Existing gaps in our ability to observe low-level clouds and precipitation from space drive new requirements for future missions
Lamer, K. and coauthors (2020). Ground-based radars insight into warm marine boundary layer clouds for shaping future spaceborne radar missions. IEEE Radar Conference. 1-4
Battaglia A. and coauthors (2020). Mind the Gap Part 2: Improving quantitative estimates of cloud and rainwater path in oceanic warm rain using spaceborne radars. Atmos. Meas. Tech., 13, 4865–4883
Lamer, K. and coauthors (2020). Mind-the-Gap Part 1: Precise Detection of Low-level Cloud and Precipitation Boundaries using Spaceborne Radars. Atmos. Meas. Tech., 13, 2363–2379
Battaglia A. and coauthors (2020). Space-borne cloud and precipitation radars: status, challenges and ways forward. Reviews of Geophysics, e2019RG000686.
Kollias P. and coauthors (2018). The EarthCARE Cloud Profiling Radar (CPR) Doppler measurements in deep convection: challenges, post-processing and science applications. Remote Sensing of the Atmosphere, Clouds and PrecipitationVII. (Vol. 10776, p. 107760R). International Society for Optics and Photonic
Spaceborne radars are often preferred over ground-based and airborne ones because of their ability to cover vast areas of the globe [Battaglia et al., Submitted]. The first spaceborne Cloud Precipitation Radar (CPR) designed to detail the vertical structure of clouds was launched in 2006 onboard CloudSat [Stephens et al., 2002]. The CloudSat-CPR is still operational however its long powerful pulse also generates a surface clutter echo which tends to partially mask signals from cloud and precipitation forming below circa 1 km [Marchand et al., 2008]. For this reason, the CloudSat-CPR’s actual ability to document shallow low-level clouds and precipitation remains uncertain.
It is not uncommon to rely on observations collected by highly sensitive airborne and ground-based millimeter radar observations to assess the performance of coarser less sensitive radars (e.g., [Burns et al., 2016; Lamer and Kollias, 2015]).
Along those lines, our recent studies have relied on the use of instrument forward simulators, observations collected by the ground-based Ka-band ARM Zenith radar (KAZR) and the ceilometer and large eddy simulations with the goal of:
quantifying the CloudSat-CPR’s ability to estimate the coverage and vertical distribution of shallow low-level clouds and precipitation as well as its accuracy in determining the location of cloud tops and cloud/virga base;
identifying which property (thickness, reflectivity, vertical location) of clouds and precipitation mostly complicate their detection from space ;
evaluating the performance of alternative radar configurations designed for an optimum characterization of clouds and precipitation.
Our work using ground-based remote sensing observations suggests that 50% of warm marine boundary layer (WMBL) hydrometeors occur below 1.2km and have reflectivities < -17dBZ, thus making their detection from space susceptible to the extent of surface clutter and radar sensitivity.
Surface clutter limits the CloudSat-Cloud Profiling Radar (CPR)’s ability to observe true cloud base in ~52% of the cloudy columns it detects and true virga base in ~80%, meaning the CloudSat-CPR often provides an incomplete view of even the clouds it does detect. Using forward-simulations, we determine that a 250-m resolution radar would most accurately capture the boundaries of WMBL clouds and precipitation; that being said, because of sensitivity limitations, such a radar would suffer from cloud cover biases similar to those of the CloudSat-CPR.
Overpass observations and forward-simulations indicate that the CloudSat-CPR fails to detect 29-43% of the cloudy columns detected by the ground-based sensors. Out of all configurations tested, the 7 dB more sensitive EarthCARE-CPR performs best (only missing 9.0% of cloudy columns) indicating that improving radar sensitivity is more important than decreasing the vertical extent of surface clutter for observing cloud cover. However, because 50% of WMBL systems are thinner than 400 m, they tend to be artificially stretched by long sensitive radar pulses; hence the EarthCARE-CPR overestimation of cloud top height and hydrometeor fraction.
Thus, it is recommended that the next generation of space-borne radars targeting WMBL science shall operate interlaced pulse modes including both a highly sensitive long-pulse and a less sensitive but clutter limiting short-pulse mode.