8th INTERNATIONAL WORKSHOP ON PRECIPITATION IN URBAN AREAS Motivation and Methods Results Discussion and Conclusions 8th INTERNATIONAL WORKSHOP ON PRECIPITATION IN URBAN AREAS Comparison of recording precipitation gauges: tipping-bucket ANETZ and electronic weighing system MPS No: so, ehm Presipitation, metsod, speiscial, imiges, arkanso, eriia, mesuement (to mesue), thaT meanS, concluscion, to groW, craitiria NO HAND IN THE POCKET! use the hands to underline diversity (one hand, other hand and so on) LOOK AT THE PEOPLE and NOT at the screen M. Savina, B. Schäppi, P. Molnar, P. Burlando and B. Sevruk Institute of Environmental Engineering Swiss Federal Institute of Technology – ETH Zürich savina@ifu.baug.ethz.ch St. Moritz, December 2009 1

SwissMetNet in Zermatt: place of the comparison Motivation and Methods Results Discussion and Conclusions Introduction Lambrecht tipping-bucket (LTB) Vs. MPS weighing (MPS) raingauge in snowfall cases SwissMetNet in Zermatt: place of the comparison In the context of the ETH project APUNCH (Advanced Process U of Complex Natural Hazard)  calibration of a X-band radar 2

Motivation 1 MPS LTB Timing of snowfall: Amount of snowfall: Motivation and Methods Results Discussion and Conclusions Motivation 1 w tb MPS LTB Heated Timing of snowfall: the LTB must melt the snow before to record the equivalent in water  delay Amount of snowfall: the big heated surface of LTB may cause a water loss (sublimation + evaporation)  underestimation despite the recording system include advantage/disadvantage (e.g. citations!!), here I am mainly talking about the effect of the heating, by means of the differences between the heated surface of the two gauges ring of the orifice, funnel and outlet evaporation losses no water splash effect becvause we consider snowfall Joss and Guttermann, 1980: 9% water loss due to heating Sevruk, 1983: 30% deficit if compared to fresh snow obs. (Weissfluhjoch, 6 years average) Sevruk and Zweifel, 2002: 14-58% of deficit for the ANETZ gauge at high Alpine sites 3

Timing of precipitation Amount of precipitation Motivation and Methods Results Discussion and Conclusions Motivation 2 5km 10km 15km Vispa Valley In order to perform the calibration of a X-band radar (Kl. Matterhorn) it is important to evaluate and minimize the heterogeneity between ground observations: Timing of precipitation Amount of precipitation 4

Gauges LTB MPS Measuring system: tipping-bucket Motivation and Methods Results Discussion and Conclusions Gauges LTB Measuring system: tipping-bucket Prec. collection: funnel Sampling surface: 200 cm2 Accuracy: < 2% Resolution: 0.1 mm h-1 Sampling time: 10 min (source: Lambrecht documentation) MPS Measuring system: weighing Prec. collection: open collector Sampling surface: 200 cm2 Accuracy: 0.1% Resolution: 0.001 mm h-1 Sampling time: 1 min (source: MPS documentation) 5

Mean wind speed = 1.3 ms-1 (std = 0.7 ms-1) Motivation and Methods Results Discussion and Conclusions Case study Zermatt (SwissMetNet field, 1638 m a.s.l.), 71 days between January and April 2009 Date [month/day] Precipitation [mm h-1] Wind Temperature Precipitation Wind speed [ms-1] to read the extreems biggest contribution: event from 4 to 6 March, LTB 20mm MPS 22mm (-9.1%), meanT=-4C and meanWS=1.75ms-1 Mean wind speed = 1.3 ms-1 (std = 0.7 ms-1) Mean temperature = -3.6˚C (std = 2.1˚C) Precipitation amount = 81.2 mm (LTB) Vs. 102.8 mm (MPS) 6

LTB and MPStb differ only in terms of: Motivation and Methods Results Discussion and Conclusions Statistical Approach Individuation of 20 events (i.e. period of precipitation longer than 60 min) post-processing of the MPS observations in order to simulate the tipping-bucket mechanism (0.1 mmh-1 of resolution and 10 min sampling time)  aggregated observation MPStb LTB and MPStb differ only in terms of: collection mechanism heating procedure MPS LTB MPStb Precipitation [mm h-1] Time [hh:mm] Precipitation [mm h-1] Time 22 Feb 2009 7

