Presentation on theme: "Echo Tops Fairly accurate at depicting height of storm tops Inaccurate data close to radar because there is no beam angle high enough to see tops. Often."— Presentation transcript:
Echo Tops Fairly accurate at depicting height of storm tops Inaccurate data close to radar because there is no beam angle high enough to see tops. Often has stair-stepped appearance due to uneven sampling of data between elevation scans.
Precipitation Estimates Storm Total Precipitation ● Total estimated accumulation for a set amount of time. ● Resets storm total whenever there is no rain detected for an hour.
- Updated once per volume scan. -Shows accumulated rainfall for the last hour. -Useful for determining rainfall rate of ongoing convection. One Hour Precipitation Total
Precipitation Estimate Advantages and Limitations ●Great for scattered areas of rain where no rain gauges are located ●Provides a graphical ‘map’ of rainfall for an entire region ●Data can be overlaid with terrain and watersheds to predict reservoir and waterway crests ●Estimates based on cloud water levels and not ground level rainfall ●‘Hail Contamination’ causes highly inflated values ●High terrain causes underestimates ●Useful as a supplement, not replacement for ground truth information
Interpreting Doppler Signatures Display examples provided by: National Weather Service Steve Davis - Lead Forecaster Milwaukee/Sullivan National Weather Service Forecast Office
Weak inbound, weak outbound Rotation too small to be resolved Stronger inbound than outbound Strong inbound, strong outbound Azimuth 3 Azimuth 2 Azimuth 1 Enlarged image along a radial. Individual “blocks” represent one sample volume. This graphically shows the radar resolution. closer a rotation the more likely it will be identified correctly rotation smaller than the beam width (possible at long ranges) > rotation is average of all velocities in sample volume Previous 1 0 beam width improved by super-resolution Azimuth Resolution Considerations Rotational couplet identification can be affected by azimuth resolution.
When the wind velocity is parallel to the radial, the full component of the wind is measured When the radial is perpendicular to wind direction, the radar displays zero velocity - This “zero zone” is called the “Zero Isodop”. What percentage of actual wind will the radar detect? 0 0 = 100% - Parallel 15 0 = 97% 30 0 = 87% 45 0 = 71% 60 0 = 50% 75 0 = 26% 90 0 = 0% - Perpendicular The Zero Isodop “Problem”
Large Scale Winds “S” Shape “S” shape of the zero isodop indicates veering winds with height. Veering may imply warm air advection. The combination shape of the zero isodop indicates both veering and backing winds with height. Combination Backward “S” Shape Backward “S” shape of the zero isodop indicates backing winds with height. Backing may imply cold air advection. Use the Zero Isodop to assess the vertical wind profile.
Large Scale Winds Uniform Flow Straight Zero Isodop indicates uniform direction at all levels. Uniform Flow with Jet Core Straight Zero Isodop indicates uniform direction at all levels >> inbound/outbound max’s show a jet core aloft with weaker winds above and below.
Example from KMKX 88D Low level jet max January 5, 1994 Steady snowfall
The VAD Wind Profile (Velocity Azimuth Display)
Small Scale Winds - Diffluence/Confluence - Diffluence Often seen at storm top level or near the ground at close range to a pulse type storm Confluence would show colors reversed
Small Scale Winds - Cyclonic Confluence/Diffluence - Anticyclonic confluence/ diffluence would show colors reversed in each panel. Cyclonic ConfluenceCyclonic Diffluence
Small Scale Winds - Pure Cyclonic Rotation - Pure Cyclonic Rotation Anticyclonic rotation would show colors reversed
Small Scale Velocity Example
Rotation with tornado
Storm Relative Velocity - SRV vs Base Velocity SRV: Subtract estimated velocity of thunderstorm from the Doppler radial velocity– Make the storm stationary When diagnosing rotational characteristics, use SRV motion of the storm masks subtle rotations within the storm When diagnosing Straight Line Winds (bow echo, microbursts), use Base Velocity straight line winds are sum of the winds produced by the storms, plus storms movement
SRV vs. Base Velocity - strong rotation - Storm Relative VelocityBase Velocity rotation in tornadic thunderstorm
SRV vs Base Velocity - subtle rotation - Base VelocityStorm Relative Velocity Janesville F2 tornado. June 25th, 1998 ~ 700 PM Interesting note: These scans are at elevation. The elevation showed little rotational information.
SRV vs Base Velocity - subtle rotation Base VelocityStorm Relative Little/no rotation seen at lowest elevation
SRV vs Base Velocity Base VelocityStorm Relative Velocity
SRV vs Base Velocity - straight line winds - Base velocity shows max inbound winds of 55 to 60 kts. SRV shows max inbound winds of 30 to 40 kts.
Bow Echoes Detecting and Predicting Downbursts oBow echoes are caused by severe downbursts, accelerating part of a line of thunderstorms ahead of the rest. oThe strongest downbursts occur under and just north of the apex of the bow, but can occur elsewhere too oSurface winds can exceed 70mph in strong bow echoes. oBow echoes can move at over 50 mph. oHighest reflectivities and strongest velocities are found at the apex. oLook for a tight gradient of reflectivity.
Review Clear-Air Radar
VHF UHF 10 cm
VHF UHF 10 cm
Clear-Air Wind Profilers
Wind Profiler Specifications Frequency (MHz) Wavelength (m) Maximum Altitude (km) Antenna Size (m) TargetBand Designation x 100Clear AirVHF x 15Clear Air and Heavy Precipitation UHF 915~ x 5Clear Air and Precipitation UHF 1036~ x 5Clear Air and Precipitation UHF