Abstract. In a standard atmosphere, there is a single lapse rate tropopause (in what follows, tropopause) that separates the troposphere below from the stratosphere above. However, in certain situations, such as in regions of strong vertical wind shear or associated with certain weather phenomena, a second tropopause layer may form above the standard tropopause. The presence of a double tropopause (DT) can have implications for atmospheric and climate studies, as it may be associated with dynamic and complex weather patterns. Based on 14 years of temperature profiles retrieved by GNSS radio occultation and the resulting DT, a possible relationship between the spatio-temporal distribution of the relative number of DT to simple tropopauses (NDT) (or dependent variable) and a set of monthly climate indices (or features) is explored with a focus on the methodological approach. A cluster analysis is applied to geographically associate the DT occurrences with the climate indices. Then a multivariate linear regression is constructed using a progression of different models to identify the relevant features for the occurrence of DTs. On a global scale, from a hierarchical cluster analysis six sub-regions with different location and spread characteristics are identified. In addition to the condition of linearity in the residuals, the performance of each model in the train and test populations is evaluated to discard possible overfitting. The required conditions of non-collinearity, stationarity and cross-correlation between the features and the relative number of NDT after the removal of the climatological mean for each month (NDT’) are checked. Mean squared errors, adjusted coefficient of determination (adjusted R2) and number of degrees of freedom (F-statistic) parameters are evaluated for each model obtained. Taking into account the constraints of the present analysis, the most relevant climatic indices for the distribution of NDT' are identified.

Gravity waves (GW) transport momentum flux (MF) and energy across the lower, middle and upper atmosphere. Global Navigation Satellite System (GNSS) radio occultation (RO) is one of the measuring techniques used onboard satellites to provide vertical temperature profiles with global and permanent coverage. These retrievals may be applied in the study of GW. Most of the analysis methods provide absolute GWMF but are missing its net direction. This happens because the procedures can deduce the orientation but not the propagation sign of GW and hence the full direction of MF. We apply here a method that allows the net calculation with four close in space and time RO soundings (quartets). We use about 10,000 daily retrievals from 1 March 2022 to 28 February 2023 to study the seasonal and latitudinal characteristics of net GWMF in the height interval from 20 to 35 km. About 600 quartets were found. The calculated zonal MF and drag exhibited negative minima at middle and high latitudes during winter in the Southern Hemisphere. This well‐known characteristic is usually mainly assigned to orographic sources. A similar intensity in zonal MF and drag is found during the same season at low latitudes. Meridional components are generally less significant. Besides finding the correct sign of the GWMF and the corresponding forcing on the mean flow, the quartets method also allows the determination of the horizontal and vertical wavelengths, the amplitude and sign of the vertical wave phase velocity and the intrinsic frequencies. The global statistics of these parameters are shown and each one exhibits a similar distribution shape across latitude bands. Large differences in the frequency of cases in vertical phase velocity sign appear only at low and high positive latitudes. The most even distribution of GW intrinsic frequency is found at low latitudes. We estimate that absolute MF calculations by methods assuming only upward GW propagation may produce a bias not larger than 40%. The increase of satellite measuring devices achieved in the last years due to the release of new missions led to a high spatial and temporal density of profiles that may allow the attainment of net GWMF climatologies over a seasonal time scale and about 4,000 km latitude bands but this performance may be even improved if the amount of retrievals continues to rise.

Gravity waves (GW) transport momentum flux (MF) and energy across the lower, middle and upper atmosphere. Global Positioning System (GPS) radio occultation (RO) is one of the measuring techniques used onboard satellites to provide vertical temperature profiles with global and permanent coverage. These retrievals may be applied in the study of GW. Some usual analysis methods provide absolute GWMF, but are missing its net direction. This happens because the procedures cannot deduce the propagation sense of GW and hence the direction of MF. We therefore recall here a recently presented method that allows the net calculation with four close in space and time RO soundings (quartets). We use about 10,000 daily retrievals from March 1, 2022 to February 28, 2023 to study the seasonal and latitudinal global characteristics of net GWMF in the height interval from 20 to 35 km. About 600 quartets were found.

