Atmospheric Rivers and The Onset of Sea-Ice Melt (ARTofMELT)

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Arctic climate is warming much faster than that of the world as a whole. Global models show the general trend of enhanced warming, but generally fail to reproduce the observed dramatic reduction in sea ice. This results in part from significant biases in the surface energy budget. The largest source of uncertainty in the energy budget results from problems in the representation of low level clouds and the resulting solar and infra-red radiation fluxes at the surface, and of turbulent heat fluxes within the near-surface atmosphere. The model biases are greatest under the extreme conditions of strong forcing by warm air intrusions - regional events where warm, humid air is transported in over sea ice, cooling close to the surface to form shallow, stably stratified layers within which fog and cloud form. The shallow layers and steep vertical gradients of temperature, humidity, and turbulence are difficult for global models to resolve. Their evolution with increasing time over the sea ice, as the vertical thermodynamic profiles, cloud liquid and ice water contents, and turbulent structure change, depends strongly on small scale processes that must be parameterized within the models. Existing parameterizations are based on measurements at lower latitudes, and fail to adequately represent polar conditions. The resulting model biases can be 100s of W/m2 during warm air intrusions.ARTofMELT is the first observational study focussed on warm air intrusions. Support for an international project, proposed by our partners at Stockholm University, has been approved by the Swedish Polar Research Secretariat, and a research cruise is scheduled for spring 2023 on the Swedish icebreaker Oden. This provides a unique opportunity to make the measurements needed to provide new understanding of the processes controlling air mass modification during these events, and develop new parameterizations for global models appropriate to polar environments. Building on several previous collaborative projects we will work as a single team with our partners in Stockholm to instrument Oden for extensive in situ and remote sensing measurements of the lower atmosphere, clouds, and the components of the surface energy budget. Directed by dedicated forecasts made at ECMWF, Oden will be positioned to sample warm air intrusions entering the Arctic from the Atlantic or Eurasian sectors. We anticipate sampling 3-5 events during the cruise, and use the observations from outside the event periods as a baseline to evaluate the impact of warm air intrusions on lower-atmosphere structure, clouds, and the surface energy budget. Other participants will assess the response of sea ice to the atmospheric forcing.