Guest Post by Willis Eschenbach
I’ve been looking again into the satellite rainfall measurements from the Tropical Rainfall Measurement Mission (TRMM). I discussed my first look at this rainfall data in a post called Cooling and Warming, Clouds and Thunderstorms. There I showed that the cooling from thunderstorm-driven evaporation is a major heat-regulating mechanism in the tropics. This is another piece of evidence for my hypothesis that the global temperature is regulated by emergent phenomena, including tropical thunderstorms. This regulation keeps the temperature within a very narrow range (e.g. ± 0.3°C over the entire 20th century).
In that post, I looked at averages over the period of record. For this post, instead of averages over time I’ve looked at the changes in rainfall amounts over time. To begin the temporal investigation, Figure 1 shows the month-by-month variations in the average rainfall.
Figure 1. Movie loop of the monthly averages of the tropical rainfall, Dec 1997 – Mar 2015. The coverage of the mission only extends from 40°N to 40°S. Note that this covers about two-thirds of the surface of the planet. Units are mm/month.
Note how the rainfall amounts clearly delineate the Inter-Tropical Convergence Zone (ITCZ) that runs along and generally just above the Equator. As the name implies, the winds of both the northern and southern tropics converge near the equator. Where the winds meet there is intense rainfall, along with the deep thunderstorm convection that drives the global atmospheric circulation.
It is interesting to see the waves of precipitation wash over places like India. It’s like the earth breathing—in the summer when India gets hot, the hot air rises. When the air rises, it draws in the moist air from off of the Indian Ocean, which pours down as the monsoon rain.
Brazil, on the other hand, was a surprise in that I never knew that all of Brazil but the far north has a long dry period from July to January or so. And when it rains, the rain comes down from the north. Always more to learn.
Now, when I look at a timeseries record, I want to be able to separate out the regular seasonal changes from the rest of the data. Figure 2 shows the month-by-month rainfall averages for the area 40°N to 40°S, decomposed into the seasonal and residual components.
Figure 2. Decomposition of the monthly rainfall record (red line, top panel) into two components—a repeating seasonal component (blue line, middle panel) and a residual component (bottom panel) which is the data minus the seasonal component. The p-value is adjusted for autocorrelation by using the Hurst exponent to calculate the effective degrees of freedom. See here for details of the adjustment.
The main thing that stands out for me in this record are the two biggest El Nino/La Nina episodes, one in 1997-1998, and one in 2009-2010. We can see that during these episodes the tropical rainfall went up. There is also an overall trend, but it is not statistically significant.
Now, we can convert the rainfall data into evaporative cooling data. To do so, we utilize the rule that what comes down must go up. So if a half meter of rain falls in a month, a half meter of water must have been evaporated during the month.
And we know that it takes about 75 watt-years of energy to evaporate one cubic meter of seawater. This lets us convert the rainfall data to evaporative cooling data. Figure 3 shows that result. Of course it is identical in shape to the rainfall data, only the units are changed.
Figure 3. As in Figure 2, showing the decomposition of the monthly evaporative cooling record (red line, top panel) into two components—a repeating seasonal component (blue line, middle panel) and a residual component (bottom panel) which is the data minus the seasonal component.
As mentioned above, I’ve shown that as the temperature goes up, so does the thunderstorm-driven evaporative cooling. In other words, the variations in thunderstorm evaporative cooling are a response to the temperature variations.
Note the size of the variations in cooling, which can change by up to eight watts per square metre in a single month. This can be compared with the estimated changes in CO2 which are expected to be about four watts per square metre in a century …
This dependence of thunderstorm evaporative cooling on temperature be seen more clearly by looking at the deep tropics, what are sometimes called the “wet tropics”. The graph below shows the area from 10°N to 10°S. You can see in the bottom panel that the evaporative cooling was high during the 1997/8, the 2002/3, the 2006/7, and the 2009/10 El Nino/La Nina episodes, and decreased during the subsequent La Nina episodes
Figure 4. As in Figure 1, but for the deep tropics from 10°N to 10°S. This shows the decomposition of the monthly thunderstorm evaporative cooling record (red line, top panel) into two components—a repeating seasonal component (blue line, middle panel) and a residual component (bottom panel) which is the data minus the seasonal component.
The first thing that caught my eye is that at 120 watts per square metre, the evaporative cooling in the deep tropics is about 50% stronger than in the full TRMM 40°N/S dataset.
You can also see the El Nino/La Nina pump in operation. The “La Nina” portion of the El Nino/La Nina pump is much clearer in this deep tropical data. We can also see the smaller El Ninos of 2002/3 and 2007/8 along with the subsequent La Ninas.
Now, here is the interesting part. I wanted to compare the evaporation with the surface temperature. To start with, I used the HadCRUT4 surface temperature for the deep tropics. Figure 5 shows the two datasets, one of temperature, and the other of evaporative cooling.
Figure 5. Temperature and evaporation in the deep tropics 10°N to 10°S latitude. The upper panel shows the HadCRUT4 surface temperature data. The lower panel shows the evaporative cooling calculated from the TRMM rainfall data.
