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Polarimetric Doppler Radar
IDENTIFYING PROBLEMS AND FINDING SOLUTIONS:
Detecting Weather Hazards
PROBLEM: HAIL - Where is hail falling, how big is it, and where has it been? Conventional radar can only identify the probability of hail at any particular location within the storm.
SOLUTION: Polarimetric radar can identify the specific location of hail, estimate hail size, and track hail swaths.
SUCCESSES: Validation of polarimetric hail detection (Heinselman and Ryzhkov): NSSL scientists recently showed that a simplified version of NSSL's polarimetric hydrometeor classification algorithm (HCA) outperforms the NEXRAD Hail Detection Algorithm (HDA) currently in use by the NWS. They compared rain and hail reports collected by storm-intercept teams (mostly during the Joint Polarimetric Experiment) with data provided by the HDA and HCA to verify the accuracy of the algorithms. Scientists found the HCA had significantly fewer false alarms. This work was done in anticipation and support of the polarimetric upgrade to the WSR-88D network.
Hail size discrimination (Heinselman) Dual polarization radar not only supplies information for hydrometeor classification, but may also prove useful for gauging hail size within the storm. Past research indicates relationships between polarimetric variables that might allow for hail size to be categorized. Investigating the relation of polarimetric variables to hail size can be beneficial in improving warnings for hail producing storms, understanding the physical processes that lead to hail formation, and determining possible early signatures associated with hail formation and growth. NSSL scientists found hail size was best distinguished using a pairing of Z DR (a logarithmic ratio of the returned horizontal and vertical power) and a correlation coefficient (a statistical number that indicates the degree of similarity among particles within a cloud).
WHAT'S NEXT: Detailed microphysical information (Brandes, Schuur, Ryzhkov, Zhang, Ikeda) Research by NSSL and NCAR scientists showed that polarimetric radar provides detailed microphysical information that could be helpful in cloud models.
PROBLEM: CLOUD ELECTRIFICATION - When does the lightning threat begin or end?
SOLUTION: Polarimetric radar information on ice crystals could warn us of lightning.
SUCCESSES: Electrification in storms (Bruning, Rust, MacGorman, Schuur, Straka) Polarimetric radar signatures within the melting layer were examined in the context of electric field observations from balloon soundings and maps of total lightning flashes during the late stages of a multi-cellular storm. Preliminary analysis indicated a positively charged layer below 0°C, followed by a large negative layer just above the melting level, with another positive and negative layer above.
WHAT'S NEXT: Oriented ice crystals: The horizontal or vertical orientation of ice crystals can be detected by polarized radar and may provide clues that could tell us when the threat of lightning begins or ends.
PROBLEM: TORNADOES - Conventional radar cannot pinpoint a tornado's location when it touches down and may miss tornadoes:
SOLUTION: Polarimetric WSR-88D radar has the unique capability to detect a strong (F3) or violent tornado (F4) when it touches the ground and creates lofted debris.
SUCCESSES: NSSL scientists have also discovered the evolution of the 3D pattern of polarimetric variables prior to tornado touchdown reveals quite unusual and intriguing polarimetric signatures aloft that might be related to the subsequent tornado. Understanding and interpretation of these signatures could provide more insight into microphysical aspects of tornadogenesis.
WHAT'S NEXT:
- define the maximum range that a tornado can be detected
- find wind strength for debris lofting
- obtain more data for tornadic bow echoes
- collect/analyze more data looking for a tornado precursor signature that may exist within the developing hook echo precipitation distribution
PROBLEM: AVIATION WEATHER HAZARDS - Conventional radar cannot detect and assess all weather hazards in the vicinity of an airport terminal.
SOLUTION: Polarimetric radar provides extra information to help detect icing conditions, lightning, and precipitation that could reduce visibility.
SUCCESSES: Aviation weather hazard detection (Brandes, Ikeda, Elmore, Ryzhkov, Schuur) Better detection and assessment of weather hazards in the terminal area are possible with polarimetric radar, ultimately improving airport capacity. NSSL and NCAR scientists have discovered improvements in quantification of winter (frozen) precipitation, detection of some mixed-phase icing conditions (especially situations involving embedded convection), the estimation of precipitation-impacted visibility, detection of lightning precursors, and improved microphysical parameterization in numerical forecast models.
WHAT'S NEXT: Polarimetric measurements should also be useful for eliminating regions of the atmosphere where particular hazards are not likely.
