Climatology
Saeed jahanbakhsh; saeideh ashrafi; Hosein Asakereh
Abstract
Introduction Cyclones constitute one of the major factors determining climatic conditions, especially precipitation in the middle latitudes. Changes in the properties of cyclones in a region may lead to variations in the precipitation conditions of that region. Therefore, studying major aspects in cyclones ...
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Introduction Cyclones constitute one of the major factors determining climatic conditions, especially precipitation in the middle latitudes. Changes in the properties of cyclones in a region may lead to variations in the precipitation conditions of that region. Therefore, studying major aspects in cyclones can clarify variations in precipitation conditions. In this research, changes of cyclones associated with precipitation in the Zard Rud basin (a Sub-basin of Jarahi basin in khozestan) has been reviewed decadal during a period ranging from the hydrological year of 1976-1997 to 2013-2014. In this research, daily precipitation maps during the studied period (13879 days) by using kriging method has been provided. Data and methods So the long-term precipitation mean of all days were extracted and by using 50 percentile, rainfall season detected. Upon identifying the precipitation season, Cyclones detected for this period. For identifying cyclones 1000 hPa hourly maps (NCEP/NCAR) were utilized. Two conditions were used to detect available cyclones: (1) the height values in each pixel of the 1000 hPa height map should be smaller than those of it 8 neighboring pixels and (2) the gradient mean of the height of the selected pixel and its 8 neighboring ones that was equal or smaller than 100 m/1000 km was regarded as the cyclone center. After identifying the cyclones on the map, the center of each cyclone was identified in consecutive maps to track the cyclone path. It was hypothesized that precipitation in the basin of the Zard Rud would be affected by the cyclones dominating the area as well as the trough of the cyclones that were far from the area, but could influence the region. Discussion Cyclones associated with precipitation in the basin were identified in the light of the presence of the cyclone or its troughs over the region during the occurrence of a precipitation. The results show that The extent of the area and frequency of cyclones in studied decades and consequently frequency of cyclonic rainy days and annual cyclonic precipitation in Zard Rud basin have decreased. Reduction in the frequency of cyclonic precipitations can be attributed to the place where the cyclones are formed. Indeed, in comparison with the past, a larger number of cyclones are formed over Saudi Arabia and Iraq, a phenomenon which has led to the entrance of dry or less humid air into the studied region. Masoudian (2012: 15-33) also indicated that a cyclonic center was formed over Iraq. Results Longitudinal extent area of cyclones decreases from 72.5˚ in first decade to 55˚ in fourth decade and Latitudinal extent decreases from 30˚ in first decade to 25˚ in fourth decade. Annual review on cyclones entry point to Iran show that minimum latitudinal extent from 1986-1987 hydrological year and maximum latitudinal extent from 1991-1992 hydrological year had fluctuation. So that, in 2011-2012 hydrological year, latitudinal extent of cyclones entry to Iran has reached the narrowest of its paths. Examining mean differences in the cyclone frequency of two halves of period (first half: 1976-1977 to 1994-1995 hydrological year and second half: 1995-1996 to 2013-2014 hydrological year) also revealed a noticeable shift in cyclones frequency. Result of surveying of cyclonic precipitation show that cyclonic total precipitation decreased during the studied decades. However, frequency of cyclones is less than first decade but second decade has the maximum amount of precipitation. It is may resulted of continuity of cyclones in this region. Taken together, a change was observed in geographical extent and frequency of cyclones associated with precipitation in the Zard Rud basin, which in turn affected precipitation in the area
Climatology
Hossein Asakereh; Sepideh Barzaman; Ali shahbaee kotenaee
Abstract
Introduction
Rainfall is amongst the most important climatic elements with a lot of spatial and temporal changes; in contrast to the other climatic phenomena, rainfall features more notable movement complexity. The studies performed in this regard indicate that such a climatic element as rainfall features ...
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Introduction
Rainfall is amongst the most important climatic elements with a lot of spatial and temporal changes; in contrast to the other climatic phenomena, rainfall features more notable movement complexity. The studies performed in this regard indicate that such a climatic element as rainfall features a non-stationary behavior with a vast part of this non-stationariness being the result of the rainfall’s being influenced by the spatial properties and the complex pattern of the spatial organization causes the emergence of complex behaviors in the precipitations. The importance of the rainfall as the country’s water resource and the daily increasing reduction of the country’s water reservoirs demands the study of the rainfall’s behaviors. In the present study and by the assistance of the methods of spatial statistics, the spatial pattern of the spring rainfalls in the northwest of Iran will be elaborated.
