Introduction to Weather Radar Interpretation: Some Useful Insights for the TV Meteorologist. Rich Kane National Weather Service, NOAA Pittsburgh, PA Version 2.1 April 12, 2004

This sure beats operating that radar in severe weather!!!

-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 Operation How Weather Radar Works -Radar transmits a wave of Electromagnetic energy (pulse) -Energy scatters in all directions -A very small portion is reflected back from the target (raindrop) Collectively, the energy is scattered back to the radar from millions of droplets to generate the radar reflectivity

Basic Radar Operation How 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 Operation Radar 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 Operation Radar Beam Characteristics and Beam Limitations

Basic Radar Operation Radar Beam Characteristics and Sampling Limitations Range (nm) Beam Diameter (nm) Why does that thunderstorm cell look a little different, or stronger when I view it from Cleveland’s radar rather than Pittsburgh’s radar. Well it might be…..

Basic Radar Operation Radar 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 Operation Comparison 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) G 2 L 2 x N 512xPi 2 xR 2 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 radar N = 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 the rainfall rates in all tropical systems are extremely high. So, where are the 50 dBZ echoes (red) and above? This is something you need to know about tropical rainfall.

Radar Rainfall Estimates Reflectivity (dBZ) = Z (the amount of energy returned) Which box will give the higher Z? Z (reflectivity) = Sum D 6 /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 = 300R 1.4 Tropical Z-R is Z = 250R 1.2 This has important ramifications on estimated radar rainfall in tropical systems!!!!!!!!! Radar Rainfall Estimates

Do Tropical Rainfall Rates and the warm rain process occur outside of the tropics? Absolutely ! Shadyside, OH 1990 Radar Rainfall Estimates

ABR (in) 14 June :12 PM to 9:48 PM EDT Wegee Creek Pipe Creek Cumberland Run Ohio River Belmont County, OH Shadyside 1-hour FFG

ABR (in) 14 June :12 PM to 9:48 PM EDT Wegee Creek Pipe Creek Cumberland Run Ohio River Belmont County, OH Shadyside 1-hour FFG inches with reports up to 5.5 inches

Radar Rainfall Estimates Bright Banding (melting) cause elevated Z -Melting snowflakes or sleet (water coated ice) provide a much higher return (Z) to the radar. -Melting layer is located at a given altitude resulting in a circular band (sometimes broken) of enhanced reflectivity (Z). The radar will then “overestimate” the rainfall amount (dashed line).

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 Maximum LRM(mid) 24-33Kft Base Reflectivity Vertically Integrated Liquid (VIL) Layer Reflectivity Maximum LRM(high) >33Kft 46-50dBZ 50-57dbz >57dBZ

LRM

Cross-section All-Tilts Reflectivity

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 storm speed is subtracted to enhance the ability to see “rotation” Hook

02 June 1998 Tornado Outbreak Damage on Mt. Washington from 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)

Composite Reflectivity Base Reflectivity Hook Echo Base vs Composite Reflectivity

Three-Body Scatter Spike - A Large Hail Indicator

/////////////////////////////////////////////////////////////////////////////////////////////////////////// Animation of Three Body Scattered Spike or Hail Flare

Don’t Worry Be Happy!

The End Hope I didn’t forget anything