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Severe and Unusual Weather ESAS 1115

Severe and Unusual Weather ESAS 1115. Spotter Training and Radar Meteorology Part 2 – Introduction to Radar Meteorology. Meteorological Sensors. Two types of two types of sensors: Remote vs. In-situ Active vs. Passive Our passive eyes can only see features of the storm

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Severe and Unusual Weather ESAS 1115

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  1. Severe and Unusual WeatherESAS 1115 Spotter Training and Radar Meteorology Part 2 – Introduction to Radar Meteorology ESAS 1115 Severe and Unusual Weather

  2. Meteorological Sensors • Two types of two types of sensors: • Remote vs. In-situ • Active vs. Passive • Our passive eyes can only see features of the storm • In order to see the inner workings of a thunderstorm, and to understand it better, we need an active remote sensor – weather radar • Radar will allow us to interrogate information about the storm by detecting precipitation and wind information within ESAS 1115 Severe and Unusual Weather

  3. Radio Detection and Ranging NEXRAD – Next Generation Radar WSR-88D – Weather Surveillance Radar, 1988 Doppler ESAS 1115 Severe and Unusual Weather

  4. Important Angles • The azimuth angle is the compass angle from north (360°) • Targets are indicated by A/R (Azimuth/Range) • Birds eye view is displayed on a PPI (Planned Position Indicator) • A cross sectional display is an RHI (Range Height Indicator) • Volume scan ESAS 1115 Severe and Unusual Weather

  5. Gate Size • Reflectivity Information is displayed in pixel-like units of area called “gates” • Gate size is determined by pulse length and angular beam width ESAS 1115 Severe and Unusual Weather

  6. Reflectivity • dBm = 10log(power returned/1milliwatt) • Z is reflectivity • dBZ is a logarithmic scale similar to dBm • 1dBZ isthe power returned by a sphere of 1mm6/m3 • Doubling of power results in a linear increase of 3dBZ • Z is proportional to D6 ESAS 1115 Severe and Unusual Weather

  7. Clear Air Mode Range from -28 to 28 dBZ 16 4dBz increments ESAS 1115 Severe and Unusual Weather

  8. Precipitation Mode Range from 5 to 75 dBZ 16 5dBz increments ESAS 1115 Severe and Unusual Weather

  9. The Significance of Clear Air Mode During Snowfall ESAS 1115 Severe and Unusual Weather

  10. NIDS - Nexrad Information Dissemination Service Base Reflectivity – Various Elevation Angles ESAS 1115 Severe and Unusual Weather

  11. NIDS - Nexrad Information Dissemination Service Base (or Storm Relative) Velocity – Various Elevation Angles ESAS 1115 Severe and Unusual Weather

  12. NIDS - Nexrad Information Dissemination Service 1 Hour Precipitation ESAS 1115 Severe and Unusual Weather

  13. NIDS - Nexrad Information Dissemination Service Storm Total Precipitation ESAS 1115 Severe and Unusual Weather

  14. NIDS - Nexrad Information Dissemination Service VIL – Vertically Integrated Liquid ESAS 1115 Severe and Unusual Weather

  15. NIDS - Nexrad Information Dissemination Service Echo Tops ESAS 1115 Severe and Unusual Weather

  16. NIDS - Nexrad Information Dissemination Service VAD – Velocity Azimuth Display VWP – VAD Wind Profile ESAS 1115 Severe and Unusual Weather

  17. NIDS - Nexrad Information Dissemination Service Composite Reflectivity ESAS 1115 Severe and Unusual Weather

  18. Interpreting Doppler Radar Radial Velocity is the velocity toward or away from a radar ESAS 1115 Severe and Unusual Weather

  19. Using the Zero Isodop When the radial is perpendicular to the the wind, the radar displays zero velocity - This “zero zone” is called the “Zero Isodop”. What percentage of actual wind will the radar detect? 00 = 100% - Parallel 150 = 97% 300 = 87% 450 = 71% 600 = 50% 750 = 26% 900 = 0% - Perpendicular When the wind velocity is parallel to the radial, the full component of the wind is measured ESAS 1115 Severe and Unusual Weather

  20. Interpreting Doppler Radar • Winds will flow perpendicular to the zero isodop from green to red • Veering wind profile is denoted by an “S” on the overall winds display • Veering winds with height indicate warm air advection and hence rising air ESAS 1115 Severe and Unusual Weather

  21. Interpreting Doppler Radar • Backing wind profile is denoted by a backwards “S” on the overall winds display • Backing winds with height indicate cold air advection and hence sinking air (subsidence) ESAS 1115 Severe and Unusual Weather

