Estimating Precipitation from Radar Estimating Precipitation from Radar

Jon W. ZeitlerJon W. Zeitler

Science and Operations OfficerNational Weather Service

Austin/San Antonio Forecast Office

Science and Operations OfficerNational Weather Service

Austin/San Antonio Forecast Office

Radar Beam BasicsRadar Beam Basics

As pulse volumes within the radar beam encounter targets, energy will be scattered in all directions. A very small portion of the intercepted energy will be backscattered toward the radar. The degree or amount of backscatter is determined by target:

size (radar cross section) shape (round, oblate, flat, etc.)

state (liquid, frozen, mixed, dry, wet) concentration (number of particles per unit volume)

We are concerned with two types of scattering, Rayleigh and non-Rayleigh. Rayleigh scattering occurs with targets whose diameter (D) is much smaller (D < /16) than the radar wavelength. The WSR-88D's wavelength is approximately 10.7 cm, so Rayleigh scattering occurs with targets whose diameters are less than or equal to about 7 mm or ~0.4 inch. Raindrops seldom exceed 7 mm so all liquid drops are Rayleigh scatters.

Potential problem: Nearly all hailstones are non-Rayleigh scatterers due to their larger diameters.  

As pulse volumes within the radar beam encounter targets, energy will be scattered in all directions. A very small portion of the intercepted energy will be backscattered toward the radar. The degree or amount of backscatter is determined by target:

size (radar cross section) shape (round, oblate, flat, etc.)

state (liquid, frozen, mixed, dry, wet) concentration (number of particles per unit volume)

We are concerned with two types of scattering, Rayleigh and non-Rayleigh. Rayleigh scattering occurs with targets whose diameter (D) is much smaller (D < /16) than the radar wavelength. The WSR-88D's wavelength is approximately 10.7 cm, so Rayleigh scattering occurs with targets whose diameters are less than or equal to about 7 mm or ~0.4 inch. Raindrops seldom exceed 7 mm so all liquid drops are Rayleigh scatters.

Potential problem: Nearly all hailstones are non-Rayleigh scatterers due to their larger diameters.  

Energy ScatteringEnergy Scattering

Probert-Jones Radar EquationProbert-Jones Radar Equation

Simplified Radar EquationSimplified Radar Equation

Since we technically don't know the drop-size distribution or physical makeup of all targets within a sample volume, radar meteorologists oftentimes refer to radar reflectivity as equivalent reflectivity, Ze.

The assumption is that all backscattered energy is coming from liquid targets whose diameters meet the Rayleigh approximation. Obviously, this assumption is invalid in those cases when large, water-coated hailstones are present in a sample volume. Hence, the term equivalent reflectivity instead of actual reflectivity is more valid.

Since we technically don't know the drop-size distribution or physical makeup of all targets within a sample volume, radar meteorologists oftentimes refer to radar reflectivity as equivalent reflectivity, Ze.

The assumption is that all backscattered energy is coming from liquid targets whose diameters meet the Rayleigh approximation. Obviously, this assumption is invalid in those cases when large, water-coated hailstones are present in a sample volume. Hence, the term equivalent reflectivity instead of actual reflectivity is more valid.

Equivalent Reflectivity (Ze)Equivalent Reflectivity (Ze)

                                  (Equation 5) Reflectivity (Z) vs.Decibels of Reflrectivity (dBZ)

Reflectivity (Z) vs.Decibels of Reflrectivity (dBZ)

dBZ = 10log10ZdBZ = 10log10Z

Beam-FillingBeam-Filling

Sending vs. ListeningSending vs. Listening

Sending vs. ListeningSending vs. Listening

99.843% of the time the WSR-88D is listening for signal returns. 99.843% of the time the WSR-88D is listening for signal returns.

A low PRF is desirable for target range and power, while a high PRF is desirable for target velocity. The inability to satisfy both needs with a single PRF is known as the Doppler Dilemma. The Doppler Dilemma is addressed by the WSR-88D with algorithms.

A low PRF is desirable for target range and power, while a high PRF is desirable for target velocity. The inability to satisfy both needs with a single PRF is known as the Doppler Dilemma. The Doppler Dilemma is addressed by the WSR-88D with algorithms.

The Doppler DilemnaThe Doppler Dilemna

Range FoldingRange Folding

Subrefraction: dry adiabatic, moisture increases with height. In addition to underestimated echo heights, this phenomenon tends to reduces ground clutter in the lowest elevation cuts.

Superrefraction: temperature inversion. In addition to overestimated echo heights, increases ground clutter in the lowest elevation cuts and is the cause of what we normally refer to as anomalous propagation or AP echoes.

Subrefraction: dry adiabatic, moisture increases with height. In addition to underestimated echo heights, this phenomenon tends to reduces ground clutter in the lowest elevation cuts.

Superrefraction: temperature inversion. In addition to overestimated echo heights, increases ground clutter in the lowest elevation cuts and is the cause of what we normally refer to as anomalous propagation or AP echoes.

The Earth is Round!The Earth is Round!

Each pulse has a volume with dimensions of ~ 500 meters (~ 1500 meters) in length by ~ 1° wide in short pulse (long pulse) mode. This means that two targets along a radial must be at least 250 (750) meters apart for the radar to be able to distinguish and display them as two separate targets (i.e., more than H/2 range separation distance).

Each pulse has a volume with dimensions of ~ 500 meters (~ 1500 meters) in length by ~ 1° wide in short pulse (long pulse) mode. This means that two targets along a radial must be at least 250 (750) meters apart for the radar to be able to distinguish and display them as two separate targets (i.e., more than H/2 range separation distance).

Storms Too Close!Storms Too Close!

Storms or Bats?Storms or Bats?

Strategies to Fix ProblemsStrategies to Fix Problems

Drop Size DistributionDrop Size Distribution

Drop Size DistributionDrop Size Distribution

Rainfall RateRainfall Rate

Rainfall RateRainfall Rate

Rainfall RateRainfall Rate