GNSS remote sensing of the Australian tropopause

Radio occultation (RO) techniques that use signals transmitted by Global Navigation Satellite Systems (GNSS) have emerged over the past decade as an important tool for measuring global changes in tropopause temperature and height, a valuable capacity given the tropopause’s sensitivity to temperature variations. This study uses 45,091 RO data from the CHAMP (CHAllenging Minisatellite Payload, 80 months), GRACE (Gravity Recovery And Climate Experiment, 23 months) and COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate, 20 months) satellites to analyse the variability of the tropopause’s height and temperature over Australia. GNSS RO temperature profiles from CHAMP, GRACE, and COSMIC are first validated using radiosonde observations provided by the Bureau of Meteorology (Australia). These are compared to RO soundings from between 2001 and 2007 that occurred within 3 h and 100 km of a radiosonde.The results indicate that RO soundings provide data of a comparable quality to radiosonde observations in the tropopause region, with temperature deviations of less than 0.5 ± 1.5 K. An analysis of tropopause height and temperature anomalies indicates a height increase over Australia as a whole of ca. 4.8 ± 1.3 m between September 2001 and April 2008, with a corresponding temperature decrease of −0.019 ± 0.007 K. A similar pattern of increasing height/decreasing temperature was generally observed when determining the spatial distribution of the tropopause height and temperature rate of change over Australia. Although only a short period has been considered in this study, a function of the operating time of these satellites, the results nonetheless show an increase in the height of the tropopause over Australia during this period and thus may indicate regional warming. Several mechanisms could be responsible for these changes, such as an increase in the concentration of greenhouse gases in the atmosphere, and lower stratospheric cooling due to ozone loss, both of which have been observed during the last decades

Interannual variability of temperature in the UTLS region over Ganges-Brahmaputra-Meghna river basin based on COSMIC GNSS RO data

Poor reliability of radiosonde records across South Asia imposes serious challenges in understanding the structure of upper-tropospheric and lower-stratospheric (UTLS) region. The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission launched in April 2006 has overcome many observational limitations inherent in conventional atmospheric sounding instruments. This study examines the interannual variability of UTLS temperature over the Ganges-Brahmaputra-Meghna (GBM) river basin in South Asia using monthly averaged COSMIC radio occultation (RO) data, together with two global reanalyses. Comparisons between August 2006 and December 2013 indicate that MERRA (Modern-Era Retrospective Analysis for Research Application) and ERA-Interim (European Centre for Medium-Range Weather Forecasts reanalysis) are warmer than COSMIC RO data by 2°C between 200 and 50hPa levels. These warm biases with respect to COSMIC RO data are found to be consistent over time. The UTLS temperature show considerable interannual variability from 2006 to 2013 in addition to warming (cooling) trends in the troposphere (stratosphere). The cold (warm) anomalies in the upper troposphere (tropopause region) are found to be associated with warm ENSO (El Niño-Southern Oscillation) phase, while quasi-biennial oscillation (QBO) is negatively (positively) correlated with temperature anomalies at 70hPa (50hPa) level. PCA (principal component analysis) decomposition of tropopause temperatures and heights over the basin indicate that ENSO accounts for 73% of the interannual (non-seasonal) variability with a correlation of 0.77 with Niño3.4 index whereas the QBO explains about 10% of the variability. The largest tropopause anomaly associated with ENSO occurs during the winter, when ENSO reaches its peak. The tropopause temperature (height) increased (decreased) by about 1.5°C (300m) during the last major El Niño event of 2009/2010. In general, we find decreasing (increasing) trend in tropopause temperature (height) between 2006 and 2013

Orographic and convective gravity waves above the Alps and Andes mountains during GPS radio occultation events – a case study

