTLS H2O. At pressures (p) lower than 60 hPa, all three ECMWF products are in good agreement with CFH. Humidity measurements by RS41 show an average dry bias of 0.1–0.5 ppmv (3–9%) compared to CFH for 60–100 hPa, and a moist bias increasing with altitude for p < 60 hPa, exceeding 100% for p < 40 hPa. Key Points: ECMWF operational and ERA5 data show moist bias (14–17%) in UTLS water vapor compared to cryogenic frostpoint hygrometer (CFH)ERA‐Interim data shows dry bias (14–30%) in UTLS water vapor compared to CFHNew ERA5 reanalysis provides an improved representation of UTLS water vapor compared to ERA‐Interim reanalysis [ABSTRACT FROM AUTHOR]

As an important component of atmosphere, water vapor is highly involved in the global water cycle and energy exchange. To date, Global Navigation Satellite System (GNSS) and radiosonde techniques have been widely used to detect the water vapor content in the atmosphere. Recently, European Centre for Medium‐Range Weather Forecasts (ECMWF) has released the latest reanalysis data set, namely, ECMWF Re‐Analysis (ERA5), whose temporal and spatial resolutions have a significant improvement over its previous‐generation product of ERA‐Interim. This study aims to assess the consistency of precipitable water vapors (PWVs) derived from ERA5, ERA‐Interim, GNSS, and radiosondes during the entire year of 2017. The GNSS‐derived PWVs were obtained at 41 Crustal Movement Observation Network of China stations, while the radiosonde‐derived PWVs were acquired at the adjacent 41 radiosonde stations. The nationwide PWVs derived from the ERA5 and ERA‐Interim show root‐mean‐square (RMS) errors of 1.8 and 2.1 mm with respect to the GNSS PWVs and 2.7 and 2.8 mm with respect to the radiosonde PWVs, respectively. Besides, the RMS errors exhibit significant regional and seasonal differences. The nationwide relative RMS of the ERA5‐ and ERA‐Interim‐derived PWVs are 11.1% and 13.4% with respect to the GNSS PWVs and 16.2% and 17.8% with respect to the radiosonde PWVs, respectively. The relative RMS values show significant difference between the east and west sides of Hu line across China. Furthermore, the nationwide PWVs are obtained using GNSS data sets at over 200 Crustal Movement Observation Network of China stations to compare with the ERA5‐derived PWVs in China. Results indicate that the spatial distribution of the PWVs derived from the two data sources is quite consistent, suggesting a great application prospect of the ERA5 products in China. Key Points: Consistency of precipitable water vapors (PWVs) derived from ERA5, ERA‐Interim, GNSS, and radiosondes over China is evaluatedERA5‐derived PWV is indicated to have a higher accuracy than that acquired from its previous‐generation product of the ERA‐InterimPWV geographical distribution derived from ERA5 and GNSS is quite consistent [ABSTRACT FROM AUTHOR]

The knowledge regarding how the boundary layer height (BLH) changes over time is still poor. Here we analyze the spatial and temporal changes in radiosonde‐derived measurements of BLH over China from 1979 to 2016. A qualitatively robust and abrupt change of BLH occurred in 2004. Over the former period (from 1979 to 2003) a spatially uniform increase was found in the BLH, while over the latter period BLH decreased in a spatially nonuniform way. Second, the meteorological influence on the rising BLH was determined to have a negative association with soil moisture, lower tropospheric stability and relative humidity, and a positive association with the near‐surface temperature. Yet, there was a different influence of meteorology on the BLH over the latter period, where a negative association was revealed with Tsfc and relative humidity. These findings shed new light on the long‐term changes in air pollution across China. Key Points: Long‐term (1979‐2016) trend in the boundary layer height (BLH) over China was examined using newly released radiosonde dataRobust trend analyses revealed a significant temporal disparity in the trend of BLH, with a robust reversal occurring in 2004The meteorological influences on the rising and declining BLH were determined to be quite different [ABSTRACT FROM AUTHOR]

We correct some errors and give some more research for our paper published in Survey Review, 2016, 48(351), 385–391. This note is a correction owing to the use of the questionable surface meteorological measurements of the International GNSS Service (IGS) and also gives more sufficient studies to validate our method due to our inadequate study. Numerical weather model values of surface meteorological data were used to check the IGS surface meteorological measurements and long-time zenith tropospheric delay values at 76 radiosonde stations in Russia and near Russia were added as test data. Revised, updated and additional results were shown and analysed, which provide more evidence to show that the new proposed ISAAS model has a better accuracy than the Saastamoinen in Russia. [ABSTRACT FROM AUTHOR]

