Animated loop of
McGill University (Montreal, Canada) radar data
during passage of the July 4-5, 1999 derecho-producing convective system
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Explanation of the animation
The animated movie loop displays four visualizations of the radar data from the McGill radar made during the period of approximately 2 1/2 hours from 1:31 to 3:56 a.m. EDT on July 5, 1999. The four animated panels include:
1. Upper left: Loop of the radar reflectivity (precipitation) showing the derecho-producing bow echo system moving across southern Quebec and into northern New England.
2. Upper right: Loop of radar-measured Doppler winds (in the same format as Figure 5) as the derecho-producing bow echo system moved across southern Quebec and into northern New England.
3. Two bottom panels: These panels show vertical views or "cross sections" of the storm as it moved through the area. The vertical view is along the diagonal line shown on the upper panels, with "A" at the beginning of the line and "B" at the end of the line. "A" is in the same location as the left edge of the bottom panels, and "B" is in the same location as the right edge of the bottom panels. The height above the ground is depicted on the left side of the panels, and ranges from the surface to 14 km (surface to 46,000 feet). The funnel-shaped gray area near the center of the bottom panels is centered on the location of the radar and is called the "cone of silence." This is the area above the radar site where data cannot be collected due to limitations in the degree of radar tilt imposed by the construction and layout of the radar equipment.
3a. First bottom panel: This panel shows the vertical structure of the reflectivity (precipitation) as the derecho-producing bow echo system moved along the diagonal line from A to B (in upper panels). The colors of yellow, red, and purple show the strong, upright bow echo thunderstorm moving to the right with the gust front. Trailing in a horizonal fashion behind the bow echo thunderstorm is a stratiform (non-convective) rain layer (green and blue colors).
3b. Second bottom panel: This panel shows the vertical structure of the winds (relative to the radar location) as the derecho-producing bow echo system moved along the diagonal line from A to B (in upper panels). Note the band of severe winds that descends from an altitude of 4-5 km some distance behind (west of) the gust front to the surface immediately behind the front. This band is depicted in dark blue and green to the left (northwest) of the radar site, and in purple to the right (southeast) of the radar. This band of strong winds, known as the rear-inflow jet , is associated with the development of the trailing stratiform rain layer in the wake of the most intense convection (see panel 3a). It is this slanting band of wind that, in general, is responsible for the severe gusts that occur at the surface immediately behind a derecho's gust front.