investigation of synoptic and thermodynamic conditions of the thunderstorm leading to severe flooding in central alborz on july19, 2015

1. introduction thunderstorms or lightning storms are kinds of storms that generally are associated with the convective clouds and usually are resulted in hailing, strong winds, and flash floods. clouds of convective storms are observed in most regions, but a small percentage of the convective storms can produce severe weather conditions and flash floods that cause damages. a flash flood is a dangerous inundation that occurs over a few hours or less, and is caused by heavy rainfall over a small area. this flood is one of the most common types of hazards produced by convective storms, which are difficult to predict. this is mainly because such severe storms are local events and they usually affect smaller areas compared to tropical cyclones. basically, vertical movements in the atmosphere are the key of most characteristics of convective systems. hence, conditions leading to the development of deep convection and resulting in severe weather events (e.g., flash floods, hail etc.) are characterized generally by unstable air, high moisture at low and mid-level, and the force which stimulates convection. this force may be caused, for example, by ground heating, orography, convergence,atmospheric front, jet stream and the like, although the synoptic patterns provide favorable conditions for the development of storms.thus, to study the convective storms, synoptic and thermodynamic conditions, humidity and similar conditions should be examined.one of these deadly thunderstorms which occurred on june 19, 2015 was a great human and financial loss. this storm killed around 20 people and financial losses were huge.in this study, the synoptic and thermodynamic conditions of this thunderstorm are discussed. the aims of this study are to predict the likelihood of thunderstorm occurrence, determine the extent of possible storms and the location of the convective storm and its relationship with the synoptic systems. 2. study area the study area is located between the northern latitude of 35° and 07 minutes to 37° and 06 minutes, and the eastern longitude of 48° and 40 minutes to 51° and 34 minutes. this area includes parts of the provinces of tehran, qazvin, zanjan, gilan, and alborz provinces. this area includes the southern slopes of the alborz mountains and the plains leading to these slopes. 3. material and methods previous studies in thunderstorm distribution have emphasized that the synoptic patterns provide suitable conditions in which thunderstorms can develop easily. thus using the ncep/ncar data, synoptic conditions in the middle and lower levels of atmosphere such as sea level pressure, geopotential height, omega, moisture convergence, specific humidity, wind pattern and precipitable water were investigated. fundamental to the understanding of severe weather and flash floods is how wind, temperature and moisture vary from the surface to the top of the troposphere. some indexes have been introduced to calculate these variations which are called the instability index.whatever is very important, the instability of the atmosphere can be determined several hours before the beginning of convection.hence, skew-t diagram and upper air data were used to determine instability with instability indexes. some of the important instability indexes arecape, ko, mvv, ji, ti, tq, vgp,vt,si, k, li, tt and sweatthat we used in this study to estimate the instability of the atmosphere. theseindexes were calculated withraob software.then,the intensity and probability of thunderstorm occurrence were determined. the occurrence of thunderstorm-related severe weather (flash floods, hail, etc.) is highly correlated with the intensity of convection. using ir data one should be able to calculate parameters related to convection intensity, such as cloud top temperature.therefore, with the calculation of brightness temperature, the vertical and horizontal extent of convective clouds was determined and their relationships with synoptic systems were examined in gis software. 4. results and discussion the results showed that the favorable synoptic conditions for thunderstorm occurrence, including low pressure at sea level and mid level, moisture convergence and sufficient moisture in the lower layers of troposphere were available in this day.the precipitable water on this day has been 30.29 mm that is sufficient to produce flash floods in unstable atmosphere. results of instability indices showed that ko, ki, ji and vt predicted stronginstability and severe convective storms. six indexes of ct, li, si, tq, tt and vgp predicted moderate instability (convective storms) and two indexes (sweat index and cape) predicted weak instability. raob software predicted that maximum vertical speed reaches to 30 m/swhich indicated sharp rise and severe thunderstorm.these results are for the 00 gmt. satellite image processing for cloud top temperatureindicated that several areas have deep convective clouds. these areas are in kordestan, east azerbaijan, zanjan, guilan, qazvin, tehran, alborz and semnan provinces where cloud top temperaturesare below 240 degrees kelvin. the main core of the storm was between karaj and qazvin and it matched with the maximum negative omega at 500 hpa. cloud top temperature at this core had been below 230 degrees kelvin and it had considerable horizontal extension.vertical extension of storm (based on cloud top temperature) between alborz and qazvin had been about 12 kms that confirms the results of instability indexes and synoptic conditions. these results are for the 12:50 gmt. hence, these results verified that the probability of the severe thunderstorm occurrence has been predictable several hours before beginning in the area. 5. conclusion although there is wide agreement that all thunderstorms require warm and moist air (as the prime gradients leading to the formation of thunderstorm) in the atmosphere, other suitable conditions are needed to increase instability and, as a result, to initiate convective activity. the roles that may be played by atmospheric instability are very important factors in thunderstorm development. in this case, the favorable synoptic conditions for thunderstorm occurrence, including low pressure at sea level and mid level, moisture convergence and sufficient moisture in the lower layers of troposphere were available. in additionto using instability indexes, the likelihood and severity of storm estimated several hours before occurrence.some instability indexes predicted strong instability and severe convective storms in study area and some others predicted moderate instability. the result of the preview on synoptic pattern and instability indexes proved that synoptic condition and instability were suitable for thunderstorm occurrence. thus, accompanying suitable thermodynamic and synoptic conditions can lead to the severe convective storms. results of satellite image processing indicated that cloud top temperature is a good representative of convective storms. the minimum cloud top temperature is correlated with report of severe weather on the ground.in this case, the cloud top temperature reached less than 230 degrees kelvin and according to the thermal profile of mehrabad station, the storm had spread 12 km far away.the minimum cloud top temperature that located between alborz and qazvin, matched with minimum of omega at 500 hpa.