The observed relationships between boundary layer height (BLH) and PM2.5 on a national scale remain unclear due to the dearth of observations. Here we investigated this relationship from a unique perspective of thermodynamic stability in the planetary boundary layer (PBL), using summertime (June–August) soundings from China for the period from 2014 to 2017. For all three times of soundings (0800, 1400, and 2000 Beijing time [BJT]), positive (negative) PM2.5 concentrations anomalies were found to correlate with negative (positive) BLHs anomalies relative to daily means. The negative correlation was strongest at 1400 BJT, followed by 2000 BJT and 0800 BJT. Overall, the PM2.5 was found to nonuniformly anticorrelate with BLH across China at 0800 and 2000 BJT. The strongest anticorrelation occurred in the North China Plain at 1400 BJT, in sharp contrast to the much weaker correlation in other regions characterized by much less polluted regions. The averaged PM2.5 in neutral boundary layers was higher than that in convective boundary layers (CBLs). The CBL, where the anticorrelation was the strongest, was conducive to dissipating more aerosol in the heavily polluted area in China than neutral boundary layer. The higher CBL formed under low cloud cover, low surface humidity, and strong wind speed was favorable for the dispersion of aerosol, in contrast to the stable boundary layers that happen under the highest cloud cover. Also, positive correlation was seen between stable boundary layer and PM2.5. The findings call for attention that the thermodynamical condition of PBL should be considered when examining the aerosol‐PBL interactions. Key Points: The relationships between BLH and PM2.5 were elucidated at a national scale in China using four‐year record of radiosonde measurementsThe response of BLH to aerosol was investigated under three PBL regimes of CBL, NBL and SBLThe joint influence of meteorology for different PBL regimes was analyzed in association with PM2.5 [ABSTRACT FROM AUTHOR]

This study aims to determine the latitudinal and topographical dependencies of turbulent flow by applying Thorpe analysis to abundant high‐resolution radiosonde data sets. Results show that turbulent energy dissipation rates are lognormally distributed with average central values of 2.7 × 10−4 and 2.9 × 10−4 m2/s3 in the troposphere and stratosphere, respectively. Moreover, they exhibit similar vertical variations at different latitudes from the lower free troposphere up to the tropopause region. Dissipation rates are quantitatively comparable at all latitudes but do exhibit significant seasonal variability. Thus, these rates have apparent seasonal cycles at middle latitudes but weak cycles at low and high latitudes. The influence of terrain is an important feature of turbulent motions. Turbulent activities over mountainous topographies exhibit larger variances than those over flat terrains in the lower troposphere and more outstanding enhancements in the tropopause region. In general, these results are comparable with those of radars in terms of magnitude, and the differences between them can be related to different measurement principles. Key Points: Effects of latitude and underlying terrain on the climatological vertical distribution of turbulence are providedThe energy dissipation rate exhibits evident seasonal cycles at midlatitudes and follows a lognormal distributionThe magnitude and variance of the energy dissipation rate can be considerably influenced by underlying terrain [ABSTRACT FROM AUTHOR]

Previous modeling studies have indicated that the Oyashio front in the subarctic Pacific Ocean significantly affects the atmosphere on meso- to basin scales; however, there were no in situ observations that captured oceanic imprints on the atmosphere in this region as far as the authors know. We present in situ evidence of atmospheric responses to the Oyashio front by using a total of 103 radiosondes launched around the Oyashio front in April 2013 with continuous surface meteorology and ceilometer observations. Composite profiles showed that the lowlevel atmosphere below 1000 m was statically stable on the cold side of the Oyashio front, but unstable and mixed on the warm side. In the atmosphere on the warm side, the relative humidity dropped sharply at an altitude of around 1000 m, an indication that the mean cloud top was at this altitude. While the frequency of cloud base height peaked at 50 - 100 m in the cold areas, cloud bases were distributed at higher altitudes in the warm areas. These differences in the atmospheric boundary layer and cloud base heights across the front were clearer under conditions of southerly winds compared with those of northerly winds. Above a local sea surface temperature minimum with a width of approximately 400 km, where the ocean mixed layer depth is known to reach a local maximum, a large horizontal air temperature gradient was observed below an altitude of 1000 m. This horizontal gradient corresponded to a sea level pressure (SLP) anomaly of 1.2 hPa, comparable to observations of SLP anomalies in the Kuroshio Extension region. Furthermore, we found that narrow warm ocean streamers moistened the overlying atmosphere, affecting downward longwave radiation. Over the wide streamer located between 146.4°E and 147.0°E on 5 April, the near-surface atmospheric properties were largely different over the western half and the eastern half. [ABSTRACT FROM AUTHOR]

Long-term stability, global coverage and high resolution are characteristics that make the Global Navigation Satellite System (GNSS) radio occultation (RO) technique well-suitable to serve as anchor measurements for observing the Earth's atmosphere. The concept of occultation soundings utilizes a receiver placed on a low Earth orbit to measure the accumulated atmospheric contribution along the limb in terms of a phase delay. A high sampling rate allows to reconstruct profiles of geophysical parameters through an inversion process of occultation signals. However, such measurements require a careful processing in order to provide accurate retrievals in the neutral atmosphere. The following development describes specific aspects in radio occultation methodology implemented in the retrieval chain from phase data to profiles of dry pressure and dry temperature. Independent retrievals from nearreal time measurements are compared with occultation products provided by official processing centers to demonstrate reliability of the solution. The region within the upper troposphere and lower stratosphere (UTLS) is particularly represented by a low uncertainty being within 0.5% (K). A comparison with radiosondes shows a significant contribution of a water vapor term in the lower troposphere that comes from the dry air assumption in occultation profiles of pressure and temperature. Radiosonde measurements reproduced to refractivity profiles show very high agreement with occultation soundings, which is generally below 5%. A superior accuracy of RO refractivity is observed in the upper troposphere, where retrievals are consistent with radiosondes to 1%. [ABSTRACT FROM AUTHOR]

