Dear David,
Thanks for that response. I think your proposal needs airing more
widely, so we should go back to the geoengineering list. Cirrus cloud
removal could be very effective in the autumn/fall. But I also want to
expand further on my point 2:
/"2. Is there more OLR (out-going longwave radiation) from open water
than from sea ice or snow, when both are at freezing point? Aha! Here's
the answer - there's very little difference [1] - they both have very
close to black body radiation."/
You point out that mostly the water is warmer than the ice, so there's a
difference in practice. But I realised after I'd sent the email that
I'd missed something perhaps even more important. The thermal radiation
from the ice cools the surface of the ice but the ice acts as an
insulator to slow the heat transfer from lower layers of the ice. This
is why ice builds up only very slowly on the underside of existing sea
ice. On the other hand, thermal radiation from the water is not just
from the surface, but from the top layer - a few centimetres thick as
water is a strong absorber [2]. Furthermore, there can be cooling by
convection, so the heat transfer can take place quickly compared to the
heat transfer within ice. A thin layer of ice may then form, which
immediately slows the outgoing radiation until the ice is melted from
below.
I'd not thought of this when considering the albedo flip effect. As the
sea ice retreats there is stronger absorption of insolation (a positive
feedback) but also greater thermal radiation (a negative feedback).
As the sea ice starts to reform after the September minimum, the open
water is critical for maintaining OLR. So one way of cooling the Arctic
would be to clear away sea ice as soon as it forms! If at the same
time, you scoop up the new ice and put it onto old ice, you've thickened
the multiyear ice which will slow the retreat during the next spring.
I'm sorry this has got off the subject of cooling using cirrus clouds,
but it points to another method of cooling the Arctic. I don't know if
much thought has been given to either method in the geoengineering
community.
Cheers,
John
/[1] http://www.engineeringtoolbox.com/radiation-heat-emissivity-d_432.html
/
[2] http://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water
---
On 06/07/2011 05:35, David Mitchell wrote:
Dear John - Thanks for your email; see my comments below.
Best wishes,
David
----- Original Message -----
From: John Nissen <[email protected]>
Date: Tuesday, July 5, 2011 14:41
Subject: Re: [geo] cirrus cloud climate engineering
To: [email protected]
Cc: Andrew Lockley <[email protected]>, [email protected],
"P. Wadhams" <[email protected]>, John Nissen <[email protected]>
>
> Dear David,
>
> This is a very interesting possibility for cooling the Arctic -
neither SRM nor CDR type geoengineering but TRM - thermal radiation
management.
>
> 1. How much cirrus is there at high latitide (Arctic) in autumn,
winter and spring? Presumably in summer, when sun is shining
continuously, the reflection of sunshine by the cloud overrides the
reflection of infrared thermal radiation from the surface back to the
surface.
>
I don't know the answer to this off-hand, but the data that could
answer it may exist in the DOE ARM data archive (i.e. it may involve a
mini-research project). However, someone may have already researched
this; I'll try to find out. If GCMs try to model this approach, such
knowledge may provide a good sanity check.
> 2. Is there more OLR (out-going longwave radiation) from open water
than from sea ice or snow, when both are at freezing point? Aha!
Here's the answer - there's very little difference [1] - they both
have very close to black body radiation.
>
Yes, the radiation component of heat loss is a black-body problem, but
there is also latent heat loss. As the ice retreats and there is more
open water, water near 0 C is relatively "hot" compared to
surroundings in winter, with stong thermals lofting H2O vapor which
can condense as water or ice in clouds, releasing considerable heat.
Two talks at the IUGG meeting proposed that the last two anomalously
cold winters at northern higher latitudes might be due to this "extra"
water vapor that can then fuel storms, putting more snow on the ground
at these latitudes.
> 3. If the Arctic Ocean is warming, then its important to let this
cool by radiation - hence removing OLR-reflecting clouds would be
important in the Autumn while ice continues to retreat but insolation
is weakening.
>
Yes.
> BTW, I envisage a combination of methods will be provide greatest
assurance of success in cooling the Arctic - stratospheric aerosols,
marine cloud brightening (especially to cool water from Gulf Stream
entering the Arctic) and perhaps your method.
>
> Kind regards,
>
> John
>
> [1]
http://www.engineeringtoolbox.com/radiation-heat-emissivity-d_432.html
>
> ---
>
> On Tue, Jul 5, 2011 at 2:46 PM, David Mitchell
<[email protected] <javascript:main.compose('new',
'[email protected]')>> wrote:
> Dear Andrew and climate engineering group,
>
> The idea Andrew mentioned is described in the attached
Environmental Research Letters paper. The new material presented
at the conference was that new measurements in cirrus clouds show
homogeneous freezing nucleation is very important for synoptic
cirrus clouds (T < -40 C), meaning that the highest, coldest
clouds that you would want to affect for climate engineering
indeed appear very susceptible for modification. There was also a
suggestion to seed only the polar and mid-latitude cirrus clouds
to leverage snow-ice albedo feedback to enhance cooling.
>
> Cheers,
> David Mitchell
>
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