Validation of integrated water vapor from OMI satellite instrument against reference GPS data at the Iberian Peninsula

This paper shows the validation of integrated water vapor (IWV) measurements retrieved from the Ozone Monitoring Instrument (OMI), using as reference nine ground-based GPS stations in the Iberian Peninsula. The study period covers from 2007 to 2009. The influence of two factors, - solar zenith angle (SZA) and IWV -, on OMI-GPS differences was studied in detail, as well as the seasonal dependence. The pseudomedian of the relative differences is −1 ± 1% and the inter-quartile range (IQR) is 41%. Linear regressions calculated over each station show an acceptable agreement (R2 up to 0.77). The OMI-GPS differences display a clear dependence on IWV values. Hence, OMI substantially overestimates the lower IWV data recorded by GPS (∼40%), while underestimates the higher IWV reference values (∼20%). In connection to this IWV dependence, the relative differences also show an evident SZA dependence when the whole range of IWV values are analyzed (OMI overestimates for high SZA values while underestimates for low values). Finally, the seasonal variation of the OMI-GPS differences is also associated with the strong IWV dependence found in this validation exercise.This work was supported by the Spanish Ministry of Economy and Competitiveness through project CGL2014-56255-C2. Manuel Antón thanks Ministerio de Ciencia e Innovación and Fondo Social Europeo (RYC-2011-08345) for the award of a postdoctoral grant (Ramón y Cajal). Support from the Junta de Extremadura (Research Group Grants GR15137) is gratefully acknowledged. Work at Universidad de Valladolid is supported by project CMT2015-66742-R. Work at Universidad de Granada was supported by the Andalusia Regional Government (project P12-RNM-2409) and the Spanish Ministry of Economy and Competitiveness and FEDER funds under the projects CGL2013-45410-R and “Juan de la Cierva-Formación” program. Work at SAO is supported by NASA’s Atmospheric Composition: Aura Science Team program (sponsor contract number NNX14AF56G). Work at Universidade de Évora is co-funded by the European Union through the European Regional Development Fund, included in the COMPETE 2020 (Operational Program Competitiveness and Internationalization) through the ICT project (UID/GEO/04683/2013) with the reference POCI-01-0145-FEDER-007690

Editorial for the special issue “Remote sensing of atmospheric components and water vapor”

Producción CientíficaThe observation/monitoring of atmospheric components and water vapor in the atmosphere is today open to very different remote sensing techniques, most of them based on the radiation-matter interaction covering the full electromagnetic spectrum. This SI collects some papers regarding the retrieval, calibration, validation, analysis of data and uncertainties, as well as comparative studies on atmospheric gases and water vapor by remote sensing techniques, where different types of sensors, instruments, and algorithms are used or developedMinisterio de Ciencia, Innovación y Universidades (RTI2018-097864-B-I00)Junta de Extremadura - FEDER (IB18092

Comparison of integrated water vapor from GNSS and radiosounding at four GRUAN stations

Integrated water vapor (IWV) data from Global Navigation Satellite Systems (GNSS) and radiosounding (RS) are compared over four sites (Lindenberg, Ny-Ålesund, Lauder and Sodankylä), which are part of the Global Climate Observing System (GCOS) Reference Upper Air Network (GRUAN). Both datasets show an excellent agreement, with a high degree of correlation (R2 over 0.98). Dependences of GNSS-RS differences on several variables are studied in detail. Mean bias error (MBE) and standard deviation (SD) increase with IWV, but in relative term, these variables decrease as IWV increases. The dependence on solar zenith angle (SZA) is partially related to the distribution of IWV with SZA, but the increase of SD for low SZA could be associated with errors in the humidity sensor. Large surface pressures worsen performance, which could be due to the fact that low IWV is typically present in high pressure situations. Cloud cover shows a weak influence on the mentioned MBE and SD. The horizontal displacement of radiosondes generally causes SD to increase and MBE to decrease (increase without sign), as it could be expected. The results point out that GNSS measurements are useful to analyze performance to other instruments measuring IWV.Support from the Junta de Extremadura (Research Group Grants GR15137) is gratefully acknowledged. Work at Universidad de Valladolid is supported by project CMT2015-66742-R

Evaluation of precipitable water vapor from five reanalysis products with ground-based GNSS observations