Abstract In this study, radiosonde observations from seven sites in Saudi Arabia for the period 1985 to 2016 were utilized to investigate the interannual, monthly, and seasonal variations and trends of precipitable water vapour (PWV). The magnitudes of these trends have been characterized and tested using the Mann-Kendall (MK) rank statistics at different significance levels. A significant decrease in the annual mean PWV by about 7% is found for the entire period. A seasonal cycle of PWV with a maximum during summer time and a minimum during winter has been found and can be mostly attributed to the variations of air temperature. On a monthly basis, the PWV values revealed a decreasing trend with the rate of decrease ranging between 0.47 and 2.6 mm per 32 years. There was a decrease in PWV in all the seasons, but it was only significant for the spring season, when it was the highest (1.79 mm per 32 years). Power spectra analyses using the Fourier Transform (FT) technique were carried out for the period 1985–2016 to investigate the periodicities in the PWV time series. Several long, mid, and short-term periodicities were recognized. Short-term periodicities such as one year, six months, three months, and four months were found. On the other hand, long and mid term periodicities such as 10.8–11 years, 1.7 years, and 1.3 years were detected. The obtained periodicities are similar to those reported by several investigators and found in solar, interplanetary, and cosmic ray parameters. The spectral results suggest that the obtained PWV periodicities in Arabian Peninsula are, possibly, related to the solar activities, as well as, the effect of terrestrial meteorological phenomena. Highlights • Variations and trends of precipitable water vapour (PWV) were investigated using radiosonde observations from seven sites in Saudi Arabia. • The magnitudes of these trends were characterized using the Mann-Kendall (MK) rank statistics at different significance levels. • A significant decrease in the annual mean PWV over Saudi Arabia by about 7% is found. • Several long- and short-term periodicities were recognized. • Some of these periodicities were associated with an extra-terrestrial origin (such as solar activity and cosmic ray). [ABSTRACT FROM AUTHOR]

Atmospheric data obtained from the radio occultation (RO) technique are a well-recognized source of information for weather and climate studies. From the Challenging Minisatellite Payload (CHAMP) mission launched in July 2000 to the most recent Constellation Observing System for Meteorology, Ionosphere, and Climate follow-on (COSMIC-2) program, a continuous RO dataset of about 20 years has been collected, and a new opportunity for long-term climate analyses using multi-mission RO observations has subsequently arisen. Therefore, assessments of the long-term homogeneities of multi-mission RO data have become a necessary research task. For this purpose, in this study, we identified systematic discrepancies between the RO temperature profiles from the CHAMP, COSMIC, and Meteorological Operational Polar Satellite (METOP) missions. The results show that the temperature profiles from all three RO missions agree well in the upper troposphere and lower stratosphere (UTLS, 9–20 km altitude) regions, while some systematic discrepancies are found in the lower troposphere (2–8 km) and the high-altitude region (21–30 km). The homogeneities of long-term RO temperature climatologies were assessed by comparing them with radiosonde temperature records. The results of this comparison show obvious temporal inhomogeneities in the lower troposphere. The reasons for these temporal inhomogeneities include the systematic discrepancies between multi-mission RO profiles, the different monthly numbers of RO profiles, and the residual sampling error. The results of this study suggest that the systematic discrepancies between different RO missions should be thoroughly considered in the development of long-term multi-mission RO-based climatologies.

In this study, we validated the consistency of the GRUAN RS92 and RS41 datasets, versions EDT.1 and GDP.2, in the upper troposphere and lower stratosphere (200–20 hPa), through dual launch campaigns at the GRUAN site and using the radio occultation (RO) product and the ERA5 reanalysis from ECMWF as standards for double difference comparison. Separate comparisons with the references were also performed in order to trace the origin of the bias between the two instruments. Then, the performance of the GRUAN raw temperature correction algorithm was evaluated, from the aspects of day–night, the solar zenith angle, and the pressure level, for GDP.2 version products. The results show that RS92.EDT.1 has a warm bias of 0.355 K, compared to RS41.EDT.1, at 20 hPa, during daytime. This bias was found to mainly originate from RS92.EDT.1, based on the separate comparison with RO or ECMWF ERA5 data. RS92.GDP.2 is consistent with RS41.GDP.2, but a separate comparison indicated that the two original GDP.2 products have a ~1 K warm bias at 20 hPa during daytime, compared with RO or ECMWF ERA5 data. The GRUAN correction method can reduce the warm bias up to 0.5 K at 20 hPa during daytime. As a result, this GRUAN correction method is efficient, and it is dependent on the solar zenith angle and pressure level.

