عنوان مقاله [English]
Hail is one of the most destructive weather phenomena causing damage in many different sectors, including losses in the agricultural sector. Hail is linked to the atmospheric elements and geographical factors ; whenever atmospheric conditions are suitable for physical processes to be combined with geo-location, the intensification of this phenomenon happens. Depending on the size and severity of the storm, damage hail is different (Dong et al.,2006,p.193)Hail made more than a billion dollars damage in agriculture in China in 2004. Hail or ice grains are composed of particles with a diameter of 5 to 50 mm and sometimes more. Severe and frequent hail are resulted from vertical movement of air in the cumulonimbus clouds causing the sperm of water droplets around them to be absorbed and freezing (Alijani & kavyany, 2008,p.264). (Iran Pour et al.,2015,p.115)In relation to the synoptic and thermodynamic analysis of thunder storms in Hamadan, it has been concluded that the pressure center for cool and cold air to the middle layers of the atmosphere and the low pressure in the North West South underlying currents of warm and humid climate of the region is located in Saudi Arabia, where the growth of cumulus and cumulonimbus clouds create thunderstorms, hail, and torrential rain . Many studies have been conducted in the country in the field of hail. (Fryzbay,1961,p.350,)To study the relationship between type of damage caused by hail in the plains of synoptic patterns, United States of America showed that the speed of movement of synoptic systems has a significant impact on the degree of damage from hail on the ground. (Hough,1961,p.242)In a 50-year period, the distribution of thunderstorms, rain, maximum temperature, dew point and air fronts and their influence on the distribution of hail in Illinois have been examined.It is believed that distribution of hail in a small area can be affected by various elements of climate. (Ezzati,2003,p.121). The role physical processes in atmospheric instabilities resonance is studied.
The studied area
Hamadan province with approximately 19545/82 square kilometers, is located on (33 degrees 33 minutes) to (35 degrees and 38 minutes) north latitude, between (47 degrees and 45 minutes) meridians and (49 degrees and 36 minutes) east longitude.It is surrounded by Zanjan and Qazvin from north, Lorestan from south, Markazi from east and Kermanshah and Kordestan from west, and based on the last country division, it includes 9 states, 29 cities, 25 districts, 73 rural districts, and 1210 villages. The average height of sea level in this region is about 1800 meters. The highest point in Hamadan province is Alvand 3584 meters high and the lowest point is Omar Abad 1600 meters high. Specifications of the studied stations are given in the following table.
Table (1): Specifications of studied synoptic stations
Nojeh in Kaboudar-Ahang
Materials and Methods
To investigate the occurrence of hail, data of statistical synoptic station were used four times during a period of 23 years (1992-2015). In this study, the primary data from surface meteorological research center, Hamedan in the period 1992 to 2015 (the first half of 2015) were collected and then initial test and extract data were controlled and a database of hail from present and past weather codes were prepared (codes 87.27, 88, 99,96,94,93,90, 89 to be included) at intervals of 3 hours . Of the 100 present weather codes, the codes of hail phenomena that contain different intensities were considered to include any emergence scout in hours and three hours before . In order to identify patterns that cause hail and identify the index days , a cluster analysis was carried out on 63 days of hail using software Spss in the study area and the days were clustered. Then, of the days,and the hours when occurrence, severity and duration of hail and rain showers were remarkable, two days was chosen, as the representative days (the tenth day of April 2002 and April 2007) and then other elements of climate, such as rainfall, temperature, pressure, humidity, wind direction and velocity and study phenomena such as thunderstorms (cumulonimbus clouds that arise), the date mentioned were studied using data from NCEP site / NCAR, maps synoptic pressure at sea level (Slp), geo potential indicator of omega-Nam, especially in the levels sea, 850 and 700 hp and map wind at 850 using the software GRADS on the peak month for the sample, and were analyzed. To analyze data and maps software packages such as, Grads Excel, ArcGIS, and Spss were used.
According to statistical analysis, the frequency of hail events in the synoptic stations was different from one another.Due to non-uniform distribution of hail during the period of 23 years (1992, 2015) studied at the station, due to its geographical location and topography , the greatest hail synoptic frequency stations has been 22 times in Malayer and the lowest synoptic frequency in Nojeh 10 times . Maximum occurrence of hail occurred in April and March at 09 and 12 o`clock (all hours in this study are in GMT) and evaluating changes in hours of hail indicates that the maximum number of storms may occur in the afternoon and early evening. In this period , Code 27 and Code 89 had the highest and lowest frequency , respectively.The greatest occurrence of hail , for a 23-year period, was from 9 to 15 o`clock reaching the peak at 12 o`clock and from 15 it declines rapidly. This shows the increase of atmospheric instabilities in these hours with increased atmospheric radiation energy received by the atmosphere and land surface features, atmospheric instability is perfectly amplified and appears to emerge at 9 to 12 because of the warming climate. The highest frequency of hail occurred respectively in April, March and February and the hail was not reported in July.
In June, December, August and September there was the lowest frequency and in March and April as the transitional warm (spring) months and, because of rapid warming of the Earth's surface and lower layers of the atmosphere (the day elongation and Sun approaching to vertical line ) and high air humidity , the instability increases and convective flows are accelerated and under the right conditions, convective clouds are grown making thunderstorm (Ezzati, 1382,p.121). The frequency of hail in the region in terms of year indicates that 1994 and 2009 experienced the greatest frequency and in 1995 and 1999 there was no hail. Spring had the highest frequency of hail and summer the lowest frequency . More than half of the hail events occurred were reported in the spring, followed by winter,.
The province of Hamedan is among the areas where due to the occurrence of hail, in a lot of crops are destroyed , and there may also appear casualties. Therefore, understanding the mechanism of formation, development and prediction of hail and events resulting from it such as thunderstorms , heavy torrential rain, lightning and strong wind can provide great help in order to reduce losses. Based on the results obtained, the highest frequency of hail occurred in April and March. In the spring, sun warms the Earth's surface and sea level in the study area is also relatively and high latitude of region strengthens the conditions for south low pressure system at this time of year . Investigating the synaptic systems along with hail showed that location of low pressure center on Sudan and extension of its edge to higher latitude in eastnorth-westsouth has covered all the srea study.On hail day, an edge of high specific humidity of the water resources in the southern Red Sea, Arabian Sea and the Sudan, penetrates the high latitudes. On this day at the same time a strong negative core Omega 850 and 700 hp on the studied area is the convergence at 1000 hPa with strong divergence in the upper levels of the atmosphere is closed and warm, moist air advection divergent and the release of latent heat of abundant, high flows and create the appropriate convective instability has provided. The wind field maps of the day, hail, can be seen on the meridional wind in South and West orbital wind is weak.
Concurrently, a core of negavtive strong omega at level of 700 and 850 hectopascal was investigated on the study area where the convergence was seen in 1000 hectopascal in higher levels of strong convergence and the convicted warm and wet air becomes divergent .With the abundant latent heat released , suitable upward flows provide the ground of convection instabilities.Examination of wind field maps on hail days shows that the dominant wind is on the south equator region and the western wind is weak.
This confirms moisture advection over warm seas. With the establishment of a high pressure system in the South East of Saudi Arabia and on the Arabian Sea, warm, moist air transfer through divergent flux over Oman and Arabian Sea has been convicted to the center of the low pressure system located on the Red Sea and Sudan by the spinner on region. Penetration of warm low pressure edges from southern Iran on the West with vertical development of clouds (cumulus and cumulonimbus clouds), high humidity, convection and intense ascending and mountain conditions in the West have been an important factor for instability and hail.