The significant distortions introduced in the measured atmospheric gravity wavelengths by soundings other than in vertical and horizontal directions, are discussed as a function of elevation angle of the sounding path and the gravity waves aspect ratio. Under- or overestimation of real vertical wavelengths during the measurement process depends basically on the value of these two parameters. The consequences of these distortions on the calculation of the energy and 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 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.Fil: Hierro, Rodrigo Federico. Universidad Austral. Facultad de Ingeniería. Departamento de Ciencias Básicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Steiner, Andrea K.. Universidad de Graz; AustriaFil: de la Torre, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Austral. Facultad de Ingeniería. Departamento de Ciencias Básicas; ArgentinaFil: Alexander, Pedro Manfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Llamedo Soria, Pablo Martin. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Austral. Facultad de Ingeniería. Departamento de Ciencias Básicas; ArgentinaFil: Cremades, Pablo Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentin

Use of radio occultation for long-term tropopause studies: uncertainties, biases, and instabilities.

Journal ArticleResearch suggests that changes in tropopause structure can both indicate and impact changes in the global climate system. The Global Positioning System radio occultation (RO) technique shows tremendous potential for monitoring the global tropopause due to its precision, temporal consistency, and global measurement density. This study examines the capability of RO to monitor the global tropopause by addressing three specific objectives: (1) quantify sources of error in individual RO tropopause measurements, (2) examine absolute bias and long-term stability of RO tropopause parameters with respect to those obtained from radiosondes, and (3) distinguish between errors due to processing and RO instrument differences by comparing tropopause parameters from different RO products. In this study, we make use of data from four different RO missions, including the recent COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate)

UTLS temperature validation of MPI-ESM decadal hindcast experiments with GPS radio occultations

Global Positioning System (GPS) radio occultation (RO) temperature data are used to validate MPI-ESM (Max Planck Institute – Earth System Model) decadal hindcast experiments in the upper troposphere and lower stratosphere (UTLS) region between 300 hPa and 10 hPa (8 km and 32 km) for the time period between 2002 and 2011. The GPSRO dataset is unique since it is very precise, calibration independent and covers the globe better than the usual radiosonde dataset. In addition it is vertically finer resolved than any of the existing satellite temperature measurements available for the UTLS and provides now a unique one decade long temperature validation dataset. The initialization of the MPI-ESM decadal hindcast runs mostly increases the skill of the atmospheric temperatures when compared to uninitialized climate projections with very high skill scores for lead-year one, and gradually decreases for the later lead-years. A comparison between two different initialization sets (b0, b1) of the low-resolution (LR) MPI-ESM shows increased skills in b1-LR in most parts of the UTLS in particular in the tropics. The medium resolution (MR) MPI-ESM initializations are characterized by reduced temperature biases in the uninitialized runs as compared to observations and a better capturing of the high latitude northern hemisphere interannual polar vortex variability as compared to the LR model version. Negative skills are found for the b1-MR hindcasts however in the regions around the mid-latitude tropospheric jets on both hemispheres and in the vicinity of the tropical tropopause in comparison to the b1-LR variant. It is interesting to highlight that none of the model experiments can reproduce the observed positive temperature trend in the tropical tropopause region since 2001 as seen by GPSRO data

The Fine-Scale Structure of the Global Tropopause Derived from COSMIC GPS Radio Occultation Measurements

The spatiotemporal structure of the lapse-rate tropopause is examined by using state-of-the-art Global Positioning System radio occultation measurements from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Formosa Satellite Mission 3 mission. The high temporal and spatial resolutions of the data reveal the detailed structure of tropopause properties such as pressure (pt), temperature (Tt), and sharpness (Nt^2) and their relationships to upper tropospheric and lower stratospheric processes. The overall results are generally in good agreement with previous studies. The climatology of all three tropopause properties shows largely homogeneous structure in the zonal direction: noticeable asymmetries are found only in the tropics and the Northern Hemisphere extratropics during boreal winter owing to localized tropospheric processes. This contrasts with the seasonal cycles of tropopause properties which are significantly influenced by stratospheric processes such as the Brewer-Dobson circulation, the polar vortex, and the radiative processes near the tropopause. On intraseasonal time scales, pt and Tt exhibit significant variability over the Asian summer monsoon and the subtropics where double tropopauses frequently occur. In contrast, Nt^2 shows maximum variability in the tropics where pt and Tt have minimum variability, possibly a consequence of vertically propagating waves. The tropopause properties derived from COSMIC observations are further applied to evaluate tropopause data directly available from the NCEP-NCAR Reanalysis (NNR). Although the NNR tropopause data have been widely used in climate studies, they are found to have significant and systematic biases, especially in the subtropics. This suggests that the NNR tropopause data should be treated with great caution in any quantitative studies