As you can see, the two datasets follow each other very closely. To demonstrate that, Figure 6 below shows the evaporation, along with the linear estimate of the evaporation based solely on the surface temperature:
Figure 6. Evaporation in the deep tropics 10°N to 10°S latitude (black), along with estimated evaporation based on temperature (red).
Note that this covers the entire deep tropics from 10°N to 10°S. This is not just the El Nino region in the Pacific, but also the other oceans and the land as well. And as you can see, in the deep tropics the temperature and the evaporative cooling are quite intimately related around the globe.
Now this correlation of temperature and evaporation should be no surprise. Common experience tells us that the warmer a wet object is, the quicker it dries by evaporation. So we’d expect evaporation to increase and decrease in parallel with temperature.
The surprising part of this analysis from my perspective was the size of the variation in evaporative cooling. We get a very large variation in evaporative cooling given a small change in temperature. Evaporative cooling rises by 27 W/m2 of increased cooling for each one degree C of surface warming.
I wasn’t all that convinced that big a number was correct, so I decided to check it against the CERES surface temperature data. It turns out that the CERES data gives us about the same answer. CERES data for the deep tropics says there’s an average of a 23 W/m2 increase in evaporative cooling per degree of surface warming for the deep tropics (10°N/S). Here’s the larger picture from the CERES data:
Figure 7. Trends in evaporative cooling per degree C of warming, for each 1°x1° gridcell from 40° North to 40° S.
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.
Regardless of the unknown global average, however, in the tropics (and particularly the deep tropics) evaporative cooling generally goes up, and sometimes very rapidly, with increasing temperature. To take another look at it, Figure 8 shows deep tropical evaporation as a function of the CERES temperature data (note that the CERES data doesn’t cover the end of the 1997/8 El Nino-La Nina episode.
Figure 8. Evaporation in the deep tropics 10°N to 10°S latitude (black), along with estimated evaporation based on the CERES satellite-measured surface temperature (red).
So I got to thinking … if there were no thunderstorms, how large would we expect the change in evaporation to be for a one degree change in temperature? We expect the evaporation to go up with increasing temperature … but how fast?
To answer this, I turned to the literature. Evaporation can only be approximated, and there is more than one way to do it. I used the formula given here (Equation 5) for evaporation over the ocean, as well as the formula in the R package EcoHydRology. The two methods gave somewhat different answers for the change in evaporative cooling per degree of warming (see “Math Notes” below). One says that assuming tropical conditions gives us about 4 W/m2 per degree warming in the deep tropics. The other says about 6-7 W/m2 per degree. And no matter how I play with the variables of wind and temperature and relative humidity, I can’t fit the data with anything more than about 7-9 W/m2 increase in evaporative cooling per degree of surface warming.
On the other hand, the answer that we’ve gotten from a couple of sets of observations (HadCRUT4 and CERES) gives a value of around 25 W/m2 of increased evaporative cooling per degree of warming for the deep tropics. And the trends of individual gridcells in Figure 6 shows evaporative cooling of more than three times that per degree of warming.
To put the contrast starkly, at the average temperature of the deep tropics (~27°C), from theoretical considerations we’d expect a 1°C rise in temperature to increase evaporation by somewhat less than ten W/m2 depending on your assumptions … but the observed average increase is 23-27 W/m2, much more than the theoretical increase in evaporation from temperature alone. I hold that this is because of the thermally controlled nature of thunderstorms.
I think that the causative chain runs as follows:
Increased surface temperature ==> earlier and more daily thunderstorms ==> increased evaporation ==> increased cooling
However, I’m happy to entertain alternative hypotheses.
To recap: the unexpected finding is NOT that evaporation increases with temperature. We’d expect that. The unexpected part is that the evaporation increases by 27 W/m2 per degree C of warming, while the theoretical increase in evaporation per degree of warming is much less than that, under ten W/m2 per degree C.
How is this increase in evaporation accomplished? Well, therein lies the story of one of the under-appreciated abilities of the thunderstorm. A thunderstorm is a dual-fuel heat engine. It runs on either temperature or water vapor. And beyond that, it can create its own fuel as it runs.
Thunderstorms run off of low-density air. The low-density air rises, bearing water vapor upwards to the level where the water vapor condenses. The heat of condensation then powers the deep convection up the tower of the thunderstorm.
Now, there are two ways to get low-density air. One way is to heat the air, so it expands and rises. The other way is to increase the relative humidity of the air, because counterintuitively, water vapor is lighter than the air. So when evaporation increases, the air gets lighter and rises.
And here’s the beauty part. The thunderstorm doesn’t just depend on the existing evaporation. Instead, it generates its own increased evaporation (and thus increased evaporative cooling) in several ways.
First, once the thunderstorm forms it generates strong surface winds in front of and underneath the storm. Evaporation is a linear function of the wind speed, with a coefficient of about 0.7. So if wind speed increases from say 2 m/sec (4.4 mph) up to 10 m/sec (22 mph), you get about three and a half times the evaporation.
