Abstract
Abstract. The appearance of second-trip echoes generated by mirror images over the ocean and by multiple scattering tails in correspondence with deep convective cores has been investigated for space-borne nadir-looking W-band cloud radar observations. Examples extracted from the CloudSat radar are used to demonstrate the mechanisms of formation and to validate the modelling of such returns. A statistical analysis shows that, for CloudSat observations, second-trip echoes are rare and appear only above 20 km (thus easy to remove). CloudSat climatology is then used to estimate the occurrence of second-trip echoes in the different configurations envisaged for the operations of the EarthCARE radar, which will adopt pulse repetition frequencies much higher than the one used by the CloudSat radar in order to improve its Doppler capabilities. Our findings predict that the presence of such echoes in EarthCARE observations cannot be neglected: in particular, over the ocean, mirror images will tend to populate the EarthCARE sampling window with a maximum frequency at its upper boundary. This will create an additional fake cloud cover in the upper troposphere (of the order of 3 % at the top of the sampling window and steadily decreasing moving downwards), and, in much less frequent instances, it will cause an amplification of signals in areas where clouds are already present. Multiple scattering tails will also produce second-trip echoes but with much lower frequencies: less than 1 profile out of 1000 in the tropics and practically no effects at high latitudes. At the moment, level-2 algorithms of the EarthCARE radar do not account for such occurrences. We recommend to properly remove these second-trip echoes and to correct for reflectivity enhancements, where needed. More generally this work is relevant for the design of future space-borne Doppler W-band radar missions.
Highlights
Thanks to their unique capability of penetrating and profiling cloud and precipitation, space-borne millimetre radars are becoming an essential component of the Global Observing System
New missions are on the horizon: the ESA-JAXA EarthCARE (EC) mission (Illingworth et al, 2015) will deploy a W-band Doppler system, whereas NASA is currently in pre-phase A for the Aerosol and Cloud, Convection and Precipitation (ACCP) Designated Observable (DO) mission, which will carry dual-frequency (Doppler) systems with frequencies ranging from Ku to W bands in different orbits (Battaglia et al, 2020; Kumjian et al, 2020)
Second-trip echoes generated by mirror images over the ocean and multiple scattering tails in correspondence with deep convective cores can become an issue in W-band cloud radar space-borne observations
Summary
Thanks to their unique capability of penetrating and profiling cloud and precipitation, space-borne millimetre radars are becoming an essential component of the Global Observing System. The EC CPR is expected to operate a PRF between 6.0 and 7.2 kHz (much higher than the CloudSat PRF, ranging from 3.7 to 4.4 kHz) These benefits come at a price: the reduced unambiguous range, ru, i.e. the maximum distance at which a target can be located to ensure that the backscattered power from that target corresponds to the latest transmitted pulse. A PRF of 7.5 kHz, which corresponds to ru ≈ 20 km, seems right at the edge to avoid significant numbers of second-trip echoes Two situations, both caused by multiple scattering (MS) events (for a thorough review of the topic see Battaglia et al, 2011), that can produce significant returns from ranges much longer than the surface echo and exceeding the unambiguous range must be considered.
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