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Team Resolves Doppler Dilemma

Researchers and engineers from NSSL and the National Center for Atmospheric Research (NCAR) have successfully developed an algorithm that solves a problem plaguing weather radars called the Doppler dilemma. The Doppler Dilemma identifies an inverse relationship between the unambiguous range and the unambiguous velocity: wind speed measurements are more accurate at closer distances and less accurate farther away. Proper censoring of this contaminated or invalid data is essential for the WSR-88D to be useful for forecasters and automated algorithms.

The Doppler dilemma occurs when a radar sends out pulses very quickly and does not have time to listen for the return echo before sending out the next pulse--it cannot detect objects farther away. Slowing the frequency of the pulses sent out results in velocity measurements that are farther away but less accurate. Trying to alleviate one problem worsens the other; this is the dilemma.

The Radar Operations Center of the National Weather Service sponsored NSSL and NCAR to find methods to deal with this problem. A complex phase coding algorithm, SZ-2 (named for NSSL developers Sachidananda and Zrnic) is the result. SZ-2 makes decisions on ground clutter filtering, echoes received after the next pulse is transmitted and more. NSSL’s Sebastian Torres was able to integrate the algorithm into the surveillance and Doppler scanning modes and also incorporated ground clutter filtering and estimation of spectral moments.

SZ-2 is now undergoing Beta testing on the NWS operational radar in Oklahoma City, and will soon be implemented on the national network of WSR-88D radars.

Background:
NSSL has long been working on radar signal processing to improve the accuracy of the WSR-88D radar data. Techniques are tested and evaluated at NSSL before being deployed on NWS systems.

Significance:
The ability to accurately measure winds over larger distances and precisely censor contaminated radar data is critical to aiding meteorologists in the forecast and warning decision process.

Outcome:
Supports overall goal to provide more accurate and timely warnings of high-impact weather events that disrupt economic productivity and cause loss of life and property.

4/11/07