Research Tools: Dual-Polarized Radar
Dual-polarized (dual-pol) radar technology is truly a NOAA-wide accomplishment. NSSL spent nearly 30 years researching and developing the technology. The National Weather Service (NWS) and NSSL developed the specifications for the modification, which was tested by engineers at the NWS Radar Operations Center. The NWS Warning Decision Training Branch provided timely and relevant training to all NWS forecasters who would be using the technology.
The upgraded radars offer 14 new radar products to better determine the type and intensity of precipitation and how much may fall. It can also help tell the difference between smoke, birds, bats and bugs, and confirm that tornadoes are on the ground causing damage. Dual-pol is the most significant enhancement ever made to the nation's radar network since Doppler radar was first installed in the early 1990s.
Radars send out short pulses (bursts) of electromagnetic wave fields. The pulses bounce off particles in the atmosphere and return back to the radar dish. A computer processes the returned signals and, through algorithms, can make conclusions about what kinds of particles it “saw,” including the speed of motion (the Doppler effect) towards or away from the radar. Non-polarimetric Doppler radars (including the NOAA radars before polarimetric upgrade) transmit horizontally polarized electromagnetic waves, which only give a measure of the horizontal dimension of the cloud particles (snow, ice pellets, hail and rain). This one-dimensional picture makes it difficult to tell the difference between precipitation types.
With dual-pol technology, the picture becomes two-dimensional because the radar sends both horizontally and vertically polarized electromagnetic waves. As these perpendicular fields bounce off an object and are received back at the radar, a computer program separately processes information about the horizontal and vertical dimension of the particles. This cross-section now gives forecasters a measure of the size and shape of the object.
Dual-pol researchers at NSSL are endeavoring to:
- Develop a dual-pol hydrometeor classification algorithm that combines polarimetric radar observations with thermodynamic information from numerical models,
- Identify repetitive polarimetric signatures associated with various microphysical processes in winter storms,
- Examine the performance of a dual-pol partial beam blockage correction algorithm on radar quantitative precipitation estimation,
- Examine impacts of polarimetric radar observations on hydrologic simulation,
- Develop specs for a dual-polarized Phased Array Radar (PAR) and evaluate various design issues, and
- Test and evaluate dual-pol PAR prototypes.
The Benefits of dual-polarization radar
NSSL conducted the Joint Polarization Experiment (JPOLE) in 2002-2003 to demonstrate the operational capabilities of the polarimetric KOUN. During JPOLE, data were delivered in “real-time” to the NWS and other users. NSSL scientists aided in the data interpretation. JPOLE proved that significant improvements in rainfall estimation, precipitation classification, data quality and weather hazard detection were possible using polarized radar.
Dual-pol radar provides measurable benefits to:
- can significantly improve the accuracy of the estimates of amounts of precipitation.
- can tell the difference between very heavy rain and hail, which will improve flash flood watches and warnings.
- can identify types of precipitation in winter weather forecasts, improving forecasts of liquid water equivalent or snow depth.
- can reduce the effects of non-weather scatterers on radar data displays.
- may contribute to increased lead time in flash flood and winter weather hazard warnings, as a result of increased confidence in polarimetric radar data.
- provides critical rainfall estimation information for stream flow forecasts and river flooding.
- may be useful in water management.
- detects aviation hazards such as birds.
- can detect aircraft icing conditions.
- can identify regions of large and giant hail.
- has the potential to save the public about $700 million annually by improving precipitation estimation.
- can improve forecasts and warnings and reduce the impact of hazardous weather on our national transportation.
- better equips forecasters to issue accurate warnings, and in turn helps the public make wiser decisions about our safety.