At present, the global reliability and accuracy of Precipitable Water Vapor (PWV) from different reanalysis products have not been comprehensively evaluated. In this study, PWV values derived by 268 Global Navigation Satellite Systems (GNSS) stations around the world covering the period from 2016 to 2018 are used to evaluate the accuracies of PWV values from five reanalysis products. The temporal and spatial evolution is not taken into account in this analysis, although the temporal and spatial evolution of atmospheric flows is one of the most important information elements available in numerical weather prediction products. The evaluation results present that five reanalysis products with PWV accuracy from high to low are in the order of the fifth generation of European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA5), ERA-Interim, Japanese 55-year Reanalysis (JRA-55), National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), and NCEP/DOE (Department of Energy) according to root mean square error (RMSE), bias and correlation coefficient. The ERA5 has the smallest RMSE value of 1.84 mm, while NCEP/NCAR and NCEP/DOE have bigger RMSE values of 3.34 mm and 3.51 mm, respectively. The findings demonstrate that ERA5 and two NCEP reanalysis products have the best and worst performance, respectively, among five reanalysis products. The differences in the accuracy of the five reanalysis products are mainly attributed to the differences in the spatial resolution of reanalysis products. There are some large absolute biases greater than 4 mm between GNSS PWV values and the PWV values of five reanalysis products in the southwest of South America and western China due to the limit of terrains and fewer observations. The accuracies of five reanalysis products are compared in different climatic zones. The results indicate that the absolute accuracies of five reanalysis products are highest in the polar regions and lowest in the tropics. Furthermore, the effects of different seasons on the accuracies of five reanalysis products are also analyzed, which indicates that RMSE values of five reanalysis products in summer and in winter are the largest and the smallest in the temperate regions. Evaluation results from five reanalysis products can help us to learn more about the advantages and disadvantages of the five released water vapor products and promote their applications.Peer ReviewedPostprint (published version

Precipitable water vapor over oceans from the Maritime Aerosol Network: Evaluation of global models and satellite products under clear sky conditions

We present results from an evaluation of precipitable water vapor (W) over remote oceanic areas as derived from global reanalysis models and from satellites against observations from the Maritime Aerosol Network (MAN) for cloudless skies during the period of 2004–2017. They cover polar, mid latitude and tropical oceanic regions and represent a first effort to use MAN observations for such evaluation. The global reanalysis model products evaluated in this study are from the Modern- Era Retrospective analysis for Research and Applications Version 2 (MERRA-2), the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA I), and the Climate Forecast System Reanalysis (CFSR) model. The satellite products evaluated are from the Moderate Resolution Imaging Spectroradiometer (MODIS), the Polarization and Directionality of the Earth's Reflectances (POLDER), the Global Ozone Monitoring Experiment (GOME-2), the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY), and the Atmospheric Infra-red Sounder (AIRS). Satellite retrievals of W are based on the attenuation of solar reflected light by water vapor absorption bands, except those from AIRS that rely on brightness temperature measurements. A very good agreement is observed between the model estimates and MAN, with mean differences of ~5% and standard deviations of ~15%. These results are within the uncertainties associated with the models and the measurements, indicating the skill of the reanalysis models to estimate W over oceans under clear sky conditions. Mean differences of W between the satellite and MAN products are ~11, 6.7, 12, −7, and 3% for MODIS, POLDER, GOME-2, SCIAMACHY and AIRS respectively, while their standard deviations are 31, 29, 28, 20 and 17%. These differences reveal the need to address inconsistencies among different satellite sensors and ground- based measurements to reduce the uncertainties associated with the retrievals.Marie Skłodowska-Curie Research Innovation and Staff Exchange (RISE) GRASP-ACE (grant agreement no. 778349

Global Spatial and Temporal Variation of the Combined Effect of Aerosol and Water Vapour on Solar Radiation

