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FAQ: Albedo/Radiation Tuning in CESM-CICE

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FAQ: Albedo/Radiation Tuning in CESM-CICE

A very common question I get is how to tune the "albedo" in the new delta-Eddington radiation scheme. One actually tunes the inherent optical properties of the snow, bare ice, or pond surfaces via the r_snw, r_ice, and r_pnd parameters respectively. One can also adjust the snow melt onset temperature (dT_mlt) and the maximum snow grain radius (rsnw_mlt). These change the effective albedo in the same direction as the r_snw value by adjusting up or down by the number of standard deviations from observations. For example, here is a discussion of adjusting the snow grain radius.

The best way to think of all of these is a linear plot (attached). The x-axis is temperature and the y-axis is effective snow grain radius. First r_snw is used to determine the non-melting snow grain radius as follows:

rsnw_nonmelting = 500. - r_snw*250., where 100. <= rsnw_nonmelting <= rsnw_mlt

Where 500. um (microns) is the base value and 250. um is the standard deviation from the observations. So r_snw < 0, increases the dry snow grain radius and lowers the effective albedo of dry snow, while r_snw > 0 does the opposite. The r_snw parameter was designed to adjust the albedo of dry snow with the sign in the same direction. The r_ice and r_pnd behave similarly for the bare ice and ponded surfaces.

For the other two snow parameters, one can look at the attached plot. The linear curves have the form:
rsnw = rsnw_nonmelting + (rsnw_mlt - rsnw_nonmelting)*fT

where fT = -min(dTs/dT_mlt - 1, c0)

and dTs is Tmelt-Tsfc and is computed during the run. This is basically the temperature when the melt starts and the snow grains begin to grow in size, hence the albedo begins to drop. So, for dT_mlt = 1, when Tsfc is less than -1C then fT is zero and rsnw = rsnw_nonmelting. For Tsfc between -1C and 0C, the fT factor linearly increases from 0 to 1. The parameter rsnw_mlt is basically the maximum possible snow grain radius at 0C.

The namelist input parameters: r_snw, rsnw_mlt and dT_mlt have two defaults currently:

Solid linear curve: rsnw_mlt = 1000., dT_mlt = 1., r_snw = 1.75 (Used for most configurations with CAM5)
Long dash curve: rsnw_mlt = 1500., dT_mlt = 1.5, r_snw = 1.5 (Used for CAM4 configurations)
Short dash curve: rsnw_mlt = 2000.0, dT_mlt = 2., r_snw = 0.0 (Used in old T31 x gx3 configuration)



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