The major objective of the GRACE Follow On (GFO) mission with its two satellites GF1 and GF2 is to obtain precise global and high-resolution models for both the static and the time variable components of the Earth's gravity field. Additional goal is the continuation of the GPS radio occultation (RO) measurements from the predecessor GRACE, successfully performed between 2006 and 2017. The GRACE/GRACE FO data contribute to the global RO dataset consisting of several missions provided by different centres since the pioneering GPS/MET mission in 1995/97. Beside climate applications, GFO data are used for the assimilation in numerical weather forecast models by the leading weather service centres. After several on-board software updates and raw data reader improvements since March 2020 rising occultations from GF1 and since September 2021 setting occultations from GF2 are continuously available. Both satellites provide about 400 atmospheric profiles daily. The RO data are processed based on different measured observables: For different GPS satellites combinations of L1CA/L2P, L1CA/L2C, or L1CA/L5 amplitude and phase measurements are available. In this study results of GFO processing and validation are presented. Bending angle, refractivity, and temperature data are compared with ECMWF operational analyses. The quality of the different measured variables is evaluated for different geographical regions. In addition, GFO data are compared with co-located COSMIC-2 and Spire radio occultations.

The characteristics of equatorial plasma bubbles, which occur during the nighttime and along the course of the geomagnetic equator in the F-region of the ionosphere, are investigated using space-based GPS radio occultation measurements from the FormoSat-3/COSMIC mission. These observations provide unique altitudinal resolution on a global scale. On the other hand, the influence of pre-reversal enhancement (PRE) and gravity wave activity on the plasma bubble characteristics are studied on a long-term basis using the International Reference Ionosphere (IRI) model and TIMED/SABER measurements, respectively. The occurrence and altitudinal characteristics of plasma bubbles are investigated in four regions, i.e., America, Africa, Asia, and the Pacific. Both the attributes of plasma bubbles are compared with the PRE and gravity waves to understand the preconditioning as well as the uplift of plasma bubbles. A brief statistical study indicated a strong positive correlation between the occurrence rate of plasma bubbles and PRE, mainly in the American sector, followed by the African, Asian, and Pacific sectors. In addition, a positive correlation was detected between the PRE and plasma bubble altitudes during the solar maximum period. However, a negative correlation was observed during the solar minimum period. Furthermore, gravity wave activity was investigated to understand the influence on plasma bubble altitudes during the solar minimum period. Interestingly, the gravity wave activity seemed to have a significant influence on plasma bubble altitudes during the solar minimum years, while, the PRE controlled them during solar maximum years across different regions during different seasons.

Atmospheric water vapour content is a key variable that controls the development of deep convective storms and rainfall extremes over the central Andes. Direct measurements of water vapour are challenging; however, recent developments in microwave processing allow the use of phase delays from L-band radar to measure the water vapour content throughout the atmosphere: Global Navigation Satellite System (GNSS)-based integrated water vapour (IWV) monitoring shows promising results to measure vertically integrated water vapour at high temporal resolutions. Previous works also identified convective available potential energy (CAPE) as a key climatic variable for the formation of deep convective storms and rainfall in the central Andes. Our analysis relies on GNSS data from the Argentine Continuous Satellite Monitoring Network, Red Argentina de Monitoreo Satelital Continuo (RAMSAC) network from 1999 to 2013. CAPE is derived from version 2.0 of the ECMWF’s (European Centre for Medium-Range Weather Forecasts) Re-Analysis (ERA-interim) and rainfall from the TRMM (Tropical Rainfall Measuring Mission) product. In this study, we first analyse the rainfall characteristics of two GNSS-IWV stations by comparing their complementary cumulative distribution function (CCDF). Second, we separately derive the relation between rainfall vs. CAPE and GNSS-IWV. Based on our distribution fitting analysis, we observe an exponential relation of rainfall to GNSS-IWV. In contrast, we report a power-law relationship between the daily mean value of rainfall and CAPE at the GNSS-IWV station locations in the eastern central Andes that is close to the theoretical relationship based on parcel theory. Third, we generate a joint regression model through a multivariable regression analysis using CAPE and GNSS-IWV to explain the contribution of both variables in the presence of each other to extreme rainfall during the austral summer season. We found that rainfall can be characterised with a higher statistical significance for higher rainfall quantiles, e.g., the 0.9 quantile based on goodness-of-fit criterion for quantile regression. We observed different contributions of CAPE and GNSS-IWV to rainfall for each station for the 0.9 quantile. Fourth, we identify the temporal relation between extreme rainfall (the 90th, 95th, and 99th percentiles) and both GNSS-IWV and CAPE at 6 h time steps. We observed an increase before the rainfall event and at the time of peak rainfall—both for GNSS-integrated water vapour and CAPE. We show higher values of CAPE and GNSS-IWV for higher rainfall percentiles (99th and 95th percentiles) compared to the 90th percentile at a 6-h temporal scale. Based on our correlation analyses and the dynamics of the time series, we show that both GNSS-IWV and CAPE had comparable magnitudes, and we argue to consider both climatic variables when investigating their effect on rainfall extremes.

