Dynamics and predictability of polar lows

Polar mesoscale cyclones occur frequently in the subpolar seas and the most violent, termed polar lows, can pose a significant hazard to life and livelihoods (Renfrew, 2014). Their short life-times and relatively small scales make them difficult to forecast even with modern high-resolution Numerical Weather Prediction models and sophisticated data assimilation schemes. Individual polar lows can now be simulated with reasonable accuracy in hindcast mode (e.g. McInnes et al. 2011). However the number of polar low case studies with high quality verification data is still not large, so further studies are important. Experimental high-resolution regional Ensemble Prediction Systems (EPS) provide some prospects for improving their predictability. Indeed the first studies have had some success, but there are significant concerns too. The EPS domain position and grid-size are critical, the results are sensitive to physical parameterizations, and the ensemble spread is typically not large enough (Kristiansen et al. 2011; Aspelien et al. 2011).

In this project, an investigation of both the dynamics and the predictability of polar lows will be carried out. The study will focus on the winter of 2012/13 which was an extremely active year for polar lows in the Norwegian Sea. For one polar low, from 26 March 2013, we have comprehensive aircraft observations made as part of the ACCACIA project. This polar low developed on a horizontal shear line, south of Svalbard, as part of a prolonged outbreak of polar mesoscale cyclone activity; and is believed to be the best observed of such shear-line polar lows. Intuitive examination of forecasts during the event, suggested that the upstream presence of Svalbard in the northerly flow played a role in determining the location and strength of the shear line. This hypothesis will be tested via idealised and case study NWP simulations. The UK Met Office’s Unified Model (MetUM) could be employed for these idealised and case study simulations, with experiments to modify the orography for example (e.g. following Harden and Renfrew 2012, Kristjansson et al.  2011).

 This winter also saw the first year of Met.no’s new operational regional EPS system. Forecaster could interactively select a domain for the EPS system. Strike probability maps were routinely generated and available to forecasters operationally. An examination of the performance of this EPS system for this case study and also the winter of 2012/13 will be undertaken in collaboration with Met.no. There may be the possibility of sensitivity tests to be run the Norwegian EPS system, testing some of the known sensitivities of the forecasting system.

References:
Aspelien, T, T. Iversen, J. B. Bremnes, I.-L. Frogner, 2011: Short-range probabilistic forecasts from the Norwegian limited-area EPS: long-term validation and a polar low study, Tellus, 63A, 564-584.
Harden, B.E. and I.A. Renfrew, 2012: On the spatial distribution of high winds off southeast Greenland, Geophys. Res. Lett.,39, L14806, doi:10.1029/2012GL052245.
KRISTIANSEN, J., SØRLAND, S. L., IVERSEN, T., BJØRGE, D. and KØLTZOW, M. Ø. (2011), High-resolution ensemble prediction of a polar low development. Tellus A, 63: 585–604. doi: 10.1111/j.1600-0870.2010.00498.x
Kristjánsson, J. E., Thorsteinsson, S., Kolstad, E. W. and Blechschmidt, A.-M. 2011: Orographic influence of east Greenland on a polar low over the Denmark Strait. Q.J.R. Meteorol. Soc., 137: 1773–1789. doi: 10.1002/qj.831
McInnes, H., Kristiansen, J., Kristjánsson, J. E. and Schyberg, H. 2011: The role of horizontal resolution for polar low simulations. Q.J.R. Meteorol. Soc., 137: 1674–1687. doi: 10.1002/qj.849
Renfrew, I.A. 2014: Polar lows, Encyclopedia of the Atmospheric Sciences, Second Edition, Academic Press.