عنوان مقاله [English]
In this research, patterning of rainstorm (more than 10 mm) was conducted by instability indices in Ahvaz. In this study, to extract the patterns of rainstorm formation, at first, rainstorm occurrence in an 11-year period (2000-2015) was taken from the synoptic stations in Ahvaz. By sorting the rainfall data, precipitations more than 10 mm were extracted, classified and examined on monthly and seasonal bases. Then, data from radiosonde observations of Ahvaz was used. The advantage of this approach is that it is based on direct observations of the atmosphere. Archives of radiosonde data were obtained from the website of the University of Wyoming’s department of atmospheric science. Then, using the RAOB software, the Skew-T diagram was plotted for the radiosonde data. The analyzed instability indices include: Showalter index (Showalter, 1953), lifted index (Galway, 1956), K index (George, 1960), Total index (Miller, 1972), Severe Weather Threat Index (SWEAT), Precipitation Water (PW), Convective Available Potential Energy (CAPE) (Moncrieff & Miller, 1976), Lifting Condensation Level (LCL), and the Level of Free Convection (LFC). The results revealed 60 rainstorms for the respective period. More rainstorms occurred in January and December. Fall and winter had the most frequent days of rainstorm, while it did not occur in the summer. Maximum CAPE was observed in the second and forth patterns, while the minimum value was in the first and second patterns, respectively. Maximum TT index indicating a higher possibility of severe rainstorm was in the first and forth patterns. Lift index was negative only in pattern four showing the intensity of the lifting of the air mass. Maximum K index, typically expected for severe rainstorms, was in patterns one and four. CCL was high in patterns one and two, while the highest value was observed in pattern three. The minimum of LCL commonly used to estimate the level of a cloud base from surface-based convection was in patterns one and two. Then, with regard to instability indices, the highest probability of the rainstorm was evaluated in the first and fourth patterns. The results of classification divided rainstorms into 4 patterns and more days were in the fourth class, while the least was in second class. In the second and fourth classes, instability indices were severe and could predict possible rainstorms, but the first and third classes couldn’t predict because synoptic systems caused the occurrence of the rainstorm. In the second and fourth classes, rainstorms were convectional. The synoptic analysis showed that every time rainstorm occurred in Ahvaz, a deep trough at 500 hPa was formed in the East Mediterranean and the study area was in front of it. Also, at sea level pressure, a low-pressure system formed in Iraq and winds got humid by passing through the Persian Gulf and entered into the atmosphere of Ahvaz. Due to the unstable atmosphere, the air would rise to heights causing rainstorm.