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This study presents, for the first time ever, occulting signals of the Global Navigation Satellite Systems (GNSSs) acquired at two polarizations from a Low Earth Orbiter, and it shows that they sense heavy precipitation. The data sets are obtained from early stages of the Radio Occultation and Heavy Precipitation experiment aboard the PAZ satellite, launched in February 2018 and activated in May 2018. Preliminary calibration algorithms are applied to remove other systematic effects, and the resulting vertical profiles of polarimetric phase shift observations are compared to precipitation information from other missions. The analysis of the data shows consistency between Radio Occultation and Heavy Precipitation experiment aboard the PAZ satellite polarimetric phase shift measurements and presence of hydrometeors, with strong signatures from heavy precipitation. The polarimetric measurements also capture vertical features consistent with the vertical structures of precipitation. Plain Language Summary: When the satellites of the navigation systems (like GPS) set below the horizon, their signals can be used to measure temperature, pressure, and humidity of the atmosphere at different altitudes (an observation called radio occultation). For the first time, a satellite has collected these setting GPS signals at two polarizations, that is, separating two different orientations of the electromagnetic field (horizontal and vertical with respect to the receiving antenna). It is the Radio Occultation and Heavy Precipitation experiment aboard PAZ satellite, launched in February 2018 and activated in May 2018. Previous theoretical studies have shown that intense rain crossing the path of the GPS signals would introduce a delay of the horizontal component with respect to the vertical one. This study shows the analysis of the polarimetric delays, and it confirms that they are related to the presence of rain, in particular intense precipitation. The measurement of polarimetric delays at different altitudes is consistent with the presence of rain structures at different heights. No other technique captures profiles of both thermodynamics and hydrometeor content in intense rain phenomena, and therefore, the Radio Occultation and Heavy Precipitation experiment aboard the PAZ satellite data could provide a new tool to understand extreme precipitation, with potential to improve its difficult prediction. Key Points: We present the first spaceborne GNSS radio occultation signals acquired at two polarizationsThe measured observables sense intense precipitation and capture its vertical structureNo other technique captures both thermodynamics and hydrometeor profiling in intense rain phenomena [ABSTRACT FROM AUTHOR]

There is currently a gap in satellite observations of the moisture structure during heavy precipitation conditions, since infrared and microwave sounders cannot sense water‐vapour structure near the surface in the presence of intense precipitation. Conversely, Global Navigation Satellite System (GNSS) radio occultations (RO) can profile the moisture structure with high precision and vertical resolution, but cannot indicate the presence of precipitation directly. Polarimetric RO (PRO) measurements have been proposed as a method to characterize heavy rain in GNSS RO, by measuring the polarimetric differential phase delay induced by large size hydrometeors. Previous studies have shown that the PRO polarimetric phase shift is sensitive to the path‐integrated rain rate under intense precipitation scenarios, but there is no current method to invert PRO measurements into quantitative estimates of the path‐averaged rain rate. In this manuscript, a probabilistic inversion approach to the GNSS PRO observables is proposed, where the GPM precipitation products are used for the construction of an a priori look‐up table (LUT) database. The performance of the LUTs is assessed for use in the inversion of satellite‐based GNSS PRO observations, based on synthetically generated PRO data of actual events, which correspond to co‐locations between GNSS RO profiles and the TRMM observations. The synthetic data include end‐to‐end propagation effects of the polarimetric observables and a simple separation algorithm to isolate the hydrometeor component of the observation. The assessment results in agreement better than ±1 mm/hr between the reference LUT and the actual rain statistics of the synthetic data, proving the suitability of the GPM‐based probabilistic inversion tool. These findings indicate that the GNSS PRO products are capable of extending the current GNSS RO ones by associating indications of rain‐rate probabilities at different altitudes, at ∼250 m vertical resolution and under intense precipitation scenarios with the standard vertical thermodynamic profiles. A probabilistic inversion technique has been developed for GNSS polarimetric radio occultation (GNSS‐PRO) observables. The retrieval is based on information obtained from the Global Precipitation Measurement (GPM) mission, organized as a look‐up table (LUT) database. Confronting the LUTs and the phase shift suffered by the GNSS‐PRO signals results in a series of probabilities of rain rates at different altitudes. These vertical profiles of rain probability complement the standard GNSS‐RO thermodynamic profiles, providing a unique insight within intense precipitation scenarios. [ABSTRACT FROM AUTHOR]

