1. A Few Thoughts on Snow/Ice Crystal Microphysics Growth Regimes and Current Operational Methodologies in Forecasting Snow Ratios and Amounts Dan Miller Science and Operations Officer NWS/WFO Duluth, Minnesota NWS Duluth Minnesota 14 October 2014 Northern Plains Winter Storm Conference - Saint Cloud, MN Greg Mann Science and Operations Officer NWS/WFO Detroit/White Lake, Michigan

2. …Or… Have We Oversold the -15 C DGZ? NWS Duluth Minnesota 14 October 2014 Northern Plains Winter Storm Conference - Saint Cloud, MN

3. Preferred Ice Crystal Growth Regimes:

4. Crystal Growth Habits: 0 C/32 F There are actually 3 temperature zones where crystal growth is enhanced (not just around -15 C) Let’s examine each of these in more detail… Growth rate is highly dependent on super-saturation level (recall that we use omega from the model (not actual RH i ) as a proxy for this) Duration can be highly dependent on moisture replenishment

5. Crystal Growth Habits (-15 C): -15 C/5 F Most rapid stellar dendrite growth regime in high super-saturation over ice 2.7 μ/s growth rate in high super-saturation ~-12 C/10 F ~-18 C/0 F Plate/Dendrite growth structure

6. Crystal Growth Habits (-15 C): Movie of rapid stellar dendrite growth near -15 C near -15 C Movie of rapid stellar dendrite growth near -15 C near -15 C Close up snapshot of stellar dendrite growth structure near -15 C Close up snapshot of stellar dendrite growth structure near -15 C

7. Crystal Growth Habits (-2 C): -2 C/28 F Plate/Dendrite growth structure 1.2 μ/s growth rate in high super-saturation (~half of -15 C) Requires: 1) copious moisture replenishment 2) ice seeding from above 2) ice seeding from above

8. Close up snapshot of dendrite growth structure near -2 C -15C structure for comparison 1.2 μ/s growth rate 2.7 μ/s growth rate Crystal Growth Habits (-2 C):

9. -5 C/23 F Prism/Needle (“Fishbone”) crystal growth structure - more rapid growth rate, but less voluminous. Higher liquid equivalent. 2.0 μ/s growth rate in high super-saturation Requires: 1) significant moisture replenishment 2) ice seeding from above 2) ice seeding from above Crystal Growth Habits (-5 C):

10. Crystal Growth Habits: Close up snapshot of needle growth structure near -5 C -15C structure 1.2 μ/s growth rate 2.7 μ/s growth rate -2C structure 2.0 μ/s growth rate

11. Max Crystal Growth Habit Summary: Temperature Regime -2C 2.0 μ/s -5C -15C Growth Rate 1.2 μ/s 2.7 μ/s Snow Density Ice Seeding? Yes No Moisture Replenishment? Copious Very High Low High/ Very High Low/ Very Low In high super-saturation over ice

12. Operational Application: -12C to -18C:

14. . Look at the difference in RH over water versus over ice

15. Where are you on the DG curve?: Model Omega is used as a proxy for Super-saturation over ice

16. Where are you on the DG curve?:

18. KOKC

19. What About -5C and -2C?:

23. Deep Isothermal Layers Minimize Uncertainty: - + - 2C - 15C

24. Model Uncertainty: Models are much better at forecasting thermal (and even moisture) profiles than they are at forecasting the vertical distribution of omega

25. Model Uncertainty: Models are much better at forecasting thermal (and even moisture) profiles than they are at forecasting the vertical distribution of omega

26. Model Uncertainty: Models are much better at forecasting thermal (and even moisture) profiles than they are at forecasting the vertical distribution of omega

27. Model Uncertainty: Models are much better at forecasting thermal (and even moisture) profiles than they are at forecasting the vertical distribution of omega

28. Model Uncertainty: Models are much better at forecasting thermal (and even moisture) profiles than they are at forecasting the vertical distribution of omega

29. Model Uncertainty: Models are much better at forecasting thermal (and even moisture) profiles than they are at forecasting the vertical distribution of omega

30. Model Uncertainty-Timing: 12z

31. 12z

32. 12z

33. Deep Isothermal Layers Minimize Uncertainty: - + - 15C

34. Deep Isothermal Layers: X-Sec vs. Sounding

36. Saturation vs. Absolute Moisture: -12 to -18C -5C -2C 1.5g/kg5g/kg3g/kg2g/kg

37. Attempting to Put it All Together: EPV*/ CAPE CAPE FGEN Omega/Temp F-Vectors F-Vectors RH

38. Attempting to Put it All Together: Omega/TEMP (with -12 to -18C DGZ color curve)

39. Attempting to Put it All Together: Omega/TEMP (with new crystal growth color curve)

40. Attempting to Put it All Together: Model Forecast SkewT Quasi- isothermal layer Around -5 C

41. Thanks for your Time and Attention