Introduction to Weather Radar Interpretation: Some Useful Insights

for the TV Meteorologist.

Rich KaneNational Weather Service, NOAA

Pittsburgh, PA

Version 2.1

April 12, 2004

This sure beatsoperating that radar in severeweather!!!

-Radar will always be an extremely important tool for the broadcast meteorologist

-Vendors are offering more and more elaborate software relative to radar operation and display

-You need to be aware of some of the: * basic radar principles

* limitations of the radar * caveats associated with radar operation, products, and some of the software

OUTLINE

-Review some basic radar principles/characteristics -Winter radar scheme (what does a radar really show)-Estimated radar rainfall - Tropical systems, hail

contamination, bright banding-Look at a few radar products and NWS use-Tornado versus downburst-Anomalous Propagation (AP)-Three body scatter spike (Hail Flare)

OUTLINE

Basic Radar OperationHow Weather Radar Works

-Radar transmits a wave of Electromagnetic energy (pulse)-Energy scatters in all directions-A very small portion is reflectedback from the target (raindrop)

Collectively, the energy isscattered backto the radarfrom millions ofdroplets togenerate theradar reflectivity

Basic Radar OperationHow NWS Radar Works – Volume Coverage Pattern

WSR-88D operates continuously 24 hours a day

･ Radar antenna (dish) rotates 360 degrees and scans several elevation slices depending on the volume coverage pattern (VCP) selected. There are two primary operating modes: clear-air and precipitation.

Clear-Air Mode

- VCP 31 and 32 - Antenna scans 5 elevation angles in 10 minutes.- Typically used on days with no precipitation; can also be used for snow detection.- Most sensitive mode

Basic Radar Operation Precipitation (Storm) Mode

VCP 21 - Antenna scans 9 elevation slices in 6 minutes. Typically used for non-severe precipitation.

VCP 11 - Antenna scans 14 elevation slices in 5 minutes. Typically used for severe thunderstorm detection and analysis.

VCP 12 - Antenna scans 14 elevation slices in 4.1 minutes (New as of April 2004). Provides best performance for radar algorithms due to saturation of data at lowest levels and quicker scan times.

Basic Radar OperationRadar Beam Characteristics and Beam Limitations

-Elevation of the center of the radar beam increases with distance from the radar (earth curvature). For example, the height of the lowest elevation slice (0.5 degree) is about 5,000 ft AGL (above ground level) at 60 nm (nautical miles) from the radar, while at 120 nm the beam height is about 15,000 ft AGL

Basic Radar OperationRadar Beam Characteristics and Beam Limitations

Basic Radar OperationRadar Beam Characteristics and Sampling Limitations

Range (nm) Beam Diameter (nm) 30 0.5 60 1.0 120 2.0 180 3.0 240 4.0

Why does that thunderstorm cell looka little different, or stronger when I viewit from Cleveland’s radar rather thanPittsburgh’s radar. Well it might be…..

Basic Radar OperationRadar Beam Characteristics and Beam Limitations

Consider the same thunderstorm located only about 10 nm from the radar (left). Now it is

affected by the cone of silence.

Basic Radar OperationComparison of Precipitation Mode vs. Clear Air Mode

©2000 Oklahoma Climatological Survey

Definition:dBZ, or decibels, is the unit of the intensity of returned radar energy (reflectivity = Z).

What does the radar tell us?

Much of the general public thinks it tells us everything from:-Where is it snowing versus raining-Where it is sleeting versus raining-Where there is lightning-Extent of flooding-Exactly how much rain has fallen-Exactly how much snow has accumulated-Even when the great “Satan” is coming (El Nino - the cause of all weather evil)!!!

