Latent Heat?

[metalicwx366]

What is latent heat and how does it play a role in forecasting?

[usedtobe]

What is latent heat and how does it play a role in forecasting?

Any time water chages phase it either takes up heat (evaporation) or releases it (condensation). In forecasting, evaporation can cool the column and is one of the reasons why you sometimes see precipitation start as rain and then have the temp fall with the precipitation then changing to snow. If enough warm advection is present the precipitation may then change back to rain.

The release of heat due to condensation is a big problem as the models cannot explicitly handle the latest heat production from the development of clouds and convection because the scale at which it occurs is so much smaller than the scale of the grid.

I have an article that is coming out this week on the Capital Weather Gang website on Tuesday. Once it has run, I'll come back and try to answer your question in more detail. If I did it now, I might run into some copyright issues with that article. Once the article has been published, I can then reproduce or post parts of it anywhere I want.

[dendrite]

As Wes said...there is latent heating/cooling with every phase change of water. Anytime the change is to a higher energy state it takes energy from the atmosphere and causes latent cooling of the air. When the phase is to a lower energy state the atmosphere gains the energy and there is latent heating of the air.

These phase changes cause latent cooling of the air:

Melting (solid to liquid)...weakest cooling

Evaporation (liquid to gas)

Sublimation (solid to gas)...strongest cooling

And these 3 cause latent heating of the air:

Freezing (liquid to solid)...weakest heating

Condensation (gas to liquid)

Deposition (gas to solid)...strongest heating

[kylemacr]

This is a pretty nifty diagram that puts the above post into a graphic.

http://santasusana.org/pakelly/ES9CP/02b_water_phase_change.gif

Another huge impact of latent heat release occurs in the tropics. Tropical systems have lots of convection which often releases massive amounts of heat (energy) into the atmosphere. This process can "build a ridge" which typically results in a Rossby wave train (alternating high and low pressure systems that begin in the tropics and move Northeast). Because lots of tropical convection occurs in the Pacific ocean (due to MJO, Kelvin waves, etc.), our weather in the U.S. is very much affected by Rossby wave trains that originate in the Pacific. As was mentioned above, the models have a lot of trouble with this because their ability to simulate convection leaves a lot to be desired.

In a forecast, this is a big problem... especially a week or longer forecast. If the model can't simulate the latent heat release in the tropics correctly, it likely will not have a proper Rossby wave train, and that can cause significant problems for forecasting weather over the U.S.

[isohume]

This is a pretty nifty diagram that puts the above post into a graphic.

http://santasusana.o...hase_change.gif

Another huge impact of latent heat release occurs in the tropics. Tropical systems have lots of convection which often releases massive amounts of heat (energy) into the atmosphere. This process can "build a ridge" which typically results in a Rossby wave train (alternating high and low pressure systems that begin in the tropics and move Northeast). Because lots of tropical convection occurs in the Pacific ocean (due to MJO, Kelvin waves, etc.), our weather in the U.S. is very much affected by Rossby wave trains that originate in the Pacific. As was mentioned above, the models have a lot of trouble with this because their ability to simulate convection leaves a lot to be desired.

In a forecast, this is a big problem... especially a week or longer forecast. If the model can't simulate the latent heat release in the tropics correctly, it likely will not have a proper Rossby wave train, and that can cause significant problems for forecasting weather over the U.S.

This is true. Better proxies are needed and new ones are being investigated all the time. Latent heat is parameterized in the NWP models and in research studies by lightning, precip, llvl moisture adv, BLI, etc. The new GOES-R will greatly aide in identifying areas of CC and CG lightning and llvl moisture fluxes which should increase modeling convection skill, especially across sparse data zones such as oceans.

[Amped]

When large volumes of water condense a lot of Latent Heat is released.

Thats what the term commonly refers to on this board.

We usually say evaporation cooling instead of "Latent Heat Absorbed" Except for TyphoonTip who digs deep into his Thesaurus to confuse people..

[isohume]

When large volumes of water condense a lot of Latent Heat is released.

Thats what the term commonly refers to on this board.

We usually say evaporation cooling instead of "Latent Heat Absorbed" Except for TyphoonTip who digs deep into his Thesaurus to confuse people..

When gas condenses into a liquid...not when water condenses, there is latent heating of the surrounding air.

[Amped]

When gas condenses into a liquid...not when water condenses, there is latent heating of the surrounding air.

Yeah thats what I meant. whoops.

[usedtobe]

This is a pretty nifty diagram that puts the above post into a graphic.

http://santasusana.o...hase_change.gif

Another huge impact of latent heat release occurs in the tropics. Tropical systems have lots of convection which often releases massive amounts of heat (energy) into the atmosphere. This process can "build a ridge" which typically results in a Rossby wave train (alternating high and low pressure systems that begin in the tropics and move Northeast). Because lots of tropical convection occurs in the Pacific ocean (due to MJO, Kelvin waves, etc.), our weather in the U.S. is very much affected by Rossby wave trains that originate in the Pacific. As was mentioned above, the models have a lot of trouble with this because their ability to simulate convection leaves a lot to be desired.

In a forecast, this is a big problem... especially a week or longer forecast. If the model can't simulate the latent heat release in the tropics correctly, it likely will not have a proper Rossby wave train, and that can cause significant problems for forecasting weather over the U.S.

But it even impacts shorter range forecast. If you get the latent heating wrong with an extratropical low it may end up being either weaker or stronger than forecast. That can impact upon the riding to the east of the storm because of the changes to the advection patterns which can impact the amount or warming that is taking place which modulates the shortwave ridge. Shift the latent heating a little too far east and you might pull a coastal farthr east than forecast. Near the east coast, small differences can make big differences in the sensible weather.