Statistical Synoptic analysis of pervasive cooling waves in northwest Iran

The amount of solar energy absorbed by earth surface features and converted into heat energy is measured as temperature. General temperature values are a function of elevation and latitude. Other secondary factors such as location of water bodies and land cover properties as local agents can form temperature structure of each location. One of the exceptional temperature situations representing minimum temperature values is cold waves. Given the general circulation pattern of the northern hemisphere, Iran is located in a place influenced by a variety of air masses. One of the important climatic phenomena prevailing in most of the country is cooling and freezing condition. Northwest Iran as a cold region is always experiencing losses by the extreme cooling events. Therefore, investigation about the patterns creating the extensive cooling waves can help us understand the behavior of the climatic phenomenon.
In order to examine the synoptic patterns of the cooling waves in northwest Iran as the purpose of this study, we have obtained the minimum temperature of 42 synoptic stations from Iran Meteorological Organization. We have selected the stations with longest period of data available. After sorting and quality control of the data and test of sufficiency of the sample stations, the minimum temperature has been interpolated by kriging method using scripting. In the procedure, a matrix 42*365 has been formed for the variables of each year. After the interpolation has been done, a matrix 5082*11323 has been formed for the 30 year statistical period and used as database in next stages of the research.
In this study, we have selected the days with cooling waves that have: (1) standard score less than -1.2, (2) spatial coherence, (3) 50% of spatial coverage, (4) the cooling waves in two successive days. Using these conditions, the days with the cooling waves have been selected for the Northwest Iran. After classification of sea level pressure data, suitable site has been selected to determine the representative days. Using scripting in MATLAB, we have calculated the correlations between the maps of different days for each class (2-6 groups), in 0.5 coefficient. We have used mean daily temperature data from 44 synoptic stations in northwest Iran from 1980 to 2010. The daily temperature data have been interpolated by kriging method using scripting in MATLAB, in the cells of 15x15 km and a total of 5082 cells. Using the defined threshold, up to 1196 cold days have been selected. They have been divided into four patterns combined by clustering program to draw the maps in Surfer. For more precise analysis of the cold waves in northwest Iran, other dynamic quantities including thermal blowing, atmospheric divergence and convergence, and prevailing meridian winds.
The results of the cooling wave patterns have indicated that four synoptic patterns are influencing the cooling waves. These patterns are including low pressure belt of west Russia and North Africa with East Turkey high, East Europe low pressure, enormous high pressure of east Europe with Sudan low, Siberian high with European high pressure. The most severe cold waves occurred when the enormous high pressure of east Europe moved the high latitude cold air through a cyclone movement into the study area. As a result, a low was formed over north Europe and moved towards the east and lower latitude areas. This movement developed the high pressure on North Africa and Mediterranean Sea. It also brought Siberian high pressure over the region with a cold weather condition. There was also a negative pressure anomaly at the location of the low. The pattern formed in trough part suitable for overshooting of cold air. In this pattern, a progressive change can be observed in contours from Europe towards Iran. This provided suitable condition for descend of the cold air from northern Europe and cold air flows from higher latitudes towards Iran. In a comparative analysis of the effective patterns in cooling waves in northwest Iran, it can be argued that the influence of the patterns revealed that two patterns of east Europe low and the west Russia and north Europe low with east Turkey high have the greatest influence on creation of the cold wave in the region. This is due to extensive descending of the high latitude cold air into median and low atmospheric levels. Since the study area is located in the east part of the overshooting of warm air from low latitudes into the median atmospheric levels, the third pattern reduced the severity of the cooling wave. The fourth pattern moved parts of Siberian high and European low towards northwest areas and created a high with prevailing cold weather. This has the lightest severity of the cold wave because the cold air was not penetrated into median atmospheric levels.
This research has investigated the synoptic patterns of three days lasting cooling waves in northwest Iran in the last decades. The research has revealed some synoptic patterns for the cooling in the region. The results of the patterns have indicated that most of the prevailing cooling events in the northwest Iran are resulted from formation of Siberian high pressure near the earth surface. Arrangement of the two high pressure systems of Siberian high and western migrating highs have played the most important role in conducting the high latitude cold air into the northwest region. In most of the patterns, the most important phenomenon and the principal agent of cooling event is establishment of a deep trough over the region. In other cases, establishment of sub-polar low in northern Europe and Russia made the polar cold air masses to penetrate into lower latitudes and the northwest Iran.