Arctic Summer-time Cyclones: Dynamics and Sea-ice Interaction

As climate has warmed in response to increasing greenhouse gases, the distribution of Arctic sea ice has changed dramatically, becoming thinner over large portions of the Arctic Ocean basin in summer with a prominent reduction of the September minimum in sea ice extent. Human activity is increasing within the Arctic as the environment changes, with more residents and visitors making use of the increased window for shipping, offshore operations and tourism during summer. This has driven demand for coupled forecasts of weather, ocean and sea-ice state across the Arctic on the timescales needed to make risk-based decisions. Weather forecast skill for the Arctic is lower than for northern mid-latitudes, but the reasons why are multi-faceted and not fully known. Our hypothesis is that some aspects of the Arctic environment are not well forecast because the surface conditions beneath Arctic weather systems are more dynamic due to the movement of sea ice. Understanding of the physical processes that couple the atmosphere, ocean and sea ice is incomplete and the new frontier in prediction is to model this coupled system with fidelity and skill. Centres striving to improve capability in this area are our project partners: the Met Office, ECMWF and Met Norway.

Arctic cyclones are the dominant type of hazardous weather system affecting the Arctic environment in summer - thus a concern for all human activities. They can also have critical impacts on the Arctic environment: in particular on sea-ice movement, sometimes resulting in 'Very Rapid Ice Loss Events' (VRILEs - timescale days to weeks) which present a major challenge to coupled forecasts; and on the baroclinicity (temperature gradients) around the Arctic, influencing subsequent weather systems and forecasts of Arctic climate from weeks out to a season ahead.

Our proposed observational experiment will be the first focusing on summer-time Arctic cyclones and taking the measurements required to investigate the influence of sea-ice conditions on their development. New observations are needed comprising of turbulent near-surface fluxes of momentum, heat and moisture measured simultaneously with the sea ice or ocean surface beneath the aircraft track and along cyclone-scale transects. These fluxes dictate the impact of the surface on the development of weather systems. We will operate from Svalbard (Norway) in summer 2021, using the British Antarctic Survey's Twin Otter low-flying aircraft equipped to measure turbulence at flight level and the surface properties through infrared and lidar remote sensing.

Our US partners, have designed an observational experiment, called THINICE, looking downwards on Arctic cyclone structure from an aircraft flying above the tropopause (10 km). Our projects are co-designed for summer 2021 so that the observations from the Twin Otter will form a bridge between US airborne and satellite measurements above and the properties of the surface fluxes and sea ice beneath.

The project brings together expertise in observations, modelling and theoretical approaches to surface exchange, cyclone dynamics and sea-ice physics. We will use novel theoretically-based approaches to interrogate forecast models as they run and determine the mechanisms through which the surface properties alter cyclone growth. The new surface and turbulence data will be used to improve the parametrization of form drag in models that is central to wind forcing of sea-ice motion as well as decelerating surface winds. These aspects will be explored with state-of-the-art atmosphere and sea-ice dynamics models. Finally, we will close the loop through investigation of the effects of increased surface roughness on Arctic cyclones and their coupled interaction with Arctic temperature gradients. A major legacy of the project will be the unprecedented observations that will enable much needed evaluation and development of environmental forecast models for decades to come.

Forecast providers

The most direct impact of the project research will be at the centres engaged in operational forecasting for the Arctic and arise through four routes:

i) The chief legacy of the Arctic Summer-time Cyclones campaign will be the simultaneous measurements of turbulent fluxes in the atmospheric boundary layer and sea ice characteristics directly beneath the aircraft. Both components are needed to improve the representation of the exchange of momentum, heat and moisture above sea ice in models. Furthermore, the observations will be conducted in the vicinity of Arctic cyclones, extending model evaluation to a much wider set of atmospheric conditions than has previously been possible.

ii) Incorporation of the effects of sea ice roughness features, such as ice ridges, into the physical models of atmospheric drag above ice will inform forecast model improvement.

iii) Novel techniques developed by the project to infer weather forecast errors and their association with physical processes (such as turbulent mixing) offer a pathway to model improvement, via better model representations of these atmospheric processes.

iv) The effect of wind forcing on sea ice movement is highly uncertain and sea ice modelling at different levels of complexity will be used to evaluate the effects of sea ice representation on coupled environmental prediction forecasts.

Three leading forecast centres are project partners: the Met Office, ECMWF and Met Norway. The improved representation of atmospheric drag due to ice will be developed and tested within the Met Office prediction model framework as part of the project. The project presents a systematic approach to pull through from research into better weather forecasts for the Arctic region.

Other operational environmental prediction providers worldwide

The Arctic Summer-time Cyclones data archive will be constructed at the Centre for Environmental Data Analysis (CEDA) and will be open access to international collaborators from other forecasting centres. The archive will also be linked to the YOPP Data Portal hosted by Met Norway. This will increase the impact of the data set through its wider international use in evaluation of environmental prediction models.

The shipping and offshore industry operating in the Arctic

It is widely acknowledged that dramatically reduced ice cover in the Arctic will lead to increased shipping activity and offshore operations aiming to benefit from natural resources. This makes accurate forecasting of increasing economic importance. The project team will engage with these organizations through workshops planned by the WMO Polar Prediction Project. For example, a planned YOPP Summit in early 2022 brings together stakeholders in the Arctic with researchers and this is the most effective way to communicate new findings to the organisations that have the greatest stake.

Public engagement activities

The project will present a major opportunity to show the general public the excitement of atmospheric and polar science and how we can learn more about nature through aircraft-based observation. The project team will create a new exhibit on the science of Arctic Summer-time Cyclones with highlights of the field campaign. The intention is for the team to present this exhibit, alongside the British Antarctic Survey's Twin Otter aircraft, during the annual Battle of Britain Air Show (30,000 attendees). The exhibit could be re-used and refreshed on several occasions, drawing large crowds, and would be a new public engagement activity for BAS.