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Author Topic: When is Radiation Breakeven?  (Read 1198 times)

JimboOmega

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When is Radiation Breakeven?
« on: March 26, 2018, 07:55:10 PM »
Assume an area of open ocean, under a clear sky, at the freezing point of the water, with no interaction from the atmosphere, currents, etc.

At what amount of insolation does it breakeven - in the sense of losses to space are the same as inbound solar radiation?

Ideally I'd like some sort of general way to figure out what day of the solar year this occurs at a given latitude.

The motivation for the question is - in the winter, water opening up as Polynya causes the system to (overall) lose more heat to space and leads to more ice forming.  In the summer, the opposite is true, as expanses of open water absorb more sunlight than they lose heat to space. In the spring, like right now, I don't really have any idea.


Alexander555

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Re: When is Radiation Breakeven?
« Reply #1 on: March 26, 2018, 08:18:19 PM »
Maybe you can use this as an example. The area north of Svalbard has a 2 m above ground temperature of -10 C .And the sea surface temperature is something like 3 C . And i think it's already open for the entire winter.

sidd

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Re: When is Radiation Breakeven?
« Reply #2 on: March 26, 2018, 08:40:40 PM »
An interesting paper is Perovich on the albedo and radiant balance of sea ice. Open access.

doi: 10.5194/tc-2018-47

sidd

Tealight

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Re: When is Radiation Breakeven?
« Reply #3 on: March 27, 2018, 10:51:58 PM »
JimboOmega you can't get an exact answer to your question because even clear sky has varying amounts of greenhouse gases in it. Especially water vapor dramatically increases backward radiation.  At the same time ocean water will always evaporates and cool the surface.

When I developed my surface radiation model and estimated greenhouse gas back radiation I calculated the break-even dates based solely on incoming and outgoing surface radiation. But the atmosphere absorbs incoming radiation as well, so you would have to move the dates forwards and backwards by a several days up to a week.

60N: 13 April - 27 Aug
65N: 21 April - 19 Aug
70N: 28 April - 12 Aug
75N: 4 May - 6 Aug
80N: 7 May - 3 Aug
90N: 9 May - 2 Aug

Edit: I estimated the back radiation for the dry central Arctic. Regions close to the humid Atlantic and the Pacific will likely reach break even point earlier.

Maybe you can use this as an example. The area north of Svalbard has a 2 m above ground temperature of -10 C .And the sea surface temperature is something like 3 C . And i think it's already open for the entire winter.

This has nothing to do with incoming solar radiation, just how much heat the ocean loses to the air. Over the ice it is at around -30C and after a around 100 kilometer over water it has warmed to -10C.
« Last Edit: April 08, 2018, 06:06:55 PM by Tealight »

Sterks

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Re: When is Radiation Breakeven?
« Reply #4 on: March 28, 2018, 12:07:31 AM »
In practice, the 1st of May for the coasts of the Arctic Ocean proper, from Beaufort sea to Kara sea clockwise, plus minus 10 days

FishOutofWater

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Re: When is Radiation Breakeven?
« Reply #5 on: March 28, 2018, 02:21:44 PM »
In practice the Bering strait is rapidly losing ice right now and the Beaufort and Chukchi are not going to be far behind. There's a reason some of us have been so concerned about the advection of water vapor from the Pacific into the Arctic this winter. Water vapor is a powerful ghg. The Alaskan side of the Arctic ocean is further south than the other shores of the Arctic so it is impacted first by spring insolation.