JMSJ Highlights

Seto et al. (2021)

Seto, S., T. Iguchi, R. Meneghini, J. Awaka, T. Kubota, T. Masaki, and N. Takahashi, 2021: The precipitation rate retrieval algorithms for the GPM Dual-frequency Precipitation Radar. J. Meteor. Soc. Japan, 99, 205-237. https://doi.org/10.2151/jmsj.2021-011.
Graphical Abstract with highlights

Overview:
The standard precipitation rate retrieval algorithms (version 06A) for the Global Precipitation Measurement mission’s Dual-frequency Precipitation Radar (DPR) are developed. The DPR consists of a Ku-band radar (KuPR; 13.6GHz), which is the successor to the Precipitation Radar (PR; 13.8GHz) on the Tropical Rainfall Measuring Mission satellite, and a Ka-band radar (KaPR; 35.5GHz). The algorithm for the KuPR is similar to the standard algorithm for the PR (Iguchi et al. 2009), but the relation between precipitation rate R and mass-weighted mean diameter Dm (R−Dm relation) is used instead of the relation between the specific attenuation k and effective radar reflectivity factor Ze (k−Ze relation), which is applied in the PR standard algorithm. As R and Dm are independent of the radar frequency, the same R−Dm relation can be applied to the algorithm for the KaPR. The k−Ze relation for the KuPR has a similar constraint on the drop size distribution with the R−Dm relation, but no k−Ze relations for the KaPR can give a similar constraint with the R−Dm relation (Figure 1).

In addition to the single-frequency algorithms, the dual-frequency algorithm is developed. The dual-frequency algorithm has the same structure and uses the same R−Dm relation as the single-frequency algorithms, but it can select either KuPR or KaPR, or both radars for the precipitation echoes and for the surface echo. For light precipitation, KaPR’s surface echo is useful to surface reference technique (SRT), as the path integrated attenuation for the KaPR is several times as large as that for the KuPR. For heavy precipitation, KuPR’s precipitation echoes and surface echo are useful, while KaPR’s precipitation echoes and surface echo may disappear due to strong attenuation. For moderate precipitation, as both radars’ precipitation and surface echoes are available, the ZfKa method and dual-frequency SRT (DSRT) can be applied.

The adjustment factor e for R−Dm relation can be estimated better in the dual-frequency algorithm by the ZfKa method and DSRT than in the single-frequency algorithms, which rely on single-frequency SRT. Based on the results of the dual-frequency algorithm, the statistics of e is summarized as DSD database (Figure 2). The database is used in the single-frequency algorithms to assume a-priori probability density function of e, where the average μx and the standard deviation sx of log10e depend on region, month, land surface type, and precipitation type, while they depend only on precipitation type in the PR standard algorithm.

References:

Iguchi, T., T. Kozu, J. Kwiatkowski, R. Meneghini, J. Awaka, and K. Okamoto, 2009: Uncertainties in the rain profiling algorithm for the TRMM Precipitation Radar.J. Meteor. Soc. Japan, 87A, 1-30. https://doi.org/10.2151/jmsj.87A.1