Using Doppler Radar to “Nowcast” Winter Storms ...A Review

1. Using Doppler Radar to “Nowcast” Winter Storms...A Review MSC Winter Severe Weather Workshop Boulder February 2002

2. Outline • Doppler in Synoptic Storms is More than just a profiler. • We can use the “Non Symmetry” of radial velocity displays in Nowcasting. • Frontal Structures • Convergence, Ascending jets. • Topographical Effects • Valleys, Upslope Flow

3. Outline.. continued • Forecasting Precipitation Phase • Combining Bright Bands and radial velocity displays • Winter Quantitative Precipitation Estimation using radar.

4. Precipitation Phase • Warm Fronts • From radial velocity patterns • Depth of cold air • Nowcasting of Temperature Advections • Changes in Strength of low level flow. • From logz and cross sections • bright band • Freezing level, lowest extent of melting snow

5. Example 1 • February 15 1990. • Major event in Toronto: 20 cm of snow followed by 24 hours of ZR with 2 cm accumulations. • Models underestimated outflow and had Toronto changing to rain 18 hours too early. • Doppler useful in assessing situation.

10. Two Different Warm Fronts It’s “What Lies Beneath” That Counts.

13. Bright Bands • Most pcpn forms through ice process rather than rain process • As snow falls through FZG LVL snow melts from outside, thus initially looks like slow falling large raindrop • Results in 5-15 dBz increase in reflectivity • Relative max of reflectivity (Bright band) is just below FZG LVL. Persists til all snow has melted. • Most predominant in stratiform situations

17. Bright Band Example B.C. 13 Dec 1998

21. Bright Band- RHI display

22. Bright Bands and Forecasting • Freezing Levels • Vertical Extent of wet snow. • Very useful in varying terrain.

23. March 27 2001 • Synoptic storm moves through lower mainland. • Rain and plus 9C had been forecast. • Outflow brought colder temps (3C YVR) and wet snow in higher suburbs.

24. Sequence of Events on Radar • Low level flow initially southeast then outflow develops down Fraser Valley. • Low level air cools with outflow and freezing level drops. • Outflow weakens and freezing level starts to rise again.

25. March 27 2001 • Interesting example of combining bright band with radial velocity winds. • Bright band shows height of melting layer lowering corresponding to intensity of outflow.

26. Between 1900z and 2000z lowest level flow backs and by 2200z there is a significant outflow down the Fraser Valley. • By 0030 the outflow reaches it’s peak and then starts to weaken.

29. Relationship between outflow and freezing level • Bright band gives height of freezing level and lowest elevation of any wet snow. • Let’s look at relationship between bright band and outflow. • Windows media player.

31. Synoptic Example 2 • Dec 11 2000 • Cold frontal passage at Toronto followed 12 hours later by major precipitation event. • Models indicated snow/freezing rain line very close to Toronto. • Use Doppler pattern to monitor accuracy of models.

38. Synoptic Example 3 • March 5 2001 • Great uncertainty about snow amounts over Toronto. • Rather innocuous pattern. Strong low level convergence on Doppler indicated significant low level vertical velocities going on.

43. A similar Alberta example

48. Cold Fronts and radial velocities

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