From MIT via press release, bear in mind this is just more model output based on estimates from observations. That said, thunderstorms and hurricanes are simply heat engines, and they transport heat from the lower atmosphere to the higher atmosphere. Willis’ thunderstorm hypothesis comes to mind.
When it rains, it pours
Study estimates rate of intensification of extreme tropical rainfall with global warming.
CAMBRIDGE, Mass. — Extreme precipitation in the tropics comes in many forms: thunderstorm complexes, flood-inducing monsoons and wide-sweeping cyclones like the recent Hurricane Isaac.
Global warming is expected to intensify extreme precipitation, but the rate at which it does so in the tropics has remained unclear. Now an MIT study has given an estimate based on model simulations and observations: With every 1 degree Celsius rise in temperature, the study finds, tropical regions will see 10 percent heavier rainfall extremes, with possible impacts for flooding in populous regions.
“The study includes some populous countries that are vulnerable to climate change,” says Paul O’Gorman, the Victor P. Starr Career Development Assistant Professor of Atmospheric Science at MIT, “and impacts of changes in rainfall could be important there.”
O’Gorman found that, compared to other regions of the world, extreme rainfall in the tropics responds differently to climate change. “It seems rainfall extremes in tropical regions are more sensitive to global warming,” O’Gorman says. “We have yet to understand the mechanism for this higher sensitivity.”
Results from the study are published online this week in the journal Nature Geoscience.
A warm rain will fall
Global warming’s effect on rainfall in general is relatively well-understood: As carbon dioxide and other greenhouse gases enter the atmosphere, they increase the temperature, which in turn leads to increases in the amount of water vapor in the atmosphere. When storm systems develop, the increased humidity prompts heavier rain events that become more extreme as the climate warms.
Scientists have been developing models and simulations of Earth’s climate that can be used to help understand the impact of global warming on extreme rainfall around the world. For the most part, O’Gorman says, existing models do a decent job of simulating rainfall outside the tropics — for instance, in mid-latitude regions such as the United States and Europe. In those regions, the models agree on the rate at which heavy rains intensify with global warming.
However, when it comes to precipitation in the tropics, these models, O’Gorman says, are not in agreement with one another. The reason may come down to resolution: Climate models simulate weather systems by dividing the globe into a grid, with each square on the grid representing a wide swath of ocean or land. Large weather systems that span multiple squares, such as those that occur in the United States and Europe in winter, are relatively easy to simulate. In contrast, smaller, more isolated storms that occur in the tropics may be trickier to track.
An intensity of extremes
To better understand global warming’s effect on tropical precipitation, O’Gorman studied satellite observations of extreme rainfall between the latitudes of 30 degrees north and 30 degrees south — just above and below the Equator. The observations spanned the last 20 years, the extent of the satellite record. He then compared the observations to results from 18 different climate models over a similar 20-year period.
“That’s not long enough to get a trend in extreme rainfall, but there are variations from year to year,” O’Gorman says. “Some years are warmer than others, and it’s known to rain more overall in those years.”
This year-to-year variability is mostly due to El Niño — a tropical weather phenomenon that warms the surface of the Eastern Pacific Ocean. El Niño causes localized warming and changes in rainfall patterns and occurs independent of global warming.
Looking through the climate models, which can simulate the effects of both El Niño and global warming, O’Gorman found a pattern. Models that showed a strong response in rainfall to El Niño also responded strongly to global warming, and vice versa. The results, he says, suggest a link between the response of tropical extreme rainfall to year-to-year temperature changes and longer-term climate change.
O’Gorman then looked at satellite observations to see what rainfall actually occurred as a result of El Niño in the past 20 years, and found that the observations were consistent with the models in that the most extreme rainfall events occurred in warmer periods. Using the observations to constrain the model results, he determined that with every 1 degree Celsius rise under global warming, the most extreme tropical rainfall would become 10 percent more intense — a more sensitive response than is expected for nontropical parts of the world.
“Unfortunately, the results of the study suggest a relatively high sensitivity of tropical extreme rainfall to global warming,” O’Gorman says. “But they also provide an estimate of what that sensitivity is, which should be of practical value for planning.”
The results of the study are in line with scientists’ current understanding of how global warming affects rainfall, says Richard Allan, an associate professor of climate science at the University of Reading in England. A warming climate, he says, adds more water vapor to the atmosphere, fueling more intense storm systems.