In this work we designed and developed a cluster of light expendable radiosondes that can float passively inside warm clouds to study their micro-physical processes. This involves the tracking of both saturated and unsaturated turbulent air parcels. The aim of this new kind of observation system is to obtain Lagrangian statistics of the intense turbulence inside warm clouds and of the lower intensity turbulence that is typical of the air surrounding such clouds. Each radiosonde in a cluster includes an electronic board, which is mounted onto a small, biodegradable balloon filled with a mixture of helium and air. The cluster is able to float inside clouds for a few hours and to measure air temperature, pressure, humidity and the associated position, velocity, acceleration and magnetic field readings of each radiosonde along their trajectory. • In-field measurement system based on a cluster of radiosondes for studying physical processes in warm clouds. • Each radiosonde is able to track turbulent air parcels inside clouds and their surroundings. • Radiosondes measure temperature, pressure, humidity, and provide trajectory data. • Field tests conducted to validate sensor readings against reference values. [ABSTRACT FROM AUTHOR]

Accurate observation of clouds is challenging because of the high variability and complexity of cloud types and occurrences. By using the long-term cloud data collected during the ARM program at the Southern Great Plains central facility during 2001–2010, the consistencies and differences in the macrophysical properties of clouds between radiosonde and ground-based active remote sensing are quantitatively evaluated according to six cloud types: low; mid-low (ML); high-mid-low; mid; high-mid (HM); and high. A similar variability trend is exhibited by the radiosonde and surface observations for the cloud fractions of the six cloud types. However, the magnitudes of the differences between the two methods are different among the six cloud types, with the largest difference seen in the high clouds. The distribution of the cloud-base height of the ML, mid, and HM clouds agrees in both methods, whereas large differences are seen in the cloud-top height for the ML and high clouds. The cloud thickness variations generally agree between the two datasets for the six cloud types.

Quality controlled and homogenized radiosonde observations have been used to validate decadal hindcasts of the MPI-Earth-System-Model for Europe (excl. some Eastern European countries). Simulated temperatures have a cold bias of 1 to 4 K, increasing with height throughout the free troposphere over Europe. This implies that the simulated troposphere is less stable than observed by the radiosondes over Europe. Simulated relative humidity is 10 to 40 % higher than observed. Part of the humidity bias, 10 to 25 % relative humidity, is due to the simulated lower temperature, but the remainder indicates that modelled water vapour pressure is too high in the free troposphere above Europe. After full-field initialization with oceanic state, the atmospheric temperature bias changes over the first couple of years, with a relaxation time of 5 years near the surface (850 hPa) and less than 1 year near the tropopause (200 hPa). Anomaly correlations, mean-square error and logarithmic ensemble spread score indicate small improvements in hindcasted tropospheric temperatures over Europe when going from ocean anomaly initialisation to ocean anomaly initialisation plus full field atmospheric initialisation, and then to full field ocean initialisation plus full field atmospheric initialisation. In the stratosphere, these changes have little effect. For humidity, correlations and skill scores are much poorer, and little can be said about changes over Europe due to different initializations.

The Global Navigation Satellite System (GNSS) plays an important role in retrieving high temporal–spatial resolution precipitable water vapor (PWV) and its applications. The weighted mean temperature (Tm) is a key parameter for the GNSS PWV estimation, which acts as the conversion factor from the zenith wet delay (ZWD) to the PWV. The Tm is determined by the air pressure and water vapor pressure, while it is not available nearby most GNSS stations. The empirical formular is often applied for the GNSS station surface temperature (Ts) but has a lower accuracy. In this paper, the temporal and spatial distribution characteristics of the coefficients of the linear Tm-Ts model are analyzed, and then a piecewise-linear Tm-Ts relationship is established for each GPS station using radiosonde data collected from 2011 to 2019. The Tm accuracy was increased by more than 10% and 20% for 86 and 52 radiosonde stations, respectively. The PWV time series at 377 GNSS stations from the infrastructure construction of national geodetic datum modernization and Crustal Movement Observation Network of China (CMONC) were further obtained from the GPS observations and meteorological data from 2011 to 2019. The PWV accuracy was improved when compared with the Bevis model. Furthermore, the daily and monthly average values, long-term trend, and its change characteristics of the PWV were analyzed using the high-precision inversion model. The results showed that the averaged PWV was higher in Central-Eastern China and Southern China and lower in Northwest China, Northeast China, and North China. The PWV is increasing in most parts of China, while the some PWVs in North China show a downward trend.

Large-scale in situ observations are sorely lacking, leading to poor understanding of nationwide atmospheric turbulence over China. Nevertheless, high-resolution soundings have become available starting in 2011, providing a unique opportunity to investigate turbulence across China. Here, we calculated the mean turbulence dissipation rate ( ϵ ) from radiosonde measurements across China for the period 2011–2018 using Thorpe analysis. The atmospheric layers that had stronger turbulence indicated by larger ϵ generally came with larger Thorpe length but with smaller Brunt–Väisälä frequency. Overall, the clear-air ϵ in the free atmosphere exhibited large spatial variability with a ‘south-high north-low’ pattern. Large clear-air ϵ values were observed in both the lower stratosphere (LS) and upper troposphere (UT), especially over the Tibetan Plateau (TP) and its neighboring regions with complex terrain likely due to large-amplitude mountain waves. Particularly, less frequent but more intense clear-air turbulence was observed in both lower troposphere (LT) and UT over the TP, while more frequent, less intense clear-air turbulence was found in northern China. The all-sky turbulence considering the moist-saturation effects was much stronger in the troposphere, notably in southern China where convective clouds and precipitation oftentimes dominated. In the vertical direction, the altitude of peak clear-air ϵ in the troposphere was found to decrease poleward, broadly consistent with the meridional gradient of tropopause height in the Northern Hemisphere. A double-peak mode stood out for the profiles of clear-air ϵ at midlatitudes to the north of 30° N in winter: one peak was at altitudes of 15–18 km, and another at altitudes of 5–8 km. The strong shear instabilities around the westerly jet stream could account for the vertical bimodal structures. The seasonality of ϵ was also pronounced, reaching maxima in summer and minima in winter. Our results may help understand and avoid clear-air turbulence, as related to aviation safety among other issues.