Atmospheric climate monitoring requires observations of high quality that conform to the criteria of the Global Climate Observing System (GCOS). Radio occultation (RO) data based on Global Positioning System (GPS) signals are available since 2001 from several satellite missions with global coverage, high accuracy, and high vertical resolution in the troposphere and lower stratosphere. We assess the consistency and long-term stability of multi-satellite RO observations for use as climate data records. As a measure of long-term stability, we quantify the structural uncertainty of RO data products arising from different processing schemes. We analyze atmospheric variables from bending angle to temperature for four RO missions, CHAMP, Formosat-3/COSMIC, GRACE, and Metop, provided by five data centers. The comparisons are based on profile-to-profile differences aggregated to monthly medians. Structural uncertainty in trends is found to be lowest from 8 to 25 km of altitude globally for all inspected RO variables and missions. For temperature, it is < 0.05 K per decade in the global mean and < 0.1 K per decade at all latitudes. Above 25 km, the uncertainty increases for CHAMP, while data from the other missions – based on advanced receivers – are usable to higher altitudes for climate trend studies: dry temperature to 35 km, refractivity to 40 km, and bending angle to 50 km. Larger differences in RO data at high altitudes and latitudes are mainly due to different implementation choices in the retrievals. The intercomparison helped to further enhance the maturity of the RO record and confirms the climate quality of multi-satellite RO observations towards establishing a GCOS climate data record.

In this study, we detect high percentile rainfall events in the eastern central Andes, based on Tropical Rainfall Measuring Mission (TRMM) with a spatial resolution of 0.25 x 0.25 degrees, a temporal resolution of 3 h, and for the duration from 2001 to 2018. We identify three areas with high mean accumulated rainfall and analyze their atmospheric behaviour and rainfall characteristics with specific focus on extreme events. Extreme events are defined by events above the 95th percentile of their daily mean accumulated rainfall. Austral summer (DJF) is the period of the year presenting the most frequent extreme events over these three regions. Daily statistics show that the spatial maxima, as well as their associated extreme events, are produced during the night. For the considered period, ERA-Interim reanalysis data, provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) with 0.75 degrees x 0.75 degrees spatial and 6-hourly temporal resolutions, were used for the analysis of the meso- and synoptic-scale atmospheric patterns. Night- and day-time differences indicate a nocturnal overload of northerly and northeasterly low-level humidity flows arriving from tropical South America. Under these conditions, cooling descending air from the mountains may find unstable air at the surface, giving place to the development of strong local convection. Another possible mechanism is presented here: a forced ascent of the low-level flow due to the mountains, disrupting the atmospheric stratification and generating vertical displacement of air trajectories. A Principal Component Analysis (PCA) in T-mode is applied to day- and night-time data during the maximum and extreme events. The results show strong correlation areas over each subregion under study during night-time, whereas during day-time no defined patterns are found. This confirms the observed nocturnal behavior of rainfall within these three hotspots.