The next way that thunderstorms increase evaporation is that they are surrounded by dry descending air. Thunderstorms condense the water out of the air as it is lifted high into the troposphere. As a result, when the air exits from the top of the thunderstorm, it contains very little water. From there it descends, providing a constant source of dry air to the surface. If there is 120 W/m2 evaporative cooling in the deep tropics and the air dries from a relative humidity of 0.75 to 0.65, the evaporative cooling increases by about a third, to about 160 W/m2. So this provision of dry air is quite a large factor in the increased evaporation.
The final way that thunderstorms increase evaporation is by increasing the evaporating surface area of the water. Over the ocean, which is 83% of the deep tropics, wind-driven waves increase the oceanic surface area. Wind-driven short-period waves of say 1/2 metre height and 30 metre wavelength increase the ocean surface area by about 1%. But when those waves start to break, or when the storm winds blow the water off of the tree leaves and the grass, sending fine spray into the air, surface area increase from the spray droplets can be 5% or more.
So once the thunderstorm gets started, it manufactures low-density air that keeps it going by generating strong winds at the base, by lowering the relative humidity of the surrounding air, and by increasing the evaporating surface area. This lets the thunderstorm cool the surface to a temperature well below the thunderstorm initiation temperature.
I highlight this because it is a crucial and often overlooked fact, one than distinguishes thunderstorms from simple linear feedback. Once the thunderstorm is initiated, it operates in the exact same manner as manmade refrigerators. It uses evaporation to remove heat from the area to be cooled. And because it is generating its own fuel (low density moist air) it can continue to cool the surface to below the temperature at which it started. And this “overshoot” in turn means that it can keep a “steady state” temperature that only varies within a narrow range. When the temperature gets too high, it gets pushed down below the thunderstorm initiation temperature. Then the temperature starts to rise again, and when it does, a new thunderstorm forms, and it pushes the temperature down below initiation temperature. The cycle repeats endlessly, and the temperature of the system varies little.
And this is the reason for the large variation of evaporation with temperature. Tropical thunderstorms are a threshold-based emergent phenomena. This means that they emerge spontaneously once a certain threshold is surpassed. In the case of tropical thunderstorms, the threshold is mainly temperature-based. As a result, the evaporative cooling due to tropical thunderstorms is a function of the surface temperature.
In closing let me add this final movie. It shows the entire history of the TRMM tropical rainfall observations, month by month. To me, there’s nothing as fascinating as observational data.
My best wishes to you all,
w.
My Usual Request: If you disagree with me or anyone, please quote the exact words you disagree with. I can defend my own words. I cannot defend someone’s interpretation of my words.
My New Request: If you think that e.g. I’m using the wrong method on the wrong dataset, please educate me and others by demonstrating the proper use of the right method on the right dataset. Simply claiming I’m wrong doesn’t advance the discussion.
Math Notes: I’ve used the R package EcoHydRology to estimate the evaporative heat flows from a wet surface. Most (83%) of the deep tropics is ocean, and the rest is usually wet, so it is a reasonable approximation. The function I used is called “EvapHeat”. The package documentation says
EvapHeat : Evaporative heat exchange between a wet surface and the surrounding air
Description
Evaporative heat exchange between a surface and the surrounding air [ kJ m-2 d-1 ]. This function is only intended for wet surfaces, i.e., it assumes the vapor density at the surface is the saturation vapor density
Usage
EvapHeat(surftemp, airtemp, relativehumidity=NULL, Tn=NULL, wind=2)
Arguments
surftemp : surface temperature [C]
airtemp : average daily air temperature [C]
relativehumidity : relative humidity, 0-1 [-]
Tn : minimum dailiy air temperature, assumed to be dew point temperature if relativehumidity unknown [C]
wind : average daily windspeed [m/s]
This function gives the answer in curious units, kilojoules/m2 per day. So I convert it to watts continuous by multiplying by 1000 joules per kilojoule and dividing by 86,400 seconds per day. This is joules/second/m2, which is the same as watts/m2. I used this function with reasonable numbers for the variables in the deep tropics (surftemp ≈ 27°C, airtemp ≈ surftemp – 0.5°C, relative humidity ≈ 0.85, wind ≈ 2 m/sec.) The values for the surface and air temperatures are from the TAO buoy data.
The second way that I determined the increase in evaporation with temperature was using the formula shown here at the bottom of page 6. It gives smaller values for the increase in evaporation with a 1°C increase in surface temperature.
After much experimentation I found that regardless of the exact values chosen for the variables (surface temperature, etc.), the change in evaporative cooling per degree of surface warming is far below the ~ 25 W/m2 of evaporative cooling shown by the TRMM data. In all cases I investigated, any combination of values that gave a total evaporative cooling of ~ 120 W/m2 also gave a change in cooling of less than ten W/m2 of additional cooling for a 1° surface temperature change.
Thanks Willis, I believe there is a paper here. It should be published.
I second that. Given all the data from all the posts on this subject, there is definitely enough to fill a science journal paper.
Seconded (thirded?). Brilliant.
Lindzen?