The reality is that (currently) radar measures… -POWER/ENERGY transmitted-POWER/ENERGY returned-Time-Other variables can be calculated/derived from the above

-Dual polarization-Phased Array

Simplified Radar Equation

P(ret)P(ret) ~ P(xmit) G2 L2 x N 512xPi2 xR2

Or more simply

Power (ret) ~ Power (xmit) X Gain X Wavelength

Distance (squared)

P(ret) = Power returned P(xmit) = Power transmitted (750,000 watts)G = Gain (area of antenna) (28ft diameter)L = Wavelength (10-11 cm)R = range from radarN = other stuff (sum of cross section, volume of beam, etc)

= 88D

(MORE)

Let’s look at the fun stuff –

-applications-caveats-examples

How Do We Know When There Will Be Heavy Rain?

The notorious duck echo

Snow

Radar Rainfall Estimates

We know that therainfall rates inall tropical systemsare extremely high.So, where are the50 dBZ echoes(red) and above?

This is something you need to knowabout tropical rainfall.

Radar Rainfall Estimates

Reflectivity (dBZ) = Z (the amount of energy returned)

Which box will give the higher Z?

Z (reflectivity) = Sum D6/unit volume D = diameter of drops

-Bigger raindrops mean greater Z -Tropical and tropical type systems generally have smaller drops (warm rain process… collision-coalescence)-The radar uses a “Reflectivity-to-Rainfall” relationship to estimate rainfall

Standard Z-R is Z = 300R1.4

Tropical Z-R is Z = 250R1.2

This has important ramifications on estimated radar rainfall in tropical systems!!!!!!!!!

Radar Rainfall Estimates

Do Tropical Rainfall Rates and the warm rain process occuroutside of the tropics?

Absolutely !

Shadyside, OH 1990

Radar Rainfall Estimates

ABR (in) 14 June 1990 8:12 PM to 9:48 PM EDT

Wegee Creek

PipeCreek

CumberlandRun

OhioRiver

Belmont County, OH

Shadyside

1-hour FFG1.30

94

11

2

ABR (in) 14 June 1990 8:12 PM to 9:48 PM EDT

Wegee Creek

PipeCreek

CumberlandRun

OhioRiver

Belmont County, OH

Shadyside

1-hour FFG1.30

4 9

11

2

3-4 inches with reports up to 5.5 inches

Radar Rainfall EstimatesBright Banding (melting) cause elevated Z

-Melting snowflakes or sleet (watercoated ice) provide a much higherreturn (Z) to the radar.

-Melting layer is located at a givenaltitude resulting in a circular band(sometimes broken) of enhancedreflectivity (Z).

The radar will then “overestimate”the rainfall amount (dashed line).

Hail Contamination

Hail Contamination

Monroe County, OH

Laplata, MD

VIL

Hail Contamination

Radar Operation

A few operational products and techniques you might useful,insightful, or possibly interesting

Layer Reflectivity MaximumLRM(mid) 24-33Kft Base Reflectivity

Vertically Integrated Liquid(VIL)

Layer Reflectivity MaximumLRM(high) >33Kft

46-50dBZ50-57dbz>57dBZ

LRM

Cross-section

All-TiltsReflectivity

Radar Operation

Tornado versus Downburst

Reflectivity… Simply – how much energy is returned from the Echo…. the more returned, the heavier the precipitation

Storm Relative Velocity (SRM)… the stormspeed is subtracted to enhance the ability tosee “rotation”

Hook

02 June 1998 Tornado Outbreak

Damage on Mt. Washingtonfrom the F1

Kennywood Macroburst

Anomalous Propagation (AP)

Clear-Air Mode

Anomalous Propagation (AP) at far ranges from the radar probably resulting from trees and other ground targets?

AP closer to the radar most likely resulting from the ground and things such as insects.

The solid line of strong returns extending from west to northeast of the radar is a cold front moving southeast across the area.

Anomalous Propagation (AP)

CompositeReflectivity

Base Reflectivity

HookEcho

Base vs CompositeReflectivity

Three-Body Scatter Spike - A Large Hail Indicator

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Animation of Three Body Scattered Spike

or Hail Flare

Don’t Worry

Be Happy!

The EndHope I didn’t forget anything