1. Radar MeteorologyM. D. Eastin Fundamentals of Radar and Display

2. Radar MeteorologyM. D. Eastin Fundamentals of Radar and Display Outline Radar Components Signal Characteristics Display – NEXRAD Display – Other Types Display – Phenomena

3. A Typical Pulse Radar System: Four Basic Components Transmitter Antenna Receiver Display System is designed to transmit microwave pulses in shorts bursts from the antenna, and then activate the receiver to “listen” for any returns associated with that pulse Returns are then amplified and displayed as radar reflectivity Duplexer: Switch which allows the same antenna to transmit pulses and receive returns Radar MeteorologyM. D. Eastin Radar Components

4. Radar MeteorologyM. D. Eastin Radar Components A Typical Pulse Radar System: Transmitter A microwave tube (“Klystron”) produces pulses of power at a desired frequency (or wavelength – 10 cm – S-band) A pulse modulator controls the timing of each pulse. Typical pulse durations are ~1 μs with each pulse separated by a few milliseconds to allow time for unique returns at large ranges Pulse Repetition Frequency (PRF) Sets the timing between each pulse Fixed (operational radars) User-controlled (research radars)

5. Radar MeteorologyM. D. Eastin Radar Components A Typical Pulse Radar System: Antenna Output from the antenna is a pulse modulated microwave-frequency sine wave. Waves travel along a microwave transmission line (or “waveguide”) through the duplexer to the antenna The antenna concentrates waves into the desired shape – often a narrow cone (or “beam”) for most meteorological radars Transmitted beams travel through the environment until they strike an object (meteorological or not!) A very small portion of the beam is reflected back toward the antenna

6. Radar MeteorologyM. D. Eastin Radar Components A Typical Pulse Radar System: Antenna Sidelobes: No radar antenna is perfectly built! Small construction flaws allow for a portion of the transmitted signal to escape through “holes” as the beam is being formed Can also strike environmental targets and have power reflected back Half-power Beam Width Function of radar design and range Radius of a conical cross-section (i.e. a circle) at a given range

7. Radar MeteorologyM. D. Eastin Radar Components A Typical Pulse Radar System: Receiver The echo power is very small compared the transmitted power Echoes are first converted to an “intermediate frequency” by mixing the unique return echo frequency with the constant transmitted frequency Intermediate wave are then amplified by a known amount before being sent to the Doppler phase detector and display unit Reflectivity: Amplitude difference between echo and known amplification Doppler winds: Related to frequency difference between transmitted wave and echo (later…)

8. Radar MeteorologyM. D. Eastin Signal Characteristics Transmitted Signal: QuantitySymbolUnits Typical ValueComments Frequencyftft hertzMHz, GHz3000 MHz c = f t λ Wavelengthλmetercm10 cm c = f t λ Pulse Duration τ secondμsμs1 μs Pulse Lengthhmeterm300 m Length of pulse as it travels through the atmosphere h = c τ Pulse Repetition Frequency Fs -1 400 s -1 Pulse Repetition Period TrTr secondms2.5 ms Time between pulses T r = 1 / F Peak PowerPtPt wattkW, MW1 MW 1 MW = +90 dBm (reference is 1 milliwatt) Pulse EnergyWjouleJ1 J Integral of the average power over one complete pulse Average PowerP av wattkW400 W Power averaged over one complete pulse repetition period P av = WF

9. Radar MeteorologyM. D. Eastin Signal Characteristics Transmitted Signal: Considerations Wavelength:Choice is a function of the target to be studied and budget Larger wavelengths → Precipitation detection Require large antennas ($) Pulse Duration:Choice a function of sensitivity and range resolution Longer durations → Better sensitivity (i.e. less error in a given dBZ) Poorer range resolution (i.e. no detailed structure) PRF:Choice dictates the maximum range at which a target can be detected ( after a pulse has been transmitted, the radar must wait long enough ) ( to allow echoes from the most distant detectable targets to return ) ( “second trip echoes” → Returns observed after the next pulse ) Larger frequencies → Greater range → Multiple echoes of same target (better sensitivity) → Less motion by radar between consecutive pulses (better angular resolution of target)

