Operational Weather Radar

Featuring:WSR-88D Doppler Radar

Transmission Power – 750,000 WGain – 35,481Beam Width <2º @ 125 nmiPulse Length - 0.47 or 1.35 kmTransmission Time – 1.57 of 4.5 μs

DOWs measure record high wind speeds of 301 +/- 20 mph (135 +/- 10 m/s) in 03 May 1999 Oklahoma City tornado. 

Abstract Sensors

WSR88 – standard weather service radar 158 Operational Sites Across the Country

Common Radar Bands: K,Ku, C,X, Ka λ=1-10cm, 1-10x size of particles (rain ice) of interest Makes use of Rayleigh Scattering Phased Array Radar

Algorithms Radar Equation Attenuation

Brief Weather Radar History ~1950: RADAR discovered

RADAR: RAdio Detection And Ranging In early use, WWII military operators picked up on what

they thought was noise Really environmental returns, only noise if you want to

see airplanes 1950-1980: Continued development of reflectivity radar 1980-2000: Invention and implementation of Doppler

radar As of 1988 the US started using 10 cm wavelength

radars (WSR-88D) as their standard radar device for radar networks

2000-present: Dual Polarization Radar and Phased Array Radar

source: http://www.cocorahs.org/media/docs/radar_basics.pdf

source: http://www.cocorahs.org/media/docs/radar_basics.pdf

Introduction

Key concepts Doppler effect Rayleigh scattering Sensing the return Reflectivity, dbz Vertical cross section

Range Height Indication Scan Constant Azimuth

Improvement with WSR-88D Volume Scanning

Vary azimuth along with elevation Optimally interpolate 3d volume or reflectance

f=fo

V±V x

Radar, Doppler and Raleigh

Radar Equation Whn transmiter and reciever are in the same place,

signal decreases in power to the fourth power

Doppler Equation Where F’ = observed frequency, F = emitted

frequency, v = velocity in medium, vs = velocity of source

Raleigh Scattering EM radiation returned from a field with average

particle size smaller than the wavelength of the radiation

Reflectivity off a target Power of the return signal - reflectivity (z) z has the units mm^6/m^3 -

density of water droplets which would return the reflectivity

emissivity is implicit To cover a large range it is often described in decibels Intensity of the return echo helps determine the

precipitation rate significant precipitation

usually is above 15 dBz Reported reflectivity is an

average of at least 25 pulses from a given azimuth and elevation

Applications Nowcast/ short term forecast

useful for extreme weather Easily deployed in remote locations

Lee Rotors Precipitation Estimation Links to related topics

RUCOOL Codar Maps Radar Altimetry Police Radar Small Aircraft Detection

Benefits from radial movement of parts

Radar Bands and Uses 2-4 S GHz, 7.5-15cm. Sirius and XM radio. Long range

weather, marine weather, ATC 4-8 Ghz - C band – satellite transponders, satellite TV, raw

satellite communications, weather. 18-27 Ghz - K - police, small drizzle/fog research 27-40 Ghz Ka – police 8-12 Ghz - X – airport radar – very long range, missile

guidance

Making a Signal The “listen” time (millisecond) is 1000x the pulse duration This allows the radar an opportunity to be able to receive the

signal again without interference from previous signals Must wait until signal has reached maximum range and

returned. Knowing exact pulse travel time allows for calculation of the

horizontal distance to the target Intensity of the return, or backscatter -> target size Radial Target Motion -> Doppler shift

What happens to the returned signal?

Must orient the returns (power at certain frequencies) into an accurate measure of precipitation

Measurements over a spherical volume are sliced up into horizontally and vertically to produce 2,3 and 4d visualizations

Account for ambient background noise

Source: http://cimms.ou.edu/rvamb/Documents/Report_7.pdf

Algorithms VIL

Vertically Integrated Liquid HAIL

Estimates presence and size of hailstones

Potential Wind Gusts Uses VIL, cloud top height Estimates the winds under the cloud (ie downdraft)

Doppler measurement readily reveals wind shear storm relative velocity

Mesocyclone Detection Algorithm Tornado Detection Algorithm Wind Shear Detection (low level)

Products Base reflectivity

how much precipitation is falling precipitation type assess a storm's structure and dimension

Composite Reflectivity Scans from all elevations, imaging precipitation

intensity and storm structure Base Velocity

radial wind field, speed of fronts/strong wind range of 140 mi

Storm relative motion Track a circulation (show up well in doppler return)

over time to determine storm motion. Removing the storm relative motion from base radials

gives an estimate of the flow with respect to the storm.

“Complications” Migrating Birds Insects Aircraft, solid buildings, large aerosols for air traffic controller – rain for meteorologist – airplanes. Identification of snow, and snow type, modification of observed matter before hitting

the ground (fallstreaks) big enough sample of precip for identification? In precip estimates – Z-R relationship

Phased Array Radar

Multiple beams with variable dwell times scan continuously perform full volume scan every 20-30s track cooperative aircraft track non-cooperative aircraft perform medium dwell scan on heavy cumuliform

convection perform long dwell scan on area of suspected tornadic

development. Expected that tornado warning could in to to 45

min

Proven NAVY technology

In Closing

Take precipitation rates with caution Highly accurate under most conditions Misleading in cases of extreme precipitation The ultimate in nowcast and high resolution spatial

time series for precipitation. Extremely useful in adaptive forecasting of

extreme weather events