Doppler Weather Radar Concepts<br /><ul><li>Radar beam increases in altitude with distance from radar.
Radar beam increases in size with distance from radar.
Radar takes 4-5 minutes to complete full scan when in storm/precip mode.
Reliable doppler wind data limited to about 60-70 nm.
Radar velocity 100% when parallel, 0% when perpendicular to beam direction.</li></li></ul><li>The Zero Isodop “Problem”<br />When the radial is perpendicular to the the wind, the radar displays zero velocity - This “zero zone” is called the “Zero Isodop”.<br />What percentage <br />of actual wind <br />will the radar detect?<br />00 = 100% - Parallel<br />150 = 97%<br />300 = 87%<br />450 = 71%<br />600 = 50%<br />750 = 26%<br />900 = 0% - Perpendicular<br />When the wind velocity is parallel to the radial, the full component of the wind is measured<br />
Products<br />Base reflectivity <br />how much precipitation is falling <br />precipitation type<br />assess a storm's structure and dimension<br />Composite Reflectivity<br />Scans from all elevations, imaging precipitation intensity and storm structure<br />Base Velocity<br />radial wind field, speed of fronts/strong wind<br />range of 140 mi<br />Storm relative motion<br />Track a circulation (show up well in doppler return) over time to determine storm motion.<br />Removing the storm relative motion from base radials gives an estimate of the flow with respect to the storm.<br />
LOT<br />Height of beam above ground vs distance from radar. <br />Green 10nm<br />Yellow 15nm<br />Red 20nm<br />
What is a supercell?<br />Storms having deep, persistent, rotation about a vertical axis<br />
What is a supercell?<br />What are the biggest unanswered questions in the study of tornadogenesis?<br />Why do storms with seemingly similar structure differ in their tornado production?<br />Although most significant tornadoes are associated with supercell thunderstorms, most supercells are nottornadic<br />What’s perhaps most troubling (from the perspective of issuing warnings) is that most supercells contain low-level mesocyclones, and perhaps most supercells even have circulations that extend to the surface<br />nontornadic<br />nontornadic<br />nontornadic<br />tornadic<br />
April 9, 2009 day 1 of 2 day storm outbreak 5PM<br />0-6 km Shear Vector<br />
Tornadogenesis: Three ingredients<br />1<br />Development of a persistent, rotating updraft<br />2<br />Ingestion of enhanced SRH (occasionally, large-scale SRH is sufficient) and development of strong low-level rotation<br />3<br />Development of a downdraft partially embedded in the rotation that aids in the transport of rotation to the ground, followed by focusing of that rotation through convergence if the downdraft reaches the ground with some very uncommon properties<br />
The Need for Better Understanding of Tornadic Storms<br />Tornado warnings<br />Improvements in our understanding of tornadogenesis should better allow us to assess the likelihood of tornadoes in thunderstorms<br />Possible advances in our ability to forecast tornado intensity and longevity<br />Improvements likely due to 88D network, better training, better SPC guidance, application of VORTEX1 findings?<br />Tornado warning performance from 1986-2002 (adapted from Brooks 2004)<br />
Summary (what we know)<br /><ul><li>Supercells acquire rotation aloft by tilting horizontal vorticity (streamwise horizontal vorticity leads to net cyclonic updraft rotation)
Although most significant tornadoes are associated with supercell thunderstorms, most supercells are not tornadic (and the supercells with the strongest mesocyclones are not necessarily the most likely to be tornadic)
Tornadogenesis requires a downdraft if pre-existing vertical vorticity is absent at the surface
The temperature of the downdrafts seems to be important to tornadogenesis; downdrafts that are excessively cold apparently are unfavorable for tornadogenesis
Environments that have large ambient low-level vertical wind shear (larger than what is found in an average supercell environment) and large ambient relative humidity favor tornadic supercells over nontornadic supercells</li></li></ul><li>LSX Composite<br />