10. Radar MeteorologyM. D. Eastin Signal Characteristics Transmitted Signal: Considerations Peak Power:The power of the return echo from a target increases with the transmitted power of the pulse → large peak powers are desired Pulse Energy:Radar sensitivity increases with pulse energy → large magnitudes desired Average Power:Directly related to peak power and pulse energy → large values desired ( Quantity most often calibrated for modern radars ) ( Most radar achieve accuracies of < 0.1 dBz )

11. Radar MeteorologyM. D. Eastin Signal Characteristics Transmitted Signal: How to Express Power Ratio of two powers: (decibels) where P 1 = Observed power P 2 = Reference power (constant)

12. Radar MeteorologyM. D. Eastin Signal Characteristics Received Signal (Radar Echoes): QuantitySymbolUnits Typical ValueComments Frequencyfrfr hertzMHz, GHz~3000 MHz Differs from the transmitted frequency by the Doppler shift (usually less than a few kHZ) Wavelengthλrλr metercm~10 cm c = f r λ r Pulse Repetition Frequency Fs -1 400 s -1 Same as transmitted PRF Pulse Repetition Period TrTr secondms2.5 ms Same as for transmitted pulse Received PowerPrPr wattmW, nW10 -6 mW 10 -6 mW = -60 dBm Time of ArrivalΔtsecondms1 ms Measured from the time of the transmitted pulse

13. Radar MeteorologyM. D. Eastin Signal Characteristics Received Signal (Radar Echoes): Considerations Frequency:Difference between the transmitted and received frequencies is the “Doppler shift” → Proportional to the radial velocity of the target → More on this later… Received Power:Many orders of magnitude smaller than the transmitted power Larger values denote a greater “total” cross-section by the target(s) Minimum Detectable Signal (MDS) → weakest return power that can discriminated from the ever present background noise Time of Arrival:Used to determine target’s range (r) from the radar following:

14. Radar MeteorologyM. D. Eastin Display - NEXRAD

15. Radar MeteorologyM. D. Eastin Display - NEXRAD Plan-Position Indicator (PPI) Scanning Strategy: Single Elevation Angle Data collected on a cone are projected onto a plane Echoes close to the radar are at a low elevation Echoes far from the radar are at a high elevation

16. Radar MeteorologyM. D. Eastin Display - NEXRAD Volume Scanning Strategies: Precipitation mode scan geometrySevere weather scan geometry Saves time…fewer elevations

17. Radar MeteorologyM. D. Eastin Display - NEXRAD Clear air mode scanning geometry Fewer elevations, slower antenna rotation achieves greater sensitivity for clear air turbulence, clouds, Insects, drizzle, or light snowfall. Volume Scanning Strategies:

18. Radar MeteorologyM. D. Eastin Display - NEXRAD Radar Reflectivity: A measure of the power scattered back to the radar from objects in the path of a radar beam Proportional to the sum of the sixth power of the diameter of all the particles illuminated by a pulse provided the particles are smaller than the radar wavelength (more on this later…)

19. Radar MeteorologyM. D. Eastin Display - NEXRAD Precipitation Mode: Used once liquid precipitation is observed

20. Radar MeteorologyM. D. Eastin Display - NEXRAD Clear-Air Mode: Used for snow and detecting the onset of deep convection

21. Radar MeteorologyM. D. Eastin Display - NEXRAD Base Reflectivity: Echo intensity at the lowest PPI scan level (0.5°) measured in dBZ

22. Radar MeteorologyM. D. Eastin Display - NEXRAD Composite Reflectivity: Maximum echo intensity at any PPI scan level measured in dBZ

23. Radar MeteorologyM. D. Eastin Display - NEXRAD Storm Total Precipitation: Time integral of base reflectivity after NWS selected start time (measured in inches) Primary tool to predict flash flooding

24. Radar MeteorologyM. D. Eastin Display - NEXRAD Vertically Integrated Liquid (VIL): Integral of reflectivity (or water mass) through a column (measured in kg /2) Used to estimate the presence of hail and hail size (large VIL = large hail)

