Implementation of form drag scheme into NEMO sea ice model SI3

The efficiency of air-sea momentum depends on top and bottom sea ice surface roughness which varies with ice types and conditions, but constants are applied in most climate models. Future sea ice reduction will entail an increase in efficiency of air-sea momentum transfer. A high physical process fidelity will be a requirement for realistic model predictions. Within the CANARI project (Climate change in the Arctic-North Atlantic Region and Impacts on the UK) the form drag scheme from the sea ice model CICE is implemented into the NEMO sea ice model SI³. Based on parameters of the ice cover such as ice concentration, size, and frequency of the sails and keels, freeboard and floe draft, and size of floes and melt pond fraction, the total form drag can be computed as a sum of form drag from sails and keels, form drag from floe edges, form drag from melt pond edges, and a reduced skin drag due to a sheltering effect. Historical simulations are presented discussing the impact on sea ice dynamics and mass balance separating the contributions from modified momentum and heat transfer.