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Nowcasting Convection

Nowcasting Convection. Fusing 0-6 hour observation- and model-based probability forecasts. Collaborators: Cindy Mueller, Steve Weygandt, Jim Wilson, David Ahijevych, Dan Megenhardt. WWRP Symposium on Nowcasting and Very Short Range Forecasting Toulouse, France, 7 September 2005.

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Nowcasting Convection

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  1. Nowcasting Convection Fusing 0-6 hour observation- and model-based probability forecasts Collaborators: Cindy Mueller, Steve Weygandt, Jim Wilson, David Ahijevych, Dan Megenhardt WWRP Symposium on Nowcasting and Very Short Range Forecasting Toulouse, France, 7 September 2005

  2. Gap in Forecast Skill 1.0 Accuracy of Rainfall Nowcasts .8 Extrapolation >1 mm/h .6 CSI GRID MESH 20 km Jun-Oct 2002 Persistence .4 .2 NWP 0 1 2 3 4 5 6 Forecast Length, hours Courtesy of Shingo Yamada JMA Others have also quantified this in various ways (e.g., Golding 2000, and many others at this conference.

  3. Outline I) Probabilistic Forecasting -> National Convective Weather Forecast (NCWF) – obs-based -> RUC Convective Probability Forecast (RCPF) – model-based II) Methodology III) Results/Verification IV) Future Work See Mueller et al. poster 5.21! just saw Weygandt talk!

  4. Operational NCWF • 0-2 hr probability forecasts • Includes extrapolation, growth and dissipation • Available on Experimental ADDS (http://weather.aero/convection)

  5. Probabilities based on: Spatial coverage of convective precip predicted by the RUC-20 model Square filter of 180 km Precipitation rate threshold for convection (1-2 mm/hr) Probabilities based on: Spatial coverage of MergedGrowth (MG) Elliptical filter with time-dependent size (1 hr, 2 hr, 3-6 hr : 60 km, 120 km, 180 km) MG(VIL,ltng) thresholded for convection RUC Convective Prob. Forecast 2 h P . P Probabilistic Forecasts Systems National Convective Weather Forecast 1 h k

  6. Area-coverage RUC : overestimates coverage and likelihoods too high Initiation RUC : good (large-scale instability and frontal) NCWF : not handled Motion RUC : improves with lead time NCWF : degrades with lead time Dissipation NCWF and RUC: similar skill Summary of Strengths and Weaknesses of NCWF and RCPF

  7. Methodology

  8. Schematic of Methodology RCPF NCWF Calibration Climatological Dissipation Summing Interpolation to 4km Grid WSR-88D Climatology Merged Probabilistic Product Coverage Maps WSR-88D Validation

  9. Obs Coverage = 5% Obs Coverage = 20% Obs Coverage = 10% RUC Prob Levels, p 50% 40% 60% 25% 75% 30% 40% 50% 60% 75% 50% 2, 4, 6 hr fcsts for each p OBS COV NCWF RUC . 0% 0% 0-35% 5% 5% 35% 10% 10% 55% 20% 20% 70% Methodology Calibration of RUC Probabilities using June 2005 Validation Data

  10. After Scaling Fcst time: 2000 UTC Valid: 0200 UTC WSI - Validation WSI @ Forecast Time Methodology Original 6hr RUC Prob Fcst Fcst time: 2000 UTC Valid: 0200 UTC • Remove excessive coverage values • Shrink Area Coverage without decreasing POD

  11. t1 t0 Area Coverage > 40 dBZ F(t1)/F(t0) in 6 hrs Use diurnal climo of fractional change in WSR-88D freq of convection to incorporate dissipation. WSR-88 D climo from 6 warm seasons frequency of echo > 40 dBZ (Knievel et al. 2004) Freq40+dBZ(06 UTC) / F40+dBZ(00 UTC) Methodology June 2005 Area Coverage Diurnal Composite Trending dissipation using Climo “…a combination of rainfall statistics containing propagation information with NWP predictions may offer significant improvement in warm rain prediction.” – Davis et al. (2003)

  12. Masking of RUC Probabilities using Climo Methodology Nationally: Convective area shrinking between 19 and 4 UTC. • Regionally: • Diurnal Cycle in SE US • Propagation evident across Great Plains *Note: Moving gray box indicates 6 hr period over which fractional change is calculated.

  13. After Climo Masking After Scaling WSI - Validation At Forecast Time Methodology Original 6hr RUC Prob Fcst Fcst time: 2000 UTC Valid: 0200 UTC Fcst time: 2000 UTC Valid: 0200 UTC • Remove excessive coverage values • Shrink Area Coverage without decreasing POD • Apply climo trending by multiplying with RCFP • Reduces RCFP in areas/time where convection is not climatologically preferred (SE at night)

  14. Case Study

  15. WSI - Validation WSI @ Forecast Time 1-6 hr Probability Forecasts Extrapolation RUC Convective Probabilities Fcst Valid Times: 1600 – 2200 UTC by 120 min Fcst Valid Times: 1600 – 2200 UTC by 60 min Radar Data 1400- 1600 UTC by 30 min

  16. WSI - Validation WSI @ Forecast Time 1-6 hr Merged Probability Forecast WSI: 1400-1600 UTC by 30 min; Fcst: 1600 – 2200 UTC by 60 min

  17. Statistical Evaluation Validation Period : 01-14 Aug 2005 Comparing NCWF05-05, RCPF50-05, Merged05-05 NCWF Merged RUC Validation Region

  18. Thank You!

  19. Methodology for Merging • Calibrate RUC Probabilities to be comparable with NCWF values • - Validation of RCFP and NCWF from June 05 data • Mask RUC Probs. using climatological tendencies observed with National network of WSR-88Ds • -(Carbone et al. 2002, Davis et al. 2003) • -Create climatological diurnal cycle of fractional changes in the coverage of convection. • Interpolate RCFP to NCWF 4 km grid • Add NCWF and RUC Probabilities • Apply smoothing filter Box diagram

  20. 12 Aug 2005 Case Study WSI National Mosaic

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