The Synoptic-Dynamic Survey of Spring Extreme Rainfall Patterns in the Southern Coast of the Caspian Sea

Climate in the south of the Caspian Sea is very different from other parts of Iran. The existence of the Caspian Sea، complex terrain، and the fact that large-scale circulation patterns، as well as local، regional، and extra-regional circulations affect this region، has caused the climatic characteristics of the region to become very unique. The south part of the Caspian Sea receives a significant rainfall in autumn and winter، but in contrast، the least amounts in spring. The incidence of extreme rainfall and flooding in this season could impose damages to the region. Therefore، it is reasonable to identify mechanisms of development، strength، and laws of movement and extension of the synoptic and dynamic patterns to amid positive effects and avoid the harmful results or decrease them. Dynamical studies have a very special place in the study of climate of a region، so that by using the dynamic indices، we can analyze more accurate atmospheric phenomena. This paper attempts to apply a dynamical and synoptic approach to investigate extreme spring precipitation، zoning amounts of total rainfall of every pattern، and provide a model for dynamic and synoptic patterns in the study area. Study Area: The study area is includes the northern provinces of Iran، Gilan، Mazandaran and Golestan in the southern coast of the Caspian Sea and north of the Alborz mountain. This area is between latitudes 36 degree and 34 minutes to 38 degree and 28 minutes of north and longitude 48 degree and 50 minute to 54 degree and 2 minute of east (figure1). Also the rainiest stations and consequently perennial rivers and dense forest cover has been in this region of Iran

In this study، a statistical - synoptic method is used to investigate the dynamical patterns of the extreme spring precipitation in the southern coast of the Caspian Sea. In this regard، the daily rainfall data from 11 weather stations located on the southern shores of the Caspian Sea over a period of 50 years (1961-2010) were obtained from Iran Meteorological Organization. The selection criterion for extreme precipitation is the 24-hour rainfall in which 70% of stations would be equal or more than 20% of the long-term average of the seasonal rainfall. Using data from National Centers for Environmental Prediction / National Center for Atmospheric Research with a horizontal resolution of 2. 5°×2. 5°، the prevailing daily average atmospheric conditions were analyzed. The data include geopotential height، the zonal (u) and meridional (v) wind components، specific humidity and vertical velocity for 1000 hPa and 500 hPa levels. By using abovementioned data، the dynamical parameters of relative vorticity، vertical velocity، divergence and convergence of wind fields and vertical profiles of relative vorticity، divergence and convergence of wind fields separately for each model studied and produced by GrADS software. The maps were analyzed and finally dynamical patterns were identified.

The main objective of this study is to determine the extreme spring precipitation patterns associated with the dynamical quantities in the north coastal strip of Iran، so the weather map of severe-pervasive rainfall are extracted and dynamical analysis is done. The 500 hPa level maps show that the extreme spring rainfall in the southern coast of the Caspian Sea mainly follows four patterns. The main feature of the first pattern is the blocking system (Omega)، which is primarily located in the east of Europe. The system is created from the northward extension of high pressure of North Africa and southward development from the low level polar front in high latitude and also strengthening of a blocking system by the Mediterranean cyclone. In this pattern، Mediterranean cyclones are a major factor of spring rainfall in the southern coast of the Caspian Sea. Also، the European cyclones with amplification by low heights of the high latitude will cause a large amount of rainfall in this season in the north of the country. In addition، the extension of the Saudi high pressure to the East، and expansion of the subtropical high pressure outbreaks to above 80 degrees north latitude، makes a good condition to merge the European cyclones and Mediterranean cyclones in the study area، which eventually lead to create the heavy rainfall in the northern region of Iran. The third spring rainfall pattern is influenced by blocking systems in the Central Asia and Siberian regions. However، the expansion of the subtropical high pressure in the north-east and existence of the Saudi Arabian high pressure in the south of country، have a negative effect on the trough of low heights so that it prevents the system to create heavy and flash flood rainfall and it decreases the rainfall. In the fourth pattern، the blocking system is the dominant form of the spring precipitation in the southern coast of the Caspian Sea. This pattern is somewhat similar to the first pattern، which the different subtropical high pressure systems abnormally extend to the north in the spring. It is observed that even outbreaks reach to the 50°-60° N. In addition، low heights of the high latitude and Mediterranean cyclones also play a major role in the precipitation pattern. To provide a comprehensive view of the nature of atmospheric circulation in the southern coast of the Caspian Sea in spring، vertical profiles of divergence and convergence in patterns of along the 37 N were prepared. The results showed that the southern coast of the Caspian Sea and East Mediterranean during the active patterns of precipitation in the spring have a prevailing western streams (Eastward) at levels of 500 hPa to 150 hPa. Eastward flows through the orbital inclination have the maximum intensity in 300 hPa to 100 hPa levels and areas of divergence maxima and minima convergence and also the fields of horizontal divergence and convergence match at 1000 hPa.

The results showed that extreme spring precipitation in the southern coast of the Caspian Sea follows four major patterns. The main feature of the first pattern is the blocking- omega system that is located in Eastern Europe and Ural Mountains that lead precipitation induced systems to the southern shores of the Caspian Sea. In the second pattern، the Mediterranean cyclones are considered as a main factor of the spring rainfall in the study area. The third pattern of spring rainfall is formed when a blocking system exist over Siberia and Central Asia. Finally، in the fourth pattern، the blocking system is also the main cause of spring precipitation in the southern shores of the Caspian Sea. This pattern is somewhat similar to the first pattern، which is different from the subtropical high pressure systems abnormally extend to the north in spring. It is observed that even outbreaks of it reaches to the 50°-60° N. Investigation of the dynamical parameters showed that during the activities of the system، in all cases، the vertical velocity of the atmosphere is negative and the positive vorticity is high and the convergence zone is located on the shores of the Caspian Sea. All these factors imply that the dynamical weather condition in the region is appropriate for dynamically fast rising of particles to the middle and upper atmosphere، which eventually create heavier spring rainfall.