Guest Post by Willis Eschenbach
Impelled by my restless curiosity, I’ve returned to the TAO buoy dataset to investigate a claim by Dr. Ramanathan of a “super-greenhouse” effect. The TAO buoys are a number of moored buoys located across the Pacific. The TAO data is available here.
Figure 1. Locations of all of the sites of the TAO buoys, stretching from above Australia on the left, across the Pacific to off of South/Central America on the right.The buoys collect information on some 17 different variables. The graphic is from the data selection page linked to above. Solid blue squares show buoys which record the currently chosen variable (in this case SST). Empty blue squares show buoys which do not measure the current variable.
I am using the sites on the Equator itself because they have the widest variety of data, including rainfall, air temperature, sea surface temperature, pressure, winds, etc.
Now, here’s the statement by Dr. Ramanathan that I wanted to investigate:
The greenhouse effect in regions of convection operates as per classical ideas, that is, as the SST increases, the atmosphere traps the excess longwave energy emitted by the surface and reradiates it locally back to the ocean surface. The important departure from the classical picture is that the net (up minus down) fluxes at the surface and at the top-of-the atmosphere decrease with an increase in SST; that is, the surface and the surface-troposphere column lose the ability to radiate the excess energy to space. The cause of this super greenhouse effect at the surface is the rapid increase in the lower-troposphere humidity with SST; that of the column is due to a combination of increase in humidity in the entire column and increase in the lapse rate within the lower troposphere. The increase in the vertical distribution of humidity far exceeds that which can be attributed to the temperature dependence of saturation vapor pressure; that is, the tropospheric relative humidity is larger in convective regions.
The “convective regions” are the warmer tropical regions where convective thunderstorms are a frequent occurrence. And his claim is kind of logical, since evaporation is in part a function of temperature, with increasing temperature leading to increasing evaporation.
However, my own experience of living in the tropics led me to suspect that contrary to Ramanathan’s claim, the relative humidity (RH) would in fact be lower in the convective areas, and lower during the times of day when there are the most thunderstorms. I thought this for two reasons.
The first is my own experience of a couple of decades of working in these tropical regions. My observations are that before the afternoon thunderstorms come rolling in, the air is often “sticky” with moisture. After the thunderstorms, on the other hand, the air feels dryer. Anecdotal, I know, but I tend to trust my own experience over theory …
The other reason is that although there is a lot of moisture moving around during the thunderstorm regime, it’s mostly concentrated under and inside the thunderstorms, and that moist air is moving rapidly upwards to have the water wrung out of it by the thunderstorm. But in the much larger area in between the thunderstorms, you have dry descending air. This is air from which the water has been stripped by the thunderstorm through a combination of condensation and freezing.
And as a result, my expectation was opposite to that of Ramanatan—I expected that the more convection, the lower the relative humidity.
So, off to the data, with a few digressions along the way around and back. First, let’s look at sea surface temperatures. This is all two-minute data, that is to say the sea surface temperature (actually one metre below the surface) is recorded every two minutes.
Now, I’ve colored the data from light blue (coldest) to red (warmest). Note that this is also in order by location—the further west you go along the Equator in the Pacific, the warmer are the ocean temperatures. Note that the water temperatures rise evenly and fairly rapidly from early morning to a peak at about three pm. Then over the next sixteen hours or so, the ocean gradually cools down again.
There is kind of a subtle oddity in the daily variations. This is that the warmer the ocean overall, the less daily variation there is in the sea surface temperatures. To illustrate this, Figure 2 shows those same daily ocean temperature cycles as anomalies around their respective averages.
Figure 2. The daily variations in sea surface temperature at eight equatorial Pacific TAO buoys, expressed as anomalies about their respective means. Red shows the warmest buoys, light blue shows the coolest buoys.
Curious. The sea surface temperature in the warmer part of the Pacific don’t vary as much on a daily basis as the temperatures in the cooler part.
As might be imagined, a similar situation holds with the air temperatures. The further west you go, the warmer the air temperatures you’ll find. Figure 3 shows the air temperatures at the same buoys shown in Figures 1 & 2.
As with the sea, the temperatures increase with the distance west. However, the changes in the air temperatures are more complex, because of the emergent atmospheric phenomena of cumulus clouds and then thunderstorm clouds. This becomes visible when we look at the air temperature anomalies.
Figure 4. The daily variations in air temperature at eight equatorial Pacific TAO buoys, expressed as anomalies about their respective means. Red shows the warmest buoys, light blue shows the coolest buoys.
Figure 4 is perhaps the strongest evidence of the existence of a cloud-based temperature regulation system that I’ve found so far. Let me see if I can explain why. Here’s a graphic showing the situation at dawn …
Figure 5. The general situation in the tropical convection areas in the early morning.
