عنوان مقاله [English]
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 large-scale dynamical 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 impact on low-level 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 Â on the 2 PVU (a potential vorticity unit is equal to ) surface corresponding to the position of the dynamical tropopause. The analysis was carried out for a prolonged, acute episode of air pollution in Tehran.
Â Â Â For quantification, the previously introduced Split Flow Index (SFI) and the Rossby-wave breaking index, referred to as Â 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.
Â Â Â 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 shown that 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 Â on the 2 PVU surface. For this reason, the Â index was used as a dynamical tool to detect and identify a blocking based on the distribution of Â on potential vorticity surfaces. Results showed positive values of Â 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 Â 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.