14-08-2013, 03:44 PM
Basic Principles of Weather Radar
[attachment=57165]
Radar
RAdio Detection And Ranging
Developed during WWII for detecting enemy aircraft
Active remote sensor
Transmits and receives pulses of E-M radiation
Satellite is passive sensor (receives only)
Numerous applications
Detection/analysis of meteorological phenomena
Defense
Law Enforcement
Baseball
Weather Surveillance Radar
Transmits very short pulses of radiation
Pencil beam (narrow cone) expands outward
Pulse duration ~ 1 μs (7 seconds per hour)
High transmitted power (~1 megawatt)
‘Listens’ for returned energy (‘echoes’)
Listening time ~ 1 ms (59:53 per hour)
Very weak returns (~10-10 watt)
Transmitted energy is scattered by objects on ground and in atmosphere
Precipitation, terrain, buildings, insects, birds, etc.
Fraction of this scattered energy goes back to radar
Elevation Angle Considerations
Radar usually aimed above horizon
minimizes ground clutter
not perfect
Beam gains altitude as it travels away from radar
Radar cannot ‘see’ directly overhead
‘cone of silence’
appears as ring of minimal/non-returns around radar, esp. with widespread precipitation
Sample volume increases as beam travels away from radar
Equivalent Radar
Reflectivity Factor 2
Ze relates rainfall intensity to average returned power
‘Equivalent’ acknowledges presence of numerous scattering targets of varying:
sizes/shapes
compositions (water/ice/mixture)
distributions
Several assumptions made (not all realistic)
Scanning Modes
Clear-Air Mode
slower antenna rotation
five elevation scans in 10 minutes
sensitive to smaller scatterers (dust, particulates, bugs, etc.)
good for snow detection
Precipitation Mode
faster antenna rotation
9-14 elevation scans in 5-6 minutes
less sensitive than clear-air mode
good for precipitation detection/intensity determination
Summary
Weather surveillance radar has varied uses
short-term weather forecasting
hazardous weather warnings
hydrologic applications
tracking bird migration patterns
Must be aware of radar’s limitations
WYSINAWYG
What You See Is NOT ALWAYS What You Get!
[attachment=57165]
Radar
RAdio Detection And Ranging
Developed during WWII for detecting enemy aircraft
Active remote sensor
Transmits and receives pulses of E-M radiation
Satellite is passive sensor (receives only)
Numerous applications
Detection/analysis of meteorological phenomena
Defense
Law Enforcement
Baseball
Weather Surveillance Radar
Transmits very short pulses of radiation
Pencil beam (narrow cone) expands outward
Pulse duration ~ 1 μs (7 seconds per hour)
High transmitted power (~1 megawatt)
‘Listens’ for returned energy (‘echoes’)
Listening time ~ 1 ms (59:53 per hour)
Very weak returns (~10-10 watt)
Transmitted energy is scattered by objects on ground and in atmosphere
Precipitation, terrain, buildings, insects, birds, etc.
Fraction of this scattered energy goes back to radar
Elevation Angle Considerations
Radar usually aimed above horizon
minimizes ground clutter
not perfect
Beam gains altitude as it travels away from radar
Radar cannot ‘see’ directly overhead
‘cone of silence’
appears as ring of minimal/non-returns around radar, esp. with widespread precipitation
Sample volume increases as beam travels away from radar
Equivalent Radar
Reflectivity Factor 2
Ze relates rainfall intensity to average returned power
‘Equivalent’ acknowledges presence of numerous scattering targets of varying:
sizes/shapes
compositions (water/ice/mixture)
distributions
Several assumptions made (not all realistic)
Scanning Modes
Clear-Air Mode
slower antenna rotation
five elevation scans in 10 minutes
sensitive to smaller scatterers (dust, particulates, bugs, etc.)
good for snow detection
Precipitation Mode
faster antenna rotation
9-14 elevation scans in 5-6 minutes
less sensitive than clear-air mode
good for precipitation detection/intensity determination
Summary
Weather surveillance radar has varied uses
short-term weather forecasting
hazardous weather warnings
hydrologic applications
tracking bird migration patterns
Must be aware of radar’s limitations
WYSINAWYG
What You See Is NOT ALWAYS What You Get!