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VERIFICATION OF OPERATIONAL THUNDERSTORM NOWCASTS

E. Ebert, T. Keenan, J. Bally and S. Dance

Bureau of Meteorology Research Centre (BMRC) GPO Box 1289K

Melbourne, Australia

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Talk Outline

Data and Forecast Processes Assessment of Storm Track Modifications

• TIFS Expert System track modification• Impact of forecaster track filtering• Impact of forecaster track modification• Combined Forecaster and Expert System Impact

Overall verification and skill of tracks

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Data and Forecast Process

Track Nowcasts Based on the Thunderstorm Interactive Forecast System (TIFS) - Bally(2004)

•Operational Nowcasts from Sydney, Australia•Three years of data from 2002-2005

Two radars - Kurnell(Letterbox) Five(ten) minute

•TITAN based cell tracks as guidance•Thresholds of 35,40 and 45dBZ

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Assessment of TIFS ModificationTIFS Expert System

Cell tracks (a) before and (b) after TIFS expert system track adjustment.

• Ingests TITAN guidance• Advances storms with any latency• Corrects for obvious cell track errors

•Cell mis-association•Lack of storm history

• Removes cells below threshold

(a)

(b)

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Assessment of TIFS ModificationTIFS Expert System

Dashed (solid) line before (after) track modification by Expert System. Stratified by the height of the 35dBZ echo, all Sydney radars and all TITAN reflectivity thresholds. Numbers of cases indicated above.

• 379 cases where original guidance set cell speed and direction to zero

33% decrease in track error (significant at 95% level)

• Track error reduced in the 73% of cases where TIFS expert system employed

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Assessment of TIFS ModificationImpact of Forecaster Filtering of Tracks

Kurnell radar incorporating all TITAN cell thresholds. Dashed (solid) line is without (with) forecaster filtering. Sample sizes above refer to filtered sample sizes at each forecast interval.

Forecasters undertake filtering

• Removal of weaker storms• Modification of obvious errors

• Consideration of environment

•Left, right moving storms

Small and positive impact• 2.8% decrease in track error• Track error reduction largest with shallow storms

• Most significant impact was a 63% reduction in number of cells in TIFS products (22,781 8,492)

• Information content An important forecaster activity

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Assessment of TIFS ModificationForecaster Track Modification

Both Sydney radars, all TITAN cell thresholds. Dashed (solid) is without (with) forecaster track modification. Small number of cases.

• Only 1.1% of cell locations modified by forecaster - 95 cases

• Mixed impact with 2.2% overall deterioration (not statistically significant)

• Forecaster modification did result in a positive impact in 62% of the cases

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Assessment of TIFS ModificationForecaster and Expert System

The TIFS expert system was employed to determine the cell speed and direction in 2.5% of cases (572)

Cells were further manually modified in only 96 of the above cases (0.4% of total)

Implies little additional track modification is employed on a day-to-day basis(rates varied from zero to about 4.4% for 35 dBz storms)

Modifications did occur with significant events (11 days with significant events and warnings issued on 8 of these days)

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Accuracy and Skill of TIFS Cell Track Nowcasts

•(a)

•(b)

Solid lines are TIFS forecasts and dotted lines extrapolation benchmark nowcasts. Kurnell radar data for all TITAN thresholds combined. (a) Stratification based on storm height, and (b) stratification based on storm speed.

(a)

(b)

• Error growth linear - 8-10 km h-1

• Larger errors for deeper and fast moving storms

Storms moving at > 40 km h-1 have errors ~50% greater than storms moving < 40 km h-1

Track errors 30-50% of storm speed• TIFS nowcasts ~ 10% better than benchmark for nowcasts beyond 20 minutes for

Shallow and moderate depth stormsSlow to moderate speed storms

• Marginally less skillful forDeep (>10 km tops) - TITAN blending

requires further optimisation Fast moving (> 40 km h-1) – harder to track

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Accuracy and Skill of TIFS Cell Track Nowcasts

Spatial distribution of TIFS track errors from both Sydney radars for all thresholds.

Small spatial biases• Slow and to right of storm motion, i.e. to the southwest

• Consistent over 60 minute nowcast

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Summary Human Intervention

• Vast majority of operational nowcasts involve little human intervention on cell tracks

• Intervention focused on quality control and product form

• Little actual track adjustment although more evident in significant events

• Human adjustment positive impact in 65% of cases but no average decrease in average track error results

TIFS tracks showed small skill relative to benchmark• Shallow-moderate depth, slow-moderate speed storms

show 10% improvement over benchmark• Fast and deep moving storms showed some

degradation in skill (non-persistent flank motion, tracking effectiveness)

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Original speed and direction were 13 km h-1 @ 0.Forecaster changed speed and direction to 34 km h-1 @ 127.

Original speed and direction were 13 km h-

1 @ 0.Forecaster changed speed and direction to 30 km h-1 @ 110.

Original speed and direction were 13 km h-1 @ 0.Forecaster changed speed and direction to 34 km h-1 @ 118.

Robertson, Faulconbridge , Blaxland, Warrimoo, Castle Hill - heavy rain, hail 2-4 cm size.

Baulkham Hills - Very strong winds destroyed several houses and brought down trees.

Sydney - A person struck by lightning.Central Coast/Newcastle - Storms damaged

homes