资源环境科技发展态势分析平台

The severe convective storm (SCS) events that affect Tehran during spring and early summer have potential to cause devastating effects. To study SCSs in Tehran, a climatological analysis is carried out using surface data of Tehran's Mehrabad station from 1961 to 2015 and the NCEP/NCAR reanalysis data to compute the large-scale and the local stability indices. Based on the maximum wind speed, the events are classified into three categories consisting of convective storm (CS), severe convective storm (SCS) and extremely severe convective storm (ESCS).

The annual frequency of occurrence of all three categories exhibit decreasing trends during the period of study. Late spring and early summer (April to June) are the favorite times for the SCS and ESCS, and the whole spring is for CS events. In diurnal distribution, for the CS and SCS, the peak occurs at 12-15 UTC. Generally, in response to diurnal surface heating, CS events tend to develop during early- and mid-afternoon, and SCS events during late afternoon. For the months of April to June, while the large-scale stability indices point to a positive trend, no clear trend can be inferred from the examination of local stability indices. While a partial explanation for the decreasing trend of the ESCS is provided based on the large-scale stability indices and the negative trend in the frequency of simultaneous occurrence of the local stability indices, it is likely that the decreasing trends observed for the convective storms are also due to the local urban effects.