Yes. Two things at a casual skim strike me. First how massive is the effect of thunderstorms and secondly how extremely non linear and difficult to ‘parameterise’, they are.
This could be the face saving way out for climate scientists “We were right about CO2, but we missed the thunderstorms” would allow them to accept the reality of the situation.
Absolutely sounds like a paper for the journals.
I’d add that this leads directly to a solution for warming. Increase ocean evaporation. Giant misting devices off the coast of California, Northern Africa, and Western Australia would cool the planet and provide much needed rain to desert areas.
Willis, you’re really getting this hammered down…congratulations
“It is interesting to see the waves of precipitation… It’s like the earth breathing…”
What’s also fascinating is the areas without. I first noticed it in Southern California, and then went looking for it in the Sahara, but you can see the pulsing of moisture approach from the south, and not quite make it, and then as that retreats the pulse from the north that doesn’t quite make it…
There’s an interesting dynamic there, almost like high ground in a sea of moisture, keeping those areas dry.
Daniel, you are noticing something important. When the earth warms enough, about another two degrees as it was prior to 6600 BC, those waves will ‘break through’ and the Sahara turns green once again.
[Note: “Kent Pitman” is a sockpuppet name used by a banned commenter. ~mod.]
Nope, because the system is bounded. He’s discovered an unstable equilibrium point (talking phase plane dynamics here), that then goes to chaos when it runs into the boundary condition of the system (i.e. the thunderstorm can only get so big before it interferes with itself due to hitting the cap of the troposphere). Due to the fact that thunderstorms build up energy (evaporation) and then discharge energy (lightning/rain), it’s likely the equilibrium point is best described as an unstable spiral. Once enough power is drained from the thunderstorm (hitting land, wind sheer, other dynamics), the system dynamics moves towards the stable equilibrium node until it gets captured on the eigenvector and the storm dies.
Be interesting to know what bifurcation mechanism controls thunderstorm development and die down.
Phase plane analysis is fun and useful. For reference so that this is not a mystery to you (as claiming perpetual motion if not in jest suggests you have not heard about nonlinear dynamics like this), see http://www.math.psu.edu/tseng/class/Math251/Notes-PhasePlane.pdf
Ged
Well said.
Nonlinear pattern structures such as thunderstorms export entropy, since emergent structure reduce the level of disorder or entropy. However the second law of thermodynamics demands that systems overall increase in entropy. Thus such pattern systems balance the books entropically by providing heat to their surroundings. Since the surroundings of a thunderstorm include the upper atmosphere and space, this is why, if I’m not wrong, thunderstorms are mandated by Thermodynamics 2 to export heat to space. This is also a role of thunderstorms that Willis has proposed.
I agree with Anthony, a paper here.
Fascinating to watch the Earth breathing like that.
Also how there are areas of ocean that get very little rain compared to others.
Australia doesn’t get much either.
“Earth breathing”.
Just what I thought.
Oldseadog,
The “breathing earth” analogy applies to polar ice, too:
http://www.thisiscolossal.com/wp-content/uploads/2013/08/BreathingEarth.gif
(Notice that Greenland never really melts)
Great movie, dbstealy. If we had a comparable Antarctic movie, we’d see how less variable conditoins are over total ocean. Temp variations are greater over land because of the greater thermal inertia of a huge mass of water.
It’s not breathing, it’s weeping, pining for the fjords. (Resists posting Parrot sketch yet again).
Robert,
I was between sips of Sancerre, so I don’t have to stick you for a screen.
Priceless.
Plus a pile!
Auto
I never knew that water vapour is lighter than air.
This fits with the fact that it is always hot and humid in Sydney’s West before a cooling thunderstorm.
Also clouds are usually high, not fog, so the area involved is saturated with water vapour.
Water vapor is also invisible, clouds and visible steam are condensed from vapor to water droplets.
It makes sense though if you look at it, O2 is 32 and N2 is 28 mix them and wagging the ratios makes an average mass of about 29. Water vapor is 18 so as the proportion of water goes up, the mass of the column goes down. I think I have seen Doctor Brown do it a little more precisely in a post not too long ago.
Lewis,
That’s why home runs are easier to hit in warm, humid air than cold, dry air. 🙂
Unless that cold, dry air is at 5280 feet (Coors Field)?
roks
Not Mile High Stadium? Or is that hockey, rather than baseball?
The altitude seems enticingly accurate, I must say.
Auto
Thanks. A wonderful post. Even before reading your text I had the same impression from the animation – that I was watching the Earth inhale and exhale. Your insights from personal experience and very capable analysis deserve much wider exposure. I also wonder from this and your other posts about emergent phenomena and tropical thunderstorms, what the calculations are for the potential and kinetic energy transfers from elevating massive amounts of water into the atmosphere (increased potential energy) and then returning it to the surface (conversion back to kinetic and/or mechanical energy) where an enormous amount if work is done to reshape Earth’s surface. I presume these are largely in addition to and mostly separate from the thermal energy transfers.
Thanks. Very interesting. The water vapour cycle is thus an effective global heat pump.
