Winter Precipitation Measured with a new Heated Tipping Bucket Gauge. John Kochendorfer 1 Mark Hall 1 Timothy Wilson 1

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1 Winter Precipitation Measured with a new Heated Tipping Bucket Gauge John Kochendorfer 1 Mark Hall 1 Timothy Wilson 1 1 Atmospheric Turbulence and Diffusion Division, NOAA, P.O. Box 2456, Oak Ridge, TN 37831, USA Introduction and background: Historically, tipping bucket gauges have not measured snowfall well, but the TB3 tipping bucket rain gauge (Hydrological Services Pty. Ltd) was modified by Hydrological Services to include a temperature sensor, a snow/ice sensor, and heaters. The need to have a low cost, truly independent, backup precipitation sensor collocated with climate station weighing precipitation sensors motivated NOAA/ORAU personnel to perform a comparison between the tipping bucket sensor and the Geonor weighing precipitation gauge. At two different sites, one at Yosemite, CA and the other at Limestone, MN, 5 min data was recorded continuously from a Geonor and a TB3 installed next to each other during the winter of 27/28. A comparison was made between the Geonor weighing precipitation gauge data and the heated TB3 precipitation data. In order to conserve power, the heated TB3s were programmed to deactivate their heaters at temperatures below -1 C. Above -1 C, a snow sensor installed near the bottom of the TB3 funnel controls heaters which were designed to melt the snow which collects in the funnel of the TB3 and allow the tipping bucket gauge to accurately measure the precipitation. All parameters in the TB3 Heater including the Low Temperature Off threshold are adjustable by the user, via the TB3 Heater s integrated SDI-12 Communications Interface. It should be noted here that data collected at temperatures of -1⁰C or lower was adversely affected due to the Low Temperature Off threshold being left at the factory default setting of -1⁰C. Hydrological Services has stated that all future TB3 Heaters will be shipped with the Low Temperature Off threshold set at -2⁰C. Results Cumulative results At Yosemite the Geonor-measured cumulative precipitation for the winter was ~4 mm. The TB3 recorded 347 mm of precipitation during the same period. A calibration coefficient of 1.14 would be necessary to correct measurements up to the magnitude recorded by the Geonor. Almost all of the precipitation at Yosemite fell as snow, and

2 almost all of the significant precipitation events occurred when the temperature was above the apparent TB3 low-temperature cutoff. At Limestone, 566 mm of precipitation occurred over the winter. The TB3 cumulative precipitation was 56 mm. The calibration necessary to correct the TB3 measurement was Rain was prevalent at the beginning of the season. 16 mm of rain fell, and the Geonor cumulative precipitation matched the TB3 very well before the snow began. Isolating snow from rain, the TB3 rain calibration was ~1, and the snow calibration was Daily results Daily Precipitation lim24(1:146,13) yos24(1:143,13) 4 Geonor Precip (mm) TB3 Precip (mm) Figure 1, Daily precipitation. Yosemite results are in green, and Limestone are blue. Some of the precipitation events spanned two or more days, and in some of these cases the TB3 recorded snowfall on a different day than the Geonor due to the lag caused by the TB3 sensing and subsequent melting of the snow. 15 min results

3 min Precipitation lim15min(1:1422,13) yos15min(1:1422,13) 2.5 Geonor Precip (mm) TB3 Precip (mm) Figure 2, 15 min precipitation. Yosemite results are in green, and Limestone are blue The use of 15 min data without regard for the lag necessary for the TB3 to sense the presence of snow and melt it caused errors. These were especially apparent in the Limestone data (lim15min, in blue) as a significant portion of snow fell when the temperature was below the heater cutoff value. The resolution of the TB3 was also apparent. Precipitation event results Based on the precipitation record, the Geonor temperature of -9. C correlated well with the unrecorded low-temperature cutoff of the TB3, and the Geonor temperature was therefore used to identify precipitation events that occurred when the temperature was below the low-temperature cutoff of the TB3. At Limestone, during the entire winter, 67.3 mm of precipitation fell when the temperature was below the apparent heater cutoff temperature. The TB3 recorded only 14.6 mm during these cold periods, and the timing of the precipitation was also out of well out of sync with the Geonor. At Yosemite, temperatures were almost always above the heater cutoff and the TB3 missed no significant precipitation events due to below -1 C temperatures. Event to event comparison was strongly dependant upon the definition of event. We chose a 3 hour wait-time, the end of which would mark the end of an event if no precipitation occurred. Because the TB3 was less sensitive to low precipitation rates, TB3 events would occasionally end before Geonor events (Figure 6). Because the Geonor events would persist for a longer period of time, there more precipitation events occured in the TB3 record than the Geonor record, and direct event to event comparison was problematic when the Geonor precipitation record was used to define Geonor events and the TB3 record was used to define TB3 events. Below are some examples.

4 7 6 lim(geanor) lim(tb3) lim(ta) Precip (mm) and Temp (C) Day Figure 3, Limestone TB3 was working well in the rain. Geonor (blue), TB3 (green), temperature (red). 1 8 lim(geanor) lim(tb3) lim(ta) 6 Precip (mm) and Temp (C) Day Figure 4, Limestone TB3 was working fairly well in the snow. Geonor (blue), TB3 (green), temperature (red).

