An Improved Method for Raindrop Size Distribution Retrieval Using Combined Dual-frequency Radar DFR

Feng, Liang; Yang, Jiefan; Guo, Zeyong; Sun, Jiming; Sun, Yue; Yang, Huiling

doi:10.5194/egusphere-2026-174

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https://doi.org/10.5194/egusphere-2026-174

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23 Feb 2026

| 23 Feb 2026

Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

An Improved Method for Raindrop Size Distribution Retrieval Using Combined Dual-frequency Radar DFR

Liang Feng, Jiefan Yang, Zeyong Guo, Jiming Sun, Yue Sun, and Huiling Yang

Abstract. Accurate retrieval of raindrop size distributions (DSDs) is essential for understanding cloud and precipitation microphysics. The dual-frequency ratio (DFR) measured by dual-frequency radars is independent of the normalized intercept parameter (Nw) and depends solely on the mass-weighted mean diameter (Dm). This characteristic makes DFR a powerful parameter for DSD retrieval. However, the DFR-Dm relationship is subject to a dual-solution ambiguity, particularly for smaller particles. In this study, W-band reflectivity is synthetically incorporated through scattering simulations based on measured DSDs, and retrieval relationships are established among X/Ka-band DFR, Ka/W-band DFR, and Dm. The results show that the ambiguity in the DFR-Dm relationship is substantially reduced, and the proposed method demonstrates clear advantages over conventional approaches, especially when retrieving DSDs of weak echoes. This advancement effectively addresses a key gap in the measurement and analysis of light precipitation processes.

Received: 14 Jan 2026 – Discussion started: 23 Feb 2026

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Liang Feng, Jiefan Yang, Zeyong Guo, Jiming Sun, Yue Sun, and Huiling Yang

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RC1:
'Comment on egusphere-2026-174', Anonymous Referee #1, 27 Apr 2026 reply
Retrieving raindrop size distribution (DSD) using disdrometers has become a research hotspot in the field of weather radar in recent years, as DSD is crucial for understanding cloud and precipitation microphysical processes. This study combines disdrometer measurements with X-Ka band vertically pointing radar to perform DSD retrieval. To address the ambiguity problem in X-Ka band retrieval, the Ka-W band is further introduced as a joint constraint to reduce the ambiguous region between the dual-frequency ratio (DFR) and the mass-weighted diameter (Dm), thereby improving DSD retrieval accuracy. Multi-frequency radar retrieval is an important research direction, and the DFR-Dm ambiguity is a key issue in DSD retrieval. Although this study has practical significance, the methodology section suffers from unclear descriptions and logical issues related to circular reasoning.
Major issues:
The radar used in this study is an X-Ka dual-frequency radar. The proposed method employs the Ka-W dual-band to constrain the retrieval process of the X-Ka dual-frequency radar in order to mitigate the ambiguity problem inherent in X-Ka dual-frequency observations. However, no measured data from W-band radar are provided in the paper. It remains unclear how this constraint can be implemented in practical applications.

This study uses DSD data observed by disdrometers during three rainfall events to fit the attenuation coefficients for the X-band and Ka-band radars. These coefficients are then used to correct attenuation in the X-band and Ka-band radar observations from the same three rainfall events. Subsequently, the DFR-Dm relationship is fitted, and DSD retrieval is performed. This algorithm may suffer from logical issues of self-consistency bias.

Specific comments:
The flowchart shown in Figure 1, along with its corresponding steps and the formulas contained therein, requires additional explanatory clarification.

Table 1 indicates that the X-Ka dual-frequency radar has the capability to observe polarimetric parameters. If more polarimetric observation parameters could be used to constrain the DSD retrieval, the retrieval accuracy would theoretically be higher than that achieved using only dual-frequency parameters.

Figure 3 shows the radar reflectivity factors observed by the X-band and Ka-band radars below the melting layer. The paper does not specify how the position of the melting layer is determined. Furthermore, does the retrieved DSD correspond to the entire rainfall region below the melting layer or only to the near-surface layer? Since the algorithm is based on ground-based disdrometer observations, it is inferred that the retrieval results should represent near-surface conditions. However, the radar observations are presented as corresponding to the rainfall region below the melting layer, indicating an inconsistency.

In Figure 5, the attenuation coefficients are fitted based on disdrometer-observed drop size data and used for radar attenuation correction. Figure 8 then compares the corrected radar retrieval results with the disdrometer observation data from the same several rainfall events. This procedure may suffer from a self-consistency trap. Similar issues exist in the DSD retrieval section.

Figure 6 presents DFR results for the Ka-band and W-band obtained through simulations, but no measured W-band data are provided in the paper. In practical applications, how can the W-band be used to constrain the retrieval process?

Figure 12 shows the DFR-Dm relationship curves, indicating that DFR(Ka, W) exhibits a narrower ambiguous region. However, even if DFR(Ka, W) can effectively reduce the ambiguous region, actual observations only include X-band and Ka-band dual-frequency radar data. Under these conditions, how the W-band radar can be introduced to achieve retrieval constraints requires further explanation.

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Citation: https://doi.org/10.5194/egusphere-2026-174-RC1

Liang Feng, Jiefan Yang, Zeyong Guo, Jiming Sun, Yue Sun, and Huiling Yang

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Short summary

The dual-frequency radar can retrieve raindrop size distribution but causes ambiguity between dual-frequency ratio and raindrop diameter for small drops. To solve this, our study added a third radar frequency, using scattering simulations based on real raindrop observations. We established clear links between different dual-frequency ratio and raindrop size. Results show the ambiguity is greatly reduced and our method outperforms conventional ones, especially in light rain detection.


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