Willis – The next way that thunderstorms increase evaporation is that they are surrounded by dry descending air. Thunderstorms condense the water out of the air as it is lifted high into the troposphere.
Don’t leave out the PE / KE conversion.
Potential Energy Equations Calculator – http://www.ajdesigner.com/phppotentialenergy/potential_energy_equation_z.php
So for your M^3 of water (1000 KG) @ur momisugly 3000 meters elevation (9800 FT) you get a potential energy of 29,400,000 joules
But rain, due to friction, has a terminal velocity of under 9 m/s or Kinetic Energy – http://www.ajdesigner.com/phpenergykenetic/kenetic_energy_equation.php#ajscroll
for the 1000 KG = 40,500 joules.
My senior engineering professor taught that friction converts to heat.
So the condensed rain heats up, after less than 10 meters falling, and re-evaporates multiple times until the 29,359,500 joules are used up heating the air.
With an average rain fall of 1m around the globe per year, it comes out to 0.9W/m2.
I was expecting it to be a lot smaller.
It is interesting to see how observational data departs from values expected by theory.
Even the climate models have increased evaporative cooling as the climate system warms (as evidenced by their increased precipitation, they go hand-in-hand). That’s not the models’ shortcoming.
It is one of the models’ shortcomings. The models have too little evaporation (2-3% per deg C when it should be ~7%), hence their ECS is too high.
[ECS – Equilibrium Climate Sensitivity – the climate’s sensitivity to CO2]
Thanks, Dr. Roy. You are indeed correct that the models show increased cooling as the climate system warms. The problem is, they have it in the wrong places at the wrong times. My next post will be a brief look at that oddity.
w.
To Roy Spencer:
However, the climate models show only about 3%/C increase in evaporation. Using a typical value of 80 W/m^2 gives only about 2.5 W/m^2 per C of warming. Using simple physics for constant wind speed and wave action one gets: For constant relative humidity (RH) about 6.6%/C and for constant specific humidity about two times this. Using 80 W/m2 for a global average this gives about 5.3 and 10.6 W/m^2/C. The tropics might run 1.5 times the global average, up to about 16 W/m^2 if the specific humidity can’t increase fast enough to get constant RH, but still much below Willis’ data of about 27 W/m^2. The increase in wind speed and drop in humidity during a thunderstorm as Willis suggests could be the difference.
Incidentally only at 6.6%/C or 5.3 W/m^2/C the negative feedback from evaporation is more than the IPCC values for positive feedback from water vapor feedback. About -0.74 C/C vs. about +0.54 C/C for water vapor. If Willis is correct, this could be a major factor with much stronger negative feedbacks. It reminds me of Lindzen and Choi’s famous paper showing a strong increase in outgoing radiation with surface temperature. The tops of thunderclouds with little water vapor above look at an infrared atmospheric window of about 75%, vs. only about 25% from the surface, and thus radiate efficiently to space.
See my papers on evaporation:
http://wattsupwiththat.com/2014/04/15/major-errors-apparent-in-climate-model- evaporation-estimates/
http://edberry.com/blog/climate-clash/g90-climate-sensitivity/improved-simple-climate-sensitivity-model/
Bingo! for the IR windows at the tops of Tropical thunder clouds in my view.
The tops of thunderclouds with little water vapor above look at an infrared atmospheric window of about 75%, vs. only about 25% from the surface, and thus radiate efficiently to space.
That is a very salient fact. if indeed it is true. I had suspected as much but never found a reference to the value..
If the radiation is happening beyond most of the CO2, it makes the existing radiation calculations complete nonsense.
Richard, Roy and Willis: Issac Held discusses why climate models predict less than a 7%/K increase in precipitation at the link below. Basically, the models predict an increase in relative humidity over the ocean to suppress evaporation.
http://www.gfdl.noaa.gov/blog/isaac-held/2014/06/26/47-relative-humidity-over-the-oceans/
A 7%/K increase in precipitation translates into an increase in convection of latent heat of 5.5 W/m2/K. OLR increases by 5.4 W/m2/K (at 288 K). DLR increases 4.9 W/m2/K (at 277 K, the temperature that produces DLR of 333 W/m2 assuming an emissivity of 1). Combined this gives a response of 5.0 W/m2/K at the surface. This is much greater than the change in energy flux at the TOA: 3.7 W/m2/K for a simple blackbody at 255 degK, 3.2 W/m2/K for Planck feedback in climate models and 1.2 W/m2/K for a planet with a climate sensitivity of 3 K/2XCO2.
Our planet can’t experience a 5 W/m2/K increase in outward energy flux at the surface and a 1.2 W/m2/K increase at the TOA. For ECS to be 3, the increase in latent heat flux needs to be 0.7 W/m2/K, which translates into a 1%/K increase in precipitation – assuming constant wind and no change in albedo. If Willis is correct that evaporative cooling in the deep tropics increases 27 W/m2/K, this is big enough to influence ECS for the whole planet! And climate models must produce huge errors in precipitation in the deep tropics.
Willis, this is an excellent presentation. Serious consideration to peer reviewed publication is in order.
