Evaluation of errors in bulk aerodynamic parameterizations over snow-covered sea ice due to approximations of roughness length

Direct measurements using eddy covariance methodology of turbulent heat and momentum fluxes were observed from the Met City tower during MOSAiC. In models, these fluxes are parameterized using bulk aerodynamic algorithms for which the transfer coefficients must be found. In practice, the coefficients are constrained iteratively to resolve the co-dependence between Obukhov Length (necessary for calculating the coefficients) and the friction velocity, u* (an expression of the Reynold’s stress solved for by the algorithm). The aerodynamic roughness length, z0, is also needed to calculate the coefficients. For calculations over the global ocean, z0 is coupled to the atmosphere through u*. However, over sea ice the meteorological correlation is weak and the physical surface roughness is heterogeneous and decoupled from the atmosphere. This introduces a vulnerability into the calculation and necessitates assumptions about z0. At MOSAiC, the tower measurements of z0 show an evolution from early- to late-winter of nearly 2 orders of magnitude where 1 order nominally corresponds to a 30-40% differences in the derived fluxes. In this presentation we evaluate the error in bulk calculations due to the z0 assumption to assess what could be gained from a surface aware scheme.