From late 2019 to early 2020, forest fires in southeastern Australia caused huge economic losses and huge environmental pollution. Monitoring forest fires has become increasingly important. A new method of fire detection using the difference between global navigation satellite system (GNSS)-derived precipitable water vapor and radiosonde-derived precipitable water vapor (ΔPWV) is proposed. To study the feasibility of the new method, the relationship is studied between particulate matter 10 (PM10) (2.5 to 10 microns particulate matter) and ΔPWV based on Global Positioning System (GPS) data, radiosonde data, and PM10 data from 1 June 2019 to 1 June 2020 in southeastern Australia. The results show that before the forest fire, ΔPWV and PM10 were smaller and less fluctuating. When the forest fire happened, ΔPWV and PM10 were increasing. Then after the forest fire, PM10 became small with relatively smooth fluctuations, but ΔPWV was larger and more fluctuating. Correlation between the 15-day moving standard deviation (STD) time series of ΔPWV and PM10 after the fire was significantly higher than that before the fire. This study shows that ΔPWV is effective in monitoring forest fires based on GNSS technique before and during forest fires in climates with more uniform precipitation, and using ΔPWV to detect forest fires based on GNSS needs to be further investigated in climates with more precipitation and severe climate change.

Global Positioning System (GPS) radio occultation (RO) and radiosonde (RS) observations are two major types of observations assimilated in numerical weather prediction (NWP) systems. Observation error variances are required input that determines the weightings given to observations in data assimilation. This study estimates the error variances of global GPS RO refractivity and bending angle and RS temperature and humidity observations at 521 selected RS stations using the three-cornered hat method with additional ERA-Interim reanalysis and Global Forecast System forecast data available from 1 January 2016 to 31 August 2019. The global distributions, of both RO and RS observation error variances, are analyzed in terms of vertical and latitudinal variations. Error variances of RO refractivity and bending angle and RS specific humidity in the lower troposphere, such as at 850 hPa (3.5 km impact height for the bending angle), all increase with decreasing latitude. The error variances of RO refractivity and bending angle and RS specific humidity can reach about 30 N-unit2, 3 × 10−6 rad2, and 2 (g kg−1)2, respectively. There is also a good symmetry of the error variances of both RO refractivity and bending angle with respect to the equator between the Northern and Southern Hemispheres at all vertical levels. In this study, we provide the mean error variances of refractivity and bending angle in every 5°-latitude band between the equator and 60°N, as well as every interval of 10 hPa pressure or 0.2 km impact height. The RS temperature error variance distribution differs from those of refractivity, bending angle, and humidity, which, at low latitudes, are smaller (less than 1 K2) than those in the midlatitudes (more than 3 K2). In the midlatitudes, the RS temperature error variances in North America are larger than those in East Asia and Europe, which may arise from different radiosonde types among the above three regions.

In this paper, we assess, in the framework of Global Navigation Satellite System (GNSS) meteorology, the accuracy of GNSS propagation delays corresponding to the Saastamoinen zenith hydrostatic delay (ZHD) model and the Vienna Mapping function VMF1/VMF3 (hydrostatic and wet), with reference to radiosonde ray-tracing delays over a three-year period on 28 globally distributed sites. The results show that the Saastamoinen ZHD estimates have a mean root mean square (RMS) error of 1.7 mm with respect to the radiosonde. We also detected some seasonal signatures in these Saastamoinen ZHD estimates. This indicates that the Saastamoinen model, based on the hydrostatic assumption and the ground pressure, is insufficient to capture the full variability of the ZHD estimates over time with the accuracy needed for GNSS meteorology. Furthermore, we found that VMF3 slant hydrostatic delay (SHD) estimates outperform the corresponding VMF1 SHD estimates (equivalent SHD RMS error of 4.8 mm for VMF3 versus 7.1 mm for VMF1 at 5° elevation angle), with respect to the radiosonde SHD estimates. Unexpectedly, the situation is opposite for the VMF3 slant wet delay (SWD) estimates compared to VMF1 SWD estimates (equivalent SWD RMS error of 11.4 mm for VMF3 versus 7.0 mm for VMF1 at 5° elevation angle). Our general conclusion is that the joint approach using ZHD models and mapping functions must be revisited, at least in the framework of GNSS meteorology.

A new concept is proposed for estimating the zenith wet delay (ZWD) and atmospheric weighted average temperature by inputting the temperature, total pressure, and specific humidity from surface weather data. In addition, a new ZWD integral method is described for highly accurate calculation of the ZWD from radiosonde observation. To evaluate the advantages of the new discrete integral formula, we utilized the 8-year radiosonde profiles of 85 stations in China from 2010 to 2017 to validate the accuracy of the radiosonde-derived ZWD. The results showed that the mean accuracy of the ZWD derived from radiosonde data was 4.28 mm. Next, the new ZWD model was assessed using two sets of reference values derived from radiosonde data and GNSS precise point positioning in China. The results confirm that the new development improved the accuracy of the estimation of the tropospheric wet delay from the surface meteorological data. The performance of this new model can be seen as an important step toward accurately correcting the tropospheric delay in Global Navigation Satellite System (GNSS) real-time navigation and positioning. It can also be used in GNSS meteorology for weather forecasting and climate research.

