Surface shortwave budget

[asiddi24]

Hi all,

I am attempting to close the surface shortwave budget, focusing on the Arctic. My goal is to eventually compute the total shortwave flux absorbed by the Arctic over the 21st century using the CESM2 LENS output. I am using the monthly mean terms.

1. I start by simply computing shortwave flux that gets absorbed over sea ice using

   FSWDN*(1-sialb)*aice = FSWABS*aice.

   These two do not match. The residual is of the order 10-30 % for different months.
2. When I investigate the same over the open ocean, I compute

   FSWDN*(1-ocean albedo)*(1-aice) = SHF_QSW*(1-aice),

   where I assume the ocean albedo to be 0.07. These two also do not match. The errors are of the order 10% which seems to be too high.

I am curious to know

1. Can the open ocean albedo not assumed to be 0.07 while performing these calculations?
2. If there is documentation of closing the surface shortwave budget over sea ice?

 

[dbailey]

Monthly means are tricky here and I don't believe sialb is available for CESM2. Not sure what you are using here. The best way to compute it is to do:

(Fsdwn-Fswabs) / Fswdn

or Fswup if it is available. This is the true broadband albedo. Ideally, daily data would be more accurate. The shortwave absorbed and albedo are computed in a two-stream radiative code and the albedo is not always averaged correctly for the grid cell. For example, issues with sun above the horizon, ice or snow not present, etc.

For the ocean, I think you have the right value. Does it balance precisely where aice = 0? There is again a daily versus monthly issue here.

 

[asiddi24]

I was using sialb for sea ice albedo. It is an output for the LENS2 data. My concern was that FSWABS was not matching FSWDN(1-sialb) and when I simply compare the albedo using FSWUP/FSWDN with sialb, there is a difference in the range of 0.1, which felt considerable.

I could simply proceed with using the albedo as FSWUP/FSWDN, if that is more accurate. I'm still curious to know if this difference in sea ice albedo as computed using FSWUP/FSWDN vs that given as output (sialb) has any consequences for analyzing, for instance the sea ice albedo feedback at the surface.

For the ocean, it doesn't balance precisely where aice=0. There is a residual of about 0-5 W/m2. Is the model computing SHF_QSW using a constant albedo value of 0.07 or is the ocean albedo a dynamical quantity depending on surface winds, zenith angle, etc. Would this difference of 0-5 W/m2 just be due to monthly averaging?

Finally,

 

[asiddi24]

I was using sialb for sea ice albedo. It is an output for the LENS2 data. My concern was that FSWABS was not matching FSWDN(1-sialb) and when I simply compare the albedo using FSWUP/FSWDN with sialb, there is a difference in the range of 0.1, which felt considerable.

I could simply proceed with using the albedo as FSWUP/FSWDN, if that is more accurate. I'm still curious to know if this difference in sea ice albedo as computed using FSWUP/FSWDN vs that given as output (sialb) has any consequences for analyzing, for instance the sea ice albedo feedback at the surface.

For the ocean, it doesn't balance precisely where aice=0. There is a residual of about 0-5 W/m2. Is the model computing SHF_QSW using a constant albedo value of 0.07 or is the ocean albedo a dynamical quantity depending on surface winds, zenith angle, etc. Would this difference of 0-5 W/m2 just be due to monthly averaging?

Finally,

I haven't tried running the model yet, but I'm excited to learn this year at the CESM tutorial. In the meantime, can you please point me to where can I look up how the model treats the ocean albedo or the sea ice albedo while computing these radiative fluxes? Thanks so much