This study aims to calculate the combined and individual effects of the optical thickness of aerosols (AOT) and precipitable water vapour (PWV) on the solar radiation reaching the Earth’s surface at a global scale and to analyse its spatial and temporal variation. For that purpose, a novel but validated methodology is applied to CERES SYN1deg products for the period 2000–2019. Spatial distributions of AOT and PWV effects, both individually and combined, show a close link with the spatial distributions of AOT and PWV. The spatially averaged combined effect results in a −13.9% reduction in irradiance, while the average AOT effect is −2.3%, and the PWV effect is −12.1%. The temporal analysis focuses on detecting trends in the anomalies. The results show overall positive trends for AOT and PWV. Consequently, significant negative overall trends are found for the effects. However, significant positive trends for the individual AOT and the combined AOT-PWV effects are found in specific regions, such as the eastern United States, Europe or Asia, indicating successful emission control policies in these areas. This study contributes to a better understanding of the individual and combined effects of aerosols and water vapour on solar radiation at a global scale

A novel fusion framework embedded with zero-shot super-resolution and multivariate autoregression for precipitable water vapor across the continental Europe

Precipitable water vapor (PWV), as the most abundant greenhouse gas, significantly impacts the evapotranspiration process and thus the global climate. However, the applicability of mainstream satellite PWV products is limited by the tradeoff between spatial and temporal resolutions, as well as some external factors such as cloud contamination. In this study, we proposed a novel PWV spatio-temporal fusion framework based on the zero-shot super-resolution and the multivariate autoregression models (ZSSR-ARF) to improve the accuracy and continuity of PWV. The framework is implemented in a way that the satellite-derived observations (MOD05) are fused with the reanalysis data (ERA5) to generate accurate and seamless PWV of high spatio-temporal resolution (0.01°, daily) across the European continent from 2001 to 2021. Firstly, the ZSSR approach is used to enhance the spatial resolution of ERA5 PWV based on the internal recurrence of image information. Secondly, the optimal ERA5-MOD05 image pairs are selected based on the image similarity as inputs to improve the fusion accuracy. Thirdly, the framework develops a multivariate autoregressive fusion approach to allocate weights adaptively for the high-resolution image prediction, which primely addresses the non-stationarity and autocorrelation of PWV. The results reveal that the accuracies of fused PWV are consistent with those of the GPS retrievals (r = 0.82–0.95 and RMSE = 2.21–4.01 mm), showing an enhancement in the accuracy and continuity compared to the original MODIS PWV. The ZSSR-ARF fusion framework outperforms the other methods with R$^2$ improved by over 24% and RMSE reduced by over 0.61 mm. Furthermore, the fused PWV exhibits similar temporal consistency (mean difference of 0.40 mm and DSTD of 3.22 mm) to the reliable ERA5 products, and substantial increasing trends (mean of 0.057 mm/year and over 0.1 mm/year near the southern and western coasts) are observed over the European continent. As the accuracy and continuity of PWV are improved, the outcome of this paper has potential for climatic analyses during the land-atmosphere cycle process

Evaluation of Water Vapor Product from TROPOMI and GOME-2 Satellites against Ground-Based GNSS Data over Europe

A novel integrated water vapor (IWV) product from TROPOspheric Monitoring Instrument (TROPOMI) is validated together with a Global Ozone Monitoring Instrument-2 (GOME-2) standard product. As reference, ground-based Global Navigation Satellite Systems (GNSS) IWV data in 235 European stations from May 2018 to May 2019 are used. Under cloud free situations, a general comparison is carried out. It suggests that TROPOMI IWV exhibits less bias than GOME-2 and better results in the dispersion and regression parameters. Moreover, TROPOMI presents more homogeneous results along the different stations. However, TROPOMI is found to be overestimating the IWV uncertainties and being, therefore, too conservative in the confidence interval considered. The dependence of satellite product performance on several variables is also discussed. TROPOMI IWV shows wet bias of 5.7% or less for IWV 25 mm. In addition, relative standard deviation (rSD) increases as IWV increases. In addition, the dependence on solar zenith angle (SZA) was also analyzed, as solar radiation bands are used in the retrieval algorithm of both instruments. Relative mean bias error (rMBE) shows positive values for GOME-2, slightly increasing with SZA, while TROPOMI shows more stable values. However, under high SZA, GOME-2 IWV exhibits a steep increase in rMBE (overestimation), while TROPOMI IWV exhibits a moderate decrease (underestimation). rSD is slightly increasing with SZA. The influence of cloudiness on satellite IWV observations is such that TROPOMI tends to overestimate IWV more as cloudiness increases, especially for high IWV. In the case of GOME-2, the rSD slightly increases with cloudiness, but TROPOMI rSD has a marked increase with increasing cloudiness. TROPOMI IWV is an important source of information about moisture, but its algorithm could still benefit from further improvement to respond better to cloudy situations. View Full-Text In this paper we present a novel algorithm for the retrieval of geometry-dependent effective Lambertian equivalent reflectivity (GE_LER) from UVN sensors; the algorithm is based on the full-physics inverse learning machine (FP_ILM) retrieval. Radiances are simulated using a radiative transfer model that takes into account the satellite-viewing geometry, and the inverse problem is solved using machine learning techniques to obtain the GE_LER from satellite measurements. The GE_LER retrieval is optimized not only for trace gas retrievals employing the DOAS algorithm, but also for the large amount of data from existing and future atmospheric Sentinel satellite missions. The GE_LER can either be deployed directly for the computation of air mass factors (AMFs) using the effective scene approximation or it can be used to create a global gapless geometry-dependent LER (G3_LER) daily map from the GE_LER under clear-sky conditions for the computation of AMFs using the independent pixel approximation. The GE_LER algorithm is applied to measurements of TROPOMI launched in October 2017 on board the EU/ESA Sentinel-5 Precursor (S5P) mission. The TROPOMI GE_LER/G3_LER results are compared with climatological OMI and GOME-2 LER datasets and the advantages of using GE_LER/G3_LER are demonstrated for the retrieval of total ozone from TROPOMI

