The future of spaceborne cloud radars, and some very specific questions about raindrops and snowflakes

By: Shannon Mason

Cloud profiling radars (CPRs) provide snapshots of the journeys of many billions of hydrometeors through the column of the atmosphere: from ice particles and liquid droplets in clouds, to the snowflakes and raindrops—mostly raindrops—that reach us at the surface, whether from gloomy stratus or towering tropical storms. CloudSat’s CPR, the first instrument of the kind in orbit, has now completed its remarkable decade-long stint as part of the A-Train of satellites, during which we learned a lot about where and how frequently clouds form, their vertical structures, and how often they precipitate (Stephens et al. 2018). However—and without wanting to sound ungrateful—to better understand the processes by which hydrometeors form, interact, and fall to the surface, we still have a lot of very picky-sounding questions about all those ice crystals and snowflakes, cloud droplets and raindrops, like: how many were there, roughly, and; what sizes and shapes did they come in? These microphysical specifics are critical to pinning down important details of the global hydrological cycle and radiation budget, as well as processes at much smaller scales.

Figure 1: The Doppler CPR aboard the upcoming EarthCARE satellite will have the capability to measure the fallspeeds of hydrometeors, providing insights into the size of raindrops and the structure of snowflakes. 

The next generation of CPRs will start to improve our answers to these questions, beginning with EarthCARE, which is due to launch in 2021 and will have the additional capability to measure the fallspeed of hydrometeors from the Doppler shift of the reflected radar beam. Our two papers on EarthCARE’s retrieval algorithms (Figure 1) have focused on how this Doppler velocity information can be used to make better estimates of precipitation:

  • The fallspeed of raindrops tells us their size, so we can distinguish tiny drizzle drops that fall slowly from larger, faster-falling raindrops. This allows us to resolve the growth of drops due to collision and coalescence with cloud droplets, or their shrinking due to evaporation (Mason et al. 2017).
  • The fallspeed of snowflakes can distinguish fluffy snowflakes from faster-falling particles that have captured liquid cloud droplets (“riming”), increasing their density—and this reveals where shallow layers of supercooled liquid may be hiding within deeper ice clouds (Mason et al. 2018).

Despite the challenges of measuring Doppler velocities on the order of 1 m/s from a spacecraft 400 km above the surface and travelling at 7 km/s, our work suggests that EarthCARE will help answer some of our questions about the sizes of raindrops and the structures of snowflakes.

Figure 2: Beyond EarthCARE’s 94-GHz Doppler radar planned for launch in 2021, the configuration of subsequent spaceborne radars is still under discussion. One important consideration is how much additional information can be gained from observing ice and rain at two and three radar frequencies. 

However, we often find we can make improved estimates of rain and snow by observing them at two or more radar frequencies simultaneously. The planning for CPR missions beyond EarthCARE is happening now, and dual- and triple-frequency as well as Doppler radars are being considered (National Academies of Sciences, 2018). It remains an open question what radar configuration (Figure 2) would provide the most information about raindrops and snowflakes, and the processes by which they grow and interact.

Radar measurements at multiple frequencies are especially useful for exploring the properties of larger ice particles and snowflakes, which have different signatures depending on their sizes and structures. Using ground-based radars in Finland—where we can test our remotely-sensed estimates against direct measurements of the snow at the surface—we’re currently quantifying how much information about snowflakes we can gain using three radar frequencies. Further insights about ice particles and processes will emerge from the PICASSO field campaign last winter and this spring (Westbrook et al. 2018), in which the FAAM aircraft directly samples ice clouds over southern England while being closely tracked by Doppler radars at four frequencies from the Chilbolton observatory in Hampshire.

The details of the insides of snowflakes and the sizes of raindrops may seem insignificant, but the insights we gain from these field experiments help to sharpen the science questions and techniques that will be used with the next generation of satellites. Following the success of CloudSat, EarthCARE and its successors will help constrain global estimates of the role of clouds and precipitation in the atmospheric energy and water cycles.

References:

Stephens, G., D. Winker, J. Pelon, C. Trepte, D. Vane, C. Yuhas, T. L’Ecuyer, and M. Lebsock, 2018: CloudSat and CALIPSO within the A-Train: Ten Years of Actively Observing the Earth System. Bull. Amer. Meteor. Soc., 99, 569–581, https://doi.org/10.1175/BAMS-D-16-0324.1

Mason, S. L., J. C. Chiu, R. J. Hogan, and L. Tian, 2017: Improved rain rate and drop size retrievals from airborne Doppler radar. Atmos. Chem. Phys., 17 (18), 11567–11589. http://doi.org/10.5194/acp-17-11567-2017

Mason, S. L., J. C.  Chiu, R. J. Hogan, D. Moisseev, and S. Kneifel, 2018: Retrievals of riming and snow density from vertically-pointing Doppler radars. J. Geophys. Res.: Atmos., 123, 13807 – 13834, http://doi.org/10.1029/2018JD028603

National Academies of Sciences Engineering and Medicine, 2018: Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space. Washington, D.C.: National Academies Press. http://doi.org/10.17226/24938

Westbrook, C., P. Achtert, J. Crosier, C. Walden, S. O’Shea, J. Dorsey, and R. J. Cotton, 2018: Scattering Properties of Snowflakes, Constrained Using Colocated Triple-Wavelength Radar and Aircraft Measurements, AMS 15th Conference on Atmospheric Radiation, https://ams.confex.com/ams/15CLOUD15ATRAD/webprogram/Paper347299.html

 

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