bucket. An algorithm is then applied to the rest of the time period where the LEDWI data
was available to get the total estimated actual snowfall depth.
Within the RWFS the data used for actual liquid amounts comes from the tipping bucket
at the METAR site as well as the from the Local Analysis and Prediction System (LAPS)
data. For this demonstration a special LAPS grid was set up over the state of Iowa.
Fifteen-minute reflectivity WSI NOWRAD radar at 1 km resolution was used put through
the LAPS precipitation accumulation analysis using a fixed Z-R relationship to form an
hourly 12 km grid of Quantitative Precipitation Estimate (QPE) observations, which was
distributed hourly. If the LAPS QPE data has a higher amount for a given hour then the
METAR QPE report from the tipping bucket then LAPS was used as "truth".
10.2 Case Studies
10.2.1 Cold Rain Case 16 January 2004
Rain moved into the Ames area around 22 UTC on 16 January 2004 and continued
through 03 UTC on 17 January 2004. According to the observations at the Ames
METAR site, the precipitation fell in the form of light rain much of the time with brief,
intermittent periods of "unknown precipitation" (UP). UP may indicate that the
precipitation was just too light to be identified by the LEDWI sensor or it may indicate a
mixture of rain and snow.
The precipitation was associated with a stationary front draped down through the
southwest corner of Iowa (Fig. 10.2). The front extended from a low pressure system that
moved slowly from the northwest to the southeast corner of North Dakota. The
precipitation moved across the demonstration area from southwest to northeast (Fig.
10.3).
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