Guest post by Willis Eschenbach
This is a two-part article. The first part is to correct an oversight in my recent article titled Rainergy.
The second part uses this new information to analyze the impact of clouds on El Niño regions.
So, onto part one. In my article “Rainergy” I pointed out that it takes about 80 watts per square meter (W/m2) to evaporate one cubic meter of seawater in one year. Therefore, evaporation of approximately 1 meter of rainfall per year cools the surface by – 80 W/m2.
Then one day I thought “Dangdang! I forgot about Vega!
Virga is rain that falls from a cloud but completely evaporates before it hits the ground.
Figure 1. Virga diagram
Here's the thing. When virga evaporates, it's like evaporating from the surface. It cools the raindrops and the surrounding air.
This is what causes rain-entrained cold storm winds to hit the ground vertically and spread across the base of the storm. You can see it all happening in this stunning time-lapse video, as the vertically entrained wind hits the water, spreads across the lake, and eventually stirs up the trees in the foreground. On the left side of the video, you can also see Verga falling and vaporizing before hitting the ground.
It's not just Virgos. raindrops are all When it rains, it evaporates, which is why it's almost always so cold.
So I started looking at how much of the rain evaporated completely before it hit the ground. I can't find a lot of information on this topic, but there are several papers that say 50 to 85 percent of rainwater evaporates. For example, see sub-cloud rainfall evaporation in the North Atlantic, which represents 65%
This makes sense because the huge surface area of hundreds of thousands of tiny water droplets allows for massive evaporation.
That's why all of this is important. I estimate the evaporative cooling for one meter of rainfall per year to be -80W/m2. This is the energy required to evaporate one meter of sea water.
but I ignored the additional cooling from the evaporation of the rain itself. Considering that about half of the rainwater will evaporate, it will provide an additional 40W/m2 of cooling effect. What's more, it's not included in the rainfall data – no way, it's already evaporated.
As I said, there's not a lot of research out there, and the rate of evaporation depends on a lot of variables. So what I did was estimate not half, but A quarter of the rain evaporates before it hits the ground. This gives a conservative value for the evaporative cooling of rainfall before it reaches the ground, although the value may be higher.
This gives a revised estimate of the evaporative cooling associated with one meter of rainfall, not -80 W/m2 per meter of rainfall over the year as I imagined, but -100 W/m2 per meter of rainfall over the year.
This concludes the first part.
Based on my new estimates of the relationship between rainfall and evaporative cooling, and careful consideration of some of Ramanathan's ideas, I decided to study changes in total sea surface cloud cooling in El Niño/La Niña regions. First, the blue box below shows the location of the so-called “NINO34” region – 5° North to 5° South latitude, 170° to 120° West longitude. Sea surface temperatures in the area indicate the state of Niño/Nina alteration.
Figure 2. Average surface temperature and location of the NINO34 region. Average from March 2000 to February 2023
This is the temperature of the NINO34 region during the CERES satellite cycle. Note that this phenomenon is called “El Niño,” referring to the El Niño, because it peaks around December or November. It bottoms out a year later around December/November (blue area) when Nino/Nina completely changes. I discuss this further in my article “La Niña Pumps.”
Figure 3. Monthly sea surface temperatures in the NINO34 region. Note the large fluctuations from approximately 25°C to 30°C, making this region valuable for studying the relationship between sea surface temperature (SST) and various cloud parameters.
Now, people who are familiar with my work know that my theory is that clouds act as powerful thermoregulators of surface temperatures. When the ocean warms, my theory is that the cumulus cloud field forms earlier in the day and covers more of the surface, reflecting more sunlight back into space.
As the ocean warms further, thunderstorms can form, cooling the surface in a variety of ways. This prevents the Earth from overheating.
Let me start with the question of the increased intensity and duration of cumulus cloud fields. This is reflected in cloud area expressed as a percentage of surface area. This is that chart.
Figure 4. NINO34 monthly cloud coverage percentage and sea surface temperature.
