Synoptic analysis of tropical cyclone Gonu and its impact in the Southeast of Iran

Super storm Gonu is one of the strongest tropical cyclones which have occurred after the deadliest and most destructive hurricane Katrina (which occurred over the Gulf of Mexico during23rd August to 3rd September in 2005) in 2007. Such intense tropical cyclones have happened rarely over the Oman Sea since most storms in the Arabian Sea tend to be small and disappear quickly or making landfall on the Arabian Peninsula and/or the Indian subcontinent. According to the historical records of severe cyclonic storms formed over the Arabian Sea, severe cyclonic storms were not reported in the Arabian Sea during 1970 to 2007 but Algeria broadcast and old people of Hormozgan believed that first time, it happened in 1977. However, there is no information about it.
The Saffir Simpson scale separates hurricanes (with winds of 74 mph or greater) into five ascending categories based on the maximum sustained wind speeds, the potential height of its dangerous storm surge, and the hurricane’s central barometric pressure. Thesuper cyclone Gonu was a Category Fivetropical storm (Saffir-Simpson Scale) which occurred over the Northern Arabian Sea in June 2007. The minimum pressure of this tropical cyclone reached 920 hPa on June 4th The meteorological phenomena which occurred during the storm activity in Iran have been announced by Iranian Meteorological Organization as “cloudy sky with heavy rain and thunderstorms;”, a raging sea that the height of its waves reached to 5.8 meters.
This research surveys the characteristics of tropical storms, synoptic and dynamic factors which affect the formation of Gonu cyclone and its impact on the South and Southeast of Iran. For this purpose, charts of NCEP/NCAR reanalysis data set have been studied in this research. The region which we considered in this research was 0° to 90°E longitudes and 0° to 70°N latitudes. A survey of surface charts and upper level atmospheric charts such as 850, 700, 500 and 300 hPa from the 1st to 8th June 2007 showed that the trajectory of the tropical depression was at first toward the Indian subcontinent, which gradually became stronger, and formed a  tropical storm on 3rd June. However, an extending high pressure ridge in the south of Indian subcontinent caused the storm’s path to change from the Northeast (toward the Bay of Bengal) to the Northwest (toward  the Persian Gulf). As a result, the storm moved toward the Northwest and finally it made a landfall over the Southern region of the Oman Sea. On the sixth day, the Scandinavian high pressure ridge weakened; the core axis of storm was from the South to the North direction at that time, and had drawn it to the Southern part of Iran. In this case, the storm carried its obtained moisture from the Arabian Sea and Oman Sea to the South and Southeast of Iran by its easterly flank flows. Falling cold air behind the storm by the Scandinavian anticyclone ridge on White Russia (the north  of Black Sea), which is associated with the further strengthening storm. The European anticyclone on the seventh day was weaker than the previous day; as a result, the core of the storm had taken the southeast-northwest direction, then imported into the southeast of Iran by meridional speed of 8 ms-1. Colliding with the southern part of Zagros Mountains, the storm Gonu caused convective (potential) instability and formation of cumulonimbus clouds and thunderstorms which synoptic reports of Iranian meteorological stations confirmed it. After crossing the Southern coast of Iran, entered to the country from south, and the intensity of storm activity decreased during moving on the rugged parts of Hormozgan, Sistan and Baluchestan and Kerman provinces (the Southern Zagros Mountains), and then disappeared with moving toward Pakistan. The highest rainfall recorded during the storm activity in the Southeast Iran were received on the sixth day of June by the ports of Konarak and Jask, for which the amounts were 90 mm and 78 mm, respectively. The greatest amount of precipitation on the seventh day was reported by Nikshahr to be 120 mm; Jask also received 59 mm of rainfall during the eighth day. Nikshahr is located in the extreme Southwest of the province of Sistan and Baluchestan. The area is mountainous and 98% of its tissue consists of highlands and mountains and the remainder is covered by plains and deserts.