Friday, August 16, 2019

Wetter or drier?

Will the world become wetter or drier overall? The article https://www.aaas.org/news/drying-atmosphere-spurs-decline-vegetation-growth says: "The findings reveal that atmospheric water vapor is expected to further wane throughout the 21st century due to rising air temperatures and a decline in the evaporation of the world's oceans."
People might find it strange that as air temperatures increase the evaporation from oceans decreases, but here is an explanation: When it comes to evaporation from the sea, most evaporation occurs when the sea is hotter than the air above it. With global warming the land heats up more than the sea and so you might tend to get air that is relatively hotter than the sea blowing to sea. When hotter air is above the sea the sea cools the air above and the relative humidity (RH) of this air increases and less evaporation results into the air because RH is higher. When The sea is warmer than the air above it the sea heats the air above it and the RH of the air decreases and more evaporation occurs because RH is low. When you have cold sea near to hot land you can get "negative evaporation" - the water vapour condenses out of the hot air above the cold sea and fog occurs.
One factor that could increase the evaporation is increased downwelling sky radiation from hotter air, but the effect of increasing RH of hotter air above cold water appears to be a clear winner in certain regions at present.

Solution: A stagnant saturated layer builds up just above the sea surface so a sheet along the coast and above the sea that deflects air downwards and gives turbulence could partially solve the problem and increase evaporation. Also see https://journals.ametsoc.org/doi/full/10.1175/JCLI3519.1 
I have been dealing with evaporation equations for a while and I take the average of 5 equations (equations from Fitzgerald, Rohwer, Engineering Toolbox, Horton, Meyer) and here are some results: Suppose wind speed is 10 km per hour, RH=80% above the sea, sea temperature is 18 deg C and atmospheric pressure is 760 mm of mercury.
If we increase the air temperature above the sea we get this. 

Tair= 0 deg C then evaporation is 13 mm per day. 
Tair=10 deg C then evaporation is 9 mm per day. 
Tair= 20 deg C then evaporation is 1 mm per day. 
Tair=30 deg C then fog forms above water.

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