683 / 2024-09-19 02:05:11

On the early detection of Polar Lows: possible criteria analysis and case studies

Polar lows,identification criteria,numerical simulation,helicity,kinematic vorticity number

The Arctic is a vital component of the Earth's climate system. Arctic weather formation and effect on air mass transport issues are of increasing relevance in the context of a changing climate. One of the most important extreme weather systems – Polar Lows (PL), – mainly formed in the Arctic region that are able to cover about 100 km during the period of their existence. Besides Scandinavia, over the Barents Sea, Laptev Sea, East Siberian Sea, which traverses by the famous Northeast Passage, we noticed a quite common formation of PL as well. Early detection of PL is impeded by the lack of forecasting efficiency and the sudden, explosive nature of PL formation and very rapid development. Nevertheless, a common versatile method for the automatic identification of PL is still being searched.

We consider the possible prognostic criteria of the PL, in particular, helicity as a squared characteristic related to integral vortex formations and kinematic vorticity number using use reanalysis data and the findings of numerical modeling in the WRF-ARW model for such a calculation, FGOALS model data for diagnostics and analysis, experimental data to verify the results.

The local minima of the geopotential correlate well with local maxima of helicity during the PL. Local changes in helicity are adjacent to the front of the PL. One day before the formation of PL, a significant increase of helicity was observed clearly. The average helicity density of large-scale motions have the values of 0.3–0.4 m/s². Kinematic vorticity number is the PL size and intensity additional indicator. Maximum intensity of PL matches the kinematic vorticity number can reach values of 12–14 units.

The helicity criterion described the identification of the PL genesis area whereas the advantage of using kinematic vorticity number is in its sharp change directly in the centre of the emerging PLs, which allows to precisely indicates the limits of the most intense part of PL and thus the additional indicator of PL size and intensity.

The challenge is to make the routine forecast of PL available through the selection of the simple prognostic integral characteristics, sufficient for PL’s size and intensity detection in a usable, understandable and convenient way for a short time. The criteria described above are reflected in the PLs genesis and at the same time should be tested using a larger set of cases extending these analyses to other active PL basins.