Spatial Characteristics of Precipitation Associated with Tropical Cyclones and Their Retrieval Uncertainties from Spaceborne Precipitation Radars

In this study, a dataset based on spaceborne precipitation radars of 692 tropical cyclones (TCs) in the Northwest Pacific from 1998 to 2021 was generated to examine the spatial characteristics of TCs, their temporal variations, and retrieval issues. The uncertainties in current low-level precipitation profile models are also investigated. Based on the Joint Typhoon Warning Center best-track position data, the hourly center position of a TC was estimated using simple interpolation, and those TCs that passed within a radius of 3° from the center were extracted. Thus, 5026 orbits for Precipitation Radar (PR) onboard the Tropical Rainfall Measuring Mission satellite between January 1998 and November 2014 and 1090 orbits for Ku-band Precipitation Radar (KuPR) onboard the GPM Core Observatory satellite between April 2014 and November 2021 were extracted. Of these, 1884 orbits for PR and 290 orbits for KuPR contained the TC center within the satellite scan. Snapshots of precipitation distribution at each altitude are available on our precipitation climatology database website (https://www.rain-clim.com/gallery/tropical-cyclone.html). The characteristics of precipitation that abruptly decreases at the center of TCs and differences between periods were examined using composite maps having a resolution of 0.05°. Very deep and rapidly downward-increasing precipitation profiles were observed several tens of kilometers from the center for typhoons with maximum wind speeds of 64 knots or more south of 20°N. At an altitude of 1 km, precipitation increased fourfold to a peak at distances up to 0.25° from the center and was higher than that at the center grid at distances up to 1.4° from the center. Tropical typhoons have higher precipitation intensity and smaller precipitation areas than tropical depressions with maximum wind below 34 knots and brought more precipitation to the southwest. In the mid-latitudes north of 30°N, the asymmetric distribution of precipitation was marked, with tropical storms producing more precipitation towards the north-east and typhoons toward the north. Thus, the spatial structure investigated in the present study is broadly consistent with that reported in previous studies on radiometer-based patterns.

Changes over time are also an important object of investigation, but the effects of sensor differences and observation limits, in addition to natural variations, need to be taken into account. Compared to the first half period, precipitation in the center of the TCs decreased in the second half of the PR observation period, while precipitation in the surrounding tens of kilometers increased. On the other hand, the KuPR statistics showed a decrease in precipitation in the lower layers near the center of the TC compared to the PR. This could be attributed to differences in the retrieval of high precipitation intensities in the V06A product. At higher altitudes, KuPR precipitation increased slightly due to better sensitivity than PR. Profiles within blind zones exhibited other retrieval uncertainties. TC-related precipitation increased downwards to an altitude of approximately 1 km but was affected by a surface clutter removal filter below an altitude of 2 km, depending on the angle of incidence. The marked downward increasing trend seen near typhoon wall clouds indicates the importance of radar near-surface processing.