I have followed your many posts here @ur momisugly WUWT and I’m convinced you are on to CAGW busting theory/hypothesis. I haven’t read a convincing argument against it. And it becomes more solid with every additional post you offer. Please keep up the good work.
To Frank regarding the models predicting less evaporation increase than about 6 to 7% and only 1 to 3%. Yes, and the models are wrong. The only way to explain this low rate of increase is a reduction in wind speed (which they do not do) or an increase in relative humidity (RH) above that to maintain constant RH. However much data over many locations and years show that RH drops a little with rising temperature. The complex computer model people keep thinking that their results are the same as real data. So if their results show less evaporation increases than expected for constant RH, than that “proves” that RH must increase with temperature.
And they all still make a large error in thinking energy constraints limit evaporation. The energy for evaporation comes from the temperature of the water reduced by the partial water vapor pressure of the atmosphere. What happens is that the evaporation cooling reduces the temperature rise less than they estimate (from CO2 warming) and there is no energy constraint, energy balance is just fine. Do they also think that a bar of steel will not expand when warming by 1 C as expected because they do not know where the energy comes from. Well then do not claim it warms so much.
Richard wrote: “The only way to explain this low rate of increase is a reduction in wind speed (which they do not do) or an increase in relative humidity (RH) above that to maintain constant RH. However much data over many locations and years show that RH drops a little with rising temperature.”
According to Isaac, the future rise in relative humidity in climate models develops only over the ocean. The relative humidity over land decreases. Do you have a reference that shows relative humidity over the ocean decreases with warming?
Richard also wrote: “The complex computer model people keep thinking that their results are the same as real data.”
Actually, model data is superior to real data (observations), especially when you are looking for changes that take place over decades. Observations are always incomplete and contain measurement error of various kinds. If the observations come from re-analysis data, that can also introduce distortions. With models, you can do repeat experiments or conduct well-controlled experiments where only one variable is changed at a time. Finally, if anyone does find a disagreement between model data and observations, someone can always cherry-pick “homogenized” observations that do agree with model data. Working with observations is a waste of time. [/sarcasm]
So back a few millions of years ago, before the ice age, when temperatures were much warmer than today, can we predict from this what the tropics were like? The poles were ice free. I’m thinking some seriously bad thunderstorm activity plus….?
Just to pick a nit: That seems decidedly nonlinear to me. Maybe something like evaporation = k * speed ^ 0.78?
Hey, Joe, good to hear from you. In the example, wind speed has gone up five times. Evaporation has gone up 0.7 * 5 = 3.5 times. Linear.
Thanks,
w.
I see. You’re assuming a non-zero intercept:
If the evaporation E = 1 ml/m^2/sec for wind speed v = 2 m/sec and E = 3.5 ml/m^2/sec for v = 10 m/sec, then the linear relationship is E = 3/8 ml/m^2/sec + 5/16 ml/m^3 * v.
Linear (black) vs. nonlinear (red):
v = seq(0, 12, 0.01);
plot(NA, xlim = c(0, 12), xlab = “Wind Speed”, ylim = c(0, 4), ylab = “Evaporation Rate”);
abline(3/8, 5/16, lwd = 2);
abline(h = c(1, 3.5), v = c(2, 10), lty = 2);
a = log(3.5) / log(5);
k = 2 ^ (-a);
lines(v, k * v ^ a, col = 2, lwd = 2);
The relationship is linear, but at zero wind speed, there is still evaporation. There is a speed value where it doubles (I can’t recall the value).
Richard wrote: “The relationship is linear, but at zero wind speed, there is still evaporation. There is a speed value where it doubles (I can’t recall the value).”
If there is no wind, any water vapor immediately above the surface of the ocean can travel upward only by molecular diffusion, which is ridiculously slow. In practice, there is always a thin layer of air above water that is saturated with water vapor, and convection is required to transport it anywhere else in a sensible period of time. Wind blowing at 1 m/s over the surface of the ocean has a Reynolds number for turbulent flow in less than 1 m. This turbulence is the rate limiting step in vertical transport of water from the ocean and the saturated thin layer of air immediately above. This is why the rate of evaporation depends on wind speed (far more than it depends on temperature).
Richard Petschauer:
Thanks. That sounds plausible; i.e., I am not knowledgeable enough to be able to tell whether it’s true or not, but it’s certainly consistent with, e.g., http://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html.
I made the comment about linearity only because Mr. Eschenbach seemed to be saying that the evaporation rate is multiplied by 3.5 whenever the wind speed quintuples. That defines a logarithmic relationship, of course, not the linear one that you and the above-mentioned site specify. But I infer from Mr. Eschenbach’s response that this point is too subtle to make it likely that further attempts at explanation would prove fruitful.
Incidentally, I had attempted to digest your “Major Errors” post, but you left too many questions unanswered for me to persist.
I just noticed there’s still activity in this thread, so I’ll take the time to clean up after myself.
I erroneously said:
The relationship defined by evaporation rate
‘s being multiplied by 3.5 every time the wind speed 
quintuples isn’t logarithmic. It’s 
, as I’d previously suggested.
