عنوان مقاله [English]
The structures of thunderstorm outflows are studied using surface and sodar data at the Geophysics Station. Distinctive
features associated with gust fronts are: a sudden drop in temperature (-. 3-8 °C), a sudden increase in wind speed (10- 20 m/s), wind shift, a pressure rise (- 2-3 mb), a humidity increment and rain.
In this study, characteristics of 10 outflow samples are determined using sodar and surface meteorology data. Horizontal winds, maximum wind speed, propagation speed of gust fronts are computed using surface data. The charts of horizontal velocity, vertical velocity, direction and turbulence flow of wind (uv, uw, vw) are considered when the sodar was recording data. Vertical structure of horizontal wind is layered before the arrival of gust front on surface. In horizontal wind profiles, there are two maxima. When the horizontal component of wind is maximum, its vertical component wind becomes minimum, which is predictable through considering of vertical structure of gust front by continuity.
Time variations of vertical component of wind speed show that Kelvin-Helmhotz instability is developed along the top of the gust front head near the surface. Estimated dimension of Kelvin-Helmholtz billows is about 10-20 km which appears to be larger than the ones observed by others. This may be due to the fact that these gust fronts occur over a sloping boundary.
Contours of vertical and horizontal components of flow velocity and its horizontal direction of gust fronts show that these gust fronts have similar shapes but vary in sizes. This analysis also shows that the outflow has a complex and multi frontal structure. Further observational work is required for more detailed analysis of the gust front in vertical and horizontal extends.