Global navigation satellite system (GNSS) tomography can effectively sense the three-dimensional structure of tropospheric water vapor (WV) using the GNSS observations. Numerous studies have utilized a tomographic window to include more epochs of observations, which significantly increases the number of valid signals. However, considering the tomography grid limits, a massive number of valid signals inevitably exhibits linear dependence. This dependence makes it impossible to improve the rank score of the tomography coefficient matrix by blindly introducing a large number of valid rays. Furthermore, excessive valid signals may lead to a high condition number in the coefficient matrix (ill-condition problem), which causes unstable results using the GNSS-WV tomography. Considering these problems, we proposed an improved tomographic approach, which applies a refined tomographic window. It differs from the general tomographic window in that the window is refined to traverse the valid signals available 15 min before and after the target epoch while retaining only the linearly independent parts (characteristic signal). Compared to the conventional method, the proposed method can filter the characteristic signal, which increases the rank score of the coefficient matrix and improves the stability of the tomography model. In this paper, we used GNSS observations from the Hong Kong Satellite Positioning Reference Station Network (SatRef) to validate the performance of the proposed method over the day-of-year (DOY) periods of 130–132, 2019 and 146–148, 2019. The numerical results showed that, by using a refined tomographic window, the proposed method obtained superior WV products in comparison with that of the conventional method.

Global navigation satellite systems (GNSSs) have become an important tool to derive atmospheric products, such as the total zenith tropospheric delay (ZTD) and precipitable water vapor (PWV) for weather and climate studies. The ocean tide loading (OTL) effect is one of the primary errors that affects the accuracy of GNSS-derived ZTD/PWV, which means the study and choice of the OTL model is an important issue for high-accuracy ZTD estimation. In this study, GNSS data from 1 January 2019 to 31 January 2019 are processed using precise point positioning (PPP) at globally distributed stations. The performance of seven widely used global OTL models is assessed and their impact on the GNSS-derived ZTD is investigated by comparing them against the ZTD calculated from co-located radiosonde observations. The results indicate that the inclusion or exclusion of the OTL effect will lead to a difference in ZTD of up to 3–15 mm for island stations, and up to 1–2 mm for inland stations. The difference of the ZTD determined with different OTL models is quite small, with a root-mean-square (RMS) value below 1.5 mm at most stations. The comparison between the GNSS-derived ZTD and the radiosonde-derived ZTD indicates that the adoption of OTL models can improve the accuracy of GNSS-derived ZTD. The results also indicate that the adoption of a smaller cutoff elevation, e.g., 3° or 7°, can significantly reduce the difference between the ZTDs determined by GNSS and radiosonde, when compared against a 15° cutoff elevation. Compared to the radiosonde-derived ZTD, the RMS error of GNSS-derived ZTD is approximately 25–35 mm at a cutoff elevation of 15°, and 15–25 mm when the cutoff elevation is set to 3°.

The vertical profiles of temperature and water vapour from the Atmospheric InfraRed Sounder (AIRS) have been validated across various regions of the globe as an effort to provide a substitute for radiosonde observations. However, there is a paucity of inter-comparisons over West Africa where local convective processes dominate and radiosonde observations (RAOBs) are limited. This study validates AIRS temperature and relative humidity profiles for selected radiosonde stations in West Africa. Radiosonde data were obtained from the AMMA and DACCIWA campaigns which spanned 2006–2008 and June–July 2016 respectively and offered a period of prolonged radiosonde observations in West Africa. AIRS performance was evaluated with the bias and root mean square difference (RMSD) at seven RAOB stations which were grouped into coastal and inland. Evaluation was performed on diurnal and seasonal timescales, cloud screening conditions and derived thunderstorm instability indices. At all timescales, the temperature RMSD was higher than the AIRS accuracy mission goal of ±1 K. Relative humidity RMSD was satisfactory with deviations

The Global Navigation Satellite System (GNSS) Radio Occultation (RO) retrieved temperature and specific humidity profiles can be widely used for weather and climate studies in troposphere. However, some aspects, such as the influences of background data on these retrieved moist profiles have not been discussed yet. This research evaluates RO retrieved temperature and specific humidity profiles from Wegener Center for Climate and Global Change (WEGC), Radio Occultation Meteorology Satellite Application Facility (ROM SAF) and University Corporation for Atmospheric Research (UCAR) Boulder RO processing centers by comparing with measurements from 10 selected Integrated Global Radiosonde Archive (IGRA) radiosonde stations in different latitudinal bands over 2007 to 2010. The background profiles used for producing their moist profiles are also compared with radiosonde. We found that RO retrieved temperature profiles from all centers agree well with radiosonde. Mean differences at polar, mid-latitudinal and tropical stations are varying within ±0.2 K, ±0.5 K and from −1 to 0.2 K, respectively, with standard deviations varying from 1 to 2 K for most pressure levels. The differences between RO retrieved and their background temperature profiles for WEGC are varying within ±0.5 K at altitudes above 300 hPa, and the differences for ROM SAF are within ±0.2 K, and that for UCAR are within 0.5 K at altitudes below 300 hPa. Both RO retrieved and background specific humidity above 600 hPa are found to have large positive differences (up to 40%) against most radiosonde measurements. Discrepancies of moist profiles among the three centers are overall minor at altitudes above 300 hPa for temperature and at altitudes above 700 hPa for specific humidity. Specific humidity standard deviations are largest at tropical stations in June July August months. It is expected that the outcome of this research can help readers to understand the characteristics of moist products among centers.