Now, this is the funniest thing. As temperatures rise from about 26°C to a maximum temperature of just under 30°C, total cloud area doubles, from 40% to 80%. This greatly affects the amount of sunlight reaching the surface, as we will see in the diagram of the net cloud radiation effect below. It is clear from the close correspondence between temperature and cloud cover shown in Figure 4 that cloud cover and cloud cover intensity are clearly functions of temperature and have little other influence.
Next, cloud top height. This is an indirect measure of the number of thunderstorms in the area. The graph below shows how the number of thunderstorms changes as sea surface temperatures change.
Figure 5. Monthly cloud top height and sea surface temperature in NINO34.
Once again we see a very big change. As sea surface temperatures rise from about 26°C to just under 30°C, the height of cloud tops almost triples, from 5 kilometers to nearly 15 kilometers. Again, the number of thunderstorms is obviously a function of temperature and little else.
With these changes in mind, we can look at the cooling effects of these cloud changes. Figure 6 below shows the change in net surface cloud radiative effect. The net surface cloud radiative effect is the total effect of clouds on the radiation reaching the surface. Clouds cool the Earth's surface by reflecting sunlight back into space and absorbing solar radiation. They also heat the surface by increasing downstream longwave radiation. The net surface cloud radiative effect is the sum of these different phenomena.
Figure 6. Monthly net surface cloud radiation effect and sea surface temperature in NINO34.
Note that clouds cool the NINO34 sea surface at all sea surface temperatures. As the temperature rises, radiative cooling increases, and not just a little, with the cooling effect increasing from -10 watts per square meter (W/m2) to almost -60 W/m2.
It is also worth noting that this effect is not linear – small deviations in temperature do not result in an increase in the net radiative cooling of the surface caused by a large increase in temperature. This is shown by the fact that the large peak in the blue line extends higher than the peak in the black line.
Then we can also look at the cooling effect of rain. As mentioned above, one meter of rainfall involves surface evaporative cooling of approximately 100 W/m2. This allows us to convert rainfall data into evaporative cooling data, as shown in Figure 7 below.
Figure 7. Evaporative cooling effect of monthly rainfall and sea surface temperature in NINO34. Note that the data set is shortened by one year because rainfall data is as of 2021
Here we see that cooling increases fivefold with increasing temperature, but on a much larger scale. Rainfall evaporative cooling changes from -50 W/m2 when the NINO34 area is cool to -350 W/m2 when the area warms. And this effect is also non-linear, as shown by the peak in the blue line.
Finally, we can combine the separate effects of net surface cloud radiation changes and rainfall evaporative cooling to obtain the total cooling effect of clouds on the NINO34 region, as shown in Figure 8 below.
Figure 9. Total monthly cooling due to clouds in NINO34. This is the sum of the evaporative cooling effects of rainfall and the radiative cooling effects of surface clouds. The data set is again shortened by one year as rainfall data ends in 2021
It can be seen that clouds have a very strong cooling effect on the NINO34 area. At peak temperatures, clouds cool the surface at a rate of -400 W/m2. In addition, as the temperature increases, the cooling rate becomes faster and faster, which sets a strict upper limit on the heat of the NINO34 region.
…the alarmists are worried about the CO2 forcing change of 0.7 W/m2 over the same period?
This is lost in noise compared to the 400 W/m2 peak cloud cooling.
Finally, note that the huge increase in cloud-related cooling does not only occur in the NINO34 region. It occurs anywhere in the ocean where temperatures exceed around 25°C. Observing the NINO34 region is valuable because temperatures vary widely there, revealing a strong connection between temperature and total cloud cooling. For a larger view, here is a scatterplot of average grid cell sea surface temperature versus average grid cell total cloud cooling from 2000 to 2021. Note that in addition to the rapid increase in cooling rates above about 25°C, the influence of clouds cools all parts of the ocean.
Figure 10. Scatter plot of total cloud cooling versus sea surface temperature. The blue dots are 1° latitude x 1° longitude grid cells.
That’s the sum of what I learned today…
My warmest regards to you all,
w.
As always, I ask that you quote the exact words you are discussing when commenting. This way you can avoid endless misunderstandings.
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