Global Navigation Satellite Systems (GNSS) have revolutionized positioning, navigation, and timing, becoming a common part of our everyday life. Aside from these well-known civilian and commercial applications, GNSS is currently established as a powerful and versatile observation tool for geosciences. An outstanding application in this context is the operational monitoring of atmospheric water vapor with high spatiotemporal resolution. The water vapor is the most abundant greenhouse gas, which accounts for about 70% of atmospheric warming and plays a key role in the atmospheric energy exchange. The precise knowledge of its highly variable spatial and temporal distribution is a precondition for precise modeling of the atmospheric state as a base for numerical weather forecasts especially with focus to the strong precipitation and severe weather events.The data from European GNSS networks are widely operationally used to improve regional weather forecasts in several countries. However, the impact of the currently provided data products to the forecast systems is still limited due to the exclusively focusing on GPS-only based data products; to the limited atmospheric information content, which is provided mostly in the zenith direction and to the time delay between measurement and providing the data products, which is currently about one hour.AMUSE is a recent research project, funded by the DFG (German Research Council) and performed in close cooperation of TUB, GFZ and DWD during 2020-2022. The project foci are the major limitations of currently operationally used generation of GNSS-based water vapor data. AMUSE will pioneer the development of next generation data products. Main addressed innovations are: 1) Developments to provide multi-GNSS instead of GPS-only data, including GLONASS, Galileo and BeiDou; 2) Developments to provide high quality slant observations, containing water vapor information along the line-of-sight from the respective ground stations; 3) Developments to shorten the delay between measurements and the provision of the products to the meteorological services.This GNSS-focused work of AMUSE will be complemented by the contribution of German Weather Service DWD to investigate in detail and to quantify the forecast improvement, which can be reached by the new generation GNSS-based meteorology data. Several dedicated forecast experiments will be conducted with focus on one of the most challenging issues, the precipitation forecast in case of severe weather events. These studies will support the future assimilation of the new generation data to the regional forecast system of DWD and potentially also to other European weather services.

Temperature observations of the upper-air atmosphere are now available for more than 40 years from both ground- and satellite-based observing systems. Recent years have seen substantial improvements in reducing long-standing discrepancies among datasets through major reprocessing efforts. The advent of radio occultation (RO) observations in 2001 has led to further improvements in vertically resolved temperature measurements, enabling a detailed analysis of upper-troposphere/lower-stratosphere trends. This paper presents the current state of atmospheric temperature trends from the latest available observational records. We analyze observations from merged operational satellite measurements, radiosondes, lidars, and RO, spanning a vertical range from the lower troposphere to the upper stratosphere. The focus is on assessing climate trends and on identifying the degree of consistency among the observational systems. The results show a robust cooling of the stratosphere of about 1–3 K, and a robust warming of the troposphere of about 0.6–0.8 K over the last four decades (1979–2018). Consistent results are found between the satellite-based layer-average temperatures and vertically resolved radiosonde records. The overall latitude–altitude trend patterns are consistent between RO and radiosonde records. Significant warming of the troposphere is evident in the RO measurements available after 2001, with trends of 0.25–0.35 K per decade. Amplified warming in the tropical upper-troposphere compared to surface trends for 2002–18 is found based on RO and radiosonde records, in approximate agreement with moist adiabatic lapse rate theory. The consistency of trend results from the latest upper-air datasets will help to improve understanding of climate changes and their drivers.

An objective of the GRACE-FO mission is the continuation of GRACE radio occultation measurements successfully performed between 2006 and 2017. GRACE and GRACE-FO radio occultations contribute to the overall radio occultation dataset used in weather and climate applications. Since mid-2019 rising occultations from GF1 are available while setting radio occultations from GF2 are still disabled. After several on-board software updates and raw data reader improvements about 280 daily GF1 radio occultations are available since March 2020. Currently GF1 radio occultation data are processed on the basis of different measured variables: For different GPS satellites a combination of L1CA/L2P, L1CA/L2C, or L1CA/L5 is available. In this study first results of GF1 processing are presented. Refractivity and temperature data up to an altitude of 60 km will be compared with ECMWF operational analyses and the quality of the different measured variables will be evaluated.

Abstract A detailed analysis of double tropopause (DT) occurrences requires vertically well resolved, accurate, and globally distributed information on the troposphere‐stratosphere transition zone. Here, we use radio occultation observations from 2001 to 2018 with such properties. We establish a connection between El Niño‐Southern Oscillation (ENSO) phases and the distribution of DTs by analyzing the global and seasonal DT characteristics. The seasonal distribution of DTs reveals several hotspot locations, such as near the subtropical jet stream and over high mountain ranges, where DTs occur particularly often. In this study, we detect a higher number of DTs during the cold La Niña state while warmer El Niño events result in lower DT rates, affecting the structure of the tropopause region. Close to the Niño 3 region, this relates to a much lower first lapse rate tropopause altitude during La Niña and corresponds to an apparent narrowing of the tropical belt there.