As you can see, at this time of day clouds are uncommon. As a result, Figure 4 shows that the temperature rises very rapidly for a couple of hours after six AM. However, as the day warms up, at some point a threshold of emergence is passed and the first thermal cumulus clouds start to form, resulting in a change of atmospheric state. Within an hour or so, in place of clear skies, there will be a fully developed cumulus field covering the entire surface.
In the colder areas, the cumulus do not form as early or as strongly, so they don’t have as large an effect. But as you can see in Figure 4, in the warmer areas there are so many clouds that the temperature actually drops for three hours, from about nine o’clock to about noon. And as Figure 4 shows, the further west you go, the warmer it gets, and the stronger the cumulus cloud effect gets.
However, even in a fully developed cumulus state, there is not continuous cloud cover. The cumulus clouds can be thought of as flags, each one marking an area where there is an upwelling column of air. However, in between the upwelling air columns and their respective clouds, perforce there must be larger areas of slowly downwelling air. And these areas don’t have clouds. As a result, although the temperature rise is reduced or reversed from nine AM until noon, the sun still gets stronger over that time, and at some point around noon the cumulus shield is not enough to stop further temperature rise.
In the afternoon, with the continuing temperature rise, a new threshold is passed and we get another change of state. This one involves the formation of thunderstorms. These astounding emergent entities pipe air vertically at very high speeds, removing heat from the surface and converting it to mechanical motion. They also cool the surface in a number of other ways.
There is an oddity, which is that when the thunderstorms develop, the albedo goes down. This is because the vertical motion is so fast in the thunderstorms that they have a proportionately much larger surrounding cloud-free area of dry descending air on all sides of them.
Now, I said at the outset that there would be “a few digressions along the way around and back” to Ramanathan. So with those as the digressions along the way around, let me come back to the topic by saying that these large areas of descending dry air are the reason that I thought that Ramanathan was wrong. Remember that I’d disagreed with Ramanathan’s claim, viz:
The cause of this super greenhouse effect at the surface is the rapid increase in the lower-troposphere humidity with SST; … the tropospheric relative humidity is larger in convective regions.
And what do the TAO buoys say about the relative humidity (RH)? Well, here are the daily cycles in RH for the same eight TAO buoys …
Figure 8. The daily variations in relative humidity (RH) at eight equatorial Pacific TAO buoys, expressed as anomalies about their respective means. Red shows the warmest buoys, light blue shows the coolest buoys.
Now, the colors of the buoys are the same. Coldest is light blue, warmest is red. But instead of the RH increasing with sea surface temperature (SST) to engender a “super greenhouse effect”, the reverse is true. As the ocean temperature rises, the relative humidity falls.
How about during the course of the day? My hypothesis regarding emergent phenomena says that the relative humidity should be lowest during thunderstorm time in the afternoon. The next figure shows the RH all of the buoys once again as anomalies, so we can compare their daily variations.
Comparing this to the SST, we see that contrary to what Ramanathan claimed, when the SST is largest, the relative humidity is the lowest.
Now, all these findings shown in Figs. 8 & 9 are curious, because Ramanathan clearly believes that relative humidity is invariant under changes in the climate. He says elsewhere (emphasis mine):
A simple explanation for the water-vapor feedback among the early studies of climate sensitivity was the fact that the relative humidity of the atmosphere is invariant to climate change. As Earth warmed, the saturation vapor pressure (es) would increase exponentially with temperature according to the Clausius–Clapeyron relation, and the elevated (es) would (if relative humidity remains the same) enhance the water-vapor concentration, further amplifying the greenhouse effect. Although it is well known that atmospheric circulation plays a big role, a satisfactory answer as to why the relative humidity in the atmosphere is conserved is still elusive.
But according to Figure 8, the relative humidity in the convective zones of the Pacific varies inversely with sea surface temperature. And this is true both for long-term average sea surface temperature, as well as for the daily average temperature variation.
I can’t say that I have any great conclusions from all of this. However, it does appear that the modelers’ claim of strong water vapor feedback rests on the idea that relative humidity stays constant in the face of warming. If these TAO data findings are correct, and if relative humidity more generally is not constant with respect to temperature, it would seem that this would greatly reduce the amount of purported water vapor feedback …
In any case, it would seem to falsify the idea of a “super greenhouse effect” that is driven by relative humidity as Dr. Ramanathan claimed.
Always more to consider, always more to learn.
My best wishes to all,
PS—If you disagree with someone, please have the courtesy to quote the exact words you disagree with. In that manner, we can all understand exactly what you are disputing.