عنوان مقاله [English]
نویسندگان [English]چکیده [English]
A front is a significant instance of thermal structure asymmetry in mid-latitude cyclones. Investigating the precipitation structure of these cyclones, it is observed that there is more concentration of precipitation near the fronts. Frontogenesis is referred to the changes in the gradient of potential temperature in magnitude and direction in one level or in one layer (for example 700-850 hPa) due to changes in wind speed and direction, and in specific, it is referred to the increases in horizontal temperature gradient with time, and on the other hand frontolysis means the decrease of this gradient with time. Therefore knowledge about fronts as one of the affecting factors in the precipitation patterns in different regions, considering the destructive floodwater, intense coldness, heavy snow, and similar extreme events, has prominent importance. Unfortunately, forecasting models usually operate in synoptic and larger scales, and in mesoscale they have some inaccuracies. These days, in many high-developed countries in meteorology science the frontogenesis function is daily computed. Daily computation of this function is so essential in forecasting the atmospheric and oceanic disasters. The main object of the analysis done in this study was the dynamic effect of atmospheric high levels in the life cycle of fronts located in this region from west to east of Iran. In addition, analyzing the outputs of WRF model in dynamics of front regions is done, in order to strengthen our understanding of their nature and characteristics and then compare the initial and secondary operations of the model. Frontogenesis function in two dimensions is extremely practical because its application in weather maps is simple and provides important atmospheric specifications about weather phenomenon. In this research, two dimension frontogenesis function with distinct analysis of its sentences (shearing and confluence/influence), is applied.
The changes in High level jet stream speed and direction, cause the elimination of geostrophic equilibrium and create a geostrophic component of wind speed. A geostrophic wind causes the formation of high level convergent (divergent) cores. High level convergence (divergence) through descending (ascending) motions and low level convergence (divergence) has an interaction with one another, and these motions cause an increase or decrease of low level temperature gradient (frontogenesis or frontolysis). In this research, a frontal cyclone that was occurred in Iran on 18th of November 2009, is studied. At first step, the WRF model is run using GFS data for the selected region. In next step parameters e.g. geopotential height and horizontal components of wind speed for levels higher than 500hPa, were changed in the model input, for these levels we used the past 24 hour data instead of the original data, and for levels lower than 500hpa we used the data for determined hours. The terms of frontogenesis equation like confluence and shearing terms for both runs of the model were determined and displayed. Comparing the two runs of the model shows noticeable differences in the magnitude of frontogenesis function and its terms. The more intense difference is seen in the cold front, which has a greater temperature gradient. Overall, we found out that the contemporary of low level vertical motions with high level vertical motions, has a direct effect on the frontogenesis process, and if the high level jet stream speed changes by the horizontal components of speed, this change will be also observed in all terms of the frontogenesis function, which is a sign of the considerable effect of high level on surface frontogenesis
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