“However, it is important to note that computer projections indicate that although the rainfall increases in the wettest regions — or similarly, the wet season — the drier parts of the tropics … will become drier still,” Allan says. “So policymakers may have to plan for more damaging flooding, but also less reliable rains from year to year.”
Written by: Jennifer Chu, MIT News Office
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here’s the paper abstract:
Sensitivity of tropical precipitation extremes to climate change
Paul A. O’Gorman Nature Geoscience (2012) doi:10.1038/ngeo1568
Precipitation extremes increase in intensity over many regions of the globe in simulations of a warming climate1, 2, 3. The rate of increase of precipitation extremes in the extratropics is consistent across global climate models, but the rate of increase in the tropics varies widely, depending on the model used3. The behaviour of tropical precipitation can, however, be constrained by observations of interannual variability in the current climate4, 5, 6.
Here I show that, across state-of-the-art climate models, the response of tropical precipitation extremes to interannual climate variability is strongly correlated with their response to longer-term climate change, although these responses are different. I then use satellite observations to estimate the response of tropical precipitation extremes to the interannual variability.
Applying this observational constraint to the climate simulations and exploiting the relationship between the simulated responses to interannual variability and climate change, I estimate a sensitivity of the 99.9th percentile of daily tropical precipitation to climate change at 10% per K of surface warming, with a 90% confidence interval of 6–14% K−1. This tropical sensitivity is higher than expectations for the extratropics3 of about 5% K−1. The inferred percentage increase in tropical precipitation extremes is similar when considering only land regions, where the impacts of extreme precipitation can be severe.
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As global humidity seems to be falling (see Jimbo’s graphs) hasn’t the entire logical edifice of this ‘paper’ collapsed – closely followed by the entire AGW conjecture?
The engineers will tell you they are highly suspicious of any predicted system that is run by positive feedback. . . It would quickly go to an extreme and get stuck there.
To show how little CO2 contributes to Global Warming, step outside on an overcast (H2O) night and compare that to a night when there’s only CO2 to insulate us. H2O must be 10,000 times more effective at trapping heat than CO2.
One has to also wonder if there is a saturation point/level of water that the atmosphere can sustain. How much water can the atmosphere hold and at what rate/temp/height/level point does it reach or can it reach before it all comes back down again ?. One wonders if they are just drumming up the 100% moisture content and trying to pretend that we are really enclosed in an aquarium.
Having lived in the tropics I can assure the writers that the majority of rains that fall are from thunder clouds, ie. cumulonimbus, of high intensity. From Malaysia to the coral islands in the Indian Ocean all the sometimes daily storm was from a thunderhead delivering torrential rain which sometimes lasted only 30 mins but could last 24 hours or longer when a succession of storms passed with no apparent break between. Total rainfall varied all the time. Monsoon times were not, as the textbooks claim, set in concrete but varied by up to a week or so. Temperatures remained in a bracket of pleasant 70’s with low humidity to high 80’s and high humidity and this can happen during a day. A couple of extra degrees would make no difference.
I have never experienced the kind of drizzly rain we get in the UK in the tropics.
There is no question that the water cycle provides a negative feedback. It is simple thermodynamics. If convection were to be forbidden (jello atmosphere) with the atmosphere essentially transparent to incoming solar and mostly opaque to outgoing IR, temperatures at the surface would be above 100 C. But with convection allowed the question is ” Is the water cycle energetically favorable?” Will its operation tend to increase the entropy of the universe? Which is tantamount to asking “Will the water cycle bring the temperature of the earth closer to the temperature of space?” The very fact that the water cycle is observed is evidence that it is energetically favorable (causes an increase in entropy), results in a decrease in surface temperature, and provides a net negative feedback on radiative forcing at the surface. The insistence by “Climate Science” that water vapor feedback puts a multiplier on radiative greenhouse forcing assumes a jello atmosphere and betrays an ignorance of basic thermodynamics.
Christian_J. says:
September 17, 2012 at 11:17 pm
“One has to also wonder if there is a saturation point/level of water that the atmosphere can sustain. ”
Excellent observation. Anthony Watts would really be the one to answer this because the meteorologists have it figured out. The water carrying capacity of the atmosphere is fixed by the temperature profile of the atmosphere and the temperature that would cause the moisture it is carrying to rain out. Both profiles are controlled by the adiabatic lapse rate. Unfortunately, the adiabatic lapse rate is a path function (you have to know a point on the curve to specify the whole curve) and meridional circulation causes a chaotic mixing of cold polar air with warm equatorial air making prediction a tough job for weather center computers. Nevertheless, the overall water carrying capacity of the atmosphere is approximately constant, fixed by the lapse rate, which in turn results from the fixed mass of the atmosphere and gravity. Of course “Climate Scientists” will disagree violently; they maintain that radiative forcing will cause a change in moisture content in atmosphere that will kill us all. Actually, it may cause it to rain somewhat more, as the above article very nicely demonstrates.
