Investigating Three-Dimensional Structure of Cyclones Causing Pervasive Precipitation in Western and Northwestern Iran with Origin in the Mediterranean Sea in the Cold Spells of the Year

Introduction

Synoptic systems have a major role in the earth’s climatic changes. These systems, especially cyclones and anti-cyclones, move to transport air currents, and thus, sensible and latent energy, thus alternatively causing instability and stability in one region. Cyclones are synoptic systems in which sea level pressure is low, the air current is ascending, and the wind direction in the northern hemisphere is anti-clockwise. The vertical air movements in cyclones create clouds, precipitation, and even thunderstorms. Considering the literature on the relationship between heavy precipitation and cyclones, the frequency, temporal and spatial distribution and main paths covered by them have been examined; however, it is thus noted that few studies have ever investigated the three-dimensional structure of cyclones and their structural changes during the distance covered. For this, this study aimed to examine the three-dimensional structure of Mediterranean cyclones entering Western and Northwestern Iran through an immediate set of atmospheric variables.

Materials and Methods

This study used two different databases to identify Mediterranean cyclones entering Western and Northwestern Iran in the cold spells of the year (October-May). The first database pertains to the daily precipitation data of 15 synoptic stations of Western and Northwestern Iran for 10 years (from 2004 to 2013), obtained from Iran’s Meteorological Organization. The second database also pertained to the variables of atmospheric temperature, specific humidity, geo-potential height, sea-level pressure, vorticity, wind meridional component, and wind zonal component, all of which were derived from the European Center for Medium-Range Weather Predictions website (https://www.ecmwf.int) in the form of daily observations for 10 years (from 2004 to 2013) for the cold spells of the year (October -May).

After data were obtained, the following five-step algorithm was defined to investigate the three-dimensional structure of cyclones entering Western and Northwestern Iran with origin in the Mediterranean Sea.

Step 1: Selecting the most pervasive precipitation days in Western and Northwestern Iran

Step 2: Detecting the time and place of formation of Mediterranean cyclones at 1000 Hpa levels

Step 3: Tracking cyclones at 1000 Hpa levels during the cyclone lifespan

Step 4: Identifying cyclones at various pressure levels and their bonding together

Step 5: Investigating the three-dimensional changes of cyclones during their lifespan

Results and Discussion

The main goal of this study was to understand the three-dimensional structure of cyclones entering Western and Northwestern Iran with origin in the Mediterranean Sea. For this aim, and to determine Mediterranean cyclones and locate their formation in the Mediterranean Sea, a five-step process was introduced. According to this process, 93 cyclones causing pervasive precipitation in Western and Northwestern Iran in 10 years (from 2004 to 2013) were identified and their three-dimensional structure was determined. The most important results from investigating the three-dimensional structure of the cyclones were as follow:

• Most cyclones causing pervasive precipitation in Western and Northwestern Iran had initially shallow and middle depths then developed into deep cyclones during their paths.

• Before their central cores were established on land, most cyclones caused pervasive precipitation in Western and Northwestern Iran.

• Except for the beginning days of the birth of cyclones causing pervasive precipitation in Western and Northwestern Iran, vorticity values, being negative at some levels and positive at some other levels, revealed at all levels positive values during [cyclones’] course of movement eastwards in the remaining days.

• On days with pervasive precipitation in Western and Northwestern Iran, vorticity in cyclonic centers increased, especially at upper atmospheric levels.

• When shallow cyclones are formed on European land, there is always a kind of weak temporal inversion at lower levels of the atmosphere, which can be a major orographic feature of cyclones.

• In the course of the Mediterranean cyclones’ movement eastwards, the temperature gradient in the central core of the cyclones involves many changes. When cyclone centers are established over the seas, the temperature gradient is low, and when established on land, it is high.

• The daily changes of cyclones’ central cores during their course of movement eastwards are low at the seas and high on land.

• Wind zonal component values in the center of cyclones causing pervasive precipitation in the west and northwest of Iran were greater than those of wind meridional components.

Conclusion

An investigation of the most important changes of atmospheric variables (e.g., geopotential height, vorticity, temperature, humidity, and wind meridional and zonal components) in two horizontal and vertical dimensions during the life span of cyclones causing pervasive precipitation in Western and Northwestern Iran determined that thickness (vertical spreading), vorticity, temperature, humidity, and wind zonal and meridional components underwent large scale changes. The thickness of cyclones during their life span saw an increase, with some shallow cyclones changing into deep cyclones. Vorticity also saw positive values at all levels on all days, except for the beginning days when it held negative values at some levels, and positive values at some levels. Also, the temperature gradient in the central cyclonic cores saw changes. When deployed on the seas, cyclone cores had a low-temperature gradient, and when placed on land, they had a high-temperature gradient. During the cyclone’s life span, the specific humidity value by the time of pervasive precipitation in Western and Northwestern Iran increased at all levels, with the wind zonal component value being greater than the wind meridional component value at most levels.