  22. What Does this Represent? ESAS 1115 Severe and Unusual Weather

  23. Veering Winds on VWPVelocity Azimuth Display (VAD) Wind Profile ESAS 1115 Severe and Unusual Weather

  24. Hurricane Katrina ESAS 1115 Severe and Unusual Weather

  25. Divergence Divergence is indicated by radial shear (along the radius) ESAS 1115 Severe and Unusual Weather

  26. Rotation Cyclonic rotation is indicated by azimuthal shear (from one azimuth to another) ESAS 1115 Severe and Unusual Weather

  27. Low-level Rotation and Storm Top Divergence ESAS 1115 Severe and Unusual Weather

  28. Rotation or Convergence? ESAS 1115 Severe and Unusual Weather

  29. Automated Detection of Meteorological Phenomena • Algorithms help detect significant features • MDA – Mesocyclone Detection Algorithm • Meso - “donut” • Persistent, strong and detected over a large depth of the storm • TDA – Tornado Detection Algorithm • TVS – Tornado Vortex Signature • Strong gate to gate shear • HDA – Hail Detection Algorithm • SCIT - Storm Cell Identification and Tracking ESAS 1115 Severe and Unusual Weather

  30. Composite Reflectivity with Storm Attribute Table ESAS 1115 Severe and Unusual Weather

  31. Composite Reflectivity with Symbols ESAS 1115 Severe and Unusual Weather

  32. Gate to Gate Shear (TVS) ESAS 1115 Severe and Unusual Weather

  33. TVS ESAS 1115 Severe and Unusual Weather

  34. TVS ESAS 1115 Severe and Unusual Weather

  35. Azimuth Resolution Considerations Rotational couplet identification can be affected by azimuth resolution. As the diagram shows, the closer a rotation is to the radar the more likely it will be identified correctly. If the rotation is smaller than the 10 beam width (possible at long ranges) then the rotation will be diluted or averaged by all the velocities in that sample volume. This may cause the couplet to go unidentified until it gets closer to the radar. Azimuth 3 Weak inbound, weak outbound Rotation too small to be resolved Azimuth 2 Strong inbound, strong outbound Azimuth 1 Stronger inbound than outbound ESAS 1115 Severe and Unusual Weather

  36. Strong TVS ESAS 1115 Severe and Unusual Weather

  37. SRV vs. Base Velocity with Subtle Rotation Base Velocity Storm Relative When diagnosing straight line winds use base velocity. The strength of an advancing line of storms producing straight line winds is the sum of the winds produced by the storms, plus the movement of the storms. When diagnosing rotation, use storm relative velocity. SRV subtracts out the motion of a storm to display pure rotational characteristics of that storm. ESAS 1115 Severe and Unusual Weather

  38. FAR vs. POD • FAR (False Alarm Ratio) – An event is warned for but does not occur results in a false alarm • POD (Probability of Detection) – An event that occurs and has been warned for results in a 100% POD • A high POD is achieved at the expense of an increased FAR ESAS 1115 Severe and Unusual Weather

  39. Doppler Dilemma • Radial velocity and range is limited by PRF • High PRF’s result in short unambiguous ranges and vice versa • Low PRF’s result in velocity aliasing and vice versa • Given a PRF, the radar can determine the radial velocity within some range, + or – some velocity, called the velocity interval • Any velocity beyond that range will “fold over” into the incorrect value • Bad data must be “dealiased” • The Doppler Dilemma: There is no single PRF that maximizes both Rmax and Vmax ESAS 1115 Severe and Unusual Weather

  40. Velocity Aliasing and Dealiasing ESAS 1115 Severe and Unusual Weather

  41. Three-Body Scatter Spike (TBSS) • The “flare” appears further than the main core along the same radial as the highest core • The added distance the beam makes from stone to ground to stone (3 bodies) results in a display further than reality ESAS 1115 Severe and Unusual Weather

  42. TBSS Examples ESAS 1115 Severe and Unusual Weather

  43. Same Storm, Different Radar ESAS 1115 Severe and Unusual Weather

  44. TBSS in Velocity Data ESAS 1115 Severe and Unusual Weather

  45. Anomalous Propagation (AP) In the wake of this line of thunderstorms, a low-level inversion created by the cold pool results in superrefraction and thus AP ESAS 1115 Severe and Unusual Weather

  46. AP and Ground Clutter AP is enhanced by strong returns from buildings (ground clutter) Cooling in the evening hours results in a low-level nocturnal inversion and AP ESAS 1115 Severe and Unusual Weather

  47. Sunset Spike ESAS 1115 Severe and Unusual Weather

  48. Sunset Spikes Sunset spikes ESAS 1115 Severe and Unusual Weather

  49. Sunset Spike ESAS 1115 Severe and Unusual Weather

  50. What the Heck Is This? ESAS 1115 Severe and Unusual Weather

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