Document Type : Research Paper
Authors
1 PhD student of agricultural climatology Hakim Sabzevari University
2 Professor Department of climatology, Faculty of planning and Environmental Sciences, Tabriz University
3 MSc student of Ferdowsi mashhad University, Meteorological Department
Abstract
Introduction
Data and information from the Meteorological Department of India and the Joint Hurricane Warning Center (JHWC) were used to investigate the structural nature of Ashuba tropical storm in the Arabian Sea from June 7 to June 12, 2015. To study the atmospheric structure, the analyzed digital data were taken from the European Center for Medium-Term Forecasts and the Center for Environmental/Atmospheric Forecasts (NCEP/NCAR) for the Arabian Sea and beyond. The study area was the Arabian Sea, located between the Indian subcontinent (eastern part) and the Arabian Peninsula (western part) and northwest of the Indian Ocean. On average, 1-2 tropical cyclones form on the Arabian Sea each year. Even in some tropical regions, strong cyclonic cycles occur at the synoptic scale (Evan & Camargo, 2001: 145). Therefore, from previous years, climatologists have studied the types of storms, due to the increase in tropical cyclones in the last decade; and thereby, this issue is followed with more sensitivity. Consequently, the main purpose of this study was to explore the structural nature of Ashuba tropical storm on the Arabian Sea in order to identify one of the region's main moisture sources.
Materials and Methods
Storm data statistics were obtained from the Meteorological Department of India and the Hawaii Hurricane Warning Center. Analyzed digital data, including; Geopotential altitude (Hgt), orbital (u), meridional wind (v), sea surface pressure (SLP), air temperature and sea water temperature (SST) for standard levels at 17 compression levels with a resolution of average daily geographic degree belonged to the National Center for Environmental Prediction/Atmospheric Science and precipitated networked data were obtained from the European Center for Medium-Term Atmospheric Forecasting (ECMWF) with a resolution of 0.125 degrees Celsius for the Arabian Sea. NASA and MODIS satellite imagery were also used for the visible band for every six days. The CAPE index was applied to evaluate the energy required by the storm supplier.
Findings and Discussion
The results of study displayed that in the middle level of the atmosphere, while forming a low-altitude nucleus with very strong positive rotation, the conditions for the production of tropical storms in the region have been provided. On the other hand, on the surface, low pressure has formed in the southeast of the Arabian Sea with a central pressure of 995 hPa and has started moving westwards towards the coasts of Oman and northern Yemen. Creating a very strong convergence current on the surface and upper divergence caused the storm to reach its maximum strength in the region on June 9. However, the anomalous temperature of the water surface in the range where the storm reached its maximum intensity reaches to over than 5 degrees Celsius. The increase in water surface temperature and the transfer of heat and moisture into the storm has strengthened and, by its nature, caused heavy rainfall in the region. Finally, on June 12, as it approached the east coast of Oman, it began to disappear due to lack of moisture for its dynamic movements, and changed from a tropical storm to a tropical hurricane. Also examining the prepared maps for the amount of precipitation and the flow of the lower levels of the atmosphere, it was determined that on the first day of the storm, a cyclonic current occurred in the east of the Arabian Sea, resulting in the maximum amount of precipitation in the west of the system, which reaches more than 240 mm. On the second day, moving north of the system, the amount of precipitation was concentrated in the south, so that the southern coast of India was not unaffected by precipitation and had about 120 mm of rainfall. On the third day, with the placement of this tropical storm in the north of the Arabian Sea, the maximum precipitation was created in the east of the system, which was more than 160 mm. On the fourth day, the western half of the Indian coast was faced with a rainfall of nearly 110 mm, which was due to its location in the east of the cyclone, which in turn caused the rise of air and the transfer of moisture to the air parcel, floods in the region. On the fifth day, the maximum rainfall was close to the eye of the storm, which was close to 100 mm, and the coastal areas of the Indian subcontinent were still experiencing heavy rainfall. Examination of the 850 hPa pressure system revealed that on the first day, the maximum relative pressure system nucleus formed in the southeastern parts of the Arabian Sea. These conditions have led to very strong convergence in the lower levels. The presence of such strong convergence and amplification of rotation has caused this anomaly to reach its maximum in the region. The strong rotating nucleus then extended to the west coast of India and then moved westward on the third day to the central regions of the Arabian Sea, with a very strong rotating current extending from latitudes 10 to 30 degrees north. As the storm/hurricane approached the west coast of the Arabian Sea, it intensified to more than five pressure system units on the fourth day. On the fifth day, the positive nucleus became independent and formed a very strong rotating closed cell. On the sixth day, with the cyclone remaining on the eastern coast of the Arabian Peninsula, its power had gradually diminished.
