عنوان مقاله [English]
Weather conditions have always affected the man’s life; so he is used to adopt his life with Weather conditions, one of which is the condition of Cold waves. Cold days and cold waves are examples of extreme-temperature that include unusual notable amount of minimum temperature. Reduction of minimum temperature depends on heating degree and wind speed. Cold waves can be considered as a natural disaster or a provider of ideal weather in summer. Thus, present paper tries to analyze cold wave that mitigates weather in the warm period of year, besides cold waves of cold period in ten years.
Materials and methods
To pattern the cold waves of Iran in a ten-year period of time (2004-2013) three-hour minimum data of European Centre for Medium-Range Weather Forecasts (ECMWF) with distance of 1 * 1 degree latitude and Longitude has been used. In present study, a cold day due to relative notion of Coldness in different time and places, is defined as a day in which standard score of minimum temperature is below -1. For this reason, minimum data of all days was separated by MATLAB software; then by using the standard score those day with Anomalies below -1 known as cold days were diagnosed. In the next step all the days (569 days) that had this characteristic by MATLAB and based on sea level pressure patterns correlation were divided into five different classes includes Spring, Summer, Autumn, late Autumn, early winter and winter And interpreted them synoptic patterns. In the next step wind speed and also minimum and maximum temperature difference thresholds was used to distinction between the Radiative cool waves from the advective cool waves, so that minimum and maximum temperature difference below 8 ° C and wind speed more than Breeze threshold was used as advective cool waves criteria.
Results and Discussion
By extracting 569 days of negative anomalies between 2004 and 2013 it became clear that 2013 with 124 days and 2006 with 20 days (with negative anomalies below -1) have the most and the least negative anomalies respectively, in the mentioned research period. August, which have 83 days of negative anomalies, also contained the highest anomaly in selected 10 years. Analyzing level pressure patterns correlation between five presented patterns showed that interclass correlation of late autumn and early winter was 0.94 and more than all other groups. The highest correlation between groups was also between fourth (late autumn and early winter pattern) and fifth classes (winter pattern). In the days which cold waves entered the country, wind speed never reached to high-speed level and never exceed of the average wind speed. This speed never reached the average wind speed in summer but the frequency of light wind was higher than other seasons. While the frequency of breeze flow in the second and cold half of year is more than the first or warm half of the year; low speed of waves produce more intensive and continues cold wave when it’s accompanied by cold weather. Cold waves of Iran are usually advection and enter country from north and through west flows. In spring pattern (first pattern) west systems cause reduction of temperature and mitigation of Iran weather especially in western and northern regions of country. Minimum and maximum temperature difference in northwest of Iran was lower than 5 ° C in this pattern, while it was observed in water areas only in the other patterns. The highest difference between daily minimum and maximum temperature has been recorded in the second class (summer pattern).
Flows from the high altitude subtropical East lead to cold advection air of higher altitudes toward east and north east of Iran. But withdrawal of subtropical high pressure was the main factor of cool waves in this pattern. Siberian high pressure has caused low difference of daily minimum and maximum temperature in northeast of Iran and Alborz mountains in the autumn pattern (third pattern). Wind speed in North of Iran was more than the other region. And also stability conditions was the most in this pattern. There was low differences between minimum and maximum in south of Zagros mountains that was due to advection of cold air by African high pressure. In late autumn and early winter pattern (forth pattern) a complete advective Pattern is ruling over country. The fifth class (winter pattern) is advective- Radiative