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The LEDWI report of intensity is well correlated with the
rate of snow accumulation but is not directly related to the
visibility reduction due to snow, which observers use to
differentiate between light, moderate, and heavy snow.
ASOS processing algorithms use the visibility sensor data
to modify the LEDWI snow intensity report so that moder-
ate snow is not reported when the visibility is greater than
1/2 mile and heavy snow is not reported when the visibil-
ity is greater than 1/4 mile.
The LEDWI has an “adaptive baseline,” which adjusts
the power spectrum threshold to reduce reports of false
precipitation. For instance, wind turbulence and thermals
(e.g., the shimmer seen across an open field on a sunny
day) can induce scintillations that are near the frequencies
characteristic of snow. Therefore, ASOS sets the threshold
for snow detection above the spectral power induced by
turbulence.
This “adaptive baseline” can pose a problem when
snow increases so slowly that the baseline rises without
snow being detected. When this occurs, only a sudden in-
crease in snow may trigger the sensor to report snow. A
similar condition may occur when the LEDWI is turned
on (say after a power failure) and precipitation is falling.
The initial adaptive baseline may be set much too high to
detect precipitation correctly and will not be reset to a rep-
resentative threshold until the precipitation ends.
Rain detection is generally not a problem. Occasion-
ally, if the rain is preceded by a gradually increasing drizzle,
the rain channel adaptive baseline threshold may rise to a
point where light rain is not sensed. As a general rule, rain
lighter than 0.01 inches per hour will not be detected.
4.3.6 PI Algorithm
Every minute, the PI algorithm requests the PI sensor
data, stores the data in memory for 12 hours, and examines
the latest 10 minutes of data stored in memory. If three or
more samples are missing, the algorithm sets the sensor sta-
tus to “inoperative” and reports “missing” for that minute. If
the 10-minute memory buffer contains less than two sensor
samples of precipitation, the precipitation report terminates.
If, however, two or more samples in the latest 10 min-
utes indicate precipitation, the algorithm then determines
the type and intensity to report. In general, to report any-
thing other than “Unknown Precipitation” (UP), two of the
samples are required to be the same type. If there is a tie
between two types of precipitation (e.g., two rain samples
and two snow samples) snow is reported. This determina-
tion is based on the hierarchal scheme for reporting present
weather: liquid (-RA, RA, +RA), freezing (-FZRA, FZRA),
and frozen (-SN, SN, +SN) in ascending order.
ASOS reports only one precipitation type at a time.
For instance, if both freezing rain and snow are detected,
snow is reported. Additional precipitation elements may
be added to the report by observers. The PI algorithm per-
forms a temperature check on the PI sensor output. If -SN,
SN, or +SN is reported, and the temperature is > 38 F, then
the PI sensor output is set to “No Precipitation” (NP). The
PI algorithm also formats and reports precipitation begin-
ning and ending remarks just as the observer does.
Once the precipitation type is determined from the last
10 minutes of data, then the 1-minute samples from the
past 5 minutes are used to compute intensity. Precipitation
intensity is determined from the highest common intensity
derived from three or more samples. Common intensities
for heavy precipitation are light, moderate and heavy; for
moderate precipitation, common intensities are light and
moderate. For example, if rain is the determined precipita-
tion type, and there are three moderate rain and one light
rain detected in the past 5 minutes, then ASOS reports
moderate rain in the METAR/ SPECI report. Likewise, if
snow is the determined type and there are one light, two
moderate, and one heavy snow in the past 5 minutes, then
moderate snow is reported.
As a third example, if the most recent 5 minutes of
sensor data contains one report of light rain, and two re-
ports of moderate rain, then light rain is reported by ASOS.
If, on the other hand, there are less than three common
intensities of the reported precipitation type, then ASOS
reports the lightest intensity. For example, if rain is the
determined type, and there are only one moderate rain and
one heavy rain reported by the sensor, then the precipita-
tion intensity is set to moderate rain (RA) in the ASOS
METAR/SPECI report. If snow is the determined precipi-
tation type and there are only one moderate and one heavy
snow, then ASOS reports moderate snow.
The ASOS PI algorithm formats and reports precipi-
tation beginning and ending remarks just as the observer
does. The PI sensor output is further compared with the
freezing rain sensor output to ultimately determine the pre-
cipitation reports and remarks (see description of the freez-
ing rain algorithm for further details).
Under blowing snow conditions, particularly where
snow is blown to a height of 10 feet or more, the PI sensor
(LEDWI) can mistakenly interpret the scintillations from
blowing snow rising up and/or settling through the sensor’s
IRED beam as rain, and occasionally as snow, depending
on the vertical velocity of the snow particles.