Assessment of Environmental Stability and RUC10 Forecasts of Storm Initiation

1. Assessment of Environmental Stability and RUC10 Forecasts of Storm Initiation Rita Roberts and Jim Wilson National Center for Atmospheric Research

2. Objective To examine the capability of numerical models (e.g. RUC 10) to provide 0-6 hr prediction of precipitation initiation and evolution?

3. Model Performance • Forecasting precipitation initiation and evolution

4. Identify storm initiation episodes (area of new storms initiated by common forcing mechanism) Sample initiation episode a) 112 Identified b)

5. YES - Forecasts No - Forecasts Statistics on RUC 10km Model (3hr) Precipitation Initiation Forecasts Number of Events Spatial (50-250km) Offset No Offset Good Forecast Temporal (1-5 hr) Offset

6. Ability of RUC 10 km Model to Initiate Precipitation 83 Best forecasts are associated with fronts or near fronts 83 68 62

7. 15:00 Valid 18:00 12:00 Valid 18:00 Initiation Zone 16:40 – 18:00 FRONTS 4 June 2002 3hr RUC Forecast Yellow contours = 35 dBZ echo at forecast time 6hr RUC Forecast

8. Initiation Cold Front 4 June 2002 RUC Analysis and Observations at 15:00 UTC RUC CIN RUC CAPE Reflectivity echoes overlaid Surface-Sounding CAPE Surface-Sounding CIN

9. Evolution Cold Front 4 June 2002 RUC Analysis and Observations at 18:00 UTC RUC CIN RUC CAPE Reflectivity echoes overlaid Surface-Sounding CAPE Surface-Sounding CIN

10. Decay Cold Front 4 June 2002 RUC Analysis and Observations at 23:00 – 00:00 UTC RUC CIN RUC CAPE Reflectivity echoes overlaid Surface-Sounding CAPE Surface-Sounding CIN

11. Convergence Reflectivity echoes are overlaid

12. Ability of RUC 10 km Model to Initiate Precipitation 83 83 68 62

13. Surface Lows 28 May 2002 Initiation Zone 17:40-19:40 15:00 Valid at 18:00 Reflectivity 3 hr RUC10 Forecast Yellow contours = 35 dBZ echo at forecast time Slide 30

14. Surface Low 28 May 2002 18:00 UTC Reflectivity Convergence RUC CAPE RUC CIN

15. Ability of RUC 10 km Model to Initiate Precipitation 83 83 68 62 Surprise that the model forecasted 68% of these events

16. 13 June 2002 – 3hr RUC10 Forecasts 12:00 Valid 15:00 6:00 Valid 9:00 9:00 Valid 12:00 9:00 UTC 10:50 UTC 12:40 UTC Init Zone 6 1030-1240 Init Zone 3 0630-0900 Init Zone 5 0930-1050 Yellow contours = 35 dBZ echo at forecast time

17. 13 June 2002 RUC analysis and observations at 10:30 UTC RUC CAPE RUC CIN Reflectivity echoes overlaid Surface-Sounding CAPE Surface-Sounding CIN

18. Ability of RUC 10 km Model to Initiate Precipitation 83 83 68 62 Model doesn’t forecast these events well; 50% are late

19. 22 May 2002 -Dry Line 23:00 – 00:00 UTC RUC-CAPE Surface-Sounding CAPE GOES-Sounder CAPE Dry line Dry line segment RUC-CIN Surface-Sounding CIN GOES-Sounder CIN

20. Model Performance • Forecasting precipitation initiation and evolution • Forecasting long-lived storm complexes

21. Analysis 15-16 June 2002 case 8 hour loop L Synoptic low and trough line Evolution of storm complex dependent on the emergence and characteristics of the gust fronts

22. Verification of RUC 6hr fx for 00:00 for 03:00 for 21:00 Does not capture accurate evolution of storms Does not propagate system Need to do a better job of representing downdrafts and outflows in models

23. a) 1800 b) 2100 d) 0300 c) 0000 e) 0600 12 – 13 June 2002 Validation of 3 hr RUC forecasts

24. Conclusions RUC10 Initiation Forecasts: Best - fronts and lows Second - elevated, however often late Worse – small scale convergence lines

25. Conclusions Why is this so? • Comparison of RUC stability fields with observations show that the model is producing realistic values during the initiation period, although with somewhat higher magnitudes than the observations.

26. Conclusions RUC10 Storm Evolution Forecasts: RUC10 is unable to forecast the evolution and propagation of storm. Why is this so? Downdrafts and gust fronts have a major influence on the evolution, lifetime and motion of convective storm complexes.

27. Conclusions RUC10 Storm Evolution Forecasts: Most numerical models, including RUC10, run on grids typically larger than the scale of convection and do not represent convection explictly but rather employ a cumulus parameterization scheme. As a result, RUC10 has difficulty in either producing a convective downdrafts or generating downdrafts and outflows of the correct intensity seen in the observations.

28. Research and Operational Challenge Precipitation microphysics plays a key role in downdraft production and characteristics. This suggests the use of polarimetric radar to measure in-cloud precipitation structure associated with downdrafts. Assimilation of refractivity information into numerical models and short-term forecast systems is critical for improved accuracy in short-term thunderstorm forecasting.

29. Forecasting Challenge:Tracking the “pockets” of boundary layer moisture so critical for convection initiation and growth. Moisture gradient Moisture gradient Convergence boundaries Convergence boundaries 00:40 UTC 00:40 UTC 22:57 UTC 00:31 UTC 00:01 UTC 00:31 UTC 22:57 UTC 00:01 UTC 23:27 UTC 23:27 UTC Moisture gradients and convergence boundaries associated with thunderstorm development, June 12, 2002

30. END

31. a) 1800 b) 2100 d) 0300 c) 0000 e) 0600 12 – 13 June 2002 Validation of 3 hr RUC forecasts Fig. 15. June 12-13 RUC 6hr precipitation forecasts (solid white contour) overlaid on radar reflectivity (gray shade scale in dBZ on right) at 3h intervals (a-e). The forecasts are 3h accumulations ending at the given time. The reflectivity is the instantaneous field at the given time. The first precipitation contour represents an accumulation of 1 mm during the 3 h period; the second contour (only reached in b) is 10 mm. Boundaries are shown by thick white lines.