This study describes the first experimental observations showing that hydrometeors induce polarimetric signatures in global navigation satellite system (GNSS) signals. This evidence is relevant to the PAZ low Earth orbiter, which will test the concept and applications of polarimetric GNSS radio occultation (RO) (i.e. ROs obtained with a dual-polarization antenna). A ground field campaign was carried out in preparation for PAZ to verify the theoretical sensitivity studies on this concept (Cardellach et al., 2015). The main aim of the campaign is to identify and understand the factors that might affect the polarimetric GNSS observables. Studied for the first time, GNSS signals measured with two polarimetric antennas (H, horizontal, and V, vertical) are shown to discriminate between heavy rain events by comparing the measured phase difference between the H and V phase delays (ΔΦ) in different weather scenarios. The measured phase difference indicates higher dispersion under rain conditions. When individual events are examined, significant increases in ΔΦ occur when the radio signals cross rain cells. Moreover, the amplitude of such a signal is much higher than the theoretical prediction for precipitation; thus, other sources of polarimetric signatures have been explored and identified. Modelling of other hydrometeors, such as melting particles and ice crystals, have been proposed to explain the obtained measurements, with good agreement in more than 90% of the cases. [ABSTRACT FROM AUTHOR]

The spatial variability of the tropical tropospheric relative humidity (RH) throughout the vertical extent of the troposphere is examined using Global Positioning System Radio Occultation (GPSRO) observations from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission. These high vertical resolution observations capture the detailed structure and moisture budget of the Hadley Cell circulation. We compare the COSMIC observations with the European Center for Medium-range Weather Forecast (ECMWF) Reanalysis Interim (ERA-Interim) and the Modern-Era Retrospective analysis for Research and Applications (MERRA) climatologies. Qualitatively, the spatial pattern of RH in all data sets matches up remarkably well, capturing distinct features of the general circulation. However, RH discrepancies exist between ERA-Interim and COSMIC data sets that are noticeable across the tropical boundary layer. Specifically, ERA-Interim shows a drier Intertropical Convergence Zone (ITCZ) by 15-20% compared to both COSMIC and MERRA data sets, but this difference decreases with altitude. Unlike ECMWF, MERRA shows an excellent agreement with the COSMIC observations except above 400 hPa, where GPSRO observations capture drier air by 5-10 %. RH climatologies were also used to evaluate intraseasonal variability. The results indicate that the tropical middle troposphere at ±5-25° is most sensitive to seasonal variations. COSMIC and MERRA data sets capture the same magnitude of the seasonal variability, but ERA-Interim shows a weaker seasonal fluctuation up to 10% in the middle troposphere inside the dry air subsidence regions of the Hadley Cell. Over the ITCZ, RH varies by maximum 9% between winter and summer. [ABSTRACT FROM AUTHOR]

Global Positioning System (GPS) radio occultation (RO) has provided continuous observations of the Earth's atmosphere since 2001 with global coverage, all-weather capability, and high accuracy and vertical resolution in the upper troposphere and lower stratosphere (UTLS). Precise time measurements enable long-term stability but careful processing is needed. Here we provide climate-oriented atmospheric scientists with multicenter-based results on the long-term stability of RO climatological fields for trend studies. We quantify the structural uncertainty of atmospheric trends estimated from the RO record, which arises from current processing schemes of six international RO processing centers, DMI Copenhagen, EUM Darmstadt, GFZ Potsdam, JPL Pasadena, UCAR Boulder, and WEGC Graz. Monthlymean zonal-mean fields of bending angle, refractivity, dry pressure, dry geopotential height, and dry temperature from the CHAMP mission are compared for September 2001 to September 2008.We find that structural uncertainty is lowest in the tropics and mid-latitudes (50° S to 50° N) from 8 km to 25 km for all inspected RO variables. In this region, the structural uncertainty in trends over 7 yr is <0.03%for bending angle, refractivity, and pressure, <3m for geopotential height of pressure levels, and <0.06K for temperature; low enough for detecting a climate change signal within about a decade. Larger structural uncertainty above about 25 km and at high latitudes is attributable to differences in the processing schemes, which undergo continuous improvements. Though current use of RO for reliable climate trend assessment is bound to 50° S to 50° N, our results show that quality, consistency, and reproducibility are favorable in the UTLS for the establishment of a climate benchmark record. [ABSTRACT FROM AUTHOR]

