Websites of Interest:

“CRYSTAL-FACE Mission Overview”:http://cloud1.arc.nasa.gov/crystalface/science.html (NASA’s Ames Research Center)

“Atmospheric Science: Aerosols and Anvil Cloud Nuclei”:http://scienceweek.com/2004/sc040611-3.htm (ScienceWeek.com, June 11, 2004)

“Study Finds Thicker Storm Clouds Over Warmer Tropical Waters Affects Climate”:http://www.gsfc.nasa.gov/topstory/20020915iristheory.html (NASA Goddard Space Flight Center Press release September 18, 2002)

“Why do we have acid rain?”:http://www.virtualglobe.org/en/info/env/03/acid01.html (VirtualGlobe.org)

The following person was interviewed for today’s program. Our thanks to:

Brian Toon
Director
Atmospheric and Oceanic Sciences Program
University of Colorado
Denver, CO
NASA Environmental Sciences Programs

Author’s Notes:

Interview with Brian Toon:

ES: Thanks for talking with Earth and Sky today, Dr. Toon. Could you tell me about some of the research on clouds and climate change that you’ve been conducting lately with the CRYSTAL-FACE experiment?

BT: Most of the debate about the greenhouse effect now is all related to how will water vapor respond to a warming climate. Two responses are possible. It could be that the water vapor will increase more rapidly in the atmosphere than we expect, and amplify the greenhouse even more than we expect. Or it may be that it will do the opposite. It will retard the greenhouse warming somehow. So that’s where the big debate is in the greenhouse effect. What will water do? Will it amplify greenhouse warming due to the gasses that people put into the atmosphere, or will it retard those changes?

One of the roles of CRYSTAL-FACE, field program led by NASA several years ago, was to try to learn more about how deep convective clouds work, how do they influence the amount of water vapor in the upper parts of the atmosphere. And how do the clouds themselves, how does the convection affect the extant of the cirrus shields at their tops, how bright they are, how much sunlight they reflect, how much infrared light they radiate into space.

ES: How would you describe these anvil clouds?

BT: These deep convective clouds are very important in the tropics. Most Americans don’t see that much of these big thunderstorms, but they’re very dramatic when they occur, but they make lots of problems in terms of lightning and hail in the center of the country. In the tropics, these are the dominant cloud types. And they’re very important because of their vertical extant – they can go from the ground up to sixteen or seventeen kilometers above the surface. And they can pump air from the ground to that huge altitude in a few tens of minutes.”

Because these things are pumping air from the ground, upward, they serve as a big pump, moving things from near the surface up to the upper atmosphere which then carries it to mid-latitudes and further poleward. So there’s sort of a giant circulating belt in the Earth’s atmosphere in which there’s rising motions in the tropics, and then air moves poleward to mid-latitudes and then slowly sinks down at higher latitudes. And in the rising part of this overturning circulation, you find all of these deep convective clouds. So people who live along the equator have a lot of thunderstorms all the time. The northern part of South America is covered by intense thunderstorms frequently, as is Indonesia and Africa and large parts of the oceans. So these clouds dominate the tropics, they do a lot of the vertical motion in the atmosphere. And because they have these huge, bright, ice crystal clouds at their tops, they are also very important to the brightness of the Earth, its reflectivity of sunlight. So that’s why we’re interested in them because of their ability to pump water upward.

