CH 15 Weather Radar

15 Weather Radar

15.1 Introduction

  • Radar is the primary tool for detecting precipitation, standing for "radio detection and ranging."

  • Utilized since the 1940s, enhancements in radar technology have improved the precision of precipitation detection and display.

15.2 Principles of Weather Radar

  • The Weather Surveillance Radar—1988 Doppler (WSR-88D) is used by the National Weather Service (NWS).

  • Prototype radar developed in 1988, and understanding its principles is essential for accurate interpretation of WSR-88D images.

  • Comparison made between WSR-88D and aircraft radar principles to illuminate strengths and limitations.

15.2.1 Antenna

  • The WSR-88D features a parabolic-shaped antenna that alternately emits and receives radio waves.

  • Radio wave pulses can strike targets, returning part of that energy back to the antenna, shaping the radar beam.

15.2.2 Backscattered Energy

  • Definition: Backscattered energy is the amount of radar energy reflected back to the antenna after striking an object.

  • Targets of backscattered energy can include:

    • Precipitation

    • Clouds

    • Dust

    • Birds and insects

    • Buildings

    • Air mass boundaries

    • Terrain features

    • Wind farms/turbines

  • Reflectivity: Measurement of the amount of backscattered energy, which appears as an echo on radar images.

15.2.3 Power Output

  • WSR-88D has a peak power output of 750 kilowatts (kW), enhancing its ability to detect low-reflectivity targets.

  • Most aircraft radars have a peak power output of less than 50 kW, making smaller targets harder to detect.

15.2.4 Wavelengths

  • Definition: Wavelength is the distance between two crests or troughs of a radio wave.

  • WSR-88D has a wavelength of 10 cm, while most aircraft radars have a wavelength of 3 cm.

  • Shorter wavelengths detect smaller targets more effectively but are more significantly attenuated than longer wavelengths.

15.2.5 Attenuation

  • Definition: Attenuation refers to any process that reduces energy within the radar beam, affecting backscattered energy.

15.2.5.1 Precipitation Attenuation

  • Definition: Precipitation attenuation is the reduction of radar energy due to absorption and scattering by precipitation particles.

  • Precipitation near the radar absorbs energy, limiting visibility beyond the initial area of precipitation.

  • Wavelength affects precipitation attenuation:

    • WSR-88D's 10 cm wavelength experiences minimal attenuation.

    • Aircraft radars (3 cm wavelength) face significant attenuation issues, often displaying only the leading edge of intense echoes.

15.2.5.2 Range Attenuation

  • Definition: Range attenuation is the loss of intensity in radar energy as the beam distance from the antenna increases.

  • WSR-88D compensates for this, but airborne radars usually compensate only out to 50-75 NM, leading to misrepresentations for distant targets.

15.2.6 Resolution

  • Definition: Resolution is the radar's ability to distinguish targets separately.

15.2.6.1 Beam Resolution

  • Beam resolution relates to the ability to identify targets at the same range but different azimuths.

  • For WSR-88D:

    • Beam width: 0.95°

    • At 60 NM, targets must be separated by at least 1 NM; at 120 NM by at least 2 NM.

  • For aircraft radars (average beam width of 5°):

    • At 60 NM, targets must be separated by at least 5.5 NM; at 120 NM by at least 10 NM.

  • Better beam resolution is a strength of WSR-88D compared to aircraft radar.

15.2.7 Wave Propagation

  • Radar beams are not linear due to atmospheric density variations affecting beam speed and direction, commonly bending in areas of different densities.

15.2.7.1 Normal (Standard) Refraction

  • Under standard conditions, atmospheric density decreases with height, bending the beam downward but less than Earth's curvature.

15.2.7.2 Subrefraction

  • Subrefraction occurs when the atmosphere’s density decreases more than normal, causing the radar beam to climb higher, potentially missing targets such as distant thunderstorms.

15.2.7.3 Superrefraction

  • Superrefraction happens when atmospheric density decreases less than normal or increases with height, causing the beam to bend more towards the Earth, detecting stronger thunderstorms.

15.2.7.4 Ducting

  • Ducting occurs when the beam bends equal to or greater than Earth's curvature, allowing the beam to backscatter to the ground, leading to false echoes such as anomalous propagation (AP).

15.2.8 Radar Beam Overshooting and Undershooting

  • Overshooting: When the radar beam height exceeds precipitation tops, especially in stratiform precipitation, leading to missed precipitation information.

  • Undershooting: When precipitation occurs above the lowest radar beam, often seen with high-cloud-based precipitation near radar sites.

  • The region where radar cannot detect precipitation due to beam height is called the "cone of silence."

15.2.9 Beam Blockage

  • Beam blockage happens when terrain obstructs the radar beam, particularly in mountainous areas, showing as areas without echoes on radar images.

  • This effect can be minimized by using mosaic images.

15.2.10 Ground Clutter

  • Ground clutter arises from returns of radar echoes from stationary objects such as trees and buildings near the radar, which can obscure precipitation signals but is usually filtered out by WSR-88D imagery.

15.2.11 Ghosts

  • Ghost echoes are diffused radar returns in clear air caused by insects or refraction phenomena, appearing less than 15 dBZ of reflectivity and expanding rapidly without movement animation.

15.2.12 Angels

  • Angels are donut-shaped echoes from undetectable phenomena like bats or birds, visible primarily in Clear Air Mode due to their low reflectivity.

  • Appear during specific times (morning for birds, evening for bats).

15.2.13 Anomalous Propagation (AP)

  • AP represents extended ground echoes due to superrefraction of the radar beam, potentially misidentified as thunderstorms due to their irregular patterns of motion.

15.2.14 Other Non-meteorological Phenomena

  • Wind Farms: Can interfere with radar signals nearby, causing beam blockage and reflecting high values that can mimic storm activity.

15.2.15 Precipitation

  • The intensity of precipitation is measured by the volume of energy returned from precipitation particles, known as reflectivity.

15.2.15.1 Intensity of Precipitation

  • Determined by:

    • Size of precipitation particles

    • State of precipitation (solid/liquid)

    • Concentration of particles

    • Shape of precipitation

15.2.15.1.1 Intensity of Liquid Precipitation

  • Larger droplets backscatter more energy. For example, a 1/4-inch particle reflects energy equivalent to 64 particles of 1/8 inch.

  • Reflectivity measured in dBZ, typically greater than 15 dBZ for liquid precipitation, lower values associated with cloud particles or smaller airborne particles.

15.2.15.1.2 Convective Precipitation

  • Characterized by:

    • Forming as lines or cells

    • Strong reflectivity gradients

    • Varied intensities from moderate to extreme

    • Rapid changes in echo patterns.

  • Hazards include turbulence, LLWS, strong winds, icing, hail, lightning, tornadoes, and low visibility below clouds.

15.2.15.1.3 Stratiform Precipitation

  • Characterized by:

    • Widespread coverage

    • Weak reflectivity gradients

    • Light to moderate intensities (39 dBZ or less)

    • Gradual changes in echo patterns.

  • Hazards include potential icing, low ceilings, and reduced visibility.

15.2.15.1.4 Intensity of Snow

  • Radar is unreliable for determining snowfall intensity, but higher reflectivity generally indicates increased snowfall rates.

15.2.15.1.5 Bright Band

  • A band of enhanced reflectivity marking the freezing level, created when radar passes through precipitation containing ice coated with liquid water, showing greater energy reflections than areas above and below the freezing layer.