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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]

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-rangeWeather Forecast (ECMWF) Re-Analysis Interim (ERA-Interim) and the Modern-Era Retrospective analysis for Research and Applications (MERRA) climatolo gies. 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, which are noticeable across the tropical boundary layer. Specifically, ERA-Interim shows a drier Inter Tropical Convergence Zone (ITCZ) by 15-20% compared both to 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 cap ture 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]

The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission will provide a calibration laboratory in orbit for the purpose of accurately measuring and attributing climate change. CLARREO measurements establish new climate change benchmarks with high absolute radiometric accuracy and high statistical confidence across a wide range of essential climate variables. CLARREO's inherently high absolute accuracy will be verified and traceable on orbit to Système Internationale (SI) units. The benchmarks established by CLARREO will be critical for assessing changes in the Earth system and climate model predictive capabilities for decades into the future as society works to meet the challenge of optimizing strategies for mitigating and adapting to climate change. The CLARREO benchmarks are derived from measurements of the Earth's thermal infrared spectrum (5-50 μm), the spectrum of solar radiation reflected by the Earth and its atmosphere (320-2300 nm), and radio occultation refractivity from which accurate temperature profiles are derived. The mission has the ability to provide new spectral fingerprints of climate change, as well as to provide the first orbiting radiometer with accuracy sufficient to serve as the reference transfer standard for other space sensors, in essence serving as a 'NIST [National Institute of Standards and Technology] in orbit.' CLARREO will greatly improve the accuracy and relevance of a wide range of space-borne instruments for decadal climate change. Finally, CLARREO has developed new metrics and methods for determining the accuracy requirements of climate observations for a wide range of climate variables and uncertainty sources. These methods should be useful for improving our understanding of observing requirements for most climate change observations. [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 article presents a study which investigates the impact of large-scale ionospheric structure on the accuracy of radio occultation (RO) retrievals. A climatological model of the ionosphere and an ionospheric data assimilation model comparing quiet and geomagnetically disturbed conditions is provided. The study suggests that monitoring ionospheric conditions is critical for long-term climate observation with RO. Also, it indicates that orbit altitude has a significant impact on the bias.