The 7% Solution

Arthur Conan Doyle,

Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.
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It is also is also:

The relationship between precipitation and temperature is founded in science. Simply put, warmer air holds more moisture. Scientists can even tell you how much. A widely used theorem in climate science called the Clausius-Clapeyron equation dictates that for every degree the temperature goes up, there is an approximately 7 percent increase in the amount of moisture the atmosphere can hold. The intensity of extreme precipitation, which is proportional to atmospheric moisture, also increases at a scaling rate of approximately 7 percent, in the absence of moisture limitations.

The problem is that when scientists ran computer models predicting the likelihood of extreme precipitation in the future, and compared those results with both present day observations and the temperature scaling dictated by the so-called "C-C equation," the numbers were off. In many cases, the increase in extreme precipitation relative to surface temperature over land was closer to 2 to 5 percent, rather than 7 percent. . . . ​

Source: https://www.sciencedaily.com/releases/2017/03/170307100337.htm

Now this is one of the conundrums of climate modeling, the predictions do not match the weather observations. But these become opportunities (usually for graduate students) to write a paper or thesis. <grins> The summary goes on:

However, Wang says the peaks seen in the observational data and climate models simply reflect the natural variability of the climate. As Earth warms, her team found, the entire curve representing the relationship between extreme precipitation and rising temperatures is moving to the right. This is because the threshold temperature at which rain intensity peaks also goes up as temperature rises. Therefore, extreme rainfall will continue to increase, she says.
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One of the things the climate models appear to show are extremes of drought and flood. For example:

• Texas
• California

It is as if a switch were thrown taking each state from severe drought to floods. But this is not unique to North America, India and SouthEast Asia has had similar extremes. So what did they find?

. . . says Wang. "Our study suggests that this is a wrong question to ask. If you want to relate rain intensity to temperature using the C-C relationship as a reference, you have to relate to the temperature at which the rain event occurs, not the mean temperature, which is the long term average."

. . . Trenberth explains the findings this way:

"In general, extreme precipitation increases with higher temperatures because the air can hold more moisture -- although that depends on moisture availability. But beyond a certain point, it is the other way round: the temperature responds to the precipitation, or more strictly speaking, the conditions leading to the precipitation, [such as extensive cloud cover or surface moisture]. The most obvious example of this is in a drought where there is no precipitation. Another example is in cloudy, stormy conditions, when it is wet and cool. By relating the changes in precipitation to the temperature where the relationship reverses -- instead of the mean temperature as in previous studies -- we can make sense of the differences and the changes. Moreover, it means there is no limit to the changes that can occur, as otherwise might be suspected if there were a fixed relationship."​

Now I all but memorized the Steam Tables, actually the unique phase changes of water. What this tells me is to understand precipitation and ice, one has to understand the non-linear effects of water phase changes including both fresh and salt water. So let me translate this in 'Bob' (not always a good thing.)

• polar ice - driven by salt-water freezing and fresh water precipitation that deposit snow
• drought vs flood - driven by some interesting, global air patterns that seem to find binary states over various surface area. These air motions (I can not call them circulations) mean there are 'resonate states' that provide geographical patterns.

Bob Wilson

 

Interesting, I need to do a little more study in that area. However, there is some associated information from another, lay source:
Global warming is increasing rainfall rates | John Abraham | Environment | The Guardian

The graph in the article suggest that the rain threshold changes with the temperature:

The idea is shown in the sketch below. Details vary with location but, as the world warms, there is a shift from one curve to the next, from left to right. The result is a shift such that more intense precipitation occurs at higher temperatures in future, while the drop-off moves to even higher temperatures.
. . .
The authors find that in some parts of the globe, the relationship is even stronger. For instance, in the tropics, there’s more than a 10% increase in precipitation for a degree Celsius increase in temperature. This is not unexpected because precipitation releases latent heat, which can in turn invigorate storms.

From a practical standpoint, this helps us plan for climate change (it is already occurring) including planning resiliency. In the United States, there has been a marked increase in the most intense rainfall events across the country. This has resulted in more severe flooding throughout the country.

In my state, we have had four 1000-year floods since the year 2000! Two years ago, Minneapolis, Minnesota had such flooding that people were literally fishing in the streets as lakes and streams overflowed and fish escaped the banks. No joke, I actually observed fish swimming past me as I waded up a street. This occurrence is being observed elsewhere in my country and around the world.​

Bob Wilson

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