Tropical cyclone forecast verification - southern hemisphere 2011-12

1. Introduction

A summary of tropical cyclone activity in the Southern Hemisphere for the 2011-12 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+168, 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)2(1)2(2)0(0)4(3)
Tropical storms (>34 knots, <64 knots)7(1)6(6)0(1)13(8)
Hurricanes/typhoons (>64 knots)4(2)2(8)0(0)6(10)
Total13(4)10(16)0(1)23(21)

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 2010-11 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+144T+168
Cases verified10988705240302317
Detection rate (%)1001001009698979694
Mean AT-3-68-185-280-313-264-357-401
Mean CT0-31-33107812122390
Mean skill (%)*****394031********************
2010-11 skill (%)*****324256********************
Mean DPE28133269400560687790971
* 2010-11 DPE 37117219262411546694643
Intensity skill (%)*****39493520404829

* DPE for all south-west Indian storms in 2010-11 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

After a quiet season in 2010-11, this season saw the highest number of tropical cyclones for nine years. Track forecast errors were a little higher than last season, but similar to the previous few seasons. Skill scores against CLIPER were close to last season's values. There was a slow bias in forecasts. The intensity tendency skill had a value of 39% 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+144T+168
Cases verified675138302519148
Detection rate (%)1001001001001001009373
Mean AT-6-62-85-139-258-223-104-146
Mean CT1-9-23-19528868102
Mean skill (%)*****605541********************
2010-11 skill (%)*****676750********************
Mean DPE32111185293401529593470
* 2010-11 DPE 33119218201522669579499
Intensity skill (%)*****49534752265750

* DPE for all Australian storms in 2010-11 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

2011-12 saw the fewest number of tropical cyclones in this region for over 20 years. Track forecast errors were mostly lower than seen in the last two seasons. Skill scores against CLIPER were a little lower than in the last few seasons. There was a slow bias in forecasts. The intensity tendency skill was 52% overall. The charts show the long term trend in forecast errors and skill.

3.3 Combined Statistics for whole Southern Hemisphere

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144T+168
Cases verified1761391229365493725
Detection rate (%)1001001009898989586
Mean AT-4-66-150-228-292-248-261-320
Mean CT0-23-3006810816494
Mean skill (%)*****454434********************
2010-11 skill (%)*****636460********************
Mean DPE29125239361499626716811
* 2010-11 DPE 34119218302500642608527
Intensity skill (%)*****42503932355136

* DPE for all southern hemisphere storms in 2010-11 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.

Activity for the southern hemisphere as a whole was near average, although the activity was skewed towards the South-West Indian region. Track forecast errors were mostly close to values seen in the last few seasons, although skill scores against CLIPER were lower than some previous seasons. The slow bias seen in previous seasons continued. The overall intensity tendency skill score was 44%, which is the lowest value for several years.

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