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How to calculte melt pond fraction from CCSM4-CICE output data

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jyangdas@...
How to calculte melt pond fraction from CCSM4-CICE output data

In the CCSM4-CICE output, there are apond001, apond0002, ..., apond005 and aicen0001, aicen0002, ..., aicen005. I find that aicen001 + aicen002 + ... + aicen005 is the total ice fraction (aice). But, when I sum the apond001 + apond002 +...+ apond005, it is up to 190%, also I have try another way: (apond001*aice001 + apond002*aice002 + .... + apond005*aice005)/100.0, it is only about 8% for north summer. I guess both of the two ways are wrong. Does it have any userguide for this calculate. Thanks very much.

Jun Yang

University of Chicago

jyangdas@...

How to calculate the sea-ice/snow albedo? In the output, there are albice, albsno and albpnd, but as I sum them, the maximum value is only about 0.4, which is too low. So I believe this way is not correct.
Thanks very much.

Jun Yang

University of Chicago

dbailey

I realize this is not documented terribly well. The pond fraction for each category, i.e. apond00n is exactly as it sounds. It is the fraction of ponds in that ice category. So, to get the pond fraction on the ice covered portion of the cell it is:

aice001*apond001 + aice002*apond002 + ... + aice005*apond005

To get a pond fraction relative to the whole gridcell, you would then divide by aice. In 1850 control integrations we see very little pond coverage in the Arctic. This is because most of the snow is sticking around through the summer. In the latter part of the 20th century runs we get better fractions of up to 0.2 in the Beaufort Sea. It is usually still low in the central Arctic. There is some internal tuning that can be done here. The runoff fraction (rfrac) is one of the main things that improves pond coverage. Currently it is set to a minimum of 15% for aice=~1, and a max of 85% for aice=~0. The other tuning in ice_meltpond.F90 is how much of the ponds affects the albedo when there is snow in the gridcell. When the snow depth exceeds 3cm, the ponds do not show through at all.

Dave

dbailey

Those albedos (albpnd, albice, albsno) are buggy and I haven't had time to fix them. If you want the broadband albedo for the sea ice covered portion of the cell:

alb = aice*fswup/fswdn

I guess you do need to divide aice by 100 in this and the pond calculations.

Dave

jyangdas@...

Hi Dave,

Frist, thanks very much. Your reply is very useful for me.

Does your paper, which includes the analysis of melt pond have been published? Now, I am analyzing the impact of melt pont on the initiation of a "Snowball Earth". I am so very troubled to cannot find some references except the Technical note of Briegleb and Light.

Thanks again.

Jun

Jun Yang

University of Chicago

dbailey

The manuscript has bee submitted for the CCSM4 special issue.

Dave

jyangdas@...

dbailey wrote:

I realize this is not documented terribly well. The pond fraction for each category, i.e. apond00n is exactly as it sounds. It is the fraction of ponds in that ice category. So, to get the pond fraction on the ice covered portion of the cell it is:

aice001*apond001 + aice002*apond002 + ... + aice005*apond005

To get a pond fraction relative to the whole gridcell, you would then divide by aice. In 1850 control integrations we see very little pond coverage in the Arctic. This is because most of the snow is sticking around through the summer. In the latter part of the 20th century runs we get better fractions of up to 0.2 in the Beaufort Sea. It is usually still low in the central Arctic. There is some internal tuning that can be done here. The runoff fraction (rfrac) is one of the main things that improves pond coverage. Currently it is set to a minimum of 15% for aice=~1, and a max of 85% for aice=~0. The other tuning in ice_meltpond.F90 is how much of the ponds affects the albedo when there is snow in the gridcell. When the snow depth exceeds 3cm, the ponds do not show through at all.

Dave

Hi Dave,

I have carefully considered the calculations for melt ponds, and find that there is an error in our previous discussion.

In the output data, aice001, apond001, ..., aice are in unit of %. And,
******************
Relative to the whole grid cell, the melt pond fraction (in unit of %) should be:

(aice001*apond001 + aice002*apond002 + ... + aice005*apond005)/100.0*(aice*0.01)
=> it multiplies by aice, rather than divides by aice.

Is it right?

**********************
By the way, relative to the sea-ice covered area, the melt pond fraction (in unit of %) should be:

(aice001*apond001 + aice002*apond002 + ... + aice005*apond005)/100.0
**********************

Thanks very much.

Jun Yang

University of Chicago

dbailey

Each of the aice00n are just the fraction of ponds within each ice category. So, to get the total pond fraction on the ice covered portion of the grid cell:

apond = aice001*apond001 + aice002*apond002 + ... + aice00n*apond00n / (100*100)

This will give you fraction. All of these quantities are in percent on the history file. You can remove one of the 100s to get percent of ponds covering the ice. To get this relative to the whole grid cell, you need to divide by aice/100. I believe I forgot that everything is in percent in the last post.

Dave

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