A new, simple parameterisation scheme for scalar (heat and moisture)
exchange over sea ice and the marginal ice zone is tested in a numerical
weather and climate prediction model. This new “Blended A87”
scheme accounts for the influence of aerodynamic roughness on the
relationship between momentum and scalar exchange over consolidated sea
ice, in line with long-standing theory and recent field observations,
and in contrast to the crude schemes currently operational in most
models. Using aircraft observations and Met Office Unified Model
simulations of cold-air outbreak (CAO) conditions over aerodynamically
rough sea ice, we demonstrate striking improvements in model performance
when the Blended A87 scheme replaces the model’s operational
treatment for surface scalar exchange, provided that the aerodynamic
roughness over consolidated ice is appropriately prescribed. The mean
biases in surface sensible heat flux, surface latent heat flux,
near-surface air temperature and surface temperature reduce from 25 to
11 W m-2, 22 to 12 W m-2, 0.8 to 0.0
K, and 1.4 to 0.8 K, respectively. We demonstrate that such impacts on
surface exchange over sea ice can have a marked impact on the evolution
of the atmospheric boundary layer across hundreds of kilometres downwind
of the sea ice during CAO conditions in the model. Our results highlight
the importance of spatiotemporal variability in the topography of
consolidated sea ice for both momentum and scalar exchange over sea ice;
accounting for which remains a challenge for modelling polar weather and
climate.