Considering the water temperature in the region, which is an average of 6 days, it showed that the water temperature in most parts of the Arabian Sea was high, so that these conditions reached more than 32 degrees Celsius in the coasts of India and the center of the Arabian Sea. These conditions were less only in the northern regions of the sea than in other regions. To understand the water surface temperature, its anomaly was also calculated for six days with the storm. Its output indicated that the eastern, northern, western and southwestern regions of the Arabian Sea were associated with a positive anomaly of 2 to 3° C. Negative anomalies only reached -1.5 degrees Celsius in the north and south of the sea. Occurrence of maximum positive anomalies in the region was one of the main reasons for the intensification of cyclones in the region, so that the western regions of the Arabian Sea had the maximum positive anomalies and on the other hand the maximum area of tropical cyclone activity.
The 12-hour reports from the Indian Meteorological Agency and the Hawaii Hurricane Warning Center were used to route the tropical storm. In these two centers, there were several data methods for routing and the origin of the storm. Geographical coordinate data with a 12-hour separation was used, which from the beginning of the storm to its decline, its characteristics and longitude and latitude were recorded as a text file. The onset of the storm was from the eastern part of the Arabian Sea, which migrated northward to higher elevations and deviated in its path due to the dominance of the Coriolis to the west of the region and disappeared off the coast of Oman.
Conclusion
Ashuba tropical storm/hurricane formed on June 7, 2015 in the Arabian Sea and disappeared on June 12, 2015. This investigation revealed that on the first day, a low-lying cell was formed in the eastern part of the Arabian Sea, during which a positive rotating nucleus or vortex was formed in the mentioned area and strengthened in the following days. The role of the Arabian Sea and abnormal changes in its water surface temperature in the occurrence of hurricanes has been mentioned in the researches of Ghavidel Rahimi (2015: 31) and Lashkari and Kaykhosravi (2010: 19). On June 9, as the subtropical anticyclone expanded further east, the Arabian Sea's low-pressure cell became oval in a circle, contributing to the deepening of the system, creating another bond at the heart of the closed cell with a height of 5,810 geopotential meters. In the last days, as the coasts of Oman and Yemen approach, the intensity of this cell decreases and its extinction stage was reached. On the surface, in parallel with the mentioned period, a low-pressure core with a central pressure of 995 hPa formed on the southeast of the Arabian Sea and the creation of a very strong positive rotation indicates the occurrence of hurricanes in the region. The central pressure of the storm reached less than 993 hPa on days 9 and 10, which was the peak of the storm. As it approached the shores, the intensity of this cyclone was greatly reduced, turning it from a tropical storm into a tropical turbulence. Examination of the water surface temperature showed that the average water surface temperature in these 6 days in most parts of the Arabian Sea was more than 29 degrees Celsius. Inspection of water surface temperature anomalies also disclosed that the maximum positive anomalies corresponded to several places in the sea, including the southern coasts of Pakistan to western India, eastern Oman and a very strong core corresponding to the southwest of the Arabian Sea with an average temperature of more than 5° C. The maximum rainfall inside the cyclone indicated that on the first day of the storm, the maximum rainfall in the southwest was 240 mm. In the following days, with the transfer of this core to the south, southeast and finally to the east, the maximum rainfall would be on the west side of the Indian coast. Only in the last days it was observed that while the maximum rainfall occured in India near the eastern part of the eye of the storm, a maximum precipitation center with an average of 100 mm has been created. In this study, two indicators, CAPE and SWEAT, were used to assess the location of storm formation. The results showed that these two indicators well showed the formation and severity and weakness of the storm during different stages. Thus, on the first day in the south of the Arabian Sea, the amount of CAPE was more than 5000 Jules/kg, which indicates the amount of convective energy available. On the other hand, the values of the SWEAT index have reached more than 380, which specify that the probability of a hurricane in this region is very high. Also, with the increase of water surface temperature in the region and the increase of anomalies in it, the necessary energy is provided for the production of cyclones in the region, which with the increase of energy within the air mass system and the presence of buoyancy energy in it, and on the other hand, instability indicators in monitoring and tracking these types of storms showed that they are a suitable tool for tracking and are able to navigate it while being aware of the intensity of the storm.
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