KEYWORDS: Radar, Signal attenuation, Reflectivity, Ku band, Ka band, Algorithm development, Detection and tracking algorithms, Satellites, Image classification
Development of precipitation retrieval algorithms for spaceborne radar began with the launch of the Tropical Rainfall Measuring Mission which carried the world first Precipitation Radar (PR). The standard Algorithm for the Dual-frequency Precipitation Radar (DPR) onboard the GPM satellite was developed based on the experience of the TRMM PR algorithm1. The latest DPR algorithm works well and satisfies the mission requirements. Nevertheless, there are still several output variables whose quality can be improved. For example, the threshold for precipitation detection can be lowered to detect more light precipitation without much increase of false detections by adopting a better filtering method than the current method. Removal of surface clutter is related to the detection of precipitation and expected to be improved too. Attenuation correction is another important area of improvement. The current attenuation correction method uses the surface reference technique (SRT) when the attenuation is large. There is a possibility of using radiometric noise for this purpose. The directions of the high sensitivity beams of the DPR’s Ka-band radar, which is called KaPR, were changed to cover the outer swath in March of 2018. Ku and Ka matched beam data are now available over the full swath so that the dualfrequency algorithm can be applied to the entire data. The new scan pattern is expected to improve not only the precipitation retrieval algorithm but also the classification algorithm. This paper summarizes these possible improvement areas in the DPR algorithm.
The observation from spaceborne precipitation radar has been contributed to better understanding of earth climate system. Global Precipitation Measurement (GPM) core satellite Dual-frequency Precipitation Radar (DPR) provides us 3- dimentional information of precipitation by the scan width of about 250 km, but there has been an argument that to bring systematic impact on the weather forecasting and monitoring, wider swath observation is necessary. Based on those discussions, the scan pattern of GPM/DPR was experimentally changed for 1 day from 13UTC on September 26th. In this experiment, the scan angle was changed to observe from nadir to about +34° assuming future spaceborne precipitation radar with wider swath width, while in the normal observation DPR scans ±17°. The height and strength of the surface echo clutter with larger incident angle were assessed statistically to examine the possibility of the rainfall retrieval with wide swath observation by DPR. For the observation with the Ku band, the result shows that the clutter top height at the larger incident angle over ocean is somehow suppressed at around 4 km while over land it increases almost linearly up to around 5 km. The same tendency is found on the Ka band observation, but it has lower clutter top height of around 2.5 km and 3.5 km, over ocean and land respectively. The results also indicate that relatively intense rainfall can be retrieved while shallow rainfall with weak echo power may not be acceptable for retrieval because it should be masked by the surface clutter.
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