Nowcasting ConvectionNowcasting Convection

Fusing 0-6 hour observation- and model-based probability forecasts

WWRP Symposium on Nowcasting and Very Short Range ForecastingToulouse, France, 7 September 2005

Collaborators: Cindy Mueller, Steve Weygandt, Jim Wilson, David Ahijevych,Dan Megenhardt

0 1 2 3 4 5 6 Forecast Length, hours

.2

.4

.6

.8

1.0

Accuracy of Rainfall Nowcasts>1 mm/h

GRID MESH 20 km Jun-Oct 2002

Courtesy of Shingo Yamada JMA

ExtrapolationP

ersistence

NWP

CS

I

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 (http://weather.aero/convection)

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 h2 h

P

National Convective Weather Forecast RUC 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

MergedProbabilistic

Product

Calibration

RCPF

Summing

WSR-88DClimatology

ClimatologicalDissipation

Coverage Maps WSR-88D

Interpolation to 4km Grid

MethodologyCalibration 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

WSI @ Forecast Time

WSI - Validation

Fcst time: 2000 UTC Valid: 0200 UTC

After Scaling

Fcst time: 2000 UTC Valid: 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)

10

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 ScalingAfter Climo MaskingOriginal 6hr RUC Prob Fcst

At Forecast Time

WSI - Validation

Fcst time: 2000 UTC Valid: 0200 UTC

•Remove excessive coverage values•Shrink Area Coverage without decreasing POD

Fcst time: 2000 UTC Valid: 0200 UTC

Case Study

1-6 hr Probability Forecasts

Extrapolation RUC Convective Probabilities

WSI @ Forecast Time

WSI - Validation

Fcst Valid Times: 1600 – 2200 UTC by 60 min

Radar Data 1400- 1600 UTC by 30 min

Fcst Valid Times: 1600 – 2200 UTC by 120 min

1-6 hr Merged Probability Forecast

WSI @ Forecast Time

WSI - Validation

WSI: 1400-1600 UTC by 30 min; Fcst: 1600 – 2200 UTC by 60 min

Statistical Evaluation

Comparing NCWF05-05, RCPF50-05, Merged05-05

Validation Region

Validation Period : 01-14 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