Data and Method
The study area of the present study is the regions in the northwest of Iran (Azerbaijan-e-Gharbi, Azerbaijan-e-Sharghi, Ardabil, Kurdistan and Zanjan Provinces) and, to perform the study, use has been made of the monthly precipitation data acquired from 121 synoptic stations as well as climatological investigations and precipitation studies for a period between 1994 and 2014.
In order to perform the spatial analysis of the precipitations, use has been made of the digital map of the elevation in the environment of ArcGIS software for extracting slope and dip. In the next part and in order to analyse the spatial structure of the rainfall and investigate the degree of similarity between the data acquired from 121 station points, use has been made of the half pseudo-variance spatial correlation index. The Semivariogram has been estimated based on the arithmetic mean of the intervals.
In the present study, use was made of the longitude and latitude of every station point and the rainfall rates of every point for delineating the empirical Semivariogram for three months, namely April, May and June in the environment of Variowin Software, version 2.2; then, various theoretical models were estimated in terms of their goodness of fit and the exponential model was selected as the best model for every month.
In order to analyse the spatial factors influencing the spring rainfalls in the northwest, the balanced geographical regression model was estimated in terms of its goodness of fit with its output being the indicator or indicators influencing the occurrence of spring rainfalls according to the explanatory variables of slope, dip, elevation and latitude.
Results and Discussion
Following the investigation of the data related to April, it was made clear that the effect of the elevations on the rainfall variations is significant in this month in the entire parts of the region. The highest significant effect of the elevation has been in the central parts of the region for such a reason as the large density of the mountainous masses in this part and the passing of precipitation systems from these regions. In parts of the region’s north (north of Azerbaijan-e-Gharbi, Azerbaijan-e-Sharghi and Ardabil), the significance rate of elevation is reduced because these regions are plains and plateaus and lower in elevation than the other areas. Latitude has been found having a significant effect in the southwestern sections of the region (south of Azerbaijan-e-Gharbi, Kurdistan and Zanjan) and, in a more scattered manner, in the north of Urmia Lake and it seems that the reason for such a significance is the passing of the precipitation systems from the southern sections of the region.
As in April, the effect of the elevation on the rainfall has been also found significant in all the sections of the region in May. The highest rate of significance has been found centered in the western and central sections of the region (particularly in the central parts) and this is completely due to the existence of the mountainous air masses. Considering the gradual displacement of the western winds towards the northernmost parts of the region and the vertical irradiation of the sunlight onto the sun-facing foothills, the role of the elevations becomes more accentuated in the creation of convectional rainfalls and the regions with lower elevation would receive lower precipitations.
In June, as well, except the south-eastern section of the region (eastern half of Zanjan Province), the other regions have been found with the significant effect of elevation on the creation of rain. In this month, the conditions fit the occurrence of foothill convection in the studied area. The highest effect of the rising and falling lands on the creation of the rain has been evidenced for the north-western sections and this is in match with the path through which the western winds pass on these days; that is because the rain-causing winds are present in this section in this month and, considering the region’s elevations, cause the occurrence of rainfall. The effects of latitude in June is like those in May and the presence of the western winds and setting of the ground for the foothill convention causes rainfalls in the northern and central section of the study region.
Conclusion
Elevation has been found influential in the entire studied region on the rainfall because the high density of high grounds causes the ascension and condensation of the humid air that causes rainfall. Besides elevation, the dip also influences the rainfall in Kurdistan and south of Azerbaijan-e-Gharbi because the orientation of the foothills in this section sets the ground for the dynamic ascension of the humid air. In Ardabil and north of Azerbaijan-e-Sharghi, slope is also an effective factor. The high slope of these regions causes the acceleration of the humid air masses’ ascension. The effects of the latitude on rainfall during spring are different and mostly related to the presence of the western winds; in April in Kurdistan and in May and June in Azerbaijan-e-Sharghi, latitude has been found with the highest effect on the rainfall.