The atmospheric boundary layer height is important for constraining air pollution and meteorological models. This study attempted to validate the MODIS-estimated atmospheric boundary layer height (ABLH), and variation in the ABLH in Uganda was evaluated. The ABLH was estimated from MODIS data using the mixing ratio profile gradient method and compared to the ABLH estimated from radiosonde data using three different methods. Unlike in studies in other regions of the world, correlations between ABLH estimated using MODIS and radiosonde data were weak, implying limited usefulness of MODIS data for determining ABLH. However, the diurnal variation in MODIS-derived ABLH and particulate matter (PM10) was consistent with the expected inverse relationship between PM10 mass concentration and ABLH, and the mean MODIS-derived ABLH values were significantly lower during wet seasons than dry seasons, as expected.

We investigated the accuracy of operational medium-range ensemble forecasts for 29 Atlantic hurricanes between 2007 and 2019. Upper-level troughs with strong wind promoted northward movement of hurricanes over the mid-latitudes. For hurricanes with upper-level troughs, relatively large errors in the prediction of troughs result in large ensemble spreads, which result in failure to forecast hurricane track. In contrast, for hurricanes without upper-level troughs, mean central position errors are relatively small in all operational forecasts because of the absence of upper-level strong wind around troughs over the mid-latitudes. Hurricane Irma in September 2017 was accompanied by upper-level strong wind around a trough; errors and ensemble spreads for the predicted upper-level trough are small, contributing to smaller errors and small ensemble spreads in the predicted tracks of Irma. Our observing system experiment reveals that inclusion of additional Arctic radiosonde observation data obtained from research vessel Mirai in 2017 improves error and ensemble spread in upper-level trough with strong wind at initial time for forecast, increasing the accuracy of the forecast of the track of Irma in 2017.

Conventional radiosondes can be used to measure the relative humidity over liquid (RHL) by assuming a saturated vapor pressure over the liquid. However, this assumption results in significant errors with respect to measurements in the upper troposphere, where the effect of ice is dominant. Therefore, this study presents a novel method that considers the effects of ice to determine the relative humidity from radiosonde RHL data for the last 40 years (1979–2018) over the upper layers of the Korean peninsula. Even though the relative humidity obtained from the reanalysis data was significantly different from the radiosonde-based RHL, the difference was much reduced when relative humidity was calculated using the novel method proposed in this study. Such improvements in the estimated relative humidity could be attributed to the consideration of the ice effect at temperatures above freezing level. Additionally, the validity of the relative humidity estimated in this study was established based on a two-week case analysis of data from Boseong station. Furthermore, two peak relative humidity modes for the lower and upper layers were clearly identified in the mean climatology profiles, which indirectly suggested the absence of mid-level clouds around the 700-hPa level and 500-hPa level in winter and summer, respectively. This study is meaningful as it is the first study to determine the relative humidity distribution over the Korean peninsula using radiosonde observations. The scientific value obtained can potentially be expanded by applying the proposed method to other radiosonde observation networks, which are widely distributed worldwide.

The planetary boundary layer height (PBLH) is a vital parameter to characterize the surface convection, which determines the diffusion of air pollutants. The accurate inversion of PBLH is extremely important for the study of aerosol concentrations, in order to predict air quality and provide weather forecast. Aerosol lidar, a powerful remote sensing instrument for detecting the characteristics of atmospheric temporal and spatial evolution, can continuously retrieve the planetary boundary layer (PBL) and obtain high resolution measurements. However, multi-layer conditions, including one or more layers of aerosol, or cloud above the PBL, can seriously interfere the accuracy of PBLH determined by lidar. A new technique of maximum limited height initialization and range restriction (MLHI-RR) is proposed to eliminate the impact of multi-layer conditions on PBLH determination. Four widely used methods for deriving PBLH are utilized, in addition to the MLHI-RR constraint. Comparisons demonstrate that the proposed technique can determine the PBLH in multi-layer conditions with higher accuracy. The proposed technique requires no affiliate information besides lidar signals, which provide a convenient method for PBLH determination under complicated conditions.

The Global Navigation Satellite System (GNSS) tomographic technique can be used for remote sensing of the three-dimensional water vapor (WV) distribution in the troposphere, which has attracted considerable interest. However, a significant problem in this technique is the excessive reliance on constraints (particularly in large GNSS networks). In this paper, we propose an improved tomographic method based on optimized voxel, which only considers the voxels passed by GNSS rays. The proposed method can completely prevent the tomographic algorithm interference of constraints that originated from empirical functions. Experiments in Nanjing in the periods of day-of-year (DOY) 182–184, 2019, and 244–246, 2019, show that the mean absolute error (MAE) and root mean square error (RMSE) of the WV density profile obtained using the proposed method are 0.9 and 1.3 g/m3, while those obtained using the conventional method are 1.3 and 1.8 g/m3, respectively, with respect to the radiosonde (RS) method. The numerical results show that the proposed method is reliable and has a superior accuracy to that of the conventional method.