Observational tests of hurricane intensity estimations using GPS radio occultations

This study presents a novel approach to estimating the intensity of hurricanes using temperature profiles from Global Positioning System radio occultation (GPSRO) measurements. Previous research has shown that the temperature difference between the ocean surface and the eyewall outflow region defines hurricanes' thermodynamic efficiency, which is directly proportional to the storm's intensity. Outflow temperatures in the eyewall region of 27 hurricanes in 2004–2011 were obtained from GPSRO observations. These observations, along with ocean surface temperatures from NASA Modern Era-Retrospective Analysis for Research and Applications, made it possible to estimate hurricane intensities using a simplified hurricane model. Our preliminary results are quantitatively consistent with best-track values from the National Hurricane Center within 9.4%. As a by-product of our study, we present for the first time GPSRO vertical temperature profiles in the vicinity of the eyewall region of hurricanes, which we compared with collocated temperature profiles from the European Centre for Medium-Range Weather Forecasts Reanalysis Interim (ERA-Interim). Some of the GPSRO data sets reveal a double tropopause in the vicinity of the eyewall—a characteristic that we do not see in ERA-Interim. We conclude that GPSRO observations can be of supplementary assistance in augmenting existing data sets used in hurricane intensity estimation. GPSROs' cloud-penetrating capability and high vertical resolution can be useful in providing soundings in the area close to the eyewall region of hurricanes revealing detailed information about their thermal structure, potentially advancing our current knowledge of their dynamics, evolution, and physics

UTLS temperature validation of MPI-ESM decadal hindcast experiments with GPS radio occultations

Global Positioning System (GPS) radio occultation (RO) temperature data are used to validate MPI-ESM (Max Planck Institute – Earth System Model) decadal hindcast experiments in the upper troposphere and lower stratosphere (UTLS) region between 300 hPa and 10 hPa (8 km and 32 km) for the time period between 2002 and 2011. The GPSRO dataset is unique since it is very precise, calibration independent and covers the globe better than the usual radiosonde dataset. In addition it is vertically finer resolved than any of the existing satellite temperature measurements available for the UTLS and provides now a unique one decade long temperature validation dataset. The initialization of the MPI-ESM decadal hindcast runs mostly increases the skill of the atmospheric temperatures when compared to uninitialized climate projections with very high skill scores for lead-year one, and gradually decreases for the later lead-years. A comparison between two different initialization sets (b0, b1) of the low-resolution (LR) MPI-ESM shows increased skills in b1-LR in most parts of the UTLS in particular in the tropics. The medium resolution (MR) MPI-ESM initializations are characterized by reduced temperature biases in the uninitialized runs as compared to observations and a better capturing of the high latitude northern hemisphere interannual polar vortex variability as compared to the LR model version. Negative skills are found for the b1-MR hindcasts however in the regions around the mid-latitude tropospheric jets on both hemispheres and in the vicinity of the tropical tropopause in comparison to the b1-LR variant. It is interesting to highlight that none of the model experiments can reproduce the observed positive temperature trend in the tropical tropopause region since 2001 as seen by GPSRO data

Tropical Temperature Variability in the UTLS: New Insights from GPS Radio Occultation Observations

