Institute of Geophysics, University of Tehran, P.O. Box 14155-6466, Tehran, Iran
Instability indices are some parameters that could be used to determine the instability of different parts of the atmosphere. These indices are used to forecast convective activities and are calculated using thermodynamic diagrams derived from radiosound data.
In this work, some of the instability indices are calculated for the Esfahan area during a 12 year precipitation period (1991-2002). These indices are: precipitable water of cloud (PWC), Showalter index (SI), K index (KI) and convective available potential energy (CAPE). The precipitation data are obtained by averaging observations from Esfahan airport (east of Esfahan) and Najaf Abad (west of Esfahan) stations. A suitable threshold for convective cloud seeding decision has been estimated using the best fit diagram of the index. This threshold has been found to be 9 mm for precipitable water of cloud, 2 degrees centigrade for Showalter index, 24 degrees centigrade for K index and 100J/kg for CAPE.
The results confirm previous cloud physicists, findings that precipitable water of cloud is the best index to estimate precipitation amounts for the Esfahan area. In addition, high values of K index and CAPE with negative and low values of Showalter index may indicate intense precipitation.