Physics of auroral phenomena : proceedings of the 36th Annual seminar, Apatity, 26 February – 01 March, 2013 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2013. - 215 с. : ил., табл.

Simulation study o f the mechanism o f theformation o fpolar mesoscale cyclones at high latitudes o f the northern hemisphere It is known from observations that, in an arctic front, a zonal flow of air is westward at more northern latitudes relatively to the centerline of an arctic front. On the contrary, a zonal flow of air is eastward at more southern latitudes relatively to the centerline of an arctic front. In an arctic front, a meridional wind velocity directs towards the centerline o f an arctic front at altitudes less than approximately 2,5 km and directs from the centerline of an arctic front at levels higher than approximately 2,5 km. A vertical wind velocity in an arctic front is upward. Therefore, an arctic front may be considered as a system consisting of two air streams moving in opposite directions in the ambient atmospheric gas, with strong velocity shear taking place close to the centerline of an arctic front. In our calculations, we define the initial and boundary conditions as consistent with observational data and for the situations when the arctic front intersects the simulation domain in the west-east direction. / / / / / / / / / / / / / / / / / / / / / / / / V Г У / / / ■ / / / /,./ / / /У / Hi JJ J l i f t / / / / / / / / / / / i )• / / / / J / / / / • / / > ' / / / > ' / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / *3 70 / / /. / / / / / / / / V ■' '■ ^ S ^ s > X S S > S ' ' S S S s ' s /■ '' •' X •' •' у ^ S ^ у S S S S s у j. 10 15 20 25 30 75 / / / / / / / / / / / I I I I I I I I I I I I I i I I i 4 i i I i I I I I I I I 1 < I I I I I I I I I . I . / / ' I I I I I I I I <J I 4 I J .1 I I : <■' \ t I II I 1 > •' I I I I / / sw / / / / / / / . 4 ' I и I I I I.I \ \ \ \ \ \ \ \ V ЙЛ \ \ \ \ N ''" ' f e - Ч \ \ Ч - - " " / / / / / / / / I I I I / / / ! I / I I I I 1 ‘ I I I I I i I t » v: ' I I j Ш ЩЩI -------------.v . W 4 . V \ i 1 ( / / I I .11* . ' t f t t / Z / S S s r S S ' / / / / / . ' / . ■ r i I / / / / / ' ./ r / S ' ' ' ' g> 70 £ 65 3 1 1 1 i ' J / / ■ / / / ' / / * / / / / / / ' / / У K:/ / / / 55 7 7 7 7 7 / 7 7 77 / / / / / / / / / / / / / / / / / / , / / / s s / s s / S /s " ' ' z' ✓^ ^ ^ — / / / У ' / 7 / я -t i i / / ' ш N W W W ' \ ч \ \ s ч ' \ \ S 4 / / / / / / У / / / / / I I . jf у ,, й ; £ ....... V. 4 - \ v 4 \ \ , , . , \Л \ \ \ \ \ П w w w w w w - \ \ \ \ \ \ \ w N\ \ \ \ \ \ \ 4 f / t t t / r / ___ / ' S . ' S - ---- --— . ___ _ -■* .................................. , / , > У V 10 15 20 25 Longitude (deg) Ц 6 Fig. 2. The distributions of horizontal component of the air velocity at the altitude of 600 m, assigned at the initial moment (top panel), computed 20 hours after the beginning of calculations (middle panel), and computed 40 hours after the beginning of calculations (bottom panel). The results are obtained for the second initial configuration of the arctic front, with the modules of the zonal wind velocities at more northern latitudes relatively to the centerline o f the arctic front being less than those at more southern latitudes relatively to it. The degree of shadowing of the figures indicates the module of the velocity in m/s. Initially, let us consider the first case when, at the initial moment, the arctic front contains a convexity in the south direction, with the deviation achieving a value o f one hundred of kilometers. It may be emphasized that, at the initial moment, the fields of the module of the horizontal velocity are asymmetric relatively to the centerline of the arctic front not only inside it but also beyond the arctic front. The initial form of the arctic front may be easy seen from the top panel of the Fig. 1, where it is like a dark curved band between two light curved bands. The time evolution of atmospheric parameters was numerically simulated using the mathematical model during the period for about two days. The results of time-dependent modeling are partly shown in Fig. 1. It can be seen from this figure that, in the course o f time, the initial distribution of horizontal component of the air velocity was considerably transformed. The simulation results, presented in Fig. 1, show that, at the presence at the initial moment of the part of the arctic front bent to the south, when the fields of the module of the horizontal velocity are asymmetric relatively to the centerline of the arctic front, eastward from this part, approximately in 20 h, a polar mesoscale cyclone is formed which moves to the south and to the east with a velocity of approximately 11 km/h. The maximum wind velocity within the polar mesoscale cyclone is reached approximately 20 h after the simulation beginning, and then it begins to decrease slowly. The radius of this polar mesoscale cyclone is about 600-800 km. 195