Tropopause temperature (TPT) is a useful indicator and a key component of climate change. Well simulating its value and seasonal‐to‐decadal variability by climate models is still challenging. How the vertical resolution influences the representation of TPT and its response to a climate forcing is largely unknown. This study investigates TPT responses to sea surface temperatures using a series of model simulations in various vertical resolution. With high vertical resolution (HV‐Res), the model gives a better representation of tropical TPTs in absolute values and seasonal variations. The corresponding changes in TPTs associated with sea surface temperature anomalies (El Niño–Southern Oscillation and Pacific Decadal Oscillation) are 30% stronger and more realistic in the HV‐Res model. Such improvements may get benefits from better representations of equatorial waves with more realistic structure and stronger interannual variations. A proper vertical resolution is therefore essential to well simulate the stratosphere‐troposphere coupling and should be used in climate change assessment.

Gravity wave (GW) activity is analyzed using temperature (T) data retrieved from a Rayleigh light detection and ranging (lidar) at Rio Gallegos, Argentina (51.6 degrees S, 69.3 degrees W). GW characteristics are derived from 302 nights of observations providing more than 1,018hr of high-resolution lidar data between 20- and 56-km height from August 2005 to December 2015. T measurements are performed by a Differential Absorption Lidar instrument. This lidar was the southernmost outside Antarctica until the end of 2017. Rio Gallegos is an exceptional place to observe large amplitude GW. Every lidar measurement is classified according to its relative position to the polar vortex. The lidar measurements are compared with collocated Sounding of the Atmosphere using Broadband Emission Radiometry and Global Positioning System-Radio Occultation data. The different instruments show different windows of the GW spectrum, providing complementary observations. In general, the geometric mean of the specific GW potential energy (PE) is larger during winter and spring than during summer and autumn. The largest geometric mean of PE is found inside the vortex and decreases monotonically at its edge, outside it and when there is no vortex. The same behavior is observed with satellite data. On average, it can be seen that lidar observations provide larger PE values than limb sounding measurements. From a Morlet continuous wavelet transform analysis, three distinct modes are captured from Sounding of the Atmosphere using Broadband Emission Radiometry and from Global Positioning System-Radio Occultation data at the upper and lower stratosphere, respectively. In particular, a systematic 3.5- to 4-year oscillation, possibly related to El Nino-Southern Oscillation is observed.

The interactions between atmosphere and steep topography in the eastern south–central Andes result in complex relations with inhomogenous rainfall distributions. The atmospheric conditions leading to deep convection and extreme rainfall and their spatial patterns—both at the valley and mountain-belt scales—are not well understood. In this study, we aim to identify the dominant atmospheric conditions and their spatial variability by analyzing the convective available potential energy (CAPE) and dew-point temperature ( T d ). We explain the crucial effect of temperature on extreme rainfall generation along the steep climatic and topographic gradients in the NW Argentine Andes stretching from the low-elevation eastern foreland to the high-elevation central Andean Plateau in the west. Our analysis relies on version 2.0 of the ECMWF’s (European Centre for Medium-Range Weather Forecasts) Re-Analysis (ERA-interim) data and TRMM (Tropical Rainfall Measuring Mission) data. We make the following key observations: First, we observe distinctive gradients along and across strike of the Andes in dew-point temperature and CAPE that both control rainfall distributions. Second, we identify a nonlinear correlation between rainfall and a combination of dew-point temperature and CAPE through a multivariable regression analysis. The correlation changes in space along the climatic and topographic gradients and helps to explain controlling factors for extreme-rainfall generation. Third, we observe more contribution (or higher importance) of T d in the tropical low-elevation foreland and intermediate-elevation areas as compared to the high-elevation central Andean Plateau for 90th percentile rainfall. In contrast, we observe a higher contribution of CAPE in the intermediate-elevation area between low and high elevation, especially in the transition zone between the tropical and subtropical areas for the 90th percentile rainfall. Fourth, we find that the parameters of the multivariable regression using CAPE and T d can explain rainfall with higher statistical significance for the 90th percentile compared to lower rainfall percentiles. Based on our results, the spatial pattern of rainfall-extreme events during the past ∼16 years can be described by a combination of dew-point temperature and CAPE in the south–central Andes.

