Field study and physical modeling of a few downbursts in the Tehran area


Downbursts are small-scale, short-lived and energetic events in the atmosphere. They usually occur from the beginning of spring to summer when the weather is more humid and the convective weather conditions are suitable. Because of the small-scale property of the downburst, it has sometimes been observed only in a meteorological station and other stations have not observed considerable changes.
The structure of the downburst outflow is examined by means of surface meteorological data at the Institude of Geophysics and Mehrabad Airport stations and skewt charts. Distinctive features associated with downburst are: a sudden drop in temperature, sudden increase in wind speed, wind shift, pressure rise and humidity increment. Strong vertical and horizontal shears are also observed according to sodar data.
In this study the characteristics of 12 outflow samples between 2003 and 2005 using surface meteorological data, and 5 outflow samples using skewt charts and one downburst sample using sodar data, are considered. Using sodar data for the downburst, the horizontal and vertical speed and wind direction relatiue to altitude have been analysed before and after the downburst occurauce. Using these profiles, and changes and maximums have been calculated. Skewt charts have been used for 5 instances, the temperature laps rate in the environment and buoyancy force have been calculated for each one, and compared to other similar instances worldwide. Downbursts have occurred when laps rates of temperature in the environment had been more than 9 (around dry adiabatic lapse rate). When the temperature laps rate in the environment was closer to the dry adiabatic lapse rate, the downdrafts were found to be more powerful and is independent from humidity mixing ratio.
The best downburst detectors are Doppler radars and lidars. Because of the unavailability of such instruments a physical model of downburst was setup in the laboratory. Vertical release of a known volume of dense fluid is simulated in an isotropic and in a density stratified environment. As soon as the released fluid reaches the surface, it forms the shape of a vortex ring and spreads out in all directions like a gravity current. Typical Froude numbers of the flow in 0.7. The structure of the flow is monitored by conductivity probes showing inhomogeneous turbulent structure. Because vertically moving flow in the atmosphere can lead to danger to flying aircraft near the ground it must be studied and monitored near the airports using fast response monitoring systems. This study may be a commencement for gathering downburst climatology data in the Tehran district.