| 30 Jun 2026
Characteristics of multiple-trip echoes observed by EarthCARE Cloud Profiling Radar
Abstract. Observations from the EarthCARE Cloud Profiling Radar (CPR) frequently contain spurious cloud signals caused by mirror images, multiple-scattering (MS) tails, and satellite mirror images (SMIs). These multiple-trip echoes are produced when transmitted radar pulses follow longer-than-nominal propagation paths and return within the reception window of subsequent pulses. Distinguishing and removing them is essential for scientific analyses using CPR observations. This study characterizes global properties of the multiple-trip echoes and evaluates the performance of the identification methods implemented in the JAXA Level 2A CPR one-sensor Echo product (CPR_ECO). For mirror images and MS tails, we adopt modelling approaches previously proposed for CloudSat-based analyses, whereas for SMIs we introduce a new method that exploits their characteristic altitude and Doppler-velocity signature associated with line-of-sight satellite-velocity contamination. Evaluations using collocated Atmospheric Lidar (ATLID) measurements, which provide CPR-independent cloud-top information, objectively show that the method properly identifies most of the multiple-trip echoes. Global statistical analyses using the identification flag reveal distinct geographical distributions, seasonal variations, vertical structures, and surface-state-dependent occurrence conditions among the three echo types. Mirror images are the most frequent type, and their distribution broadly follows cloud occurrence, with a preference for ice-free ocean. MS tails are concentrated in tropical and subtropical convective regions under strongly attenuating conditions. In contrast, SMIs occur almost exclusively over surfaces with near-saturated backscatter, such as melting sea ice and land with surface-water cover. These results provide a basis for improving multiple-trip echo identification and for addressing overlap cases in which spurious echoes contaminate genuine cloud signals.
This paper provides a clear and comprehensive description of second- and multiple-trip echoes in EarthCARE observations. It proposes practical methods to identify and screen these echoes while thoroughly characterizing their vertical and zonal distributions. Very interesting to see also the discussion about the satellite mirror, the first time this is seen in cloud spaceborne radars. Overall, the manuscript is very well written, with clear figures, a logical structure, and well-supported arguments. I have only a few minor comments that I believe would further improve the paper.
1) Figure 1 and explanation of SMI height
In Figure 1, the term nR_u appears, but the parameter n is not defined. Please clarify its meaning in either the figure or the caption. In addition, in Lines 215–218 you explain why, over ocean surfaces, the SMIs consistently appear near H_SMI = 2.4 km regardless of the pulse repetition frequency (PRF). Since this is not immediately intuitive, especially for readers who are not familiar with radar principles, I think a simple timing diagram (chronogram) would be very helpful to illustrate this behavior.
2) Line 23, The phrase "surfaces with near-saturated backscatter" is somewhat vague for readers who are not familiar with Cloud Profiling Radar (CPR) observations. It would be helpful to provide a more quantitative description, for example by indicating an approximate σ⁰ threshold corresponding to near-saturated backscatter.
3) Figure 13: I suggest placing slightly more emphasis on the feature shown in Figure 13e over the tropical region. The persistence of reflectivities around −20 dBZ near cloud top, rather than decreasing toward the CPR sensitivity limit, appears to be a clear indication that second-trip echoes have not been completely eliminated or separated from the genuine cloud signal. Have the authors considered subtracting the extrapolated multiple-scattering tail from the observations? It would be interesting to assess whether such a correction recovers a more expected behavior in the figure.