Radial Velocities and Cold Fronts Cold Fronts Usually

Radial Velocities and Cold Fronts

Cold Fronts Usually marked by quite sharp change in wind direction. This will mean sharp change in radial component of wind l Will show up as a discontinuity on Doppler. l

Diagnosing Wind Pattern Visually we see a northeast to southwest discontinuity running through the centre of the screen. l Let’s assess the flow either side of the line. l

East of discontinuity Zero line along the 110 degree azimuth. l Winds from 200 degrees. l We can see a jet structure with speed max of about 30 m/s between 1 and 1. 5 km above the radar. l

West of Discontinuity Look along the 360 degree azimuth. l The zero line runs right along this with inbound to the left and outbound to the right. This implies winds from 270 degrees. l West wind west of the discontinuity, southwest ahead of it. l

West of Discontinuity continued l Beyond 60 km (800 m. above the radar) along the 360 azimuth we see away velocities again. The beam has gone back into the warm sector.

Calculate Slope of Front At 360 degrees at 60 km the cold air is 800 m thick. The perpendicular distance to this point from the front is about 35 km. l So the slope of the front is about 1: 45. l

Why does the front look like this on Doppler Just east of the discontinuity the radar beam is looking directly into the wind (200 degree wind) l Sharp frontal trough- wind will quickly shift to west and the inbound component drops rapidly. l We get a minimum of inbound wind right at the front even if the actual wind speed across the front don’t change. l Thus the discontinuity. l