Tuesday, 5 October 2004
MESOCYCLONE INDUCED SEVERE WINDS WITHIN DERECHO PRODUCING MESOSCALE CONVECTIVE SYSTEMS (DMCSs)
Straight-line winds within an MCS have typically been associated with downbursts or rear inflow jet induced winds succeeding the gust front. The latter has been correlated to radar imagery depicting a bowing line echo wave pattern; more specifically near the apex of a bowing line or line segment. However, it has been noticed that in numerous instances, severe wind damage also occurs northwest of the apex, frequently causing damage over a broad area. A proportion of these severe “straight-line” winds may transpire from surfaced based mesocyclones. Recently, numerical modeling studies support mesocyclone induced severe winds not associated with downbursts or bowing lines.
Analysis of WSR-88D Doppler radar data indicates mesocyclones may indeed be accountable for some severe surface winds. Analyses were performed on selected MSCs in which severe winds occurred. Only events within a 120 km radius of a WSR-88D site were used, as beyond this range surface features cannot be determined. Using the 2 dimensional radial velocity fields, visual inspection of WSR-88D data was used to determine if mesocyclones existed at or near the time of severe wind reports. In a number of instances, severe winds were found to take place in absence of a bowing line but congruent with a mesocyclone at or near the surface.