Tropical cyclone forecast verification - southern hemisphere 2010-11

1. Introduction

A summary of tropical cyclone activity in the Southern Hemisphere for the 2010-11 season is presented below together with an assessment of the performance of the Met Office global model in predicting the tracks of these tropical cyclones.

Tropical cyclones are experienced in the South Indian Ocean, tropical seas to the west, north and east of Australia and the western and central South Pacific Ocean. For the purpose of tropical cyclone verification the Southern Hemisphere is usually divided in to two basins; the South-West Indian (west of 90° E) and Australian (east of 90° E). Mean error statistics for each basin are presented together with a table of statistics for the whole Southern Hemisphere. The global model produces a six-day forecast every 12 hours. Verification is performed at 12-hour intervals up to forecast time T+144, although statistics are only presented at 24-hour intervals in this report.

The global model resolution in operation during most of the season was 0.3515625°x 0.234375°x 70 levels. This is equivalent to a horizontal resolution of 39 km x 26 km at the equator.

An explanation of error statistics and their sign conventions.

Advisory positions from RSMCs La Réunion and Nadi, Fiji, Bureau of Meteorology, Australia and JTWC Hawaii are used as verifying observations of storm location. Best track data from these centres will be obtained once they become available. Past experience shows that use of best track rather than real time data usually only makes minor differences to seasonal track error statistics. Some mean error statistics for last season are also included for the purposes of a comparison. Forecast tracks are only verified when a depression reaches tropical storm status.

2. Tropical cyclone activity

Tropical cyclone activity
 SWIAUSSATTOTAL
Tropical depressions (<34 knots)1(0)2(0)0(0)3(0)
Tropical storms (>34 knots, <64 knots)1(6)6(8)1(1)8(15)
Hurricanes/typhoons (>64 knots)2(5)8(7)0(0)10(12)
Total4(11)16(15)1(1)21(27)

Basin name abbreviations:
SWI : South-West Indian (west of 90° E)
AUS : Australian (east of 90° E)
SAT : South Atlantic

The number in brackets indicates the figure for the 2009-10 season.

N.B. 10-minute averaged wind speeds from RSMCs used when available. Data from JTWC (1-minute averaged) is only used when other data is unavailable and maximum wind speeds are scaled to make them equivalent to the RSMC 10-minute averages.

3. Summary of all southern hemisphere storms

3.1 South-west Indian basin storms

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144
Possibly verified25191310864
Detection rate (%)100100100100100100100
Mean AT-9-53-87-225-380-318-21
Mean CT-3-19-61-109-15375570
Mean skill (%)*****324256***************
2009-10 skill (%)*****-12-13-42***************
Mean DPE37117219262411546694
* 2009-10 DPE 391543165007791012*****
Intensity skill (%)*****6854802533100

* DPE for all south-west Indian storms in 2009-10 season

Plot of the observed tracks of all storms in the south-west Indian basin
Forecast positional errors in the south-west Indian basin
Forecast skill in the south-west Indian basin

The 2010-11 season was one of the quietest on record with just three named storms. After last season's sharp rise in mean track forecast errors, the errors dropped sharply again this season to levels close to the lowest ever seen.

Similarly, there was a rise in skill scores close to the level seen two seasons ago. There were slow and right-of-track biases in forecasts at short lead times, but these biases declined and even reversed at longer lead times. The intensity tendency skill had a value of 68% overall. The charts show the long term trend in forecast errors and skill.

3.2 Australian basin storms

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144
Possibly verified1321037651322112
Detection rate (%)10010010010010010092
Mean AT-6-41-82-139-243-147-74
Mean CT4-16-32-308461-55
Mean skill (%)*****676750***************
2009-10 skill (%)*****585252***************
Mean DPE33119218210522669579
* 2009-10 DPE 36145303450565570439
Intensity skill (%)*****636176448183

* DPE for all Australian storms in 2009-10 season

Plot of the observed tracks of all storms in the eastern Australian basin
Plot of the observed tracks of all storms in the western Australian basin
Forecast positional errors in the Australian basin
Forecast skill in the Australian basin

Tropical cyclone frequency this season was very similar to last season. Track forecast errors were lower than last season at all lead times up to 96 hours. Skill scores also rose and at 24 and 48 hours were the highest ever achieved. There was a slow bias in forecasts. The intensity tendency skill was 62% overall. The charts show the long term trend in forecast errors and skill.

3.3 South Atlantic basin storm

A plot of the observed track of the one storm in the South Atlantic basin

The South Atlantic saw a storm named Arani this season following on from last season's Anita. It was only briefly classified as a tropical storm and just one 12-hour forecast was verified.

3.4 Combined Statistics for whole Southern Hemisphere

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144
Possibly verified1591228961402716
Detection rate (%)10010010010010010094
Mean AT-6-43-83-153-270-185-61
Mean CT3-16-36-4371131101
Mean skill (%)*****636460***************
2009-10 skill (%)*****373230***************
Mean DPE3411921830250642608
* 2009-10 DPE 34111185266307377482
Intensity skill (%)*****646077407088

* DPE for all southern hemisphere storms in 2009-10 season

Forecast positional errors for the whole southern hemisphere.
Forecast skill for the whole southern hemisphere.
Along-track errors for the whole southern hemisphere.
Cross-track errors for the whole southern hemisphere.
Intensity skill for the whole southern hemisphere.

Due to the exceptionally low south-west Indian activity, tropical cyclone activity was the lowest for 20 years in terms of storm numbers. Track forecast errors were lower than last season's values, and at some lead times only second to the record values of 2008-9. Both cross track and along track errors were lower than last season, but there was still a notable slow bias in particular. The overall intensity tendency skill score was 63%, which is the highest value recorded since intensity verification started in 2001.

Further tropical cyclone information

The Met Office pages contain information on tropical cyclone forecasting at the Met Office. Monthly updates of tropical cyclone activity and forecasts are made, together with observed and forecast track information of recent storms, track prediction error statistics, lists of names and real-time tropical cyclone forecast guidance.

Tropical Cyclone information can be found on the home page under the "Weather/World" heading.

Seasonal summaries of tropical cyclone activity and forecasts have been issued since the 1994-5 Southern Hemisphere season. To obtain these or any further information on tropical cyclone forecasting email the Met Office.

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Acknowledgements

Mr.S.Lord, NCEP, Washington, USA and Mr.C.Mauck, FNOC, Monterey, USA supplied CLIPER models for various basins.
Mr.S.Lord and Dr.M.Fiorino supplied GrADS software used to produce track plotting charts.

Last updated: 17 December 2013