1. Rita Roberts and Jim Wilson National Center for Atmospheric Research
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. Statistics on RUC 10km Model (3hr) Precipitation Initiation Forecasts
YES - Forecasts
No - Forecasts
Number of Events
No Offset
Good
Forecast
Spatial
(50-250km)
Offset
Temporal
(1-5 hr)
Offset

6. Ability of RUC 10 km Model to Initiate Precipitation
Initiation Mechanism
Number
of Events
% Time Precipitation is forecast
% Time Precip. Area too large
% Time Fcast is late
Fronts 23 40 30
Surface Lows 4
100 75 25
Elevated (frontal) 11 56
Elevated (isolated) 39 35
Convergence Boundaries 32 50 83
Best forecasts are associated with fronts or near fronts 83 68 62

7. FRONTS 4 June 2002 3hr RUC Forecast 6hr RUC Forecast Initiation Zone
15:00
Valid 18:00
Initiation Zone
16:40 – 18:00
FRONTS
4 June 2002
12:00
Valid 18:00
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
Initiation Mechanism
Number
of Events
% Time Precipitation is forecast
% Time Precip. Area too large
% Time Fcast is late
Fronts 23 40 30
Surface Lows 4
100 75 25
Elevated (frontal) 11 56
Elevated (isolated) 39 35
Convergence Boundaries 32 50 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
Initiation Mechanism
Number
of Events
% Time Precipitation is forecast
% Time Precip. Area too large
% Time Fcast is late
Fronts 23 40 30
Surface Lows 4
100 75 25
Elevated (frontal) 11 56
Elevated (isolated) 39 35
Convergence Boundaries 32 50 83 83 68 62
Surprise that the model forecasted 68% of these events

16. 13 June 2002 – 3hr RUC10 Forecasts 6:00 Valid 9:00 9:00 Valid 12:00
Init Zone 3
9:00 UTC
Init Zone 5
10:50 UTC
Init Zone 6
12:40 UTC
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
Initiation Mechanism
Number
of Events
% Time Precipitation is forecast
% Time Precip. Area too large
% Time Fcast is late
Fronts 23 40 30
Surface Lows 4
100 75 25
Elevated (frontal) 11 56
Elevated (isolated) 39 35
Convergence Boundaries 32 50 83 83 68 62
Model doesn’t forecast these events well; 50% are late

19. 23:00 – 00:00 UTC 22 May 2002 - Dry Line 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. Does not capture accurate evolution of storms
Verification of
RUC 6hr fx
for 21:00
for 03:00
for 00:00
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. Validation of 3 hr RUC forecasts
12 – 13 June 2002
Validation of 3 hr RUC forecasts
a) 1800
b) 2100
c) 0000
d) 0300
e) 0600

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.
00:31 UTC
00:40 UTC
00:01 UTC
23:27 UTC
Convergence boundaries
22:57 UTC
Convergence boundaries
22:57 UTC
Moisture gradient
Moisture gradients and convergence boundaries associated with thunderstorm development, June 12, 2002

30. END

31. Validation of 3 hr RUC forecasts
12 – 13 June 2002
Validation of 3 hr RUC forecasts
a) 1800
b) 2100
c) 0000
d) 0300
e) 0600
Fig. 15. June 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.