So there’s been a big debate about the greenhouse effect, and a lot of this revolves around how well these clouds put water into the upper atmosphere. There’s a lot of different issues here, but you can imagine that a simple as possible situation is, if the Earth warms up, and there’s more convection because it’s hotter at the surface, presumably you’ll make more convection or it will be more intense. And your first thought is, well, you have to pump more water into the upper atmosphere, because what these clouds are doing, is carrying air from the ground up to high altitude, and the water at the ground is very moist, and it’s very cold at high altitudes, so you get a lot of clouds forming there when the air is going vertically. On the other hand, you can imagine if the convection is more intense, the clouds might be more efficient at making hail, and rainfall, and precipitation in general. And if that’s the case, then the clouds may actually remove more water as they’re air moving vertically, and you may end up with a drier upper troposphere. So it’s critical to understand, are these clouds making the upper atmosphere moister, or are they making it drier? And in the simplest kind of thinking about this, you can imagine it going in either direction. So obviously we want to study these clouds, and try and figure out how they work, how does the size of the cirrus anvil depend on the intensity of the updraft making the convection? In general, is this making the atmosphere drier or moister? A very interesting part of this is the stratosphere. The stratosphere is incredibly dry, the humidity there is only a couple of percent throughout most of the atmosphere. And we don’t really understand why the atmosphere is so dry. So we also don’t understand, in a future climate, if the Earth is warmer, will the stratosphere be moister, or drier, or just the same? And this matters, not only just because water added to the stratosphere can make the greenhouse effects stronger, but also because there’s a lot of chemistry in the stratosphere involving ozone that’s sensitive to water. So we can affect the ozone layer by adding water or subtracting water from the stratosphere. So we’d also like to know, how is the water getting into the stratosphere, and how so many of these convective clouds, which are so violent, are able to rise so rapidly that they can actually punch into the stratosphere and deposit water and other chemicals into the stratosphere.

So when one wants to understand, what does it mean when the satellite sees a certain cloud brightness, or what does it mean if the satellite sees a certain cloud color in the infrared. What does it tell you about how big the particles are, what are they made out of, and whether they’re going to rain or not. So part of the goal here was to use the airplanes to learn about the clouds, and the other part was to use the airplanes to better understand what satellites see so we can use the satellites to investigate clouds worldwide and all the time.

What that means is that the clouds are lofting ice to high altitude, and the ice is evaporating and adding water into the upper troposphere. So the cloud was not removing all of the water in the column by precipitation, it was putting a lot of it at high altitude, which was spreading out and evaporating and moistening the air loft. So that was a surprise.

And this matters because the satellites are using the particle sizes to figure out bright the clouds are, and how much light they reflect, and how much light they emit to space. One wants to be confident that the satellites are doing the retrievals properly, and we still don’t know who’s right – whether the satellite is correct or whether the aircraft is correct.

It was still remarkable, technologically, that somebody could fly through a cloud, catch the ice crystals, take them apart, and analyze the little dust grain inside the ice crystal and tell you it’s a micrometeorite.

ES: How were the ice crystals forming in these clouds?

BT: Though freezing in the clouds was influenced by dust and clay minerals and special particles, most particles formed just by water drops freezing because they were so super cooled that they spontaneously froze. Some of the ice crystals in the anzils formed just by water drops freezing and making them as opposed by ice forming on dust grains.

Another surprising observation here has to do with the balance between ice and vapor. So what we expect to happen in a cloud full of ice is that all of the vapor will condense to make ice crystals, until there’s a balance in which the ice basically is in balance with its vapor. There’s a well-known amount of vapor that you expect at a given temperature above an ice crystal. And we observed in this field program, and going back to other ones earlier, and analyzing them more carefully we can actually see this as happening in other situations, what was observed is that there’s actually more vapor left in the vapor phase than there should be, based on currently existing theory about how clouds grow and form. So this is a big surprise. There’s still quite a bit of debate going on about it. But, one suggestion has been that nitric acid, which is present in the gas phase in the atmosphere, is condensing in the ice crystals. By condensing in the ice, it affects their surfaces so that more water stays in the gas phase than one would expect. And if this turns out to be correct – and there are a lot of laboratory studies that need to be done to verify this, it means that the chemistry of the atmosphere is having a direct effect on cloud physics, and in particular it’s affecting the amount of vapor that the clouds are leaving behind in the gas phase when ice crystals fall out as precipitation. So, it’s hard to tell quantitatively how important this is at the moment. But it’s definitely a new link between atmospheric chemistry and climate that we hadn’t anticipated.

These observations suggest that the clouds are moistening that atmosphere more than we thought. And by doing so, they’re enhancing the greenhouse effect. They’re putting more water into the upper atmosphere.

ES: So how did you go about to actually measure the inside of a cloud?

