نشریه نیوار
پیاپی 98-99 (پاییز و زمستان 1396)

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

Evapotranspiration is an important factor of hydrological cycle that must be estimated in irrigation and drainage projects. Studies have shown that in the absence of lysimeter, FAO Penman-Monteith method provide acceptable estimate of evapotranspiration values, but this method requires a lot of meteorological parameters. Therefore other hydrological evapotranspiration methods that needs few parameters should be evaluate for each region. In this study reference crop evapotranspiration calculated by Ref-ET software for Sepidan, Fars, by using Sepidan synoptic stations meteorological data during years 2004 to 2015. The results of methods compared with FAO Penman-Monteith standard method. The efficiency of all methods was evaluated using Root Mean Square Error (RMSE) and Mean Absolute Error (MAE) statistic parameters. The results showed that Methods based on Penman-Monteith (ASCE st PM, ASCE PM rs, ASCE PM), respectively, with the RMSE of about 0.03, 0.079, 0.079 and with the MAE of about, 0.021, 0.068, 0.069 appropriate methods for estimating reference evapotranspiration are considered. FAO 24Rd Pan and FAO Pan Methods, respectively, with the RMSE of about 1.426, 2.338 and with the MAE of about 1.186, 2.072 has the least compliance with the Penman-Monteith FAO method. So in situation of less data for FAO Penman-Monteith method in Sepidan, it is possible to use of other methods that needs few hydrological parameters.

In order to identify the structure and determine the source of dust, a regional climatic model, RegCM-4, which was combined with a dust plan and 40 km spatial step was run from March to May 2011. The output of the model indicates that the southern parts of Turkmenistan adjacent to Iran (northeastern Iran) initially were the location of dust particles to air, then with increasing gradient of pressure and increase the wind force, it's increase the particle depth of The dust in the Karakum desert of Turkmenistan and the northern parts of Afghanistan. The transfer of dust to the area was affected by the increase pressure on Turkmenistan, the Caspian Sea and Khorasan, and the low pressure deployment on the eastern part of Iran and Pakistan, which resulted in an increase in wind speed and changes Direction to northern.

Chilling and freezing are the most important limiting factors of product and distribution in plants that because of their damage reduce the production potential of agricultural and horticultural products in vulnerable areas every years. The goal of this research is statistical analysis of Kashan frosts, the level of its effect on agricultural products and presenting some suggestions for agricultural Sustainable development. In doing so, data of daily minimum temperature of Kashan synoptic weather station in statistical term of 43 years (01/01/1971 to 31/12/2014) obtained from Iran weather station and sorted basic Shamsy calendar. Normal and random data were recognized and were observed significant weak decreasing trend in number frost day by using linear Regression at a significance level of 95%. Often freezing temperatures that occurred in Kashan station often sensitive plants, vegetables and trees bloom have harmed. The average period of frost at the station is 46 days and probability of frost period is not earlier than 28 Mehr and no later than the 24 Farvardin certainly. Kashan's fiercest frost during the study period, with a temperature of 17 degrees, occurred in the winter of 1386 that a total damaged 148,000,000,000 Tomans to agricultural sector of the city. Often Kashan frosts are Winter So in the case of products like as evergreen trees such as citrus and olive, it is recommended that to be used from varieties more resistant to cold.

In order to achieve risk management to reduce losses caused by extreme rainfall in the West of country and to identify the thermodynamic conditions of this phenomenon during a period of 50 years (2010-1961), Skew-T log P climate graphs extracted from the website of the University of Wyoming America were used. Moreover, atmospheric instability indices, SI, LI, Sw, KI, C.T, V.T, T.T, PWC, of Kermanshah province station, in order to full coverage of the region (about 21 cases of extreme precipitation), were used and the results of all the indicators above the standards of atmospheric instability were compared and evaluated. In this study, discrepancies between observed and predicted values were observed and finally regional instability indices have been determined as it follows: Si ≤ ¡71/20 LI ≤ ¡63/16Sw ¡25/16 ≤ KI ¡23/1 ≤ C.T ¡50/8 ≤ V æ 90/18≤ T.T33≤ the increase of SW, KI, C.T, V.T and T.T to larger numbers than what were listed in the table and the reduction of SI and LI to smaller numbers are a representative of an extreme increase of instability and probability of extreme precipitation. The results showed that the conditions of potential instability in the event of extreme precipitation on the vertical profile of the atmosphere had become dominant. Also, the western system during periods of extreme precipitation is associated with a Mediterranean low-pressure system on the ground and trough in the upper level that is an appropriate model for extreme rainfall and provides humidity for precipitation.

Climate change refers to changes expected in climate behavior of a region than behavior over a long period of time for the information recorded in the area. Drought can occur in any geographic area and it could be have a major impact on the region as an environmental phenomenon known and an integral part of climate change. Scientists know climate phenomena particularly, consecutive droughts and extreme as most important cause of desertification and its extension. In this study, the risk of climate change and drought in Khorasan Razavi were assessed during a 23-year period (1989-2011). Climate change was obtained at the provincial level using Do marten and temperature changes indexes. To evaluate drought, it was used two indicators as PNPI & SIAP. Based on risks of climate change, results showed that 76 % of the province affected by very severe risk, 11.1% severe risk, and 12.8% no risk. The risk of drought based on SIAP, 4.9 % of the province affected by severe risk, 64.4% moderate risk and 30.7% low risk. Based on PNPI, 49.3% of the province affected by severe risk, 19.9% moderate risk and 30.7% low risk. It seems that Khorasan Razavi province has more area about drought risk based on PNPI.

In this study, maximum, minimum and mean temperature trends were investigated inShahrekord station from 1984 to 2013. Shahrekord station was transferred to the new[A1] location of airport in 2001 but this displacement was inconsiderable. The statistical significationof temperature trend was evaluated by three methods, comparing the mean of two sequential half, Mann-Kendall test and sequential Mann-Kendall test in the terms of monthly, quarterly and annually. The results of comparing the mean of two sequential half showed that from 1984 to 1998, the mean minimum temperature reduced about -0.81 and the mean maximum temperature increased about 0.98in comparing the results from1999 to 2013.According to Mann-Kendall test, the maximum temperature shows significant increasing trend in March and October. These three methods confirmed that maximum annual temperature increased in Shahrekord station but the trend in mean annual temperature was not significant. Graphical Mann-Kendall test revealed significant decreasing trend in average of annual minimum temperature. Also, by using Mann-Kendall test it has been shown that there is an insignificant increasing trend in maximum temperature and insignificant decreasing trend for minimum temperature in all seasons.