Tropical cyclone forecast verification - southern hemisphere 2009-10

1. Introduction

A summary of tropical cyclone activity in the Southern Hemisphere for the 2009-10 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 0.5625°x 0.375°x 70 levels. This is equivalent to a horizontal resolution of 62 km x 42 km at the equator. The horizontal resolution was increased to 0.3515625°x0.234375° on 9 March 2010.

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)0(1)0(0)0(0)0(1)
Tropical storms (>34 knots, <64 knots)6(8)8(13)1(0)15(21)
Hurricanes/typhoons (>64 knots)5(2)7(3)0(0)12(5)
Total11(11)15(16)1(0)27(27)

Categories assigned based on 1-minute averaged winds

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 2008-9 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 verified61412816620
Detection rate (%)1001009388100100*****
Mean DX-6-111-214-280-303-160*****
Mean DY-4-67-203-393-628-801*****
Mean AT-5-30-88-207-555-628*****
Mean CT-3-64-191-303-382-576*****
Mean skill (%)*****-22-16-43***************
2008-9 skill (%)*****495744***************
Mean DPE311503125067831010*****
* 2008-9 DPE 43116214310418573794
Intensity skill (%)*****46624367100*****

* DPE for all south-west Indian storms in 2008-9 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 total number of tropical cyclones was similar this season compared to last, but there were more strong cyclones. After a couple of seasons of significant drops in mean track forecast errors, there was a sharp rise this season to levels not seen for a long time.

Similarly, there was a sharp drop in skill scores to levels not seen since the 1990s after record high scores last season. There were marked slow and right-of-track biases in forecasts this season. Despite the poor track errors, the intensity tendency skill had a value of 50% 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 verified118886345322215
Detection rate (%)100100100100100100100
Mean DX-6-74-158-214-285-261-214
Mean DY-7-55-156-252-369-209-105
Mean AT-12-66-152-253-316-302-126
Mean CT2-41-100-112-122-107-165
Mean skill (%)*****594950***************
2008-9 skill (%)*****615771***************
Mean DPE36145303450565570439
* 2008-9 DPE 41104150199279429493
Intensity skill (%)*****68374719557

* DPE for all Australian storms in 2008-9 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 similar to last, but the proportion of more intense cyclones was higher. After record low track forecast errors last season, this season saw a rise to levels closer to the average for the last decade. Despite the higher track errors this season, skill scores were only slightly lower than last season. As with the South-West Indian Ocean there was a slow and right-of-track bias in forecasts. The intensity tendency skill was 45% overall. The charts show the long term trend in forecast errors and skill scores.

3.3 South Atlantic basin storm

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144
Possibly verified6420000
Detection rate (%)100100100********************
Mean DX3-2-40********************
Mean DY80-7********************
Mean AT10-17-37********************
Mean CT70-16********************
Mean DPE387760********************
* 2008-9 DPE 42109175237329482568
Intensity skill (%)*****500********************

* DPE for all Southern Hemisphere storms in 2008-9 season

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

The South Atlantic saw its first storm since 2004, which lasted a couple of days and was generally well predicted by the model.

3.4 Combined Statistics for whole Southern Hemisphere

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144
Possibly verified1851339361382415
Detection rate (%)1001009897100100100
Mean DX-6-83-171-230-287-253-214
Mean DY-5-57-166-286-326-258-105
Mean AT-9-53-131-242-354-329-126
Mean CT0-47-124-157-163-146-165
Mean skill (%)*****383129***************
2008-9 skill (%)*****555761***************
Mean DPE35145303463599607439
* 2008-9 DPE 42109175237329482568
Intensity skill (%)*****61434626587

* DPE for all southern hemisphere storms in 2008-9 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.

Tropical cyclone activity was similar to last season in terms of storm numbers, but more active in terms of longevity of storms. There were also a greater proportion of strong storms than last season. Track forecast errors were lower than last season's record values, but still higher than the two seasons prior to that. Both cross track and along track errors were increased compared to last season. The overall intensity tendency skill score was 46%, which is the second 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.

________________________

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