The typical atmospheric boundary layer (ABL) over the southeast (SE) Pacific Ocean is featured with a strong temperature inversion and a sharp moisture gradient across the ABL top. The strong moisture and temperature gradients result in a sharp refractivity gradient that can be precisely detected by the Global Positioning System (GPS) radio occultation (RO) measurements. In this paper, the Constellation Observing System for Meteorology, Ionosphere & Climate (COSMIC) GPS RO soundings, radiosondes and the high-resolution ECMWF analysis over the SE Pacific are analyzed. COSMIC RO is able to detect a wide range of ABL height variations (1-2 km) as observed from the radiosondes. However, the ECMWF analysis systematically underestimates the ABL heights. The sharp refractivity gradient at the ABL top frequently exceeds the critical refraction (e.g., -157 N-unit km-1) and becomes the so-called ducting condition, which results in a systematic RO refractivity bias (or called N-bias) inside the ABL. Simulation study based on radiosonde profiles reveals the magnitudes of the N-biases are vertical resolution dependent. The N-bias is also the primary cause of the systematically smaller refractivity gradient (rarely exceeding -110 N-unit km-1) at the ABL top from RO measurement. However, the N-bias seems not affect the ABL height detection. Instead, the very large RO bending angle and the sharp refractivity gradient due to ducting allow reliable detection of the ABL height from GPS RO. The seasonal mean climatology of ABL heights derived from a nine-month composite of COSMIC RO soundings over the SE Pacific reveals significant differences from the ECMWF analysis. Both show an increase of ABL height from the shallow stratocumulus near the coast to a much higher trade wind inversion further off the coast. However, COSMIC RO shows an overall deeper ABL and reveals different locations of the minimum and maximum ABL heights as compared to the ECMWF analysis. At low latitudes, despite the decreasing number of COSMIC RO soundings and the lower percentage of soundings that penetrate into the lowest 500-m above the mean-sea-level, there are small sampling errors in the mean ABL height climatology. The difference of ABL height climatology between COSMIC RO and ECMWF analysis over SE Pacific is significant and requires further studies. [ABSTRACT FROM AUTHOR]

Trends in monthly average, zonal average temperature in the stratosphere as retrieved from highly accurate modern satellite data are intercompared and compared with climate reanalyses from 2003 through 2014. The data sets used are those of Atmospheric Infrared Sounder and Global Positioning System (GPS) radio occultation, and the reanalyses are those of MERRA and ERA‐Interim. Trends produced by all data sets agree to within 0.02 K/year in the lower stratosphere and 0.05 K/year in the middle stratosphere. A number of retrieval errors are found that introduce incorrect trends and seasonal anomalies. Adding microwave data to the infrared retrieval changes trends by approximately 0.01 K/year, thus improving agreement with the other data sets. The signature of the quasi‐biennial oscillation in temperature and the annual cycle of temperature over Antarctica as retrieved from infrared data contain null‐space errors of more than 3 K due to erroneous priors used in retrieval. Nonuniformity in GPS radio occultation gives rise to errors because changes in received GPS signal strength alter the upper boundary initialization in radio occultation retrieval. Finally, an incorrect specification of atmospheric water vapor introduces an erroneous seasonal cycle of temperature as retrieved from GPS radio occultation data in the upper troposphere. All of these time‐dependent retrieval errors can be corrected with future research and improvements to spectral infrared and GPS radio occultation retrieval systems. Plain Language Summary: Satellite instruments that have been observing the atmosphere since 2003 are highly calibrated and particularly rich in information. Two of them have been measuring temperature trends in the atmosphere between 7 and 30 km altitude, and their results differ by an uncomfortable amount. Because the measurement techniques have nothing in common, it is possible to trace the causes of the differences to fairly well‐known problems in their formulations that translate basic measurements to air temperature. The results of this paper will direct those responsible for retrieving temperature from modern satellite data to fix these problems and thereby bring multiple, independent observations of temperature trends into better agreement with each other. Key Points: Temperature trends in the lower and middle stratosphere as retrieved from modern satellite data differ by 0.02 and 0.05 K/yearNull‐space error affects temperature anomalies in infrared data, and receiver performance affects trends in GPS RO dataTime‐dependent biases inherent to infrared and GPS RO retrieval systems can be removed by modification of the retrieval systems [ABSTRACT FROM AUTHOR]