Spatio-temporal variable drag for the sub-ice-shelf melt parameterisation in NEMO, ocean model

Ice-shelf basal melting in NEMO, as in most ocean models, is parameterised based on a friction velocity calculated from a drag coefficient that is constant in space and time, usually tuned to approach observed melt. But the drag between the ice and the ocean should depend on the roughness at different scale. This means that roughness evolution in space and time is not taken into account in today's model. In recent decades, some ice shelves, particularly in the Amundsen Sea Embayement (ASE), have experienced an increase of their damage, associated with more surface and basal crevasses so their sub-shelf environment is rougher. There is good chances that this phenomenon is to happen more in the future and in an extended number of ice shelves. Here we present a study using a spatially variable coefficient of drag, which depends on the topography and is applied on the first wet cell height. We use the ice shelf parameterisation of NEMO4.2 on a configuration of ASE at 12th of a degree.