Guest Post by Willis Eschenbach

I got to thinking again about the question of evaporation and rainfall. I wrote about it here a few years ago. Short version—when the earth’s surface gets warmer, we get more evaporation and thus more rainfall. Since what comes down must go up, we can use the Tropical Rainfall Measuring Mission (TRMM) satellite rainfall data to calculate the corresponding rainfall-related evaporation.

From that TRMM data, we can also calculate how much the evaporation changes with additional warming. Figure 1 shows the trends in evaporative cooling with respect to temperature, in units of W/m2 of additional evaporative cooling per degree of additional warming

Figure 1. Amount of additional evaporative cooling per additional degree of temperature. Red areas have the greatest rainfall and thus the greatest evaporative cooling. The area of greatest cooling is the Inter-Tropical Convergence Zone (ITCZ) just above the Equator. Note that this includes three additional years of CERES and TRMM data compared to my earlier analysis.

As you can see above, the change in evaporative cooling per degree C ranges from a drop in evaporation of -70 watts per square metre per degree of surface warming (W/m2 per °C), all the way up to an increase in cooling of well over one hundred W/m2 per °C of surface warming.

Now, I’d gotten that far in my previous analysis, but I was stymied by the incomplete coverage. At the time I said:

As noted above, the TRMM data covers about two-thirds of the surface area of the Earth. From appearances, unlike in the tropics, the correlation of evaporation and temperature is negative in the unsurveyed areas of both the northern and southern extratropics. The grey line at about 30°N/S shows where the relationship goes negative. This is no surprise. In the extratropics, rain is associated with cold fronts instead of being associated with thermally driven tropical thunderstorms. As a result, although the overall average change in cooling shown in Fig. 6 is 11.7 W/m2 per degree of warming, I suspect this be largely offset once we have precipitation data for the currently unsurveyed areas.

So my quick guess at the time was that overall the value might be around zero. However, this question continued to bother me. So I started thinking about how I could estimate the trends in the areas not covered by the TRMM data. I began by looking at the average evaporative cooling by degree of latitude. Figure 2 shows that result.

Figure 2. Latitudinal averages of evaporative cooling (W/m2 per °C), in 1° wide latitude bands.

This was quite encouraging. I had previously assumed that as we went towards the poles, the trend would continue to go more negative. But both in the northern hemisphere (positive latitude) and the southern hemisphere (negative latitude), the trend is heading back towards zero as we go towards the poles. This would indicate that the values nearer to the poles might be around zero.

Next, I took a different look at the data. Figure 3 shows a scatterplot of the evaporative cooling trends versus the average surface temperature. It shows on a gridcell-by-gridcell basis the relationship between the evaporative cooling trend for that gridcell versus the long-term temperature of that gridcell. There are 28,800 gridcells shown in Figure 3.

Figure 3. Scatterplot of the evaporative cooling trends versus the average surface temperatures, 40° North to 40° South.

Now, this is quite revealing. It shows that at the cold end of the temperature scale, the evaporative cooling trend is quite small. Where the surface temperature is 0°C to 5°C, the average trend is -0.05 W/m2 per °C. For a surface temperature of 5°C to 10°C, the average trend is -0.6 W/m2 per °C.

SO … I think we can reasonably estimate that the average trend in the unmeasured areas of the globe shown in Figure 1 is on the order of -0.5°C. Recalling that the area from 40°N to 40°S is about 2/3 of the globe, and that the average for that area is 10.7 W/m2 per °C, that means that the global average is (1/3) * -0.5 + (2/3) * 10.7 = 7.0 W/m2 per °C.

Now, if this estimate is high and the actual value in the white unsurveyed areas in Figure 1 is say -3.0 W/m2 per °C, that would give a value of 6.1 W/m2 per °C. And if the estimate is low and the actual average value in the unsurveyed areas is 3.0 W/m2 per °C, that gives us an average of 8.1 W/m2 per °C.

So it appears that a likely global value for the trend in the evaporative cooling for each °C of additional warming is on the order of 7 ± 1 W/m2 per °C.

Next, note that in the tropics the evaporative cooling trend goes up rapidly with temperature. The average in the tropics is 16.7 W/m2 per °C, with some areas cooling at over 100 W/m2 per °C.. Since the tropics is the area where the most energy enters the system, this provides a strong mechanism to prevent overheating.

Next, it is important to understand that this strong cooling effect is not applied at random. Instead, in general it occurs preferentially where the surface is warmer than the surrounding areas. As a result of the targeted application of the cooling, it has a larger effect than if it were applied willy-nilly.

Finally, in IPCC terms this would be classed as a feedback. That is to say, if the surface is warmed (by increased forcing or any other cause), when it warms the increased number of thermally-driven thunderstorms act to increase the evaporation and cool the surface back down. However … as far as I can find it is not included in the IPCC analysis of all feedbacks.

Regards to all,

w.

My Usual: If you comment please QUOTE THE EXACT WORDS YOU ARE DISCUSSING, so we can all understand your subject.

Further Reading On Evaporative Feedback:

Dr. Judith Curry

Dr. Roy Spencer