AbstractGlobal positioning system (GPS) radio occultation (RO) observations, first made of Earth's atmosphere in 1995, have contributed in new ways to the understanding of the thermal structure and variability of the tropical upper troposphere–lower stratosphere (UTLS), an important component of the climate system. The UTLS plays an essential role in the global radiative balance, the exchange of water vapor, ozone, and other chemical constituents between the troposphere and stratosphere, and the transfer of energy from the troposphere to the stratosphere. With their high accuracy, precision, vertical resolution, and global coverage, RO observations are uniquely suited for studying the UTLS and a broad range of equatorial waves, including gravity waves, Kelvin waves, Rossby and mixed Rossby–gravity waves, and thermal tides. Because RO measurements are nearly unaffected by clouds, they also resolve the upper-level thermal structure of deep convection and tropical cyclones as well as volcanic clouds. Their low biases and stability from mission to mission make RO observations powerful tools for studying climate variability and trends, including the annual cycle and intraseasonal-to-interannual atmospheric modes of variability such as the quasi-biennial oscillation (QBO), Madden–Julian oscillation (MJO), and El Niño–Southern Oscillation (ENSO). These properties also make them useful for evaluating climate models and detection of small trends in the UTLS temperature, key indicators of climate change. This paper reviews the contributions of RO observations to the understanding of the three-dimensional structure of tropical UTLS phenomena and their variability over time scales ranging from hours to decades and longer

Precision estimation in temperature and refractivity profiles retrieved by GPS radio occultations

The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) is a six-satellite Global Positioning System (GPS) radio occultation (RO) mission that started in April 2006. The close proximity of these satellites during some months after launch provided a unique opportunity to evaluate the precision of GPS RO temperature and refractivity profile retrievals in the neutral atmosphere from nearly collocated and simultaneous observations. In order to work with nearly homogeneous sets, data are divided into five groups according to latitude bands during 20 days of July. For all latitude bands and variables, the best precision values (about 0.1%) are found somewhere between 8 and 25 km height. In general, we find that precision degrades significantly with height above 30 km and its performance becomes there worse than 1%. Temperature precision assessment has been generally excluded in previous studies. Refractivity has here, in general, a precision similar to dry temperature but worse than wet temperature in the lower atmosphere and above 30 km. However, it has been shown that the better performance of wet temperature is an artificial effect produced by the use of the same background information in nearly collocated wet retrievals. Performance in refractivity around 1% is found in the Northern Hemisphere at the lowest heights and significantly worse in the southern polar zone above 30 km. There is no strong dependence of the estimated precision in terms of height on day and night, on latitude, on season, or on the homogeneity degree of each group of profiles. This reinforces the usual claim that GPS RO precision is independent of the atmospheric conditions. The roughly 0.1% precision in the 8–25 km height interval should suffice to distinguish between day and night average values, but no significant differences are found through a Student t test for both populations at all heights in each latitude band. It was then shown that the present spatial density of GPS RO does not allow to analyze smaller latitudinal bands, which could lead to smaller dispersions associated with the day and night means, where it would then be potentially possible to detect significant statistical differences among both categories. We studied the uncertainties associated with the background conditions used in the retrievals and found that their contribution is negligible at all latitudes and heights. However, they force an artificial improvement of wet temperature precision as compared to the dry counterpart at the lowest and highest altitudes studied. In addition, we showed that there is no detectable dubious behavior of COSMIC data prior to day 194 of year 2006 as warned by the data providers, but our result applies only to the precision issue and cannot be extended to other features of data quality. Regarding accuracy, we estimated an average bias of 0.1 K for GPS RO temperature between about 10 and 30 km height and somewhat larger at lower altitudes. We expect a roughly −0.5 K bias above 35 km altitude. Regarding refractivity, a −0.2% bias of the measurements was estimated below about 8 km height.Fil: Alexander, Pedro Manfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: de la Torre, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Austral. Facultad de Ingeniería; ArgentinaFil: Llamedo Soria, Pablo Martin. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Austral. Facultad de Ingeniería; ArgentinaFil: Hierro, Rodrigo Federico. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Austral. Facultad de Ingeniería; Argentin