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WAFC CAT verificationObjective verification of GRIB CAT forecasts Dr Philip G Gill, WAFS Workshop on the use and visualisation of gridded SIGWX forecasts, Paris, 14 September 2009

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Contents

This presentation covers the following areas

• Introduction

• SIGWX forecast comparison

• Aircraft data

• Verification methodology

• Verification results

• Summary

• Further improvements

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Introduction

• What – Objective verification of gridded binary (GRIB) and significant weather (SIGWX) Clear Air Turbulence (CAT) forecasts

• Where – Global verification

• When – November 2008 to May 2009

• Why – To demonstrate the quality of the new GRIB forecasts using objective verification.

• How – Verification against aircraft observations from the Global Aircraft Data Set (GADS)

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Manual SIGWX BUFR and GRIB CAT forecasts

• Manual SIGWX chart

• Example field from new GRIB forecast

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Comparison of SIGWX BUFR charts

UK US UK&US

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SIGWX BUFR CAT forecast comparison

• One month UK-US comparison (January 2009)

• Average coverage of globe UK ~6%, US ~3%

• Percentage overlap of all forecasts between UK and US ~20%

Areas forecast by both UK and US

Areas forecast by UK but not USAreas forecast by US but not UK

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GRIB forecast comparisonproduced by HKO

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Global Aircraft Data Set

• Archive of aircraft data set up by Joel Tenenbaum (State University of New York)

• British Airways fleet of Boeing 747-400 aircraft

• Global coverage, but flights mainly over northern hemisphere

• Automated aircraft observations every 4 seconds

• Indicator of turbulence derived from vertical acceleration, aircraft mass, altitude and airspeed called the derived equivalent vertical gust (DEVG).

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GADS Data coverage

10-day sample of GADS data

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Verification methodology

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Forecast assessment

• Turbulent/non turbulent event defined on 10min aircraft track ~120km - approx grid size

• Forecast turbulent event – CAT potential >= Threshold

• Observed turbulent event –

Slight or greater turb (DEVG>=2m/s)

Moderate or greater turb (DEVG>=4.5m/s)

• Construct 2x2 contingency tables for each threshold

• Sum entries in contingency tables over the verification period

• Produce a Relative Operating Characteristic (ROC) curve by plotting the Hit rate against False alarm rate for each threshold.

Turbulence observed

No turbulence observed

Turbulence forecast

Hit False alarm

No turbulence forecast

Miss Correct rejection

2x2 contingency table

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Results UK GRIB and SIGWXmoderate or greater turbulence

ROC curve UK WAFC GRIB v BUFR SIGWX CAT moderate or greater turbulence (DEVG>=4.5m/s) Jan 2009 - May 2009 verified against global GADS data

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

False alarm rate

Hit

rat

e

UK T+24 GRIB

UK T+24 BUFR

~500,000 events

~300 turbulent events (devg>=4.5m/s)

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Variation with forecast rangeROC curve January 2009 UK GRIB against Global GADS data

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

False alarm rate

Hit

rat

e

T+6

T+12

T+18

T+24

~100 000 events

Devg>=2m/s

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Latitudinal variation UK GRIBmoderate or greater turbulence

ROC curve UK WAFC GRIB v BUFR SIGWX CAT moderate or greater turbulence (DEVG>4.5m/s) Jan 2009 - May 2009 verified against GADS data 50N-90N

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Flalse alarm rate

Hit

rat

e

UK T+24 GRIB

UK T+24 BUFR

ROC curve UK WAFC GRIB v BUFR SIGWX CAT moderate or greater turbulence (DEVG>=4.5m/s) Jan 2009 - May 2009 verified against GADS data 20N-50N

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

False alarm rate

Hit

rat

e

UK T+24 GRIB

UK T+24 BUFR

ROC curve UK WAFC GRIB v BUFR SIGWX CAT moderate or greater turbulence (DEVG>=4.5m/s) Jan 2009 - May 2009 verified against GADS data 20S-20N

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

False alarm rate

Hit

rat

e

UK T+24 GRIB

UK T+24 BUFR

ROC curve UK WAFC GRIB v BUFR SIGWX CAT moderate or greater turbulence (DEVG>=4.5m/s) Jan 2009 - May 2009 verified against GADS data 50S-20S

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

False alarm rate

Hit

ra

te

UK T+24 GRIB

UK T+24 BUFR

20S to 20N~60,000 events

~100 turb events

90N to 50N~200,000 events

~50 turb events

20N to 50N~200,000 events

~200 turb events

50S to 20S~12,000 events

~10 turb events

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Latitudinal variation US GRIBmoderate or greater turbulence

20S to 20N~30,000 events

~50 turb events

90N to 50N~280,000 events

~200 turb events

20N to 50N~100,000 events

~100 turb events

50S to 20S~6,000 events

~3 turb events

ROC curve US WAFC GRIB v BUFR SIGWX CAT moderate or greater turbulence (DEVG>=4.5m/s) Feb 2009 - Apr 2009 verified against GADS data 50S-20S

0

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

False alarm rate

Hit

rat

e

US T+24 GRIB

US T+24 BUFR

ROC curve US WAFC GRIB v BUFR SIGWX CAT moderate or greater turbulence (DEVG>=4.5m/s) Jan 2009 - May 2009 verified against GADS data 20S-20N

0

0.1

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

False alarm rate

Hit

rat

e

UK T+24 GRIB

UK T+24 BUFR

ROC curve US WAFC GRIB v BUFR SIGWX CAT moderate or greater turbulence (DEVG>=4.5m/s) Jan 2009 - May 2009 verified against GADS data 20N-50N

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0.1

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0.5

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

False alarm rate

Hit

ra

te

US T+24 GRIB

US T+24 BUFR

ROC curve US WAFC GRIB v BUFR SIGWX CAT moderate or greater turbulence (DEVG>4.5m/s) Feb 2009 - Apr 2009 verified against GADS data 50N-90N

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0.1

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Flalse alarm rate

Hit

ra

te

US T+24 GRIB

US T+24 BUFR

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UK and US Nov 2008 and Jan 2009 ROC curve

~200 000 events

Devg>=2m/s

ROC curve global CAT T+24 forecasts verified against GADS data Nov 2008 and Jan 2009

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

False alarm rate

Hit

rat

e

EGRR GRIB T+24

KKCI GRIB T+24

EGRR SIGWX BUFR T+24

KKCI SIGWX BUFR T+24

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UK GRIB and SIGWX

~300 000 events

Devg>=2m/s

SIGWX automated object - UK automated SIGWX chart production system based on GRIB data

ROC Curve UK WAFC, UK SIGWX and CAT object verification against global GADS data

November 2008-January 2009

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False alarm rate

Hit

rat

e

GRIB T+24

SIGWX BUFR T+24

SIGWX SWAM T+24 moderateCAT

SIGWX SWAM T+24 severe CAT

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Summary of results

• Both UK and US GRIB products show more skill than the manual SIGWX products.

• Global UK and US GRIB CAT forecasts score similarly

• Slight difference in scores as forecast range increases

• Some differences in scores at individual latitude bands –best performance between 20N and 90N.

• UK CAT coverage on manual BUFR SIGWX charts greater than the US

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Further improvements

• Automate verification process

• Improve consistency of forecasts by analysing verification data and altering production systems.

• Use verification to test future model upgrades and re-tune algorithms

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Questions and answers