عنوان مقاله [English]
Since the spatial and temporal distribution of rainfall in Iran is influenced by distribution of global circulation systems, the slightest change in its pattern can lead to severe weather abnormalities. Therefore, spatial and temporal abnormalities of rainfall and extreme changes in rainfall intensity and differences in precipitation types are one of the main characteristics of Iranian rainfall. Climate change is one of the problems of human society and is a threat to the planet Earth. The increase in the temperature of the earth has caused profound and extensive changes in the Earth's climate, causing changes in the time and place of precipitation, which has caused a lot of damage, especially in the last decade. The purpose of this study was to identify and study the changes in the heavy rainfall components of Iran in relation to changes in the middle tropospheric systems.
Materials and methods
In this study, we received daily rainfall data of 53 synoptic stations from Iran Meteorological Organization from 1984 to 2013. The cluster analysis and zoning were conducted using Euclidean distance method and Ward's method. For zoning based on cluster analysis, we also used seven variables of consecutive rainy days. The final result was the division of the country into 3 regions according to the number of days. In the following, data from the upper atmosphere levels were used to investigate changes in the mechanisms of rainfall forming. The data are including discovery data of geopotential heights, u wind, v wind, omega, in 500 hpa level, and specific humidity at level 700 in daily average, and 1 * 1 degrees from the European Center for Medium Term Forecast (ECMWF). The statistical period of study was divided into two periods of 15 years (first study period 1984-1998 and second study period 1999-2013), the peak days for each area were determined in each of the two time periods. For each time period, the peak rainfall was 5 days. Then, the synoptic parameters were analyzed and compared in each study period.
Results and discussion
In the second study period, the range of trough decreased and the depth of trough increased. The central core of geo-potential heights in the first time period is greater than the second period of study. In other words, the core in the first period is 5400 geo-potential meters, which in the second interval it decreased to 5350 geo-potential meters. In area 2, in the second period of study, the high elevation in the convergence region of the Arabian Sea is more shallow and in a more inappropriate position than the first pattern. The area 3, in the second study period, is located in northwest Iran behind the trough, and cold weather is falling with low amount of rainfall. In 3 areas, in the second period of study, the amount of moisture has decreased and caused the rainfall of the second interval to decrease relative to the first interval.
In the second period of study (1999-2013), the range of trough has decreased and the depth of trough has increased. The Omega component showed that in the second study period, the mean omega-negative peak of the area was reduced. The Omega component study showed that in the second study period, the mean omega-negative in the target area was reduced. It was also found that the direction of the wind streams in the second period was reduced due to the flow. The amount of moisture in the second period of study has also decreased and has caused the second period rainfall to decrease over the first period. In the study of the synoptic components of second area, it was found that in the second period of studies (1999-2013), located on the convergence area of the Arabian Sea, is more shallow and in a more inappropriate position than the first pattern. This caused the maximum moisture content to fall to 5 grams per kilogram. Investigation of the synoptic components of the third region, revealed that in the second study period, the study area, especially the northeast Iran, is somewhat ahead of the trough, and the average annual precipitation in this is increasing in the second period. It can also be understood that the northwest Iran is located behind the trough with a fall in the cold weather. The rainfall in this part has decreased during the period.