Hello everyone, at AWI we were having yet another look at the SST biases of the coupled climate simulations made with OpenIFS 43r3 - FESOM2, but also at ECE3(IFS36r4 - NEMO3.6), and the ECMWF IFS43r1 - NEMO3.6 HighresMIP contribution. All of these models show the well known warm biases over the SO among other places. First we calculated the Spearman's rank correlation coefficient of the SST bias and the magnitude of the wind stress, which turned out to be significant and positive motivating further digging.
ECMWF IFS43r1 - NEMO3.6 HighresMIP SW cloud radiative forcing bias compared to OpenIFS 43r3 - FESOM2 ocean surface wind-stress:
In the higher latitudes (>30°) there seems to be a positive correlation, and in the lower latitudes a negative one. This is supported by the literature on the effect of surface roughness on surface albedo with varying latitudes. At low latitudes the rough surface decreases the solar incidence angle, leading to more reflection and higher albedo. At high latitudes the solar solar incidence angle is increased leading to lower albedo, as illustrated by the following graphic from [1]. This is underpinned by observed and computed values from [2].
We were then wondering if scaling of the ocean surface albedo with the wind speed might be too strong in the model. we had a look through the code and found a reference in surfrad_ctl_mod.F90 that such a scaling was added some 25 years ago:
PJANSSEN/JJMORCRETTE ECMWF 96/11/07 WIND DEPENDENT SEA ALBEDO
But the actual calculation in line 418:
ZAPTI1=MAX(0.037_JPRB/(1.1_JPRB*PMU0(JL)**1.4_JPRB+0.15_JPRB),REPALB)
does not include the wind intensity PWND, only the cosine of the incidence angle PMU0 considered. Was the formula added in 1996 moved to somewhere else? If so, where too? Or has it been dropped entirely? That would be bad news I guess, not only because the effect would be missing, but also because including it again would make things even worse in terms of radiations biases.
We also found a second body of literature that is concerned with breaking waves rather than non-breaking ones. These waves produce whitecaps that increase the direct and diffuse albedo in regions where storm systems move through frequently [3]. We where thinking of adopting such a scheme, assuming it is not yet part of OpenIFS.
Literature:
[1] https://doi.org/10.1029/JC090iC04p07313
[2] https://doi.org/10.1175/1520-0469(1972)029<0959:AOTSS>2.0.CO;2
[3] https://doi.org/10.5194/gmd-11-321-2018