The Cloud Radiative Effect, Take Two

Guest Post by Willis Eschenbach

Well, in my last post I took a first cut at figuring the cloud radiative “feedback” from the CERES dataset. However, an alert commenter pointed out that I hadn’t controlled for the changes in solar radiation. The problem is that even if the clouds stay exactly the same, if the solar radiation increases, the net cloud radiative effect (CRE) increases due to increased reflection … and I hadn’t thought about that, had I? Dang … so my post was wrong.

So, to control for solar radiation, I did a multiple linear regression. The dependent variable was the net CRE, and the independent variables were the surface temperature and the solar radiation. As you might expect, this gave smaller results than my first analysis. I believe that this method is correct, but I’m always willing to be shown wrong. Not happy to be … but willing to be.

Figure 1. Net CRE as a function of surface temperature, after controlling for solar radiation. The gray lines are contour lines at zero W/m2 per °C. I suspect that the blue around Antarctica is an artifact due to the presence of the sea ice edge. 

Note that there are several areas in the tropical oceans which have a strong negative change in radiation with respect to temperature. These are the areas of the Inter-Tropical Convergence Zones, about ten degrees both north and south of the Equator. It is in these areas that much of the regulation of global temperature takes place, by means of the combined effect of cumulus clouds and thunderstorms.

In addition, there is a large area of the Southern Ocean where the clouds oppose the temperature rise.

The area of clouds off of the coast of California and northern Mexico is an area of persistent stratus that also strongly opposes warming. (See here for a discussion of this location in the literature).

Finally, I note that the global average change in net cloud radiation for each degree of surface warming is positive, at 0.7 W/m2 per degree. On reflection, it seems to me that we need to compare that to how much we’d expect the cloud radiation to increase if the surface temperature goes up by 1°C.

And I don’t know the answer to that … still pondering on that one.

Finally, it’s worth bearing in mind that the radiative effect of clouds is only the beginning of a long list of ways that clouds cool the surface. These include:

• Physically transporting heat from the surface directly to the upper troposphere where it radiates easily to space. Since the heat is transported either as latent heat, or as sensible heat inside the thunderstorm tower, it doesn’t interact with the large amount of water vapor, CO2, and other GHGs in the lower atmosphere.

• Wind driven evaporative cooling. Once the thunderstorm starts, it creates its own wind around the base. This self-generated wind increases evaporation in several ways, particularly over the ocean.

a) Evaporation rises linearly with wind speed. At a typical squall wind speed of 10 mps (20 knots), evaporation is about ten times higher than at “calm” conditions (conventionally taken as 1 mps).

b) The wind increases evaporation by creating spray and foam, and by blowing water off of trees and leaves. These greatly increase the evaporative surface area, because the total surface area of the millions of droplets is evaporating as well as the actual surface itself.

c) To a lesser extent, surface area is also increased by wind-created waves (a wavy surface has larger evaporative area than a flat surface).

d) Wind created waves in turn greatly increase turbulence in the boundary layer. This increases evaporation by mixing dry air down to the surface and moist air upwards.

e) Because the spray rapidly warms to air temperature, which in the tropics is often warmer than ocean temperature, evaporation also rises above the sea surface evaporation rate.

• Wind driven albedo increase. The white spray, foam, spindrift, changing angles of incidence, and white breaking wave tops greatly increase the albedo of the sea surface. This reduces the energy absorbed by the ocean.

• Cold rain and cold wind. As the moist air rises inside the thunderstorm’s heat pipe, water condenses and falls. Since the water is originating from condensing or freezing temperatures aloft, it cools the lower atmosphere it falls through. It also cools the surface when it hits. In addition, the falling rain entrains a cold wind. This cold wind blows radially outwards from the center of the falling rain, cooling the surrounding area.

• Increased reflective area. White fluffy cumulus clouds are not tall, so basically they only reflect from the tops. On the other hand, the vertical pipe of the thunderstorm reflects sunlight along its entire length. This means that thunderstorms shade an area of the ocean out of proportion to their footprint, particularly in the late afternoon.

• Modification of upper tropospheric ice crystal cloud amounts (Lindzen 2001, Spencer 2007). These clouds form from the tiny ice particles that come out of the smokestack of the thunderstorm heat engines. It appears that the regulation of these clouds has a large effect, as they are thought to warm (through IR absorption) more than they cool (through reflection).

• Enhanced nighttime radiation. Unlike long-lived stratus clouds, cumulus and cumulonimbus generally die out and vanish as the night cools, leading to the typically clear skies at dawn. This allows greatly increased nighttime surface radiative cooling to space.

• Delivery of dry air to the surface. The air being sucked from the surface and lifted to altitude is counterbalanced by a descending flow of replacement air emitted from the top of the thunderstorm. This descending air has had the majority of the water vapor stripped out of it inside the thunderstorm, so it is relatively dry. The dryer the air, the more moisture it can pick up for the next trip to the sky. This increases the evaporative cooling of the surface.

Finally, since they are emergent phenomena that only arise where the surface is warmer than its surroundings, clouds and thunderstorms preferentially cool mainly the warmer areas in a way which is not well represented in bulk averages. In other words, the averages of the bulk measurements of say temperature and relative humidity in a gridcell containing thunderstorms gives little idea of the high-speed movements of massive amounts of energy which are taking place.

Anyhow, that’s take two on the CRE … I’m still ruminating on what I can learn from the CERES data, it’s far from mined out.

Best to all,

w.