Field experiment for GPM ground validation using the dual Ka-band radar system

Global Precipitation Measurement (GPM) started as an international mission and follow-on mission of the Tropical Rainfall Measuring Mission (TRMM) project to obtain more accurate and frequent observations of precipitation. Japan Aerospace Exploration Agency (JAXA) is in charge of developing GPM/Dual-frequency Precipitation Radar (DPR) algorithms as the sensor provider and producing and delivering hourly global precipitation map to make useful for various research and application areas. In order to secure the quality of precipitation estimates, ground validation (GV) of satellite data and retrieval algorithms is essential.

End-to-end comparisons between instantaneous precipitation data observed by satellite and ground-based instruments are not enough to develop and improve the algorithms. In order to estimate the error of various physical parameters in the precipitation retrieval algorithms of DPR, we have started to conduct intensive observations using several instruments for measuring precipitation. DPR algorithm estimates particle size distribution (DSD) parameters of precipitation using the difference of scattering and attenuation feature. The equivalent reflectivity factors (Ze) and attenuation factors (k) are independently calculated in the algorithm. The characteristics of the Mie scattering effect and attenuation of the actual precipitation particles for radar beams are essential for construction of the DPR algorithm, especially for Ka-band. The folding observation of two ground-based Ka-band radars can directly measure Ze and k regardless of solid and liquid phase. The simultaneous observations using the two Ka-radars and precipitation particle observation systems will provide the combination of the information of the precipitation particles and scatter and attenuation. These information are very useful for the development of DPR and GMI algorithms as scattering tables. Ka-radar observation with C/X-band radar can verify the dual-frequency algorithm of GPM/DPR.

JAXA has developed a dual Ka-band radar system for the GPM/DPR algorithm development. This system consists of two identical FMCW type Ka-band radars. In order to achieve the folding observation, the radars need to satisfy the following conditions:

- to observe the precipitation echo between the long distance,

- to detect the weak echo with the attenuation by precipitation particles

- to avoid the radio wave interference each other,

- to avoid the ground clutter and so on.

As the result, the specifications of the radar are:

- observation range: more than 15 km,

- minimum detectable reflectivity: -20 dBZ at 10 km from the radar,

- sidelobe level: less than -22 dBZ

- Nyquist velocity: 10 m/s

- beam width: 0.58 degree

- different transmission time between the two radars using GPS

We started to conduct the intensive observation in Okinawa Island using the Ka-band radars and a mobile precipitation particle observation system for system evaluation and rain observation. We confirmed that the both Ka-radars satisfied the established specifications for the algorithm validation activity. The comparison reflectivity value of the Ka-band radars with that of the C-band radar of National Institute of Information and Communications Technology (NICT) and disdrometer indicated good correlation.

These information are very useful for the development of DPR and GMI algorithms as scattering table. After the observation in Okinawa Island, we will continue to collect the data of the dual Ka-radars in Nagaoka, Sapporo and Mt. Fuji targeted to several types of the precipitation (rain, dry snow, wet snow and melting layer) using the Ka-radars.