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Freezing Rain Comes to ASOS

This paper is a condensed version of a presentation made at the Mt. Washington Observatory's "FOCUS 2000: Wind, Ice, and Snow" Conference, June 1995. For the full paper, contact the ASOS Program Office. Direct questions or comments to Hughes STX Corporation (ATTN: Al Ramsay), 43872 Weather Service Road, Sterling, VA 20166; (703/260-0341);

Where Did It Come From And How Does it Work?

Since the late 1980s, the NWS has been evaluating a new sensor to report freezing rain. The technology has a long and successful history as an aircraft ice detector. After going several modifications in the early 1990s, the sensor performed so well in the winter of 1994-1995 that NWS decided to deploy it on the ASOS. This article will give you a picture of how it works, how well it works, and where it may have problems in detecting and reporting icing conditions.

The new sensor, Rosemount Aerospace Corporation's Model 872C2, detects ice accumulation by monitoring the resonant frequency (nominally 40,000 hertz) of a vibrating magnetostrictive metal tube. The tube is made of a special metal alloy in which the atoms actually elongate when subjected to a magnetic field ("magnetostriction'); the tube is subjected to an oscillating electromagnetic field, and special circuits determine the natural resonant frequency of the tube, which depends on the amount of mass made to vibrate. The resonant frequency decreases with increasing accretion of ice, frost or wet snow. The sensor's de-icing system is activated by the ASOS algorithm when ice, frost or wet snow drive the frequency below a certain value. The sensor is de-iced through internal heating elements, which heat the probe and strut above the boiling point of water for a short time.

ZR Sensor

Frequency values are reported to the ASOS once each minute. The system combines information from the sensor with data from other ASOS sensors to generate the required reports of freezing rain. The ASOS does not report data that indicate the presence of icing without detectable precipitation.

A "sensor" event begins (or continues after a de-ice) when the vibration frequency shows ice 0.005 inches of ice accretion and the rate of frequency decrease exceeds about 0.002 inches in 15 minutes. The event ends whenever the frequency indicates an equivalent of less than 0.005" of ice or whenever the rate of ice accretion is less than about 0.002" in 15 minutes. A "system" freezing rain event is reported from the ASOS only after combining the sensor output with data from the ASOS precipitation identifier:

The Light Emitting Diode Weather Identifier (LEDWI) is an optical sensor that reports precipitation falling through its sensing volume. It is able to discriminate between rain and snow. The LEDWI must provide a positive indication of precipitation ("P" or "R") before ASOS can transmit a report of freezing rain. This algorithm protects the ASOS from falsely reporting rime icing as freezing rain but results in the loss of 0.6% of true freezing rain minutes when precipitation is too light to be detected by the LEDWI (less than about 0.01" per hour).

If the LEDWI is reporting snow (S), any output from the freezing rain sensor will be overridden, and the ASOS will report snow. This algorithm protects the ASOS from falsely reporting wet snow as freezing rain, but results in the loss of 2.3% of true freezing rain minutes when the freezing rain is mixed with snow.

Will It Catch Every Minute of Freezing Rain?

A measure of ASOS's ability to detect freezing rain is provided by the number of minutes in which both ASOS and an observer reported freezing rain. During the 1994-1995 test, observers reported 11318 minutes of freezing rain; ASOS reported 12234 minutes. Although the total numbers of minutes are close (and have been over the last three testing seasons), researchers noted that the ASOS and human do not necessarily report freezing rain at the same time. The number of coincident minutes was 7428, or about 66 percent of all human minutes. Detailed analyses of the 3890 minutes of un-reported freezing precipitation are summarized here. A significant, but indeterminate, fraction of the minutes lost to slow or no accretion are attributed to three causes:

Will It Lie to Me?

Researchers found that, of the 12,234 minutes of freezing rain reported by the ASOS, 9771 minutes (80%) were confirmed by an observer as having either freezing rain or freezing drizzle in progress. An additional 467 minutes (4%) were reported during ice pellets accreting on surfaces, but which did not technically constitute a "freezing rain" condition. Sixty-seven minutes (0.5%) can be called true false alarms, directly attributed to wet snow accreting on the probe, briefly mis-identified as "R" or "P" by the ASOS weather identifier, and therefore erroneously reported as freezing rain.



      CAUSE                                                           # MINUTES
                                                                      (% OF ALL
                                                                    ZR MINUTES)

     UNKNOWN   (ASOS 1-minute data were not available)              330  (2.9%)

     SLOW RECOVERY FROM DE-ICE                                      204  (1.8%)

              AT BASE OF PROBE)                                     178  (1.6%)

     SNOW OVERRIDE                                                  258  (2.3%)

     LEDWI UNABLE TO DETECT PRECIP                                   67  (0.6%)

     SLOW OR NO ACCRETION                                          2853 (25.2%)

The remaining 1929 unverified ASOS freezing rain minutes are believed to have occurred in three conditions:


Unreported Freezing Rain Minutes
     CATEGORY                                                      MINUTES

     FREEZING RAIN                                                   12234

          HIGH ACCRETION RATE (NOT TRANSMITTED BY ASOS)              13557

     WET SNOW (NOT TRANSMITTED BY ASOS)                               1276

          (NOT TRANSMITTED BY ASOS)                                 >40000


                                                    TOTAL           >68000

How About Ice That Isn't From Freezing Rain?

Including the 12,234 icing minutes actually reported by the ASOS as freezing rain, the 872C2 sensor responded to more than 68,000 minutes of icing from all sources, summarized in the table above.

What's Happening?

Testing in 1994-1995 indicated that the combination of the 872C2 ice detector and the Light-Emitting Diode Weather Identifier provides an effective capability for the ASOS to identify periods of freezing rain, with minimal false alarms. Sensors will be installed as quickly as they can be manufactured, beginning in November-December 1995 and continuing until more than 300 are installed at NWS, FAA and Navy sites in the United States. Although this ASOS data might be able to differentiate among glaze, rime, frost and wet snow, neither the NWS nor the Federal Aviation Administration sees a need to provide this information and have planned no formal development activities.

December 1995


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