Integrated water vapor over the Arctic: Comparison between radiosondes and sun photometer observations

Producción CientíficaThe amplification of global warming because of the feedbacks associated with the increase in atmospheric moisture and the decrease in sea ice and snow cover in the Arctic is currently the focus of scientists, policy makers and society. The amplification of global warming is the response to increases in precipitation originally caused by climate change. Arctic predominant increases in specific humidity and precipitation have been documented by observations. In comparison, evapotranspiration in the Arctic is poorly known, in part, because the spatial and temporal sparsity of accurate in situ and remote sensing observations. Although more than 20 observations sites in the Arctic are available, where AERONET sun photometer integrated water vapor (IWV) measurements have been conducted, that information have been barely used. Here, we present a comparison of IWV observations from radiosondes and AERONET sun photometers at ten sites located across the Arctic with the goal to document the feasibility of that set of observations to contribute to the ongoing and future research on polar regions. Sun photometer IWV observations are averaged for three-time windows; 30 min, 6 and 24 h. The predominant dry bias of AERONET IWV observations with respect to radiosondes, identified at tropical and midlatitudes, is also present in the Arctic. The statistics of the comparison show robust results at eight of the ten sites, with precision and accuracy magnitudes below 8 and 2% respectively. The possible causes of the less robust results at the other two sites are discussed. In addition, the impact of selecting other temporal coincidence windows in the average sun photometer IWV used in the comparison were tested. Auto-correlation in diurnal sun photometer IWV could produce appreciable bias in the statistics used for the comparison. We suggest using only one pair of values per day, consisting in the daily mean IWV sun photometer and the IWV radiosonde observation value. This feature should be valid also for comparison of IWV from sun photometer and other instruments. Maximum 10% error level of IWV from sun photometer observations, when compared with radiosondes, have been found for the Arctic. It is in the same order of magnitude than at tropical and middle latitudes locations. It has been demonstrated the feasibility of AERONET IWV observations in the Arctic for research on this variable. AERONET standard instruments and its centralized-standard processing algorithm allow its IWV observations to be considered a relative standard dataset for the re-calibration of other instrumental IWV observations assuming radiosondes as the absolute standard dataset.Ministerio de Ciencia, Innovación y Universidades (grant RTI2018-097864-B-I00)Junta de Castilla y León (grant VA227P20)Junta de Extremadura - Fondo Europeo de Desarrollo Regional (grant GR21080 and project IB18092

Atmospheric Moisture Effects on Deep Convection in the Western Mediterranean

Improving the understanding and representation of heavy precipitation is crucial to prevent its hazards. A powerful means to reduce errors is to assimilate high-resolution humidity GPS observations. Here, novel experiments employing sub-hourly atmospheric GPS, in-situ observations and nudging are used to study the impact of moisture corrections on convection. This work adds new explanations on the sensitivity of extreme precipitation to moisture changes and on the added value of nudging GPS