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

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

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 Outline just saw Weygandt talk! See Mueller et al. poster 5.21!

–0-2 hr probability forecasts –Includes extrapolation, growth and dissipation –Available on Experimental ADDS ( Operational NCWF

Probabilistic Forecasts Systems 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 P. k 1 h 2 h P National Convective Weather ForecastRUC Convective Prob. Forecast

Summary of Strengths and Weaknesses of NCWF and RCPF 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

Methodology

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

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

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

Use diurnal climo of fractional change in WSR-88D freq of convection to incorporate dissipation. Freq40 + dBZ(  UTC) / F40 + dBZ(  UTC) WSR-88 D climo from 6 warm seasons frequency of echo > 40 dBZ (Knievel et al. 2004) Methodology “…a combination of rainfall statistics containing propagation information with NWP predictions may offer significant improvement in warm rain prediction.” – Davis et al. (2003) 11 00 F(  1 )/F(  0 ) in 6 hrs Area Coverage > 40 dBZ Trending dissipation using Climo June 2005 Area Coverage Diurnal Composite

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

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

Case Study

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

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

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

Thank You!

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

12 Aug 2005 Case Study WSI National Mosaic