As Mr. Petschauer observed, though, it appears that the evaporation rate is not actually multiplied by 3.5 every time the wind speed quintuples but instead bears the linear relationship to wind speed that the link in my previous comment sets forth.
The movies are hypnotic in a good way. What surprised me is the very high rainfall in the relatively small areas of the oceans. Maybe this can be used to collect fresh water in the future.
“Envisions wind powered ships with BIG funnels collecting the rainwater.*
Frank says:
“If there is no wind, any water vapor immediately above the surface of the ocean can travel upward only by molecular diffusion, which is ridiculously slow.”
I have seen equations by ASHRAE (Society off Heating, Refrigeration and AC Engineer) based on data on indoor swimming pools that show evaporation based on water and air temp and RH that shows evaporation with no wind. I also have seen data on evaporation from outdoor swimming pools that also include wind speed. Extrapolating the curve to zero wind speed still shows some evaporation.
At the water air interface, water vapor is lighter than air so it rises refreshing the interface to some extent. The surface water cools and sinks doing a similar thing. But whatever happens for constant wind speed and wave action for a change in water temperature, the same conditions will occur before and after the warming. Evaporation will increase as a function of the differences in the water vapor pressure of saturated air at the water temperature and the water vapor pressure of the air that depends on its temperature and relative humidity. Water vapor pressure increases from about 6 to 7% per C depending on the actual temperature in the ranges of interest on the surface and lower atmosphere.
Joe Born said to Richard Petschauer
“Incidentally, I had attempted to digest your “Major Errors” post, but you left too many questions unanswered for me to persist”
Joe is referring to:
http://wattsupwiththat.com/2014/04/15/major-errors-apparent-in-climate-model- evaporation-estimates/
Sorry Joe I did not make it more clear. Maybe I should rewrite it.
The gist of the paper is: The climate model people (including Trenberth in an Email to me) attempt to justify their very low increases in evaporation by saying that there is “not enough energy” to support the expected increased in evaporation. The problem is that they assume the water temperature rise is predetermined by their calculations and hence its outward longwave radiation is fixed and in order to retain the total energy leaving the surface and match the incoming energy, latent heat leaving the surface from evaporation is constrained. They ignore the fact that evaporation cools the surface and reduces the longwave outward radiation so that more can come from latent heat. But they cannot admit the surface warms less than the models show. The equations I used for energy balance at the surface, atmosphere and at the planet level solves this problem.
The real reason it seems the climate models underestimate evaporation (I think they use the correct equations) is that they over estimate surface relative humidity (RH) and have it increase more than enough to maintain constant RH. They claim it is nearly constant. But a slight change makes a large difference. They also underestimate precipitation, that is very hard to estimate, which reduces humidity and increases evaporation (and thunderstorms more as Willis suggests). Globally evaporation and precipitation must be nearly equal. It is interesting to note that evaporation is easier to estimate but hard to measure (except from a closed container), while the reverse is true for precipitation.
I appreciate the reply. I hope, but doubt, that I’ll get time to revisit that post.
Great graphics. One nit is your equating precip with evaporative cooling locally, the ITCZ gathers moisture from the north and south boundaries of the Hadley circulation.
“the ITCZ gathers moisture from the north and south boundaries of the Hadley circulation”
I was wondering if anyone would note this.
To continue your thought Steve:…and deposits the dry air emerging at the top, not in the vicinity of the ITCZ, but centered around 30 degrees North and South…the B climate zone desert belts…thousands of miles away.
As noted by Willis, this is the primary circulation pattern of the Earth.
That is true of the big picture, but the whole system has bit of a fractal feel. As you peer down at individual systems you see the same patterns emerge at smaller scales until you get to the individual thunder cell where it stops (or at least becomes way less defined – there may be smaller columns within a cell, just because I’ve never heard of it doesn’t mean it isn’t there.) Air around a line of thunderstorms throws dry air up to contribute to the ITCZ pattern but some returns around the storm locally as well due to dry air being denser and the air below being swept into the thunderstorm at its base. This is observed with individual cells within the line as well. The cumulative effect is as you have stated, but much more complex when looked at closely.
i agree, but there is the larger scale contribution. Consider, are the continents a net source or sink of atmospheric water? Insofar as their rivers flow into the ocean they are a sink.
Thanks for this massive effort, Willis. This would make a paper of real interest to Science. Compared to this we see a lot of substandard stuff published these days.
Fascinating (and crystal clear) read! Even more fascinating Earth “respiration”!
So, reversely, historical average precipitation data for the tropics could be a proxy for historical (world) temperatures?
The Earth sweats to cool off, sort of like we do. Interesting.
‘Always more to learn.’
I say that constantly when I argue climate change…. and the alarmists sneer in ignorance.
So, if global temperatures ‘tried’ to go up, the Earth would respond with more rainfall? Is what happened to North Africa / Sahara a consequence of Earths cooling in the past? Less heat, less rain?
Is what happened to North Africa / Sahara a consequence of Earths cooling in the past?
