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. is the minimum value and rsnw_mlt is the maximum value.
Melt occurs based on the following formula:
dTs = Tmelt - Tsfc
fT = -min(dTs/dT_mlt-1,c0)
rsnw = rsnw_nonmelting + (rsnw_mlt - rsnw_nonmelting)*fT
This is the straight line in the attached plot, where rsnw = rsnw_nonmelting below -dT_mlt and then linearly increases until it reaches a maximum of rsnw_mlt. So, one can adjust the snow grain radius values at the end points, or the temperature at which the snow grain radius begins to increase.
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. is the minimum value and rsnw_mlt is the maximum value.
Melt occurs based on the following formula:
dTs = Tmelt - Tsfc
fT = -min(dTs/dT_mlt-1,c0)
rsnw = rsnw_nonmelting + (rsnw_mlt - rsnw_nonmelting)*fT
This is the straight line in the attached plot, where rsnw = rsnw_nonmelting below -dT_mlt and then linearly increases until it reaches a maximum of rsnw_mlt. So, one can adjust the snow grain radius values at the end points, or the temperature at which the snow grain radius begins to increase.
Last edited: