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Polarimetric Doppler Radar
IDENTIFYING PROBLEMS AND FINDING SOLUTIONS:
Estimating Rainfall
PROBLEM: The number and sizes of drops that make up rainfall varies greatly by storm, time, and location. Hail can also contaminate rainfall estimates.
SOLUTION: Polarimetric radars provide more information on the characteristics of rainfall:
- Can tell the difference between rain echoes and other scatterers like snow, ground clutter, insects, birds or chaff
- Can reduce the impact of different sizes of drops in different parts of the storm on the quality of rainfall estimation.
- Are immune to radar miscalibration and attenuation (the radar beam being absorbed or scattered) in precipitation, and partial blockage of the radar beam
SUCCESSES: The Joint Polarization Experiment (JPOLE) demonstrated the operational capabilities of the polarimetric KOUN, and proved that significant improvements in data quality, rainfall estimation, hail detection and rain/snow discrimination were possible using polarized radar. Part of the JPOLE project was to compare radar data with actual measurements taken on the ground. NSSL research found:
- Errors of 1-hour precipitation accumulation were reduced
- Significant
improvement in estimating areal rainfall and heavy precipitation (often
mixed with hail). Practical implications include:
- River flash flooding forecast and management that require reliable measurement of areal rain accumulations regardless of rain intensity
- Urban flash flooding forecast that requires accurate estimation of heavy rain with high spatial resolution
- Rain gauge comparison proves more accurate: NSSL scientists compared polarimetric rainfall estimation with rain gauge data from the Agricultural Research Service (ARS) micronet (a dense rain gauge network in Oklahoma ) during different seasons and different types of rain. A new rainfall algorithm that uses all of the polarimetric variables has been developed for rainfall estimation as a result, and errors in the estimates of hourly rain were significantly reduced when compared with conventional non-polarimetric radar
WHAT'S NEXT:
- Exploring how improved precipitation estimates from polarimetric rainfall measurements can be used to initialize hydrologic models over small watersheds
- Looking into how assimilation of radar data into local numerical weather prediction models could help predict tornadoes, strong winds, hail, and other hazards
