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 One Hour Precipitation Total Updated once per volume scan Shows accumulated rainfall for the last hour Useful for determining rainfall rate of ongoing convection Precipitation Estimate Advantages and 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 Limitations 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 Azimuth Resolution Considerations closer a rotation the more likely it will be identified correctly rotation smaller than the 0 50 beam width possible at long ranges rotation is average of all velocities in sample volume 0 Previous 1 beam width improved by super resolution Enlarged image along a radial Individual blocks represent one sample volume This graphically shows the radar resolution Azimuth 3 Strong inbound strong outbound Rotational couplet identification can be affected by azimuth resolution Range 0 Weak inbound weak Rotation too outbound small to be resolved example 120 nm Azimuth 2 Azimuth 1 Stronger inbound than outbound The Zero Isodop Problem When the radial is perpendicular to wind direction the radar displays zero velocity This zero zone is called the Zero Isodop 0 100 100 When the wind velocity is parallel to the radial the full component of the wind is measured 0 What percentage of actual wind will the radar detect 00 100 Parallel 150 97 300 87 450 71 600 50 750 26 900 0 Large Scale Winds Use the Zero Isodop to assess the vertical wind profile The combination shape of the zero isodop indicates both veering and backing winds with height Combination S Shape Backward S Shape S shape of the zero isodop indicates veering winds with height Veering may imply warm air advection Backward S shape of the zero isodop indicates backing winds with height Backing may imply cold air advection Large Scale Winds Uniform Flow Uniform Flow with Jet Core Straight Zero Isodop indicates uniform direction at all levels 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 Confluence Cyclonic Diffluence Small Scale Winds Pure Cyclonic Rotation Anticyclonic rotation would show colors reversed Pure Cyclonic Rotation Small Scale Velocity Example 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 Velocity Base Velocity rotation in tornadic thunderstorm SRV vs Base Velocity subtle rotation Base Velocity Storm Relative Velocity Janesville F2 tornado June 25th 1998 700 PM Interesting note These scans are at 3 40 elevation The 0 50 elevation showed little rotational information SRV vs Base Velocity subtle rotation 3 40 Base Velocity Little no rotation seen at lowest elevation 0 50 Storm Relative SRV vs Base Velocity Base Velocity Storm 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 l 4 10 cm l 1 3 UHF VHF l 2 6 4 N 6 n a i i i 1 10 cm UHF Cn2 l 1 3 VHF Clear Air Wind Profilers Wind Profiler Specifications Frequency MHz Wavelength m Maximum Altitude km Antenna Size m Target Band Designation 50 6 20 100 x 100 Clear Air VHF 449 0 75 15 15 x 15 Clear Air and Heavy Precipitation UHF 915 0 3 5 6 5x5 Clear Air and Precipitation UHF 1036 0 3 5 5 6 5x5 Clear Air and Precipitation UHF