عنوان مقاله [English]
The study of the simultaneous occurrence of cyclones and the changes of Tropopause Pressure's level (TPL) can provide useful insights into the characteristics of the precipitation, especially the widespread precipitation (WP) over Iran; as mid-latitude cyclones are one of the most critical factors associated with WP in Iran. Understanding the mechanisms and the features associated with the cyclones can be crucial for estimating and predicting cyclones and their consequences with precision. To this end, in the current study, we underlined the relationship between tropopause and cyclones affecting WP in the country.
In the current study, two data sets were adopted. These data sets include daily precipitation data of Asfazary national data set (version 3) and atmospheric data (including temperature and geopotential height (GH) data of ERA-Interim base from the European Centre for Medium-Range Weather Forecasts (ECMWF)) with spatial resolution of 0.25 degrees for an area comprised 0 to 80° N and -10 to 120° E. The main aim of selecting the aforementioned area and the data was to identify all the cyclones which are originated from or pass through the Mediterranean Sea and are associated with WP over Iran. Accordingly, the associated pressure levels of the tropopause were examined.
The Asfazary database from 1979 to 2015 was adopted to identify days with WP based on precipitation anomalies covering more than 10% of the country. Accordingly, a total of about 1189 days with WP was extracted for the intended period.
In this study, regional variations of GH at the level of 1000 hPa have been used to identify cyclone centers. To this end, the GH of the pixel was evaluated in relation to the eight neighboring pixels; when the GH was lower than the neighboring ones, and the gradient of the GH was at least 100 geopotential meters per thousand kilometers, the pixel was considered as the center of the cyclone. Cyclones were tracked with respect to the days with WP, and their characteristics were investigated based on the day of cyclone activity and the day of WP.
Using the thermal criterion defined by the World Meteorological Organization (WMO, 1957)), the tropopause was identified.
The 1189 days with WP have been studied visually. Since it is not feasible to present all the days in this brief paper, a few samples were selected to identify the association of tropopause with cyclones on days with WP. The days were selected based on the highest percentage of the area covered for different months. Accordingly, for the entire period, 8 days were selected to represent January, February, March, April, June, October, November, and December. In May, July, August, and September, days with WP were not observed. In the present study, to investigate the relationship between tropopause and cyclones in eight WP samples, the features of tropopause and cyclones on the starting days and on the days with WP were considered.
The spatial distribution of the TPL on the day of cyclone activity and the day with WP showed that on the day of cyclone activity, tropopause had certain characteristics; at this time, the tropopause pressure level showed larger values than those in the surrounding areas. Even on days when WP was observed in Iran and within the cyclone activity range, this anomaly was observed in the TPL. The tropospheric condition of the country compared to the day of the cyclone activity had significant differences; at the time of precipitation, tropopause level showed a larger numerical value in most areas compared to the beginning of the cyclone, especially in areas with heavy precipitation intensity. Tropopause at the time of the formation of the cyclone with WP on April 7, 2013, was different from other under study cases. In this case, at the beginning of the cyclone activity on the cyclone formation area, the tropopause did not have a significant anomaly; while on the day of WP in the south of Iran, the anomaly was significantly prominent. It seems that this difference can be due to the differences in the origin and the mechanism of cyclones in different areas. This probably explains the difference in the characteristics of tropopause on the day of cyclone activity. In the whole area under study, at latitudes above 30 degrees, in geographic locations where the cyclones emerged at the 1000 hPa, tropopause was broken. At this time, tropopause pressure levels showed larger values than the surrounding areas. Given this fact, it seems that there is a relationship between the two phenomena, cyclones and TPL.
Based on the findings, in all eight samples of WP days, tropopause had special characteristics in the same area of cyclone; in addition, tropopause pressure levels in these areas were higher than their counterparts at the same geographical situation.