Tropical cyclone forecast verification - southern hemisphere 2012-13

1. Introduction

A summary of tropical cyclone activity in the Southern Hemisphere for the 2012-13 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 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 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 (<35 knots)0(2)1(2)0(0)1(4)
Tropical storms (35-63 knots)3(7)6(6)0(0)9(13)
Hurricanes/typhoons (>63 knots)7(4)7(2)0(0)14(6)
Total10(13)14(10)0(0)24(23)

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 2011-12 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 verified92695337211152
Detection rate (%)1009910010098979694
AT error (km)
3-41-84-83-72-96-43-205
 CT error (km)
-12-111066112246452479
Track skill (%)*****555454********************
* 2011-12 skill (%)*****394031********************
DPE (km)
4579156217303447544542
* 2011-12 DPE (km)28133269400560687790971
Intensity skill (%)*****454051529600

* DPE and skill for all south-west Indian storms in 2011-12 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

Despite a quieter season than 2011-12 in terms of total storm numbers, there was a higher proportion of storms reaching hurricane strength. In 25 years of assessment, track forecast errors were the lowest ever achieved at all but one lead time. Skill scores against CLIPER were also amongst the highest ever achieved. There was a small slow bias and left-of-track bias in forecasts. The intensity tendency skill had a value of 46% 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 verified97715136231572
Detection rate (%)100100100100100100100100
AT error (km)
-11-54-95-135-168-202-317-515
CT error (km)
-3-1054010098-77-503
Track skill (%)*****564741********************
* 2011-12 skill (%)*****605541********************
DPE (km)
57123183265360499563792
* 2011-12 DPE (km) 32111185293401529593470
Intensity skill (%)*****526172748743100

* DPE and skill for all Australian storms in 2011-12 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

After a quiet season in 2011-12, this season saw activity back closer to the recent average. Track forecast errors were close to values seen in the last couple of seasons as were skill scores against CLIPER.  There was a marked slow bias in forecasts. The intensity tendency skill was 61% 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 verified189140104734426124
Detection rate (%)10099100100100100100100
AT error (km)
-4-48-90-108-122-157-203-360
CT error (km)
-7-10853105161144-12
Track skill (%)*****565250********************
* 2011-12 skill (%)*****454434********************
DPE (km)
51101170241333477555667
* 2011-12 DPE (km) 29125239361499626716811
Intensity skill (%)*****49506264545050

* DPE and skill for all southern hemisphere storms in 2011-12 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, with a higher proportion of storms reaching hurricane strength than last season. Up to 120-hours, track forecast errors were the lowest ever achieved in 25 years worth of assessment. Skill scores against CLIPER were higher than most previous seasons. The slow bias seen in previous seasons was reduced a little, but there was a left-of-track bias this season. The overall intensity tendency skill score was 53%, which is higher than last season.

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