The statistical and synoptic analysis on widespread hailstorm events in northwest of Iran

Document Type : Research Article


1 Tehran University

2 Alkharazmi University

3 University of Tehran


Hazards can be divided into two main groups: one based on origin and other based on speed of occurrence. From view point of speed, the hazards divided into catastrophe and gradual types. For example earthquake, flooding, landslide, avalanche and intense rainfall especially hail are classified into catastrophe events. Gradual events include Drought, global warming and soil creeping. In gradual events per form considerations can effectively decrease the pains and damages. But in catastrophe ones there isn't enough opportunity for adaptation or response and pains and damages are intensive. Therefore pre-identifying the potential risk of catastrophe hazards can help us to decrease their pains and damages.
Hail is one of the destructive atmospheric phenomenon which causes wide destruction in different economic section including agriculture. Hail often occurs from thunderstorm systems especially cumulonimbus clouds in springtime. Hail is a result of severe convection in thunderstorm systems. The occurrence of heavy rainfalls in spring and summer may tend to flooding and damage the crops and roads. The target of this research is recognition of the synoptic patterns and determination of the quantitative dynamic thresholds such as relative vorticity, divergence and convergence of humidity, vertical motion and also thermodynamic quantities such as specific humidity and streamline from 48 hours before the storm begin to start until it reaches to peak.
Study Area
The northwest of Iran is the main study area. The selected stations belong to tree provinces: Ardebil, east and west Azerbyejan. The geographical location is within 35° 57 to 39° 46' North and 44° 13' to 48° 55' East.
Material and Methods
From 100 metrological codes, numbers of 99, 96, 89 and 27are related to hail with various severities. Then with concern to frequencies the hourly and monthly graphs of hail occurrence for stations were drawn. And reanalysis data for hailstorm cases are obtained from; NCEP/NCAR. The data of mean sea level pressure, geopotential height of 700 and 500 HPA levels, specific humidity and wind components of 850 HPA, convergence and divergence in surface level, relative vorticity and omega of 700 HPA and the cross section of omega and relative humidity for 38o 5’ N (Tabriz) are used from 48 hours before storm generated with 6 hour interval in an area from equator to 70o N and UTC meridian to 70o E. This research performed by using statistical and synoptic software, Excel and GrADS.
Results and Discussion
Investing 3-hour data, the most frequent hail is seen in hours 9 to 18 UTC. Most monthly frequencies of hail is seen on April, May, March and June, respectively. The maximum frequency of hailstorm in selected stations reported in Maraghe. For synoptic analysis of hailstorm, the daily map of May 1997 was analyzed from 48 hours before storm generation. On May 4th at 03 UTC in Tabriz, 06 UTC in Sardasht and at 21 UTC in Kalkhal the severe hailstorm was reported.
With concern to mechanism and behaviour of weather systems in formation of hailstorm for northwest of Iran in may 4th 1997, the following results were concluded by synoptic analysis:
- In mean sea level charts by backward motions of Azores high center to the west and Siberian high center to the east, the Icelandic Low through southeast extension is caused to its trough to penetrate into south coasts of Caspian Sea. Also with backward motion of STHP cell from north of Arabian peninsula to south, the influence field of Sudan Low and Red Sea trough reach to north and middle east especially to Mesopotamia in Iraq. In eastern boundaries of Iran the Pakistan low with westward motion expands over most parts of Iran in low levels. Position of low and high centers 700 and 500 HPA levels are similar and in northwest of Europe the eastward motion of Icelandic low associated with its strengthening.
- In central parts of Mediterranean Sea with movement of low centers the trough axis direction become northeast to southwest.
- Circumpolar Vortex expands to near 30o N and affects the northwest of Iran from east part of the trough.
- In 700 HPA level from 48 hours before rainfall occurrence, the amount of vorticity maximum is 4 to 6 unit (x10-5 rad/sec) and its minimum is -4 to -6 unit and also moisture convergence amounts are 2 to 5 and -2 to -4 respectively. The maximum of specific humidity in 850 HPA is 8 to 10 (gr/kg).
- The cross section of Omega on a day before rainfall occurrence shows negative amounts in west part of study area. This pattern moves toward Iran by eastward movement of atmospheric waves and the maximum omega reaches to -0.35 (Pascal/sec).


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