5 3 25 lim(geanor) lim(tb3) lim(ta) Precip (mm) and Temp (C) Day Figure 5, Limestone TB3 was working poorly at low temperatures. Geonor (blue), TB3 (green), temperature (red). 4 yos(geanor) yos(tb3) yos(ta) Figure 6, Yosemite TB3 events. Geonor (blue), TB3 (green), temperature (red). One of the only low temperature events at Yosemite is in Fig (6). The lower sensitivity of the TB3 to low precipitation rates for snow events is responsible for the first event discrepancy in Fig (6), and low temperatures may be responsible for later discrepancies. If the event buffer had been longer (8-12 hr), these three separate TB3 events would not be distinct, and would be more comparable to the Geonor event. Using the Geonor to define distinct events, below the Geonor precipitation events are compared to TB3 measured precipitation which occurred during the Geonor events.

6 8 7 limevent(1:114,13) yosevent(1:114,13) Figure 7, Comparison of all precipitation events. Limestone (blue), Yosemite (green). We separated these events out by temperature. 8 7 limevent(1:114,13) yosevent(1:114,13) Figure 8, Comparison of low-temperature (minimum temperature < -9. C) precipitation events. Limestone (blue), Yosemite (green).

7 As expected, in Fig (8), when the TB3 heaters were disabled by the sensor s lowtemperature power-saving algorithm the TB3 performed poorly. 8 7 limevent(1:114,13) yosevent(1:114,13) Figure 9, Comparison of warm-temperature (minimum temperature > -9. C) precipitation events. Limestone (blue), Yosemite (green) Figure 1, Comparison of rain-only (minimum temperature >. C) precipitation events. Limestone (blue), Yosemite (green).

8 8 7 limevent(1:114,13) yosevent(1:114,13) Figure 11, Comparison of snow-only (minimum temperature > -9. C, maximum temperature <. C) precipitation events. Limestone (blue), Yosemite (green). 8 7 limevent(1:114,13) yosevent(1:114,13) Figure 12, Comparison of mixed, snow and rain events (minimum temperature <. C, maximum temperature >. C) precipitation events. Limestone (blue), Yosemite (green). Qualitatively, the snow-only results were very good. Limestone snow-only data was a little better than the Yosemite data. The pervasive pattern at Yosemite is that temperatures were above freezing until just before precipitation began, and they dropped

9 to below freezing just before precipitation events begin. We don t have enough data to create site-specific calibration coefficients for rain vs. snow (particularly rain only events at Yosemite). Snow-only regression results: Geonor = TB3* mm. R 2 = 1.. Mixed event (~ C) regression results: Geonor = TB3* mm. R 2 =.99. Rain-only regression results: Geonor = TB3* mm. R 2 =.98. Table 1, Event regression calibration results, combining the data from both sites together. Discussion In these preliminary tests from Limestone, ME and Yosemite, CA the TB3 captured winter precipitation well, and differences between the weighing sensor and the tipping bucket sensor can be corrected for reliably by using calibration coefficients that are specific to precipitation type. The performance of the heated TB3 justifies further testing for its prospective use as a low-cost, independent backup to a weighing precipitation sensor. From TB3 data alone (without using the Geonor data to identify the end and beginning of events) it was difficult to determine when actual precipitation events began and ended. Using a 3 hour buffer, or waiting-time, there were significant differences between the number of Geonor and TB3 events. See fig (6) for an example. In the event analysis presented here, the Geonor data was used to determine the beginning and ending of events. This approach was chosen to circumvent the challenges of defining event. In reality, an ideal measure of precipitation will not be available when the data from the backup sensor is needed, and more careful analysis is needed in order to best approximate the precipitation event record from TB3 data. The sensitivity and the errors involved in choosing the definition of event will vary widely depending upon the type of climate analysis that the data is used for. The greatest potential errors from winter TB3 data would occur in precipitation intensity analyses that are highly-sensitive to event duration and number of events. With regards to the low temperature cutoff of -1 C, further climate analysis is needed to evaluate the importance of low-temperature precipitation. Available wintertime precipitation data from representative locations across the United States could be analyzed to carefully choose the low temperature cutoff and quantify the potential magnitude of precipitation missed due to low temperature. Test-bed wish list (in an ideal world, with unlimited resources): 1) Replication (several Geonor and TB3s). 2) Imagery. Photos of one or more of the TB3 funnels would be helpful for evaluating the effectiveness of the heater and snow sensor. 3) TB3 heater load. Is it on or off? If we are considering reprogramming the sensor for lower temperatures, we need to understand power use and heater effectiveness for lower temps. A temp sensor near one of the heaters might be sufficient. The

10 heater load is controlled predictably by the sensor, but it would be preferable to measure this directly. 4) Wind Speed. 5) Snow depth/liquid content of snow. How sticky is the snow? Does this affect snow blown out of the sensor by the wind or how well the TB3 senses and melts the snow? 6) More precipitation data. Although significant differences between rain and snow calibration coefficients, if all of the precipitation available for cross-comparison is snow it is hard to tell if errors are general calibration errors or snow related. Dry snow, wet snow, etc. 7) One sensor with a disabled/lowered low-temperature cutoff. It might be interesting to know if the TB3 is capable of melting snow at lower temps. At Limestone, for example, if the cutoff had been -12 or -15 C the TB3 may have captured all the important precipitation events.