We estimate precipitable water vapor (PWV) from data collected by the low-cost Global Navigation Satellite System (GNSS) receiver at a vessel. The dual-frequency GNSS receiver that the vessel ISABU is equipped with that is operated by the Korea Institute of Ocean Science and Technology. The ISABU served in the Pacific Ocean for scientific research during a period from August 30 to September 21, 2018. It also performs radiosonde observations to obtain a vertical profile of troposphere on the vessel’s path. The GNSS-derived PWV is compared to radiosonde observations and the Atmospheric Infrared Sounder (AIRS) on NASA’s Aqua satellite output. A bias and root-mean-square (RMS) error between shipborne GNSS-PWV and radiosonde-PWV were −1.48 and 5.22 mm, respectively. When compared to the ground GNSS-PWV, shipborne GNSS-PWV has a relatively large RMS error in comparison with radiosonde-PWV. However, the GNSS observations on the vessel are still in good agreement with radiosonde observations. On the other hand, the GNSS-PWV is not well linearly correlated with AIRS-PWV. The RMS error between the two observations was approximately 8.97 mm. In addition, we showed that the vessel on the sea surface has significantly larger carrier phase multipath error compared to the ground-based GNSS observations. This also can result in reducing the accuracy of shipborne GNSS-PWV. However, we suggest that the shipborne GNSS has sufficient potential to derive PWV with the kinematic precise point positioning (PPP) solution on the vessel.

Global Navigation Satellite System (GNSS) tomography has developed into an efficient tool for sensing the high spatiotemporal variability of atmospheric water vapor. The integration of GNSS top signals and side rays for tropospheric tomography systems using a novel height factor model (HFM) is proposed and discussed in this paper. Within the HFM, the sectional slant wet delay (SWD) of inside signals (the part of the side signal inside the tomography area), which is considered a key factor for modeling side rays, is separated into isotropic and anisotropic components. Correspondingly, two height factors are defined to calculate the isotropic and anisotropic part of tropospheric delays in the HFM. In addition, the dynamic tomography top boundary is first analyzed and determined based on 30-year radiosonde data to reasonably divide signals into top and side rays. Four special experimental schemes based on different tomography regions of Hong Kong are performed to assess the proposed HFM method, the results of which show increases of 33.42% in the mean utilization of rays, as well as decreases of 0.46 g/m3 in the average root mean square error (RMSE), compared to the traditional approach, revealing the improvement of tomography solutions when the side signals are included in the modeling. Furthermore, compared with the existing correction model for modeling side rays, the water vapor profiles retrieved from the proposed improved model are closer to the radiosonde data, which highlights the advantages of the proposed HFM for optimizing the GNSS tomography model.

The turbulent mixing and dispersion of air pollutants is strongly dependent on the vertical structure of the wind, which constitutes one of the major challenges affecting the determination of boundary layer height (BLH). Here, an adaptive method is proposed to estimate BLH from measurements of radar wind profilers (RWPs) in Beijing (BJ), Nanjing (NJ), Chongqing (CQ), and Wulumuqi (WQ), China, during the summer of 2019. Validation against simultaneous BLH estimates from radiosondes (RSs) yielded a correlation coefficient of 0.66, indicating that the method can be used to derive BLH from RWPs. Diurnal variations of BLH and the ventilation coefficient (VC) at four sites were then examined. A distinct diurnal cycle of BLH was observed over all four cities; BLH gradually increased from sunset, reached a maximum in the afternoon, and then dropped sharply after sunset. The maximum hourly average BLH (1.426 ± 0.46 km) occurred in WQ, consistent with the maximum hourly mean VC larger than 5000 m2/s observed there. By comparison, the diurnal variation of VC was not strong, with values ranging between 2000 and 3000 m2/s, likely owing to the high-humidity environment. Furthermore, surface sensible heat flux, latent heat flux, and dry mass of particulate matter with aerodynamic diameter ≤2.5 µm (PM2.5) concentrations were found to somehow affect the vertical structure of wind and thermodynamic features, leading to a difference between RS and RWP BLH estimates. This indicates that the atmospheric environment can affect BLH estimates using RWP data. The BLH results from RWPs were better in some specific cases. These findings show great potential of RWP measurements in air quality research, and will provide key data references for policy-making toward emission reductions.

In this study, sensors mounted on a new type of round-trip airborne flat-floating sounding system (RTAFSS) were used to obtain the observation data of the three stages of "rising, flat-floating and falling". This new sounding method has a good application prospect. We performed spectrum analysis on the normalized temperature fluctuation, and the vertical wavenumber spectrum from the rising and falling stages and the horizontal wavenumber spectrum from the flat-floating stage were obtained. This is the first time the complete gravity wave spectrum characteristics were obtained from three consecutive stages: rising, flat-floating and falling. The results show that the gravity wave spectrum of the three stages can be well obtained by RTAFSS. For the horizontal wavenumber spectrum, the spectral slope is basically around −2, and the difference in the spectral structure of the horizontal wave number spectrum may be due to the intermittent turbulent activity and the variable intensity of the gravitational wave during its propagation. This study aims to make experimental exploration of the spectrum characteristics of gravity waves by this new type of observation data. It is expected to reveal the spectrum characteristics of horizontal wavenumber in the stratosphere region of China, providing a theoretical basis for spectrum analysis in a wider space–time range after further network observation of RTAFSS.

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.