Entering the Cloud “Twilight Zone”

Clouds: We see them as objects with clear-cut shapes and outlines. The satellites that collect cloud data and the climate models built on this data work on this supposition as well. But the line between “cloudy” and “clear” is much hazier than we think. Researchers at the Weizmann Institute of Science have presented new findings on the cloud “twilight zone” – the barely visible cloud cover outside those neat shapes – showing that it may be making a significant, previously unaccounted-for contribution to planetary warming. 

Even the role of the clouds that are visible in satellite images has been open to interpretation. Not only are clouds transient phenomena but their contribution to the total energy budget – how much and of which sort of radiation is let in or out – has been difficult to pin down. Clouds can contribute to both cooling and warming the Earth’s system, depending on their type, location and height in the atmosphere. They cool by reflecting part of the solar (shortwave) radiation back into space, and they warm the system as an efficient component of the global “greenhouse” – absorbing some of the longwave radiation emitted from below the cloud level. So it is little wonder that the regions of cloud that are not seen in the normal satellite data, or by the naked eye, have been thought to be of little significance in terms of their contribution to either effect.

But Prof. Ilan Koren of the Weizmann Institute’s Earth and Planetary Sciences Department thinks they may be key to understanding the role that clouds play in climate. “When we analyze cloud images, we can see pockets or haloes of cloud. The nice outlines disappear: Clouds don’t actually confine themselves to neat shapes,” he says.

What makes a twilight zone? It can be made of clouds in various stages of forming or dissipating, clouds or parts of clouds that simply have a weak optical signature, clouds that are too small for the satellite’s resolution, or it can be haze forming in pockets of high humidity. Koren and others had identified the cloud twilight zone over a decade ago, suggesting that it interacts with cloud formation in complex ways.

In the current study, the scientists – research student Eshkol Eytan in Koren’s group, who led this study together with Staff Scientist Dr. Orit Alteratz Stollar; Dr. Alex Kostinski of Michigan Technological University; and Dr. Ayala Ronen of the Israel Institute for Biological Research, then a visiting scientist in Koren’s lab – set out to devise a way to check whether the seemingly negligible clouds specifically contribute to warming, as a greenhouse gas does, by trapping the Earth’s heating, long-wave energy on their undersides. Their idea was to search for evidence that might be hiding within the existing satellite data.

To understand how these overlooked twilight components might contribute to the energy balance, the researchers had to find a way to uncover their information. That entailed reordering the optical data of cloud fields over the oceans in an unusual way, by “taking their temperature” and plotting it as a function of the distance from the nearest cloud. If an effect was found for these “unseen” parts of clouds, it could be assumed to be valid for the other components of the cloud twilight zone. This reordering revealed a strong signature in the longwave – that is, warming – in the twilight areas.

The researchers found that satellite measurements of certain regions near visible clouds revealed temperatures that were two or more degrees Celsius cooler than the farther-away areas that were truly clear. On a map, the coolest areas were within ten kilometers of the clouds, dropping off at a distance of about thirty kilometers from the cloud “edge.” “Cool temperatures above this twilight cloud cover means that the long-wave radiation from the Earth is not passing through, and the atmosphere is that much warmer underneath,” says Koren. 

These invisible twilight clouds play an important role in our planet’s natural warming mechanism

Next the team zoomed out to see if the warming effect they had identified in single, small areas could be extended to the satellite information spanning the rest of the globe. Applying the analytical methods they had developed, they looked at hundreds of pixels of satellite data during a period of six days. Looking at the the heat signatures within these pixels and analyzing them through the lens of this model suggested that significant areas of the sky that we – and satellites – label as clear, are actually filled with thin clouds that warm the planet.

How significant is this effect? The short answer, says Koren, is “very.” Take the fact that this study presents strong evidence that invisible, twilight clouds cover at least 50 percent of supposedly clear skies, and this heating is active day and night. Applying the model the team developed for calculating this contribution to warming, they came up with a figure expressed in terms of energy: around 0.75 watts per square meter. “If you compare that to all of manmade global warming, which is estimated to be around 2-3 watts per square meter, that figure is quite significant. And that is the conservative figure. The true one could be higher,” he adds. “That means these invisible twilight clouds play an important role in our planet’s natural warming mechanism.”

This new model will help solve some of the current mysteries as to the effects of clouds on the planet’s energy budget, as well as help climate modelers fill out the models and improve their accuracy.