Philip Bradley says:
September 17, 2012 at 3:54 pm
“…they could issue the same weather forecast every day.“It might rain today. On the other hand it might not.”
And if so, it wil happen at 4:00 pm if Nigeria is typical
Aren’t models adjusted until they can hindcast with some degree of fit (even if the parameters are wrong). It isn’t saying anything at all that observations agree with models – they try to make them that way. But, because they have the wrong cluster of parameters and the weights on them are wrong, they don’t do well into the future. Isn’t that a terminal shortcoming?
“””””……Lightrain says:
September 17, 2012 at 10:40 pm
To show how little CO2 contributes to Global Warming, step outside on an overcast (H2O) night and compare that to a night when there’s only CO2 to insulate us. H2O must be 10,000 times more effective at trapping heat than CO2…….”””””
Well not exactly. It’s a very safe bet;sure to win you a beer at the bar, that it was much warmer during the day before those warm cloudy nights than it was on the days before those cooler cloudless nights; and it’s a safe bet that in both instances it still cooled down after sunset, cloud or no cloud.
It was the conditions during the day before, that produced the conditions at night, including producing the clouds.
It isn’t rocket science; in fact more like hot air balloon science.
Higher daytime Temperatures in the presence of water lead to more evaporation (see “How much more rain will global warming bring?” Wentz et al, SCIENCE july 7 2007). See also Clausius-Clapeyron equation.
Hotter moist air (H2O molecular weight 18) rises in air (molecular weight 28.8). When it gets high enough the Temperature due to the lapse rate has fallen to the dew point and clouds start to form. The higher that daytime temperature was, the higher will be the dewpoint altitude for a given starting relative humidity. The higher the starting (surface) relative humidity, the lower will be the dewpoint altitude for a given starting surface Temperature.
Those clouds can form during the late day hours due to lower sun angle which reduces insolation due to obliquity and greater line of site air mass, or will start to form at night after sunset as the atmosphere cools.
If you need a third beer, bet that the Temperature will not go up after sundown ( unless there’s a large warm air mass moving in from somewhere else. Replacing the atmosphere after the bets are in is cheating.
So the daytime Temperature and relative humidity CAUSES the clouds AND the night time Temperatures; it is NOT the clouds that cause the night time Temperature.
“””””…..geo says:
September 17, 2012 at 10:40 pm
The engineers will tell you they are highly suspicious of any predicted system that is run by positive feedback. . . It would quickly go to an extreme and get stuck there……”””””
Not necessarily. The vast majority of ordinary drum type automobile brakes employ a “Two leading shoe” design, which is a positive feedback friction mechanical system , and they don’t often (if ever) lock up at an extreme braking level. Since the positive feedback coefficient, is directly proportional to the brake material coefficient of friction, then if you overheat the brakes, so that resin binder melts and forms a slick surface, the friction goes down and the positive feedback disappears in a hurry. It’s called “Brake fade.” Two “trailing shoe” drum brake systems (rare) are negative feedback; since the more friction, the more the mechanical sytem tries to unwrap the shoes from the brake drum, so they don’t fade when they get hot; but having a lower mechanical advantage gain, they need any of various kinds of “boosters” to lower brake pedal pressure requirements. Were only used on expensive luxury cars.
For positive feedback to cause a system to go to an extreme and get stuck there, it has to be a DC feedback system, and the loop gain has to exceed one at some frequency. Usually the limit stop conditions result in zero loop gain, so the system can recover on its own when the feedback goes away (AC systems)
Pretty hard to find those conditions anywhere in the climate system, given that the earth rotates, so the sun goes out occasionally.
Correction:
“A cold rain’s a-gonna faallll.” The coldness of the rain will nullify the warmness of the ground, which will send less water aloft to get cold. It’s all very simple. Or something like that.
Understanding is complete when the models agree with each other. When they don’t, actual observations are used to adjust the models and force agreement.
Now the models all agree, therefore complete understanding of the climate has been achieved.
How could anyone question the brilliance of this logic?