عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Divergence and vorticity are two most attractive and popular quantities in the study of sever atmospheric phenomena. On the other side, the role of deformation, especially total deformation – expect for a few cases related to the frontogenesis theory – has received less attention of scientists. This study is trying to fill this gap a little. It is trying to examine the role of deformation in general and the stretching deformation, shearing deformation and total deformation in particular on precipitation. In doing so, the GFS (the Global Forecast System) wind components data in five years (2003-2005) were used to compute the stretching deformation, shearing deformation and total deformation in a network covering the Iranian and influential surrounding area.
In order to evaluate the effect of the deformation field on the atmospheric variable here we used the collective 24 hours precipitation reported by the synoptic observatory around the concerned domain. Due to the irregularity of the observatories in the beginning, the reported data of the rainfall from synoptic stations were moved to regular grid points applying Cressman objective analysis method. The distribution maps of precipitation and contours of stretching deformation, shearing deformation and total deformation also were prepared.
Comparison of the precipitation maps and the maps of stretching deformation, shearing deformation and total deformation in six, twelve and eighteen hours before the occurrence of precipitation, showed that in the concerned region the flux of total deformation pattern (dot product of the total deformation and the wind) was more consistent with the precipitation maps in 6 hours later. Therefore, the flux of the total deformation has a predictive value for precipitation and could be used as a precursor of precipitation.
Besides the general matching of the patterns of the total deformation flux and precipitation, the location of the maximum precipitation occurrence is reasonably compatible with the location of maximum flux of total deformation in six hours earlier. In the grid points of significant values of the precipitation, the total deformation fluxes were calculated for comparison. Also for more precise investigation, the values of the cumulative rainfall were divided into three groups (0-10), (10-20), (20-30) mm and the values of the total deformation flux were computed for each of these intervals. It has been verified that the interval of 0-10 has the maximum frequency of happening regardless of value of deformation flux magnitude. The probability that precipitation occurs between 0-10 is 26%. Also the probability of occurrence of larger deformation flux decreases exponentially as the value of deformation flux increases regardless of value of precipitation.
The results of the computation show that the total deformation flux is varying between 5e-5 and 10e-5 for all intervals of the three groups of rainfall. The elongation of the dilatation axis of the total deformation for all available data was computed. Based on the results of this computation the axis of dilatation in severe rain falls are elongated in the direction of West - East, Southwest - Northeast and North – South depending on the raining area and the core of maximum precipitation.