A statistical–dynamical analysis of severe convective storms in Ahvaz

Thunderstorms often appear with a strong, energetic and short-term air flow. These types of storm are caused by the ascent of hot and humid air in an unstable atmosphere and can lead to heavy rainfall if there is sufficient moisture, and otherwise they will lead to a dust storm. The purpose of this study was to investigate the storms in Khuzestan province, which usually experiences gusty winds, and convective storm events during spring and early summer. With regard to the frequent occurrence of dust storms in Khuzestan in recent years, it has become ever more important to study the convective storms. In order to identify the storm events in Khuzestan, Ahvaz station was selected due to much longer record of data and more accurate information than other stations in the province. A statistical analysis has been carried out on the long-term data of Ahvaz synoptic station between 1981 and 2016. After a thorough analysis of the long-term data, five cases of severe convective storms were selected. The cases include the 16th of October 1981 (case 1), the 27th of August 1985 (case 2), the 18th of December 1985 (case 3), the 24th of April 1992 (case 4) and the 8th of May 2000 (case 5). Finally, the characteristics of the cases were determined by numerical simulations using the Weather Research and Forecasting (WRF) model. The simulations were performed using four nested domains D1 to D4 with horizontal resolutions of 81, 27, 9 and 3km, respectively. The NCEP/NCAR reanalysis data were used for the boundary and initial conditions.Statistical analysis shows a decrease in the intensity and frequency of occurrences of the convective storms with maximum wind speed of greater than 12m/s during the 36-year period. Monthly analysis reveals that the most (least) frequent severe convective storms occur in April (October). Another important finding is that the severe convective storms occur mainly in spring and in the interval between 12UTC and 15UTC. From the early hours of the day until 13:00 UTC, the percentage of events increases, and then until the end hours of the day, the percentage of events decreases as the surface heating is maximized in the afternoon. The low frequency of occurrence in autumn and winter is due to reduced surface heating as a result of reduced solar energy received from the sun. The results of numerical simulations demonstrate that the position of the maximum wind speed is different in the considered cases. The comparison of the results of simulation of the selected cases with observational data shows that the model of simulated storm has a time delay of about 1 to 3 hours with respect to the actual storm occurred in the area. Tracking the storms from 30 min before to 30 min after the occurrence of the maximum wind speed along the path in the fourth domain of the model points out that the cases 1 to 3 evolve in the northeast and the cases 4 and 5 evolve in the northwest of Ahvaz station.