چکیده:
The Central Zagros cold season with abundant snowfall that other event such as climate, influenced by the atmosphere circulation patterns and distribution of surface pressure of Earth. In this study, to determine weather patterns and dynamical conditions of heavy snowfall in the mountain roads, daily data of synoptic station: Borougen Branch, Koohrang and Lordegan for the year 1987- 2010 of Iran Meteorological Organization received. In the following, days of heavy snow to use of index measure rainfall more than 15 mm and below-freezing temperatures and check the monitor code 70-75 (Code scout snow), 136 days of heavy snow was determined. In order to determine weather patterns, principal component analysis matrix array S on 500 hPa geopotential height data was used. The results showed that heavy snowfall in the mountainous roads of Zagros cause to seven synoptic patterns. As trough of the east Mediterranean is west of Iran and study area is close to the region's extreme volatility. At the same time low pressure gradient in north and south of the study area Caucasus is severe, respectively, associated with strong positive vorticity in the study area, and strong negative vorticity on the East of Oman in the south and Caucasus area in north. However, low pressure center and the North East with strong positive vorticity and high pressure of Siberia to the eastern border of Iran is the continuation and sustainability under the dynamic conditions in the study area. During heavy snowfall strong high pressure of Siberia combined from the north of Iran and the Caspian Sea with high-pressure of Caucasus in relation to Polar vortex in the middle levels of the atmosphere Caucasus cold air streams in the study area. The humid currents from the Black Sea and the Mediterranean by the cyclone flow in the West of Iran through the Persian Gulf region flows to the Zagros Mountains. In all atmospheric patterns, the temperature of zero degrees in the south on the Persian Gulf region and the arrival of masses of cold and wet down to widths And heavy snowfall in the roof of Iran.
خلاصه ماشینی:
In this regard, many studies have been conducted in different regions, including Birkland 5 (1996: 281), who used 500 hPa geopotential height data and Principal Component Analysis (PCA) to identify atmospheric circulation patterns related to heavy snowfall in Montana, USA, and showed that in all heavy snow day patterns, the study area is behind a deep trough and under the influence of northwesterly currents.
Synoptic results showed that the occurrence of snowfall in this region is related to the meridional movements of the westerly wind, the negative phase of the North Atlantic Oscillation, low 700 hPa surface height in eastern USA, the Canadian cold air mass, and the passage of a cyclone from the south of the study area, while the opposite of the above synoptic conditions brings a decrease in snow in this region.
(2010: 135), in the synoptic analysis of 6- Hartley & Keables 7- Steban 8- Hui 9- Bednorz heavy snowfall in northwestern Iran using daily temperature and precipitation data during the 1976-2008 statistical period and the Principal Component Analysis method, identified six synoptic patterns, most of which were introduced as the main cause of heavy snowfall in the northwest region due to the influx of moist southern currents through the activation of the Sudanese cyclone system and its combination with the Mediterranean cyclone.
(Refer to the page image) Figure 1: Map of the synoptic station and the study area (Chaharmahal and Bakhtiari) - In the second stage, in order to determine atmospheric patterns and dynamic conditions of heavy snowfall occurrence, upper air numerical data with a spatial resolution of 2.
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