Surface Heat and Moisture Exchange in the Marginal Ice Zone: Observations and a New Parameterization Scheme for Weather and Climate Models

Aircraft observations from two Arctic field campaigns are used to characterise and model surface heat and moisture exchange over the marginal ice zone (MIZ). We show that the surface roughness lengths for heat and moisture over unbroken sea ice vary with roughness Reynolds number (R_✶; itself a function of the roughness length for momentum, z₀, and surface wind stress), with a peak at the transition between aerodynamically smooth (R_✶<0.135) and aerodynamically rough (R_✶>2.5) regimes. The conceptual model of Andreas (1987) accurately reproduces this peak, in contrast to the simple parameterizations currently employed in two leading numerical weather prediction models, which are insensitive to R_✶. We propose a new, simple parameterisation for surface exchange over the MIZ that blends the Andreas (1987) conceptual model for sea ice with surface exchange over water as a function of sea ice concentration. In offline tests, this Blended A87 scheme performs much better than the existing schemes for the rough conditions observed during the ‘IGP’ field campaign. The bias in total turbulent heat flux across the MIZ is reduced to only 13 W m^-2 for the blended A87 scheme, from 48 and 80 W m^-2 for the Met Office Unified Model and ECMWF Integrated Forecast System schemes, respectively. It also performs marginally better for the comparatively smooth conditions observed during the ‘ACCACIA’ field campaign. However, the benefit of this new scheme is dependent on the representation of sea ice topography via z₀; a key remaining source of uncertainty in surface exchange parameterisation over sea ice.