Climatology
Naser Mansourei Derakhshan; Bohlol Alijani; Majid Azadi; Mehry Akbary
Abstract
Introduction The weather fronts are known for their large vorticity, dense, moisture, and statical Stability gradients, and their longitudinal scale is one unit greater than their width. The width of the front is known as the baroclinical zone, in which the front lines have a very large ...
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Introduction The weather fronts are known for their large vorticity, dense, moisture, and statical Stability gradients, and their longitudinal scale is one unit greater than their width. The width of the front is known as the baroclinical zone, in which the front lines have a very large temperature gradient, which is determined by the angle between pressure and temperature lines. Position of a front is located in warm side of the extreme temperature gradient, inside the heat transfer zone and intensity of the front is determined by the size of the horizontal or quasi-horizontal temperature gradient.Even the numerous expert synopticians disagree with each other in the position of the fronts, their types and intensity, in the manual drawing method of the fronts. So their drawn fronts are very different While objective front is based on numerical methods and its purpose is to avoid applying people''s tastes in their manual method. The advantages of objective front metod in comparison with subjective front method are high speed front detection, the possibility of determining front frequeny, moving, and feedback of fronts with land side effects. So far, various methods have been developed for objective front method. They performed objective front method using numerical methods and the first and second derivatives of the temperature parameter on a regular grid points with a relatively low resolution of about 100 km. Inside the country, there has been no study about automatic and numerical front methods. On the other hand more than 90 percent of heavy rainfall in the tropics is associated with the fronts. Therefore, considering Iran''s location in the middle latitudes, it is very necessary to study and identify the fronts. So the climatological study of the manual front detectin is very time consuming, expensive and practically impossible. Therefore, in this research, the, automatic and numerical front detection have been discussed for the first time in the country. Methodology In this study, grid point data from the European Center for Medium-range Weather Forecasting (ECMWF) of type (ERA - Interim) is used with gaussian grid points. In this centre, different types of data are classified into different formats and in different time intervals and different grid resolution. In order to study of the fronts, isobaric level data with 6 hour intervals and resolution of 0.75 × 0.75 degrees with grib format is used. This grid resolution is set in a regular 61×61 matrix with a grid distance of 83 km. Different quantities can be used to select the appropriate parameter to detection of fronts such as temperature, humidity, wind direction and wind speed, vorticity, thickness and thickness changes ,and temperature is on of the most important of them. On the other hand, detection of the exact location of the extreme temperature gradient, which is accompanied by the effects of heating on the warm convergence belt in the warm side of the front leads to warm weather, can be identified only by using the equivalent potential temperature. Results and Discussion The main idea for identifying frontal areas is to use a temperature parameter in two-dimensional horizontal coordinates. The line representing the front in these areas is identified using a frontal identification function. In order to identify the front, the masking conditions are applied once or several times. In other words, in this equation, the horizontal gradients of the equivalent potential temperature are used, which should not be less than the value of the K-threshold value. >K . Several indicators are considered to identify the front. The first of them is that the front must be at a turning point in the curvature of the temperature lines which is along the temperature gradient. The second indicator is the location of the maximum values of temperature gradient,and the third criterion is the point where the second derivative of the temperature gradient is zero. Various experiments have shown that the smaller the temperature derivative of the front temperature parameter, the less error there will be (J. Jenkner, 2009). Thus, the Front Termal Parameter (TFP), invented by Renard & Clarke (1965), was used as the main method of frontal reconnaissance. TFP = In this equation, second derivative of the temperature parameter has been used, which has converted the temperature gradient, which is a vector quantity, to a scalar quantity. Conclusion Examination of the results of objective fronts showed that the detection of fronts near the ground due to the interaction between the boundary layer and the fronts is very erroneous and the fronts are practically indistinguishable. On the other hand, at higher levels, shallow fronts at numerical output are not detected. Therefore, the appropriate level for automatic identification of fronts in the study area, 700 hPa level was selected. Examining the results, it is inferred that cold and warm fronts are often found at the bottom of the ridge and above the ridge of the upper surfaces, and these fronts, during the formation stage, are often discontinuous and gradually evolve during the developmental stages. Strengthening the front will take a more integrated form. Studies have shown that cold fronts produce stronger frontogenesis than warm fronts. Also, the output of objective fronts showed that TFP is a good parameter for detecting the front in this region and with the results of previous studies such as Hewson (1998: 49), Jenkener et al. (2010: 9), they show a good match. The results of this study can be used in the discussion of climatology and forecasting of fronts and can be helpful in the discussion of flood management due to heavy rainfall on the front.
Climatology
Yaghoob Rezazadeh; Bohloul Alijani
Abstract
Cold and freezinginthe North West ofthe country(Iran), has arrived allyearirreparabledamages to thetransportsectors, roadaccidents, gardens andagriculturalproductsandcrippled thenormal lifeof people incityand villagesAt first, theverycolddays of 18 stationsin the statistical periodwere extracted then ...
