analysis of instability indices during severe weathers, in mashhad metropolis

1. introduction increasing of extreme events such as flood, hailstorm, drought, thermal waves, dust storm and other dangerous climatological events, especially in local scale, could be the result of the climatic change over the globe. the characteristics and impacts of severe weather are function of place and time of occurrence. severe weathers are usually accompanied by strong winds and heavy hailstones, which are very destructive. they develop and then downfalls very fast. this is the reason that makes forecasting of these phenomena difficult. it is very important to study severe weathers from both thermodynamic and dynamic points of views. the aim of this study was to improve forecasting by investigating the relationship between upper atmosphere conditions and surface weather conditions in mashhad, iran. showalter (1953) introduced an index named si for determining the amount of instability and prediction of the thunderstorm and hailstorm. galway (1956) extracted the lifting index of li among his studies over convective events. his index is more or less similar to that of si index and their thresholds amount for occurring the thunderstorm and hail are same. george (1960) developed a simple formula for calculating k index using dry bulb and dew point temperatures in different atmospheric level. miller (1972) introduced the sweat and tt indices by combining some different indices. miller and mancriff (1976) have been finally developed a strong index of cape for evaluating the potential activity of the thunderstorm. 2. study area in this research, mashhad synoptic and upper atmosphere station, is located in the northeast of iran with 36° 16´ n latitude and 59° 38´ w longitude, was selected as the study area, and severe instabilities that occur during 1980-2009 were used for analysis. 3. material and methods in this research, we try to introduce the parameters and instability indices and try to prepare the summary of their thresholds to anticipate the kinds of instabilities. therefore, set of parameters and indices is calculated specifically for mashhad station as follows: showalter index, lifted index (li), sweat, k index (ki), total totals (tt), convective available potential energy (cape), cin, and equilibrium level pressure(el), precipitable water (pw), bulck richardson number (brn). severe instabilities which occurred during 1980- 2009 are used in this analysis. the severe instability reports were obtained from the metar, synop, and special meteorological reports, and also from synoptical notebooks. the international criteria for severe instabilities [winds greater than 50 knot (25 m/s), all kind of hail reports, all kind of thunderstorms, lightnings, squalls, funnel clouds, and tornado or waterspout] were used. all of these reports have been classified in three categories: lightning, thunderstorm, and hail. the days with no of these criteria are called no-instability. instability indices were derived from radiosonding data of mashhad upper air synoptic station that collected in wyoming university database. 4. results and discussion for most indices, the threshold that gives the best prediction was determined using statistical methods, box plots, and scatter plot diagrams. for achievement to better results, relationship between two different indices in forecasting of phenomena was analysed, and complex of each pair of indices such as cape with lifted index (li), and lifted index with equilibrium level pressure (el), that had a strong relationship, introduced as the best combination indices. table 1 lower and upper thresholds for instability indices during various severe weathers in mashhad. lightning threshold thunderstorm threshold hail threshold indices & units - cape (j/kg) - sweat si(°c) li(°c) ki(°c) tt(°c) - - plcl (hpa) tlcl (°k) - lfc (hpa) el (hpa) (g/kg) the threshold of most individual indices, which gives the best prediction for different kinds of phenomena was determined and introduced, that are summarized in table 1. then upper and lower thresholds of different indices in forecasting of severe weathers were showed in box plots, and scatter plot diagrams. finally, relationship between two different indices in forecasting of phenomena was discussed, and complex of cape with lifted index (li-cape), and lifted index with equilibrium level pressure (li-el), had a power relationship and introduced as the best combination indices. 5. conclusion a set of sounding data was carried out in mashhad station, in the period 1980-2009 were analyzed and the result is that using instability indices, has very good performance in predicting and warning of severe weathers. in this study, 12utc sounding has better result than 00utc for severe weather forecasting. for some indices such as richardson number (brn), convective inhibition (cin), and thickness results were not favorable. using these parameters individually may result in error for prediction, and it is necessary that all indices are used in combination with each other and, synoptical methods of forecasting.