introduction to weather radar interpretation: some useful insights for the tv meteorologist. rich...
TRANSCRIPT
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
-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 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)
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).
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
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
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.
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Animation of Three Body Scattered Spike
or Hail Flare