Exploring the Tropical Atmosphere with Advanced Radio Occultation: Processing, Serving, and Analyzing Radio Occultation Data to Advance Atmospheric Science

The Global Navigation Satellite System (GNSS) is the set of satellite constellations launched worldwide to provide positioning information for all forms of navigation. The system includes the Galileo constellation launched by the European Union, the GLONASS satellites from Russia, Beidou from China, and the US Global Positioning System (GPS). GPS is well known in the US as we rely on it for driving directions and an ever-expanding array of cell phone apps.

But the radio waves transmitted for GNSS positioning also have tremendous value for looking at the atmosphere: GNSS radio waves are refracted by the atmosphere to an extent which depends on the temperature and water vapor content of the lower atmosphere and the density of electric charge in the ionosphere. The refraction causes a delay in the phase of the waves when they reach a receiver, and if the reciever is placed on an orbiting satellite the phase delay can be used to measure ionospheric charge and atmospheric temperature and water vapor (although water vapor requires additional information).

Because the measurements are made during the rising and setting, or occultation, of a GNSS transmitter satellite relative to a receiver satellite, the technology is referred to as GNSS Radio Occultation (GNSSRO). GNSSRO has proven to be a highly effective observing method as the measurements can be made under all weather conditions, are unaffected by clouds and aerosols, and are absolutely calibrated (SI traceable) through the atomic clocks that support the GNSS transmitters.

They also have relatively high vertical resolution, providing a profile of atmospheric properties which can be used to identify sharp features such as the tropopause and fluctuations in temperature caused by atmospheric gravity waves.

This award supports work on the development and scientific use of GNSSRSO measurements at the University Corporation for Atmospheric Research (UCAR). UCAR has played a foundational role in GNSSRO technology starting with the GPS/MET proof-of-concept satellite mission in the 1990s, and they led the development of the Constellation Observing System for Meteorology, the Ionosphere, and Climate (COSMIC, also called COSMIC-1 or C1), a constellation of six GNSSRO receiver satellites launched in 2006.

They also managed the development of COSMIC-2 (C2, a mission led by the US National Oceanic and Atmospheric Administration), a follow-on constellation of six satellites orbiting over the tropics and subtropics (30 degrees south to 30 north) launched in 2019. The UCAR group, formally known as the UCAR COSMIC Program, has also been collecting and processing GNSSRO data from several other satellite missions, for example SAC-C (from Argentina), GRACE (a NASA mission), Metop-A and -B (EUMETSAT missions), and PAZ (from Spain).

Including all satellite misions there are now two decades of continuous GNSSRO observations, a long enough record to be valuable for research on climate trends.

Work here emphasizes the low latitudes, motivated by the high density and quality of low-latitude observations available from C2. One question to be addressed is the extent to which RO-based observations can lead to better understanding of hurricanes and other tropical cyclones (TCs), for instance the extent to which accumulation of water vapor in a deep layer near the developing TC promotes storm development.

Another is whether the discrepancy between the observed profile of tropical tropospheric warming and the profile simulated in response to greenhouse gas increases is due to inadequacies of climate models or biases in observations. The high resolution and absolute calibration of RO soundings makes them ideal address this question. Other issues to be pursued include better characterization of the tropical waves which drive the stratospheric Quasi-Biennial Oscillation and the use of RO to assess the representation of water vapor in commonly used reanalysis datasets.

The project also includes research on the low-latitude ionosphere, taking advantage of the Ion Velocity Meter (IVM) deployed as a secondary payload on the C2 satellites as well as two recently launched NASA missions, the Global-Scale Observations of the Limb and Disk (GOLD) and Ionospheric Connection Explorer (ICON). The suite of C2, ICON, and GOLD observations is used to study equatorial plasma bubbles and the effect of the equatorial dynamo on the ionosphere. Further work seeks to improve the assimilation of GNSSRO observations into ionospheric models.

In addition to these research applications the project includes research to advance GNSSRO theory and algorithms so as to maximize the value of RO for atmospheric research. One concern is the effects of random refractivity fluctuations on RO measurements, as such fluctuations are thought to contribute to the negative bias in RO-derived atmospheric moisture commonly found near the surface.

A further concern is the detection of super-refraction, in which refraction becomes strong enough to trap GNSS radio waves near the surface and prevent their use for determining temperature and water vapor. Super-refraction must be detected so that data from the ducted portion of the RO profiles is not used. Additional work develops RO methods for the ionosphere, including better methods for detecting localized charge irregularities and ray-tracing techniques which use separate ray paths for the two wavelengths transmitted by GNSS satellites.

The work has societal as well as scientific value due to the use of GNSSRO for weather prediction and climate monitoring. GNSSRO is used in most weather forecasting centers and improvements in RO algorithms developed under this award have direct application in weather prediction. The examination of GNSSRO as an observational technique for looking at hurricanes is particularly relevant given the lack of observations that can be used to initialize hurricane forecasts.

The ionospheric work is also directly applicable to efforts to predict space weather. In the area of climate monitoring, the project develops datasets from RO observations over multiple satellite missions, processed in a consistent manner so that they can be used to assess long-term trends in temperature and moisture. The award includes support for hosting, archiving, serving, and supporting access to datasets by the research community.

The datasets and data services are augmented by several education and outreach efforts seeking to facilitate use of GNSS RO data by the research community. One of these is a postdoctoral research program which supports early-career scientists seeking to work with RO observations. The project also works with the UCAR Significant Opportunities in Atmospheric Research and Science (SOARS) program, through which it provides mentorship to students from underrepresented groups.

A two-week summer colloquium is planned for 2022 to bring together experts in GNSSRO and give early career researchers an opportunity to become familiar with GNSSRO and its research applications.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.