25. Radar MeteorologyM. D. Eastin Display - NEXRAD Radial Velocity: Observed velocity component along the radar beam direction (measured in knots)

26. Radar MeteorologyM. D. Eastin Display - NEXRAD Storm-relative Radial Velocity: Velocity component with the component of the storm motion along the radar beam removed Best display for detecting mesocyclones, tornado vortex signatures, or microbursts

27. Radar MeteorologyM. D. Eastin Display - NEXRAD Combined Radar Reflectivity and Radial Velocity: Used to detect most severe weather

28. Radar MeteorologyM. D. Eastin NEXRAD Data Available Data: All NEXRAD data since 1992 has been archived and is publically available through the NCDC at: http://hurricane.ncdc.noaa.gov/pls/plhas/has.dsselecthttp://hurricane.ncdc.noaa.gov/pls/plhas/has.dsselect Level-II:Radar reflectivity and radial velocity at original sampling resolution Raw volumetric data Level-III:Derived products most used by forecasters Base reflectivity Composite reflectivity Base radial velocity Base storm-relative radial velocity Vertically-integrated liquid (VIL) Echo tops (ET - maximum height of 10 dBZ echo) Storm total precipitation and many more….

29. Radar MeteorologyM. D. Eastin Display – Other Types Range-Height Indicator (RHI) Scanning Strategy: Radar is scanned in elevation at a fixed azimuth Volume scans are accomplished by rotating slowly in azimuth while scanning rapidly in elevation

30. Radar MeteorologyM. D. Eastin Display – Other Types Examples of RHI Scans: Sequence of RHI scans showing development of shallow cumulus along the south Florida coast Note ground clutter and echo from tall buildings (echo from radar side lobe)

31. Radar MeteorologyM. D. Eastin Display – Other Types Examples of RHI Scans: Vertical cross-section through a squall line reconstructed via RHI slices through a PPI volume

32. Radar MeteorologyM. D. Eastin Display – Other Types Time-Height Scanning Strategy: Radar is pointed vertically as storm passes over

33. Radar MeteorologyM. D. Eastin Display – Other Types Horizontal cross-sections: Radar data is interpolated from cylindrical to Cartesian coordinates and displayed in Cartesian space Often done when constructing analyses from multiple Doppler radars (more on this later…)

34. Radar MeteorologyM. D. Eastin Display – Other Types Vertical cross-sections: Radar data is interpolated from cylindrical to Cartesian coordinates and displayed in Cartesian space Cartesian grid can be sliced similar to RHI scan If constructed from multiple Doppler radars, the vertical wind component can be estimated and displayed

35. Radar MeteorologyM. D. Eastin Display – Other Types Radar Composites: Composite reflectivity from multiple PPI scans are projected onto a single display to show regional or national precipitation distributions The rain-snow distinction determined by surface observations (not the radar)

36. Radar MeteorologyM. D. Eastin Display – Phenomena Beam Blockage: Caused by tall buildings, trees, water towers, and cell towers near radar… Blocked Beam

37. Radar MeteorologyM. D. Eastin Display – Phenomena Non-Meteorological Targets: Insects and bats often rest during the day and travel at night → take-off at sunset Birds rest at night and travel during the day → take-off at sunrise Birds departing At 1114 UTC

38. Radar MeteorologyM. D. Eastin Display – Phenomena Non-Meteorological Targets: Ground clutter, aircraft, etc… Columbia shuttle Break-up Ground clutter and diverted aircraft

39. Radar MeteorologyM. D. Eastin Display – Phenomena Bright Band: Enhancement of radar reflectivity at the melting level as large aggregate snowflakes develop a thin film of water on their surface before they collapse to a smaller drop

40. Radar MeteorologyM. D. Eastin Display – Phenomena Convective Storms: PPI Displays

41. Radar MeteorologyM. D. Eastin Fundamentals of Radar and Display Summary: Radar Components Signal Characteristics Display – NEXRAD Display – Other Types Display – Phenomena