WR: Could be yes. The famous rock paintings of Tassili n’Ajjer in the south of Algeria with large wild animals and crocodiles, now somewhere in the middle of the Sahara, seem to date from at most 9-10 millennia ago (Wikipedia). The Neolithic Subpluvial wet period was the period from about 9500 – 5000 BP (yes, in the warm Holocene Climatic Optimum)
Present warmer temperatures in combination with better drought resistance of plants due to extra CO2 in combination with a higher growth rate because of the same CO2 could change back the Sahel (and other semi arid parts of the world like parts of Australia) to greener times. This map shows it’s already happening: http://www.drroyspencer.com/wp-content/uploads/co2_growth.jpg
As I say, a warm planet is a happy planet.
No. The African Humid Periods are a recurrent phenomena about every 23,000 years linked to the precession cycle, modulated by the eccentricity cycle (Milankovitch) and reflected in the Mediterranean sediments sapropels (dark organic layers). They are caused by the northern migration of the Inter-Tropical Convergence Zone (ITCZ) due to the high latitudes receiving more insolation.
http://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/74570127/1_2.png
Each dark layer corresponds to an African Humid Period and their grouping reflects the interaction between the precessional and eccentricity cycles.
http://www.nature.com/scitable/knowledge/library/green-sahara-african-humid-periods-paced-by-82884405
We now happen to be near a minimum in the precession cycle (bottom in about 1000 years). In about 8,000 years the Sahara will be green again. Some predictions about future climate can be done with high confidence.
No question this is brilliant analysis, Willis and, in keeping with your posts in general, you always find the unexpected. I think of the CAGW theory now of missing these ‘unexpecteds’, largely because you can’t many of these with deterministic models. I join the many others here in pushing you to publish this. You have also grown a body of ideas through teasing out empirical data to support them- I would say a book on the planet’s heat engine would be an enormous contribution to climate science.
“…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.”
Of course, minus the ‘heat’ radiated to space from the evap/thunderstorm activity. It would be interesting to also compare this cooling to the radiant energy exiting the TOA.
Hi Willis. I enjoy your articles.
You said “And we know that it takes about 75 watt-years per square metre to evaporate one cubic meter of seawater per year.”
The units are muddled up.
watt * year * m^-2 = m^3 * year^-1 * specific heat of water
watt * second * m^-2 = m^3 * second^-1 * specific heat of water
watt * second * m^-2 = m^3 * second^-1 * Joules * m^-3
watt * second * m^-2 = second^-1 * Joules
Joules * m^-2 = second^-1 * Joules
m^-2 = second^-1
m^2 = second ??????
“watt-years per square metre” Is an amount of energy per square metre.
The evaporation of 1 cubic metre of water is an amount of energy.
So the evaporation of 1 cubic metre of water per square metre is an amount of energy per square metre.
But 1 cubic metre of water per square metre means a depth of 1 metre.
I think what you meant is:
“it takes about 75 watt-years per square metre to evaporate one meter depth of seawater”
Or, to make it clearer:
“it takes about 75 watts per square metre to evaporate one meter depth of seawater per year.”
Checking the units
“it takes about 75 watts per square metre to evaporate one meter depth of seawater per year.”
watt * m^-2 = m * specific heat of water * year^-1
watt * m^-2 = m * Joules * m^-3 * second^-1
watt * second * m^-2 = m * Joules * m^-3
Joules * m^-2 = m * Joules * m^-3
Joules * m^-2 = Joules * m^-2
That agrees
Willis, Many thanks for an interesting and extremely well illustrated post on this important subject.
If I’ve interpreted Figures 2 and 5 correctly, they suggest a 1 C change in surface temperature is associated with a circa 10% change in precipitation over 40S-40N (if the temperature change was similar in the deep tropics to 40S-40N)? That is much higher than the 1-2% per C precipitation change in CMIP3 GCMs (Vechi and Soden 2007), which seems too low. The change is higher, 2-3.3 % per C, in CMIP5 GCMs (Mauritzen and Stevens 2015), increasing to 3.5-4% per C when including a LW IRIS effect. However, your findings are quite close to an estimate for the observed increase in global precitptation over 1987-2006 of 7.4% per C (Wentz et al 2007). Whilst Lambert et al (2008) argue that Wentz’s finding was not representative of longer term changes under greeenhouse gas warming, and that it did not imply GCMs were wrong on this, I do not find their reasoning very convincing.
Nic: It is worth noting that evaporation is directly linked to ECS. If ECS is 3, then the energy flux leaving the planet increases at only 1.2 W/m2/K. If evaporation increases 7%/K, that is 5.5 W/m2/K. OLR increases 5.4 W/m2/K (at 288 K), but is nearly cancelled by the increase in DLR of 4.9 W/m2/K (at 277 K). We can’t have a response of 6 W/m2/K at the surface at 1.2 W/m2/K at the TOA. Climate models must contain a mechanism for dramatically suppressing the expected 7%/K increase in evaporation. See:
http://www.gfdl.noaa.gov/blog/isaac-held/2014/06/26/47-relative-humidity-over-the-oceans/