Comparing reflectivity measurements between satellite- and ground-based radar observations: A case study for precipitation and drought monitoring in Cyprus

Drought is a multidimensional phenomenon that is imperceptible at its early stages, it evolves slowly and cumulatively and results to adverse consequences, for example depletion of water volumes from rivers and reservoirs, decrease of carbon uptake in vegetation etc. Cyprus is characterized by semi-arid to arid climate conditions, experiencing extensive droughts that have a negative impact on the ecosystem, the economy and the agricultural production.

Existing research on drought events in Cyprus is limited to the usage of in-situ data, mainly temperature and precipitation measurements at meteorological stations. Polarimetric weather radars can offer more detailed information regarding precipitation phenomena, especially in areas with sparse network of meteorological stations or remote areas of interest.

This study compares reflectivity measurements from the two ground-based X-band dual polarization radars of the Department of Meteorology of the Republic of Cyprus with measurements obtained from NASA’s Global Precipitation Measurement (GPM) mission.

The DPR (Dual-frequency Precipitation Radar) aboard of GPM is employed in order to derive the radar reflectivity factor with a spatial resolution of 5-25 km for 120 km wide swath. The ground-based radars operate since 2017. They scan in PPI mode at eight (8) constant elevation angles, whereas their azimuth angle varies with a spatial resolution of 0.1° and the radius of each scan is 150 km. The radar stations are located in Rizoelia, Larnaca district, and Nata, Paphos district, providing a full coverage of the island.

Satellite-based radar reflectivity values are used to adjust the ground-based radar measurements. Consequently, the adjusted values of the ground-based radar reflectivity are used as input to modelling expressions for estimating the ground-based radar precipitation.

In order to ensure that the observations are spatially coincident, we have developed a collocated grid, hereafter called universal grid, on which both the ground- and satellite-based radar observations are interpolated at the same locations. The universal grid is a three-dimensional (3D) grid with grid cell size of approximately 2500 m along both horizontal directions, whereas the vertical resolution is set equal to the height resolution of GPM, i.e. 150 m. Regarding temporal resolution, GPM overpasses Cyprus approximately once a week. For the purposes of this study, we selected overflights after the beginning of the ground-based radar operation that coincide with precipitation events.

Additionally, statistical analysis of the reflectivity measurements has been conducted to understand the relationship between the ground-based and the satellite-based datasets and identify spatio-temporal patterns of precipitation.

Acknowledgements:

The authors acknowledge the ‘EXCELSIOR’: ERATOSTHENES: EΧcellence Research Centre for Earth Surveillance and Space-Based Monitoring of the Environment H2020 Widespread Teaming project (www.excelsior2020.eu). The ‘EXCELSIOR’ project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No 857510, from the Government of the Republic of Cyprus through the Directorate General for the European Programmes, Coordination and Development and the Cyprus University of Technology.

The authors acknowledge also the Department of Meteorology of the Republic of Cyprus for the provision of the X-band radar data.