نوع مقاله : مقاله تحقیقی (پژوهشی)
1 کارشناس سازمان هواشناسی، تهران، ایران
2 دانشیار پژوهشگاه هواشناسی و علوم جو، تهران، ایران
عنوان مقاله [English]
In the present study, using the ERA-Interim reanalysis data for geopotential height, horizontal wind speed and relative vorticity at 300, 200 and 150 hPa levels, the quasi-geostrophic potential vorticity, the quasi-geostrophic potential vorticity gradient, the wave activity and wave activity flux for cyclonic and anticyclonic Rossby wave breaking events that occurred over West Asia during the winter time 1979-2018 were calculated and analyzed. The mechanism of Rossby wave breaking during five days before to five days after break was analyzed.
From three to five days before Rossby wave breaking events, the formation and development of wave were initiated. In the five days before anticyclonic breaking, ridge and trough informed with vertical axis potential vorticity penetration across the axis of the trough in the 200hPa about 5PVU. From four days before breaking, the ridge penetrated to north of Europe. It caused to intensify jet and form critical latitudes. Equatorward reflection of ridge and trough caused trough to penetrate to lower latitude with anticyclonic circulation. Anticyclonic circulation reinforcement caused formation of equatorward wave activity flux and divergence of wave activity flux in the two regions of lower latitude.
Through the anticyclonic breaking, the NE-SW slope of axis of trough increased and potential vorticity rose to 7PVU. In the breaking days, the weakening of jet was intiated in the upstream of trough on the north of Europe. Through two days after anticyclonic breaking, equatorward wave activity flux in the downstream of trough was weakened and divergnce region of wave activity flux was split into areas in Europe and Mediterranean regions. From three days after breaking, weakening of jet in the downstream of trough was initiated. First the upper part of wave passed from middle latitude and then the lower part passed from subtropical latitude.
The process of cyclonic breaking was intiated approximately five days before breaking. Four days before cyclonic breaking, ridge and trough informed with NW-SE axis potential vorticity penetration across the axis of trough in 200hPa about 3PVU. In cyclonic breaking, the formation of the ridge in the midlatitude caused zonal velocity to intensify and the formation of critical latitude over Europe. During three days before wave breaking, the jet in the upstream extended to downstream of trough, the NW-SE slope of the axis of trough increased and potential vorticity rose to its maximam about 6.5PVU in the midlatitude. During these days, poleward reflection of wave activity flux caused the divergence of the wave activity flux in the downstream of trough and cyclonic circulation to increase.
Rossby wave breaking mechanism was similar in the different areas of East Atlantic Ocean until West Asia. In these regions, the meridional wave activity flux in the anticyclonic wave breaking was more than in the cyclonic wave breaking. However, equatorward (poleward) wave activity flux in the anticyclonic (cyclonic) wave breaking on the east of Atlantic Ocean and Europe was similar to (twice as many) on the east of Mediterranean and West Asia. Approximately, zonal wave activity flux in wave breaking on the downstream of troughs on East Atlantic Ocean and Europe was twice as many on East Europe and West Asia; just as the wave amplitude on Europe and east of Atlantic Ocean which was higher than on the east of Mediterranean and West Asia. Due to wave breaking, wave activity flux on the east Atlantic and Europe was stronger than on the east Mediterranean and West Asia.