Scan Strategies and Basic Products (Version 1.1)

MSC Radar Course

Paul FordLead Instructor MOIP Dartmouth

Acknowledgements

• Dave Ball and Tim Bullock for suggesting I participate

• Norman Donaldson (King Radar) for explanations and encouragement

• Paul Joe, Dave Hudak, Rob Nissen, Mike Leduc, Phil Chadwick, Mark Pilon, Steve Knott and many others over the years for discussions on radar and mesoscale meteorology

• The dozens of MSC interns and other practicing meteorologists I have worked with for their “darn good questions”

Outline

• EC MSC and NWS NEXRAD system specifications

• MSC and NEXRAD scan strategies

• Product Examples and Usage– Discussion and Personal Observations

• Summary

• Questions

Basic Specifications of MSC Radars

• Wavelength: 5 cm• Antennae: Parabolic 3.7 or 6.1m dishes• Transmitter type: Magnetron (random phase)• Peak Power: 250 kW• Pulse Lengths: 0.8 – 2 microseconds• Pulse Repetition Frequencies: 250 – 1200 pulses per

second

MSC Radar Antenna – new 6.1m dish

MSC Antennae

• Beamwidth (deg) = 70 / (Antenna diameter)• The diameter of the reflector of the new systems is 6.1m,

compared to 3.7m for the retrofit systems• Thus, the angular beamwidth (to half power) of the new

systems is narrower than in the retrofit cases (0.65 deg vs 1.1 deg)

• This improves resolution and useful range – antenna gain is higher when beamwidth is small

IDENTIFIER NAME BEAMWIDTH ANTENNA

XGO GORE 0.65 deg 6.1m

XMB MARION BRIDGE 1.1 3.7

WTP HOLYROOD 1.1 3.7

XME MARBLE MOUNTAIN 0.65 6.1

XNC CHIPMAN 1.1 3.7

XAM VAL D’IRENE 0.65 6.1

WMB LAC CASTOR 1.1 3.7

Pulse Repetition Frequency (PRF)

• The pulse repetition frequency (PRF) is the number of pulses emitted by the radar per second (pps)

• A pulse travelling to a target at range rmax and back will cover a distance 2rmax

• The pulse will make it back to the radar before the next pulse is emitted if: 2rmax=c/PRF

PRF and radar range

• Thus, the higher the PRF, the lower the effective range (ignoring second-trip echos from objects located beyond rmax)

• Recall, higher PRFs are necessary in Doppler mode to derive a greater range of velocities (Doppler dilemma)

• Dual PRF technique used in MSC system to extend Vr range to 48 m/s

rmax=c/2PRF

Rmax in selected modes

Mode PRF Rmax (km)

Reflectivity 250 600

Doppler 900 &1200

125

Clear Air 50 3000

Rmax vs PRF

0

100

200

300

400

500

600

700

PRF

R m

ax

(k

m)

NEXRAD Transmitter Specs

• Type: S-band (10 cm), coherent chain (STALO/COHO), line modulator, klystron tube amplifier (53 dB gain typical)

• Frequency: 2700 to 3000 MHz • Power: 750 kw peak at klystron output • Average Power: 300 to 1300 watts • Pulse Widths: 1.57 and 4.5 microseconds (-6 dB points) • PRF short pulse: 318 to 1304 Hz • PRF long pulse: 318 to 452 Hz

NEXRAD Antenna Specs

• Type: center fed paraboloid of revolution 28 feet (8.5m) in diameter • Polarization: linear horizontal • Gain at 2850 MHz: 45.5 dB (including radome loss) • Beamwidth at 2850 MHz: 0.925 deg • First sidelobe: -29 dB (others less than -40 dB beyond 10 deg) • Radome: fiberglass foam sandwich frequency tuned, 39 foot

truncated sphere

NEXRAD Reflectivity Product

• Reflectivity computation: linear average return power • Reflectivity estimate standard deviation: less than 1 dB typical • Number of pulses averaged: 6 to 64 • Range increment: 1000 m • Max range for reflectivity: 460 km • Signal Detection Capabilities (at 0 dB SNR)

– Minimum required signal detection, short pulse -7.5dBZe at 50 km– Typical Detection (for Ze=200*R1.6) -10 dBZe at 50 km (rainfall of 0.01mm/hr)– Minimum required signal detection, long pulse -23.0dBZe at 25 km

NEXRAD Velocity Base Product• velocity computation: complex covariance argument (pulse pair

estimator)

• velocity estimate standard deviation: less than 1 m/sec (at spectrum width of 4 m/sec)

• number of pulses averaged: 40 to 200

• range increment: 250 m

• azimuth increment: 1 deg

• max range for velocity: 230 km

Volume Scans• Definition: a series of consecutive radar scans that together sweep out a

volume of the atmosphere• MSC radars perform separate conventional and Doppler volume scans• Some data processing is done by a computer at the radar site• Image products are produced by MSC’s Unified Radar Processor (URP)

which is run on computer servers in each of the storm prediction centres (SPC) across Canada

• Users may access any of these servers remotely and request images using the Interactive Viewer or the NinJo workstation

