Document Type : Research Article
Authors
1
meteorological organizational/weather forecaster
2
Gilan Meteorological office
3
Institute of Geophysics University of Tehran
4
Teacher
5
Baqiyatallah University of Medical Sciences, Tehran, Iran.
6
Tehran Disaster Mitigation and Management Organization, Tehran, Iran
Abstract
In the warm seasons of the year, some storms occur that are difficult and complicated to predict. These strong winds that are usually accompanied by dust are known as Haboob in some countries. In the last decade, some of the storms that have occurred over Tehran area, which have caused numerous damages, have included the mechanism of the Haboob event. In this study, a method is introduced for the feasibility of predicting the potential of storms with a down burst structure, which provides defined warning levels for this phenomenon. In the introduced method, by using a post-processing on the output of a numerical weather prediction model, according to the dynamic and thermodynamic conditions of the weather, a level of warnings is issued for the potential of storm events with down burst mechanism. In the present work, an ensemble forecasting system developed for the WRF model is used to provide short term predictions over Tehran area. Five different thermodynamic indices were calculated for the grid points and the process of calculating the potential of a storm event with a down burst structure was carried out by considering the temperature conditions on the earth's surface, the thermodynamic conditions of the atmosphere, the vertical profile of relative humidity, and also checking the presence of dynamic conditions for air ascent. If the thermodynamic conditions and the instabilities of the atmosphere identified by the relevant indicators are conducive and the temperature of the earth's surface and the vertical profile of the relative humidity are fit, then the potential of a storm with a down burst structure can be considered probable. The humidity conditions were considered in such a way that the lower levels of the atmosphere have low relative humidity and the higher levels have more relative humidity so that the probability of evaporation of rain before it reaches the earth's surface is high. By combining these conditions for the output of the numerical model, in all the time steps, three warning levels of the model output for the potential of a storm with a down burst mechanism were presented in the form of yellow, orange and red color zones. Four cases of strong winds and storm, as well as the famous storm that occurred on June second, 2014, were investigated for the city of Tehran. Various forecasting maps of the output of model run include the mean sea level pressure, the thickness of the layer between the levels of 500 and 1000 hPa, wind speed, the relative vorticity of the level of 500 hPa, geopotential heights of some levels, thermodynamic indices, relative humidity in some pressure levels and The skew-T diagram was prepared at the grid points to analyze the dynamic, thermodynamic and synoptic conditions of the atmosphere. According to the information of the Meteorological Organization of Iran, winds with speeds between 80 to 120 kilometers per hour have been recorded in the meteorological stations of Tehran on these dates. In all these cases, the recorded wind speeds is significantly higher than the direct prediction of numerical weather forecasting models. In fact, it could not be recognized and predicted by the output of these models. The maps for forecasting the warning level of the feasibility of a storm with a down burst mechanism in all cases investigated in this work during the calculations related to the probability of the occurrence presented in this study. It seems the presented method has been able to predict the potential of the occurrence of strong winds and storms with down burst structure for the city and province of Tehran.
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