Hydrogen peroxide (H2O2) plays a significant role in the oxidizing capacity of the atmosphere. It is an efficient oxidant in the liquid phase and serves as a temporary reservoir for the hydroxyl radical (OH), the most important oxidizing agent in the gas phase. Due to its high solubility, removal of H2O2 due to wet and dry deposition is efficient, being a sink of HOx (OH+HO2) radicals. In the continental boundary layer, the H2O2 budget is controlled by photochemistry, transport and deposition processes. Here we use in situ observations of H2O2 and account for chemical source and removal mechanisms to study the interplay between these processes. The data were obtained during five ground-based field campaigns across Europe from 2008 to 2014 and bring together observations in a boreal forest, two mountainous sites in Germany, and coastal sites in Spain and Cyprus. Most campaigns took place in the summer, while the measurements in the south-west of Spain took place in early winter. Diel variations in H2O2 are strongly site-dependent and indicate a significant altitude dependence. While boundary-layer mixing ratios of H2O2 at low-level sites show classical diel cycles with the lowest values in the early morning and maxima around local noon, diel profiles are reversed on mountainous sites due to transport from the nocturnal residual layer and the free troposphere. The concentration of hydrogen peroxide is largely governed by its main precursor, the hydroperoxy radical (HO2), and shows significant anti-correlation with nitrogen oxides (NOx) that remove HO2. A budget calculation indicates that in all campaigns, the noontime photochemical production rate through the self-reaction of HO2 radicals was much larger than photochemical loss due to reaction with OH and photolysis, and that dry deposition is the dominant loss mechanism. Estimated dry deposition velocities varied between approximately 1 and 6 cm s−1, with relatively high values observed during the day in forested regions, indicating enhanced uptake of H2O2 by vegetation. In order to reproduce the change in H2O2 mixing ratios between sunrise and midday, a variable contribution from transport (10 %–100 %) is required to balance net photochemical production and deposition loss. Transport is most likely related to entrainment from the residual layer above the nocturnal boundary layer during the growth of the boundary layer in the morning.

The significant distortions introduced in the measured atmospheric gravity wavelengths by soundings other than those in vertical and horizontal directions, are discussed as a function of the elevation angle of the sounding path and the gravity wave aspect ratio. Under- or overestimation of real vertical wavelengths during the measurement process depends on the value of these two parameters. The consequences of these distortions on the calculation of the energy and the vertical flux of horizontal momentum are analyzed and discussed in the context of two experimental limb satellite setups: GPS-LEO radio occultations and TIMED/SABER ((Atmosphere using Broadband Emission Radiometry/Thermosphere–Ionosphere–Mesosphere–Energetics and Dynamics)) measurements. Possible discrepancies previously found between the momentum flux calculated from satellite temperature profiles, on site and from model simulations, may to a certain degree be attributed to these distortions. A recalculation of previous momentum flux climatologies based on these considerations seems to be a difficult goal.

Atmospheric gravity waves (GW) serve as an essential mechanism in the transport of energy and momentum flux from the low to the upper atmosphere. In the last decades satellite observations have become an important part in the analysis of GW due to their global and frequent coverage. Present procedures often provide GW absolute momentum flux (MF), ambiguous 3‐D propagation direction, and apparent vertical wavelengths. We here introduce a method with close sounding quartets, which allows the calculation for GW of the net MF, the definite propagation direction, and “real” wavelengths. Among the satellite observational techniques, Global Positioning System (GPS) radio occultation (RO) retrievals provide temperature profiles that after adequate processing may yield GW properties like wavelengths, MF, and energy. Our procedure is illustrated by an example under requirements that tend to ensure that four GPS RO soundings are observing the same GW. The future increase of satellite measuring devices due to new missions (including GPS RO) will lead to a higher spatial and temporal density of profiles that may eventually allow the attainment of GW climatologies of net MF, propagation direction, and “real” vertical wavelengths.