MSC Radar’s Volume Scans

• Nominal times of the scans are 0, 10, 20, 30, 40 and 50 minutes of each hour

• The conventional scan (CONVOL) is completed in the 5 minutes before the nominal time

• 4 Doppler scans (DOPVOL) are performed in the 5 minutes after the nominal time

Basic MSC Antenna “Scan Strategies”

• Conventional (CONVOL): – 24 PPI scans– top down– 24.6 to 0.3 degrees– 6 RPM– 5 minutes to complete

• Doppler (DOPVOL1a,1b,1c):

– 3 scan angles (LOLAA, 1.5 and 3.5 degrees)

– much slower (0.85 RPM) since more sample points are collected for the Doppler processing

– fills next 4.5 minutes

• Doppler (DOPVOL 2):– BALD angle

– ‘rapid’ scan, made during the last 30 seconds of the Doppler cycle, noisy (low S/N)

– fills last 30 seconds (~2 RPM)

Implications of the MSC CONVOL

• 1.5 km CAPPI becomes a PPI beyond about 130 km– Overshooting of distant shallow precipitation

• Tops of tall (e.g. cumulonimbus) cells within 20-30 km of the radar are often underestimated

– This can lead to a misdiagnosis of the extent of growth or decay of cells close to the radar

U.S. NEXRAD Scan Strategies – VCP is the “Volume Coverage Pattern”

VCP Scan Time (min) Elevation angles (°) Usage Special attributes

11 5

0.5, 1.5, 2.4, 3.4, 4.3, 5.3, 6.2, 7.5, 8.7, 10, 12, 14, 16.7, 19.5

Convection, especially when close to the radar

Has the best overall volume coverage.

12 4

0.5, 0.9, 1.3, 1.8, 2.4, 3.1, 4.0, 5.1, 6.4, 8.0, 10.0, 12.5, 15.6, 19.5

Convection, especially activity at longer ranges

Focuses on lower elevations to better sample the lower levels of storms.

121 5.50.5, 1.5, 2.4, 3.4, 4.3, 6.0, 9.9, 14.6, 19.5

Large number of rotating storms, tropical systems, or when better velocity data is needed.

Scans lower cuts multiple times with varying pulse repetitions to greatly enhance velocity data.

21 60.5, 1.5, 2.4, 3.4, 4.3, 6.0, 9.9, 14.6, 19.5 Shallow precipitation

Rarely used for convection due to sparse elevation data and long completion time.

31 10 0.5, 1.5, 2.5, 3.5, 4.5

Detecting subtle boundaries or wintry precipitation Long-pulse

32 10 0.5, 1.5, 2.5, 3.5, 4.5

Slow rotation speed allows for increased sensitivity. Default clear-air mode, reduces wear on antenna. Short-pulse

NEXRAD VCP11-Convection Mode

NEXRAD VCP21 – Shallow Precip Mode

NEXRAD VCP31-Clear Air Mode

Let’s go to MSC’s Unified Radar Processor to look at some examples of products from MSC and US radars!

MSC Conventional Products Derived from CONVOL

Composite Products

• Composed of data of similar type– PRECIP product or NEXRAD base reflectivity– CAPPI 1.0 km– CAPPI 1.5 km– ECHOTOP– 3 hr Precipitation Accumulation

• MSC URP default for composites is “maximum value”

MSC Doppler Products Derived from the DOPLVOL scans:

(DOPVOL1a,b,c and DOPVOL2)

MSC Fast Fourier Transform (FFT) Filtering of artifacts - Doppler products only

Construction of the Velocity Azimuth Display (VAD)

Shallow snow – moisture only up to ~2 km on upstream sounding

NEXRAD Products

• A lot of similarity to MSC products

• All based on PPI’s, no CAPPI’s

Base Reflectivity (Labelled CLOGZ1 in MSC URP)

NEXRAD ‘Composite’ Reflectivity (COMPZ MAXR)

NEXRAD Echo Tops

NEXRAD Base Radial Velocity: Labelled VR 1 in MSC URP

Doppler Product Usage: Bright Band Detection

Doppler Product Usage: Mesoscale Boundaries and

Circulations

Example: Eastern Ontario Convection Case

• generation of gust fronts/outflow boundaries

• mesocyclone signatures

• interaction of the mesoscale boundaries with roll clouds– convective initiation

Franktown Radar

DivergenceSignature

MesocycloneSignature

Thunderstorm Outflow Boundaries

Another Gust Front

Lake Breeze Fronts

Lake Breeze Fronts

Summary• Conventional data is used in:

– Weather surveillance

▪ CAPPIs, composites– Cross sections, severe cell identification and assessment

– Multi-level products (MAXR, ECHOTOP)

• No clutter suppression

• Coarser native resolution (sampling only every degree and km)

• Higher minimum displayed value – weaker returns not displayed

Summary

• Reflectivity and radial velocity data displayed

• Doppler products are all PPI’s– Can derive profiles of reflectivity and radial velocity

• Higher resolution and weaker returns (0.5 degree and 0.5 km sampling) from slower antenna rotation rate and better sampling

– Mesoscale boundary detection possible

• BALD (Precip and LR products) and LOLAA (lowest CLOGZ PPI) is capable of seeing shallow precipitation farther out

Thank You!

Questions??