Snow to Liquid Ratio Variation with Temperature: Whose Assumptions are These, Anyway?

1. Snow to Liquid Ratio Variation with Temperature:Whose Assumptions are These, Anyway? Robert A. Weisman and Jacob Yurek* EAS Dept., Saint Cloud State University *Current affiliation: DTN/Meteorologix, Burnsville, MN Email: raweisman@stcloudstate.edu Web: http://web.stcloudstate.edu/raweisman/

2. Snow-to-Liquid Ratios • Huge operational problem • if we could get QPF right…(Hah!) • Regional knowledge not well known 10 years ago • Depends on Cloud Physics within Each Cloud • Efficiency of Ice Crystal Growth Rate • Having saturated layer with temperature colder than -15°C • Having a feeder layer with high liquid water content • Good Review by Baumgardt, NWS LaCrosse • Effect of moist-melting layers • Effect of ground temperatures, especially in recent mild years • Compaction Issues Weisman & Jurek - NPWSC 06

3. Forecast Tools – Proxy Methods • Knowledge of individual cloud physics lacking • 10:1 or bust!...except maybe lake effect? (1995) • Baxter et al. 2005 SLU Research on mean snow:liquid by area • Mean ratio based on NWS Forecast Areas • Roebber et al. 2003 Extrapolation of Snow-to-liquid ratio based on model data • Best: Know what sounding will look like • Forecast soundings often stink! Weisman & Jurek - NPWSC 06

4. Forecast Tools – Proxy Methods(2) • Sounding Snapshots • Critical Thicknesses • 1000-500 mb 5400 m • Mostly underground away from ocean • 1000-850 mb 1200 m • Critical Temperature Structure • Being Saturated at Critical Temperatures for Ice Crystal growth • Wetzel Winter Weather Forecasting Ingredients looks at 600 mb temperatures Weisman & Jurek - NPWSC 06

5. Snow-to-liquid vs Surface Temp. • Frequently used simplification • Old “Study”? • Gives a single value conversion for surface temperature ranges • Advanced from “everything is 10:1” • Version previously posted on NWS Quad Cities, IA/IL website (not found now) Weisman & Jurek - NPWSC 06

6. Still found at ToolKit - Envirocast - the weather & watershed newsletter Weisman & Jurek - NPWSC 06

7. Jacob Yurek Senior Research Project • Frequent error in bad snow forecasts. • Single ratio doesn’t make sense from operational experience • Empirical Rules Based on Henn, Jurek, and Weisman work to come later • Determine snow:liquid for long term staffed reporting station: • Minneapolis-St. Paul International Airport, MN (KMSP) Weisman & Jurek - NPWSC 06

8. Methods • Time period: 1961-1995 • Source: Local Climatological Data for KMSP • 822 Cases • No sleet, freezing rain, nor mixed precipitation allowed in any case • Must have produced measurable liquid • Before period when ASOS used entirely for liquid equivalent Weisman & Jurek - NPWSC 06

9. Methods cont. • Stratified by surface temperature categories as noted in old “study” • 34-28°F (261 cases) • 27-20°F (240 cases) • 19-15°F (117 cases) • 14-10°F (87 cases) • 9-0°F (90 cases) • -1 to -20°F (27 cases) Weisman & Jurek - NPWSC 06

10. Mean Values • Other Statistics? • Look at Distributions Weisman & Jurek - NPWSC 06

11. 28 – 34°F 10:1 Weisman & Jurek - NPWSC 06

12. 27 – 20°F 10:1 15:1 20:1 Weisman & Jurek - NPWSC 06

13. 19 – 15°F 15:1 20:1 10:1 30:1 Weisman & Jurek - NPWSC 06

14. 14 – 10°F 15:1 20:1 40:1 25:1 30:1 10:1 Weisman & Jurek - NPWSC 06

15. 9 – 0°F 20:1 15:1 25:1 30:1 Weisman & Jurek - NPWSC 06

16. -1 – -20°F 30:1 40:1 20:1 Weisman & Jurek - NPWSC 06

17. Results • Study contaminated by observational bias • Anecdotal confirmation • In fact, look at Mean ratio based on NWS Forecast Areas • Next step: • Is Cooperative Data contaminated? • New, independent data taken without assumptions Weisman & Jurek - NPWSC 06

18. Empirical SCSU Snow Forecasting Rules • Wet snow (850 temp. of -2 to 0°C) usually 5-8:1 • Surface temperature: 32-34°F • Can be 10:1 for surface temperature of 28-32°F • Dry “dendritic snowfall” • 850 mb temp of -8 to -4°C • Saturated at -10 to -15°C in sounding • Surface temperatures generally in the 20’s°F • Saturated at -10 to -15°C in sounding • Wet snow: around 10:1 • Dry snow: 12-15:1 • Colder “dendritic snowfall” • 850 mb temp of -12 to -5°C • Saturated at -10 to -15°C in sounding • Surface temperature: 18-24°F • 15-20:1 • Even colder or lake effect: 20+:1 Weisman & Jurek - NPWSC 06

19. References Baumgardt, D., 1998: Wintertime cloud microphysics review. NWS LaCrosse, WI. http://www.crh.noaa.gov/arx/micro/micrope.php Baxter, M. A., C. E. Graves, and J.T. Moore, 2005: A climatology of snow-to-liquid ratio for the contiguous United States. Wea. and Forecasting, 20, 729-744. http://www.eas.slu.edu/CIPS/Research/snowliquidrat.html COMET module: http://www.comet.ucar.edu/class/aes_canada/06-1/html/descriptions/snowRatio.htm National Environmental Education and Training Foundation, 2003: ToolKit. EnviroCast: The Weather and Watershed Newsletter, 1(3), http://www.stormcenter.com/envirocast/2003-01-01/envirocast-article2.php Roebber, P. J., S. L. Bruening, D. M. Schultz, and J. V. Cortinas, Jr., 2003: Improving snowfall forecasting by diagnosing snow density. Wea. and Forecasting, 18, 264-287. http://sanders.math.uwm.edu/cgi-bin-snowratio/sr_intro.pl Wetzel-Seemann, S. W., and J. E. Martin, 2001: An operational ingredients-based methodology for forecasting mid-latitude winter season precipitation. Wea. and Forecasting, 16, 156-167. http://speedy.meteor.wisc.edu/~swetzel/winter/winter.html Weisman & Jurek - NPWSC 06