Improved key process in representing Arctic warming (D3.5)
Creators: name: Davy, Richard
name: Gao, Yongqi
name: Gastineau, Guillaume
name: Tian, Tian
name: Zhang, Ying
Publisher: name: Zenodo
Year: 2020

description: In the work presented in this deliverable the Blue-Action teams focused on improving the representation<br> of some of the most important physical processes which contribute to Arctic warming within the climate<br> models used by the consortium. The two processes we addressed were the effect on the atmospheric<br> state due to the presence of leads in the sea ice cover and turbulence under strongly stable thermal<br> stratification. <strong>The work done:</strong> We first analysed the results of previously performed large eddy simulations which<br> resolved the turbulence over leads to determine the effect leads have on sensible heat flux from open<br> water. Because of the effect of three-dimensional structures in the turbulent mixing above leads, the<br> heat flux coming from leads can be amplified compared to the fluxes one would get from open water<br> under the same air-sea temperature difference. The amplification effect strongly depends on the width<br> of the lead, with the largest effect occurring for leads of widths around 1.4 km. We assessed the<br> functional sensitivity of this amplification effect to key parameters used in the turbulence-resolving<br> model, including the length scale for the convective boundary layer, which characterizes the background<br> stability in the atmosphere. We combined this relation between the amplification effect of heat fluxes as a function of lead width<br> with observed distributions of lead widths. These were taken from the peer-reviewed literature.<br> Together, this gives us a scheme to describe how the presence of leads affects the surface sensible heat<br> flux, and this depends upon the concentration of sea ice and the background atmospheric stability. We<br> implemented this scheme in four climate models (NorESM, EC-Earth3, IPSL/ LMDZ6A and CAS-ESM/<br> IAP4) and tested the scale of the effect using multiple single-model ensemble simulations of historical<br> climate. <strong>The key findings: </strong>The presence of leads in sea ice dramatically alters the surface energy balance in the<br> Arctic. There is a large seasonal cycle to the effect of the presence of leads, because the flux from the<br> leads depends strongly on the background stability in the atmosphere. In the winter when the<br> atmosphere is often strongly stably stratified, the leads greatly amplify the surface sensible heat flux<br> coming from open water. In the summer there is the opposite effect and the generally weaker<br> atmospheric stability reduces the flux coming from leads. The net effect is to increase near-surface<br> temperatures over sea ice in winter, with little or no change in the summer. Therefore this scheme may<br> be used to address the long-standing winter cold bias over ice in many contemporary global climate<br> models, as shown in CMIP6. <strong>Linked Zenodo records: Guidelines for improving the representation of surface heat flux in the</strong> Arctic https://zenodo.org/record/4728073
description: The Blue-Action project has received funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement No 727852.