ROHP-PAZ
A team of researchers from the Institute for Space Studies of Catalonia (IEEC) and Spanish Research Council (ICE, CSIC) has released the data obtained by the experiment with GPS signals on board the Spanish Earth observation satellite PAZ, launched in February 2018. These data sets are the only GPS signals acquired at two polarizations from a spaceborne satellite, a new technique called polarimetric radio occultations (PRO). The researchers had previously confirmed that the recorded polarimetric signals are sensitive to heavy rainfall and other hydrometeors, hypothesis tested in the GPS experiment aboard PAZ.
The Radio Occultation is a technique to observe the atmosphere of a planet using two elements: one that transmits radio or microwave signals (the GPS satellites in this case) and another element that receives them (in this case a device installed aboard the PAZ satellite). The peculiarity of this technique is that, if the transistor and receiver elements are joined in a straight line, it crosses the Earth, that is, the elements are hidden by the Earth. Despite this, the signal continues to be received because the GPS beam bends. The bending of the rays can be measured and related to the vertical structure of the atmosphere. As a result, vertical profiles of temperature, pressure and moist are typically inferred from the GPS radio occultation data.
The novelty of the Radio Occultation and Heavy Precipitation experiment aboard PAZ (ROHP-PAZ) is its capability to quantify the effects suffered by the signals induced by large rain droplets (intense rain) and frozen particles (cloud ice, snow, graupel…). These features are captured through the vertical and horizontal components of the GPS signal, what is called ‘polarimetric observations’, measured for the first time from Space. The other spaceborne sensors measuring rain and cloud ice look the rainy scenarios from above, in the form of vertical ‘slices’, while the polarimetric radio occultation technique provides horizontal ‘slices’ of the precipitation at reasonable vertical intervals, a side-look that complements the rest of measurements. It is also the only one that can jointly sense precipitation and its thermodynamic properties, important pieces of information to understand the conditions underlying intense precipitation.
After a process of accurate calibration of the data, today the sets are being made publicly available. The set will be regularly expanded as the PAZ satellite continues its data acquistion, at a rate of approximately new 200 vertical profiles every day. “Opening the data to other researchers is a major milestone of our activity: it is the only way our research can be verified and new ideas can be tested by other researchers” explains Dr. Estel Cardellach, principal investigator of the experiment. “We are positive the polarimetric radio occultations have potential to contribute to a broad range of geophysical applications, but we cannot do it all by ourselves: scientists working on meteorology, weather forecast or climate have the expertise to generate new uses of these signals, so they need access to the sets”, adds the IEEC/CSIC researcher.
Involving other research communities
To celebrate the data release, and to boost the use of these data sets among the meteorology, weather forecast, climate and remote sensing research communities, the first ‘ROHP-PAZ Users Workshop’ is taking place on April 23, 2020. Because of the coronavirus emergency, its format was changed to become a fully on-line event. “Over 50 scientists from Europe, America and Asia have registered to the meeting, coming from different scientific backgrounds” reports Dr. Ramon Padullés, ROHP-PAZ researcher at IEEC/CSIC. “The interest towards this new technique is increasing, and we hope the on-line workshop will boost new collaborations and science return”. One of the key questions to be discussed in the meeting is how to use this new type of data to improve the weather forecast and the climate projections, as currently both of them have limited capabilities to conclude about extreme precipitation events. The workshop is organized together with researchers at NASA/Jet Propulsion Laboratory (JPL), who also participated in the calibration phase of the mission and who plan to release their own ROHP-PAZ data sets later this year.
The ROHP-PAZ experiment is an experiment funded by the Spanish Ministry of Science and Innovation, and led by the ICE, CSIC/IEEC. In addition to NASA/JPL, the experiment is possible through collaborations with the PAZ satellite owner, operator and exploiter, Hisdesat, the North-American National Oceanics and Atmospheric Administration (NOAA) and the University Corporation for Atmospheric Research (UCAR).