Underestimation of the total amount Motivation and Methods Results Discussion and Conclusions Underestimation of the total amount During the 20 events: MPS 98.3 mm MPStb 97 mm (evaporation of the water left in a tipping bucket, between two consecutive events) LTB measured 78.6 mm 20.1% and 19.1% underestimation if compared to MPS and MPStb respectively (e.g. Zweifel and Sevruk 2002) What is right ? ..probably the one that sees more precipitation ! (similar aerodynamic characteristics) 8

Underestimation of the intensity Motivation and Methods Results Discussion and Conclusions Underestimation of the intensity a) c) b) d) mean=0.015mmh-1 fd [-] mean=2.1% mean=0.042mmh-1 mean=32.5% 10 min periods when both gauges recorded precipitation 10 min periods when MPStb recorded precipitation analyzing the intensities we must distinguish between the times in which both gauges recorded precipitation from the times in which only mps recorded precipitation Figure . Relative frequencies (fd) of the differences between MPStb and LTB. a) and b) are based on 10 min time steps for the cases where precipitation is recorded in both gauges; c) and d) group the cases in which MPStb had precipitation, independently from LTB; b) and d) are normalized to the MPStb observations. no correlation between error and ws and temp ( same aerodynamics!) 9

Average of the timings for the 20 common events: Motivation and Methods Results Discussion and Conclusions Time delay Average of the timings for the 20 common events: Division of the delay: MPS Vs. MPStb 21.5 min due to the tipping-bucket mechanism (i.e. gauge resolution) MPStb Vs. LTB 11 min due to precipitation collection (i.e. melt the snow + wet the funnel and direct the meltwater to the tipping mechanism) the mean duration differs of about 54 min checking of the synchronization of the two clocks 10

Time delay MPS Vs. MPStb 21.5 min MPStb Vs. LTB 11 min Motivation and Methods Results Discussion and Conclusions Time delay LTB-MPS MPS-MPStb MPS Vs. MPStb 21.5 min MPStb Vs. LTB 11 min Figure . Histogram showing the delay of the beginning of events measured by the gauges. Only the 20 common event are considered. Mean delay LTB-MPS = 32.5 min; MPS-MPStb = 21.5 min. 11

The tipping-bucket produced the 20.1% of water loss Motivation and Methods Results Discussion and Conclusions Conclusions Over the 20 snowfall events, this study revealed clear advantage of the weighing gauge: Only -2.1% of relative error (MPS as references) for the common measurements But: The tipping-bucket produced the 20.1% of water loss Strong disagreement of the timing, 32.5 min of delay for the tipping-bucket  Necessity to take in account the lake of homogeneity both in time and mass 12

error, will be performed a comparison with fresh snowfall observations Motivation and Methods Results Discussion and Conclusions Outlook The same comparison is taking place also this year (same place and settings) In order to evaluate not only the relative incongruences but also the “absolute” error, will be performed a comparison with fresh snowfall observations Merging of snowfall observations (i.e. generation of spatial field) derived from different gauge types 13

Thanks for your attention! References: APUNCH ETH project, http://www.cces.ethz.ch/projects/hazri/apunch. Joss, J. and Guttermann, Th., 1980. Unterscheidung Regen/Schnee/Hagel und Verdunstungskorektur im ANETZ. Intern Rep., Schweizerische Meteorologische Anstalt, Zürich, Switzerland. Lambrecht, Operating instructions. Lambrecht meteorological instrument, Germany. MPS, brochure. MPS System, Slovakia. Nespor, V. and Sevruk, B., 1999. Estimation of wind-induced error of rainfall gauge measurement using a numerical simulation. J. Atmos. Ocean. Techn, 16(4), 450-464. Sevruk, B., 1983. Correction of measured precipitation in the Alps using the water equivalent of new snow. Nord. Hydrol., 14(2), 49-58. Vuerich, E., Monesi, C., Lanza, L., Stagi, L. and Lanzinger, E., 2009. WMO Field Intercomparison of Rainfall intensity Gauges. Vigna di Valle, Italy, October 2007 – April 2009. X-and radar: http://www.kmradar.ethz.ch. Zweifel, A. and Sevruk, B., 2002. Comparative Accuracy of solid precipitation measurement using heated recording gauges in the Alps. Proc. WCRP Workshop on Determination of Solid Precipitation in Cold Climate Regions, Alaska. Acknowledgements: MeteoSwiss (in particular XXX), SLF (in particular Christoph Marty) and the Canton of Wallis. 14