عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Earning easy, fast and applied methods for analysis anemometric data is an essential need for climatology until recognition regime of dust winds. Sabzevar City in Northeast of Iran is a suitable region for continual formation of Dust Phenomenon. These phenomena can have short term and long term harmful effects in this areas. Of course long term effects are a lot more harmful that short term ones. In Sabzevar, precipitation is below of 200 mm and its climate is arid. This phenomenon were taged life and financial damages in Sabzevar.
In this study, dust phenomenon was studied in terms of time and space. Therefore, data involving time and date for supervision, wind direction and speed and current condition of Sabzevar during twenty years statistical period in 1989-2008 were prepared and using these data, statistical analysis about time frequency of dust phenomenon was done in this town. Also, we separately calculated frequency of meteorological codes of Dust phenomenon (06, 07, 08, 09 and 30, 31, 32, 33, 34, 35). we used "WRplot View" software and convert data to appropriate forming for this software and using "paint" software. we draw those in 16 directions and 9 speed classes. Then "Dust Rose" diagram was drawn annually and seasonally and compared with annual and seasonal wind rose in this region. Also, annual Dust Rose was drawn for two codes contain 06 and 07 that they had the most frequency between all data.
Results and Discussion
The average occurrence of days with dust is 17.15 days in a year and dust days during said statistical period had an increased trend. Also this phenomenon more occur in spring and summer and specific in afternoon. And code 07 appropriated itself more than 90 percent daily reports of Dust phenomenon in Sabzevar station. The study of annual wind rose in Sabzevar synoptic station during statistical period 1989-2008 showed that the average wind speed was 3 m/s and had the most frequency according to speed 3 - 4 m/s. however, measurement of average speed 8.75 m/s for winds with dust of which 43% have a speed more than 10m/s, show that in some occasions in a year, condition is provided for blowing strong winds in a region. The study of seasonal wind roses show that summer season is the most windiest season and only in 20.9 percent of times, weather is quiet and the average wind speed in this season is more than other seasons (3.72 m/s). In addition, the study of seasonal dust roses show that in 77.7 % of times, dust phenomenon is occurred during warm period in a year and winds with a speed more than 10 m/s have the most frequency and spring season with 46.2 % has the most hour report for dust phenomenon.
According to annual wind rose diagram, in 73.75 % of all wind blow times in this region during twenty year statistical period, wind blow direction is from eastern north to eastern south in this town and in 11.65 % of times, it include western south to western north, and only in 14.6 %, wind blows from other direction towards this town. That is, in 85.4% of all times, wind blows in east-west direction. This result is wholly consistent with topography condition in this town and it seems that wind is channelized in this town in east-west direction. The study of seasonal dust roses results show that in a year, different atmospheric systems make dust phenomena in a region. Starting a cold period in a year, dust winds blow severely from west half of this town and maximize in winter. But with beginning of spring season and western systems recession, these winds blow less from west half towards this town so that they reach to about zero in summer and wholly blow from east half towards this region.
As a result, according to Sabzevar topography and wind channelization in east-west direction and the presence of vast regions fit for wind erosion in major wind path in this town as well as the average speed calculation 8.75 m/s for dusty winds, it seems that thiese cases fit condition for formation and severity dust phenomenon and cause dust problem in this town.