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Cold and freezinginthe North West ofthe country(Iran), has arrived allyearirreparabledamages to thetransportsectors, roadaccidents, gardens andagriculturalproductsandcrippled thenormal lifeof people incityand villagesAt first, theverycolddays of 18 stationsin the statistical periodwere extracted then of thestatistics, the number of 43 Severe and pervasive wave was selected..Finally, for Synopticanalysis ofcoldwaves, tworepresentativewaves on February1991 and January 2007were determined. The maps of Earth and the500Hpa pressuressurfaceof thefreezing from theNCEPwebsitehas been prepared andanalyzed. The results showed that in most cases of severe frost in the top level, a deep descends in the Iran west established and polar regionsCold Weatherwill leadtoIran. The establishment of above thebarrier in the East of this descends was due relativestagnation of descends and the continuity of cold inthe region intensifies. At the surface of Earth, west migrant high-pressures and the spread of Siberian high-pressure tab add to intensityat 500Hpa levels. Accordingly, we can at least 5 days prior to the deployment of these systems in the vicinity of the study area, predicted cold conditions, the managers and farmers are prepared to deal with cold.
Climatology
Bohlul Alijani; Ali Bayat; Mehdi Doostkamian; yadollah Balyani
Abstract
Precipitation is one of the most essential and variable climate components whose understanding has long been a concern for climatologists. The main objective of the current paper is to investigate and analyze the precipitation cycles in Iran. In order to realize this objective, the annual precipitation ...
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Precipitation is one of the most essential and variable climate components whose understanding has long been a concern for climatologists. The main objective of the current paper is to investigate and analyze the precipitation cycles in Iran. In order to realize this objective, the annual precipitation data of isometric station of Iran were extracted. These data have been collected by the country’s meteorological organization since the establishment of the station until 2008 which adds up to more than 40 years of statistics. Then, in order to investigate and analyze the precipitation cycles, spectral analysis (co-structural analysis) was utilized. Regarding the calculations, the programming utilities of Matlab were used and the Surfer software application was exploited for drawing operations. The results obtained from analyzing the cycles show that there are significant 2 to 3 year cycles, 3 to 5-year cycles, 2 to 6 year cycles and sometimes 11 or more- year cycles governing Iran’s precipitation patterns. Hence, in east and southeast of Iran, 3 to 5-year cycles are prevailing and in west and northwest 2 to 3-year cycles are dominant and finally in north east 2 to 6-year cycles are customary. The most numerous and the most variable cycles happen in south and south east, mainly due to the mountainous regions of Zagros as well as the proximity to Persian Gulf. The north western regions, much like the southwestern regions, indicate variable cycles due to the mammoth mountains of Sabalan and Sahand. Moreover, the presence of those cycles which have a return period equal to the statistical period has been seen in various parts of Iran, which indicates a precipitation trend in this country.
Climatology
Peyman Mahmoudi; Mahmood Khosravi; Seyed Abolfazl Masoodian; Bahlol Alijani
Volume 19, Issue 54 , February 2016, , Pages 303-327
Abstract
To identify and detect the frequency variation trend of Iran’s pervasive and semi-pervasive frost days in the current research, minimal daily temperature data of 663 Iranian climatology and synoptic stations were acquired from Iran Meteorology Organization during the time interval between 1962 ...
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To identify and detect the frequency variation trend of Iran’s pervasive and semi-pervasive frost days in the current research, minimal daily temperature data of 663 Iranian climatology and synoptic stations were acquired from Iran Meteorology Organization during the time interval between 1962 and 2004 for October to April months. Following data acquisition, Iran’s isothermal maps for each day starting from 1.1.1962 until 31.12.2004 (9116 days) were prepared using Kirging interpolation technique in order to construct the database of the county’s minimal temperature. In the next step, frosts were classified in three types based on a spatial principle: pervasive frosts (simultaneous occurrence in more than 65% of Iran’s surface area), semi-pervasive frosts (simultaneous occurrence in 25% - 65% of Iran’s surface area), and local frosts (simultaneous occurrence in less than 25% of Iran’s surface area). Then, frequency of pervasive and semi-pervasive frost days were analyzed in three scales including monthly, seasonal, and yearly using two estimation techniques of slope SENSE and linear regression.
Results indicated that frequency of pervasive frosts in Iran held a statistically significant decreasing trend in December and January months, during winter, and also, in annual basis. But, for semi-pervasive frost days, it was observed that variation was significant only in January having a positive trend. It signifies that number of days with semi-pervasive frost increased during the 43 years under study. Therefore, number of semi-pervasive frost occurrences increased while number of pervasive frost occurrences decreased in January. The same rule holds for other scales i.e. monthly, seasonal, and yearly basis.