BT: One of the goals was to try to fly through the footprint, the place where satellites were looking – and there’s a lot of deep convection in the deeper tropics than Florida. So we wanted to examine some clouds even more in the tropics than Florida is, and we wanted to fly through these satellite footprints. So we made a number of flights further to the south. There are going to be follow up missions to this one. There’s one planned now for the summer of 2005 in Costa Rica. Costa Rica is nearly on the equator – and the goal there is to look at clouds that are closer to the equator. Florida is fairly far north, and it’s barely in the tropics. And clouds there are somewhat affected by not really being in the tropics, and they’re certainly affected by all of the pollution from North America and Florida, although the air in Florida was strongly affected by the entire east coast, because the air over the east coast tends to blow out over the Atlantic and then circle back over Florida. So there’s a lot of pollution in the air there in southern Florida. So we’d like to go to the deeper tropics and look at clouds in a different environmental setting.”

You know, these kinds of missions are some of the first measurements that human beings have ever made in this part of the atmosphere. It’s extremely difficult to get to the top of a cumulus cloud. There’s only a handful of aircraft that can get there, and the people who are making instruments, trying to measure things in the atmosphere, have made numerous technological breakthroughs in the last decade or so that is really allowing us, for the first time, to understand and measure many of these properties. So when you go someplace for the first time, you find new things. So a lot of the debate is because someone has created a new instrument and made a new measurement for the first time, and what they measured didn’t turn out to be what everyone was expecting. So for example, no one expected that there would be nitric acid getting in ice and affecting the properties of the ice crystals. People really didn’t anticipate that the ice crystals would be evaporating in place and releasing a lot of heavy isotopes of water in the upper atmosphere. So there are a whole host of surprises here, and it just reflects a general situation in science where you’re dealing with very, very complicated systems that have a lot of interactions in them. And people haven’t been imaginative enough to anticipate what nature is going to do. So when you go out and you make some measurements, you often discover that nature didn’t do what you though it would do. So this doesn’t mean that we don’t understand clouds, or we can’t calculate how the climate is going to change in general, but it does mean that there’s still things out there that we don’t understand and they can surprise us and they can have significant impact on these systems and on our predictions of what’s going to happen. So that’s why there’s still debate. If there were no debate, it would be very disappointing, because one wouldn’t have discovered anything new.”

So there’s obviously a huge effort going on, not only in the United States, but also throughout the world, to better understand climate change, and change in chemistry of the atmosphere. And there’s no question that people are impacting the global climate. If you go and look at the quantities of the material that are involved in changing atmospheric chemistry, or changing the climate, people, for example only have to touch kilograms, a few pounds of chloroflourocarbons, or something like that, to affect the ozone layer over the whole Earth. The average American is responsible for releasing about five tons of carbon into the atmosphere every year, mostly through burning fossil fuels for cars, and heating, and things like that. So five tons of carbon is a lot of stuff to put into the atmosphere. So the reason for doing this is to better understand what is going to happen to our climate in the future, to better verify our modeling and understanding of this, so we can make better predictions, to better understand how satellites are making measurements, so that we can use observations to try and tell us where the earth is actually going, as opposed to predicting where it’s going. So that’s the reason for the big interest in this. We really want to understand the future better. And these systems are very complicated, there’s lots of different parts to them. You have to understand what’s going on near the ground – how does the pollution that people put into the atmosphere affect the little water clouds that start the convection. Then you have to understand what’s happening fifteen kilometers above the surface, where the deep tower bubble has risen and made a big ice sheet. How did the ice get there, what’s the ice going to do next, how much precipitation has there been? Other people are interested in predicting convection for weather, for example. How are they going to predict which way a thunderstorm is going to move, and how the thunderstorm is going to produce hail and precipitation. So there were a lot of modelers there who would like to make weather models, to be able to better predict the movement and intensity of thunderstorms. So, projects like these attract people from many different disciplines because there are many different problems that are involved, all the way from weather prediction to climate change, from the ground to the stratosphere, from the details of atmospheric chemistry to the details of cloud physics and how light moves through the atmosphere. So there’s a whole host of different chemical and physical disciplines, and people interested in the whole atmosphere from the ground to the stratosphere.”