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

Weak westerlies over high and mid latitudes cause cut-off low blockings (CB) and CBs create durable phenomenon such as frost waves. Iran, in the south of mid latitudes is engaged with annual spring frost waves. So far, most of the researches in Iran have been done with statistical and synoptic methods, but most of these studies, especially the synoptic ones, are case study samples. The apparent role of large-scale atmospheric systems such as cut-off low blockings in the emergence and durability of spring frost-waves, requires great research to undertaken throughout Iran and in a long period. So our goal was the study of effects of cut-off lows on frost durability over Iran.
Data and

For this purpose, the data of minimum daily temperature of 69 weather stations from March 20th 1968 to June 21st 2014 was collected from the meteorological organization of Iran. It was determined that 114 spring frosts with a duration between 3 to 25 days exist and in the next step 47 enduring ones based on tables of daily temperatures and fluctuation of their frequencies were determined. All of these 114 waves were determined based on the frost duration criteria which lasted at least two consecutive days and their spatial criteria occurrence in more than two stations. This allowed the determination of the most durable frost index wave of each year during spring. Analyzing the peak-days of all index waves, distinguished the waves affected by the cut- off low blocking system. The final synoptic patterns were designed for analysis by examining the 500 hPa maps of the distinct days.

The height contours on 500 hPa air-charts from ENCEP/ENCAR showed cut-off low blockings were dominant patterns during the peak days of the most enduring frosts (18 ones). The first results indicate that the appearance of this blocking which directs the cold air flows to Iran from high latitudes has minimized over the last three decades. Designed synoptic patterns show that during the peak days of spring, frosts affected by cut-off low blockings, appeared around Caspian Sea, especially its eastern basin and stayed for many days and then divided into two branches including the main one in northern latitudes and minor in southern ones. Although the southern branch of these winds appeared relatively weaker than the main and northern one, but its effects were cold and sub-zero airflow from the Scandinavian Basin, Eastern Europe, Caucasus Basin and the Ural Mountains towards northwest, north, and sometimes northeast of Iran, and penetrated till Central Iran. Lack of high summits and mountains, lower inclination of sunlight and proximity to the southern warm waters are factors that have moderated the violence of these flows in the southern coastal provinces and there have been no reports of temperatures of zero and below zero degrees Celsius. The results showed four factors including “the diagonal move of cut-off lows from Southern Scandinavia to Pamir Plateau” and “eastward movement of troughs from Mediterranean Basin” both at 500 hPa level and “the position of Zagros mountains” and “the entrance of cold high pressure tongues of Siberia” at the surface, caused the symmetric influence of durable frosts over west and east of Iran.

Based on this research, the decrease in the occurrence of cut-off low blockings during the last three decades in Iran can be justified by the tangible and searchable expansion of warm air territory from low latitudes to the high latitudes in Southwestern 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.