Simulating a Precipitation Radar Onboard a Geostationary Satellite

Spaceborne precipitation radars, such as the Tropical Rainfall Measurement Mission (TRMM) and Global Precipitation Measurement (GPM) Core Observatory, have been contributing to the societal and scientific developments. Building upon the success of TRMM and GPM Core Observatory, the Japan Aerospace Exploration Agency (JAXA) is now studying a feasibility of a satellite equipped with a precipitation radar on the geostationary orbit. The frequent observations would enhance our understanding about storms and improve numerical weather prediction (NWP).

However, it is not clear what kind of observation data will be obtained from the geostationary satellite radar because it observes precipitation obliquely at an off-nadir point. Besides, the horizontal resolution is limited by the antenna size; it is difficult to construct a large antenna in space. At this moment, the antenna size of about a 20-m-by-20-m square is considered. The tilted sampling volume and the coarse resolution would result in more contaminations from the surface clutters. Since the tilt is smaller in the tropics, we would expect the geostationary satellite radar would be more effective for tropical events, especially with strong precipitation. Therefore, this study investigates how the geostationary satellite radar would observe a typhoon, an organized severe storm in the tropics.

The results demonstrated that it would be possible to obtain three-dimensional precipitation data albeit the quality of the observation depends on the beam width and sampling span, and the position (latitude) of the typhoon. With a beam width and sampling span at 20-km horizontal resolution, the radar cannot observe low altitude or weak precipitation. This limitation was somewhat mitigated by oversampling (i.e., beam width at 20-km resolution but with a 5-km sampling span). Narrowing down the beam width and sampling span to be 5-km resolution, the surface clutter contamination was confined to near-surface levels. As for the position of the typhoon, the higher the latitude, the larger the off-nadir angle, and it becomes easier to obtain the precipitation signal.