EarthCARE reveals how atmospheric ripples boost cloud formation over Antarctica

Thanks to EarthCARE's cloud profiling radar (CPR) we can see and measure gravity waves in clouds for the very first time from space—while synergy with the atmospheric lidar (ATLID) allows us to study their direct effects on weather through enhanced ice cloud formation.

What are gravity waves?

Atmospheric gravity waves are ripples in the atmosphere that move horizontally as variations in vertical air motion, just like those made by wind-driven waves on the surface of the ocean or by a stone thrown into a puddle, which are types of gravity waves. In the atmosphere, gravity waves can be generated by air flowing over mountains, as well as from large-scale weather systems and deep convective storms. They are not to be confused with gravitational waves, which are ripples in spacetime generated during interactions between massive celestial objects like black holes or neutron stars.

Though they form near the surface, atmospheric gravity waves are most often studied in the stratosphere, where they become a major source of energy transfer and variability in the middle atmosphere. They contribute to the quasi-biennial oscillation, when the stratospheric winds over the equator switch from easterly to westerly and back every two years or so, which can influence global weather via its effects on things like the Atlantic jet stream. Gravity waves can also disturb the polar vortex, causing events such as sudden stratospheric warming.

Satellite observations of gravity waves typically focus on temperature variations in the cloud-free middle atmosphere, and measurements of vertical velocity in gravity waves have so far relied on localised measurements from ground-based remote sensing or in-situ sensors.

"Until EarthCARE, we couldn't measure the vertical motion in clouds except over a very small number of ground stations,” says Shannon Mason, who works on EarthCARE synergy products at the European Centre for Medium-range Weather Forecasts (ECMWF). "We can now do it globally, thanks to EarthCARE's unique measurements. It's really exciting."

How does EarthCARE measure the effects of gravity waves in clouds?

On 7 August 2025, the European Space Agency's EarthCARE satellite observed a large-scale cloud over West Antarctica that highlights its unique potential to measure vertical air motion in clouds, including those induced by gravity waves. In the first image we see how each of EarthCARE's instruments highlights a different, unusual aspect of the cloud.

The CPR was developed by the Japan Aerospace Exploration Agency (JAXA) and measures the internal properties of clouds, including the vertical velocity of cloud particles, snowflakes and raindrops. It shows a vertically coherent wave structure in the novel Doppler velocity measurements. ATLID, which detects cloud boundaries and layers by pulsing laser beams towards Earth and measuring the properties of the light that comes back to the instrument, shows a distinct layer of very high attenuated particulate backscatter at temperatures between -40 and -55°C (where ATLID's laser beams are less able to penetrate the cloud). We also see a region of especially low brightness temperature in the multispectral imager (MSI) thermal infrared (TIR) channels.

The gravity wave signature in the CPR Doppler velocity appears to have a wavelength (the distance between adjacent peaks or troughs) around 18 km. Corresponding wave structures are evident in CPR radar reflectivity at the cloud base, and in the shallow band of high attenuated particulate backscatter from ATLID, shown in the second image. Both features help us understand how the gravity wave interacts with the cloud.

ATLID measurements crystal clear

The shallow layer of high-backscatter ATLID signal is due to the rapid attenuation of the lidar beam. Such layers usually indicate clouds made of liquid droplets, but we know that "supercooled" liquid water is only physically sustainable at temperatures warmer than around -40°C. The ATLID depolarization ratio measurement, which can be used to distinguish spheroidal cloud droplets from more randomly oriented ice crystals, confirms that this cloud layer is not dominated by liquid droplets.

So why does the enhanced lidar backscatter appear so sharply above the -40°C level? The upward motion of the gravity wave may be driving rapid cooling of the air, leading to the formation of ice particles by the "homogeneous nucleation" process—where cloud ice particles form directly from condensing water vapour, and don't require particles to freeze onto—which preferentially occurs at temperatures colder than -40°C. The many tiny, newly formed ice crystals in this layer have a very strong signal to ATLID, which is sensitive to high concentrations of particles. Conversely, the radar reflectivity—which is sensitive to the largest particles—increases significantly at temperatures warmer than -40°C and down to the cloud-base around -20°C. This may show the combined effects of ice crystals growing and sticking together to form larger snowflakes.

How the CPR tells apart precipitation from moving air

So, can we successfully estimate the properties of this gravity wave-enhanced ice cloud layer and the growth of snowflakes below it?

One novel measurement we hope will provide information about ice clouds and snow from EarthCARE is the CPR Doppler velocity, which measures the vertical motion of ice particles. From this we hope to infer their mass or density—think about the difference between a barrage of snow pellets or hail and the gentle settling of a fluffy clump of snowflakes. However, as the beautifully clear gravity wave pattern in this case illustrates, the Doppler velocity also contains information from underlying vertical air motions, which are usually more subtle. To use the Doppler velocity to help estimate the properties of ice and snow, our challenge is: can we reliably disentangle the fall speeds of ice particles from the vertical motion of the air?

By averaging the Doppler velocity across many nearby pixels with similar radar reflectivity, the CPR Corrected Doppler (C-CD product) smooths over the small-scale variability due to measurement noise and air motion, leaving behind only the fallspeed of the snowflakes and raindrops detected by the radar, the "sedimentation velocity". By subtracting this from the measured Doppler velocity, we can indirectly estimate the vertical air motion.

EarthCARE synergy boosts global understanding of clouds

Combining the CPR sedimentation velocity along with the other measurements from CPR, ATLID and MSI, our best estimate of the properties of this cloud (the synergistic ACM-CAP product) successfully resolves the gravity wave-induced enhancement of the ice cloud. We see a rapid increase in ice water content in the layer of high lidar backscatter similar to those formed by homogeneous nucleation in the coldest ice clouds. Below the -40°C level the ice water content is more constant with height while the rapid increase in the size and decrease in the number of ice particles is consistent with many small ice crystals clumping together into larger snowflakes.

"We think this embedded layer of enhanced ice formation is directly attributable to the action of the gravity wave," says Shannon. "Over this remote part of Antarctica, without the Doppler to tell us there's a gravity wave here changing the ways ice crystals form, this would have looked like just another weird cloud."

"It will be exciting to see what we can do in the future to use these ripple signatures in EarthCARE's Doppler velocity measurements to map the global occurrence of gravity waves in the lower atmosphere, and to better understand their effects on the properties of clouds."

Source: 

European Space Agency. (2025, September 15). EarthCARE reveals how atmospheric ripples boost cloud formation over Antarctica. Earth Online.