جستجوی مقالات مرتبط با کلیدواژه « wave breaking » در نشریات گروه « زمین شناسی »

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.

With regard to the adverse effects of air pollution in Tehran on its residents، it has become vital to investigate the meteorological factors that determine conditions favorable for the establishment of critical air-pollution episodes. Among these factors، the largescaledynamical processes acting in the upper troposphere are particularly important، as they can provide a means of predicting the critical episodes using the medium-range weather forecasts. As a step in exploring such factors، this study focused on some aspects of the large-scale upper-tropospheric flow believed to have a significant impacton lowlevel flow. The upper-tropospheric jet stream with its possible split and the Rossby wave breaking as a way of detecting and measuring blocking strength were examined using various diagnostics including the distribution of potential temperature θ on the 2 PVU (a potential vorticity unit is equal to 10-6 m2 s-1Kkg-1) surface corresponding to the position of the dynamical tropopause. The analysis was carried out for a prolonged، acute episode of air pollution in Tehran. For quantification، the previously introduced Split Flow Index (SFI) and the Rossbywave breaking index، referred to as β for brevity were employed to identify the jet split and blocking formation، respectively. The Global Forecast System (GFS) data consisting of geopotential height، temperature and horizontal velocity components were used for the three-month period of 1 Nov 2010 to 31 Jan 2011. The geopotential height at 500 hPa، and the wind speed، relative vorticity and streamfunction fields at 300 hPa were analyzed to determine the synoptic structure associated with the intense air-pollution in Tehran from 22 Nov to 22 Dec 2010 (the month of Azar 1389 in Iranian calendar). The synoptic structure indicates persistent blockings in the Northern Atlantic، and the formation of deep troughs in the southern Europe together with strong ridges in their downstream sides. The high values of the geopotential height field، the negative relative vorticity and low wind speed in the middle and upper troposphere were the dominant features over Iran. To compute the SFI، three regions had to be involved: the subtropical jet (STJ)، the polar front jet (PFJ) and the gap between these two (GAP). The SFI was computed by subtracting the mean relative vorticity of GAP from the sum of mean relative vorticities of the STJ and PFJ regions. The sign and magnitude of the SFI were criteria for the occurrence and strength of a split jet. The results for the SFI were generally sensitive to the longitudinal and temporal interval used in computation of the SFI. By considering broader and longer longitudinal and temporal intervals، respectively، more accurate values for the SFI index could be obtained. With regard to the latter sensitivities، it was shownthat within the positive values of SFI in the large part of Nov. and Dec. 2010 representing non-split flow، there was the period 9 to 14 Dec. 2010 during which the formation of a transient blocking over central Asia led to negative (positive) values of SFI at the beginning (end) of this period. The blocking formation was associated with the reversal of the meridional gradient of θ on the 2 PVU surface. For this reason، the β index was used as a dynamical tool to detect and identify a blocking based on the distribution of θ on potential vorticity surfaces. Results showed positive values of β over Iran for a few days in the middle of Azar (early Dec. 2010)، indicting the the presence of a transient blocking. There was also a second case of positive values of β from 11 to 16 Dec.، which closely followed the negative values of the SFI (split-flow regime) from 9 to 14 Dec. over central Asia.