Precipitation measurements with polarimetric radio occultations

In 2009, the Spanish Ministry of Science and Innovation approved a proposal to modify the Global Positioning System (GPS) receiver and to allocate a Polarimetric (Pol) Radio Occultation (RO) antenna in the Spanish PAZ satellite. PAZ became an opportunity to test the new Pol-RO concept, which aims to capture ROs using a two orthogonal linear polarization antenna. The experiment has been named Radio Occultations and Heavy Precipitation with PAZ (ROHP-PAZ). The objective is to measure the phase difference between the horizontal and the vertical components of the incoming electromagnetic field that is induced when heavy precipitation flattened raindrops are present in the ray-path. This effect, widely studied in weather radar community, will be measured from space using GNSS signals for the first time with PAZ, which is planned to be launched in 2017. The main objective of this new concept is to enhance the RO capabilities by providing vertical precipitation information along with the current standard RO thermodynamic products (i.e. temperature, pressure and moisture). Until now, no other observing system has been able to provide simultaneous thermodynamic and precipitation information under extreme conditions. The high vertical resolution, global coverage and all-weather capability properties of the RO observations combined with vertical indication of precipitation intensity can be of great value for heavy rain characterization, and therefore for climate and weather forecast and research. The theoretical background for the technique, its feasibility and applications have been assessed in this dissertation. The theoretical basis has been developed combining electromagnetic propagation theory and cloud and precipitation microphysics. Forward scattering simulations at L-band have been obtained in order to relate the microphysics parameters with the expected Pol-RO observables. The feasibility has been addressed using coincident (in space and time) RO profiles and space-based precipitation observations. Such simultaneous observations allow for the characterization of actual RO measurements according to the coincident precipitation information. Finally, the applications have been investigated through realistic end-to-end simulations of the Pol-RO observations, which provide the anticipated Pol-RO products for different precipitation situations, regions, and seasons. Before the launch of the satellite, a field campaign has been conducted with the aim of starting the characterization of the polarimetric measurements. The engineering model of the PAZ antenna was placed at the top of a mountain peak in order to capture, for the first time, linear polarimetric GNSS signals at low grazing angle. This campaign has been useful to start identifying the hardware internal effects and unexpected precipitation features that will be affecting the Pol-RO observations. These effects have been incorporated to the simulations, hence providing valuable feedback to obtain more realistic Pol-RO products. Besides feedback, the data from the field campaign have shown the first observational evidence that precipitation and other hydrometeors induce a noticeable effect on the GNSS polarimetric signals. All these exercises yielded several relevant results. The noise level analysis from actual RO observations sensing precipitation scenarios has allowed to set a detectability threshold for the technique, indicating that a high percentage of moderate to heavy precipitation events will be detected with PAZ. Nevertheless, the integrated nature of the Pol-RO observable does not allow to distinguish between the contributions from the rain's intensity and extension, leaving an ambiguity in the provided product. In an attempt to solve such ambiguity, a tomographic approach has been proposed, which has yielded promising theoretical results. Moreover, it has been shown how the Pol-RO observables can be linked to physical precipitation parameters, such as the along-ray averaged rain rate, in a probabilistic way. The end-to-end simulation has also revealed that the ionosphere will induce a non-negligible depolarization, that will require calibration. And finally, the collocated data has shown the potential applications for Pol-ROs products