Improved key process in representing Arctic warming (D3.5)

Summary:

This Blue-Action task was focused on improving the representation of some of the most important
physical processes which contribute to Arctic warming within the climate models used by the
consortium. The two processes we addressed were the effect on the atmospheric state of the fracturing
of the sea ice cover and turbulence under strongly stable thermal stratification. The creation and
development of
The work done: We first analysed the results of previously performed large eddy simulations which
resolved the turbulence over leads to determine the effect leads have on sensible heat flux from open
water. Because of the effect of three-dimensional structures in the turbulent mixing above leads, the
heat flux coming from leads can be amplified compared to the fluxes one would get from open water
under the same air-sea temperature difference. The amplification effect strongly depends on the width
of the lead, with the largest effect occurring for leads of widths around 1.4 km. We assessed the
functional sensitivity of this amplification effect to key parameters used in the turbulence-resolving
model, including the length scale for the convective boundary layer, which reflects the background
stability in the atmosphere.
We combined this relation between the amplification effect of heat fluxes as a function of lead width
with observed distributions of lead widths. These were taken from the peer-reviewed literature where
The key findings: The presence of leads in sea ice dramatically alters the surface energy balance in the
Arctic. There is a large seasonal cycle to the effect of the presence of leads, because the flux from the
leads depends strongly on the background stability in the atmosphere. In the winter when the
atmosphere is often strongly stably stratified, the leads strongly amplify the surface sensible heat flux
coming from open water. In the summer there is the opposite effect and the generally weaker
atmospheric stability reduces the flux coming from leads.