Tropical cyclone forecast verification - southern hemisphere 2008-9
Southern hemisphere 2008-9
1. Introduction
A summary of tropical cyclone activity in the Southern Hemisphere for the 2008-9 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 into 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 6-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 the season was 0.5625°x 0.375°x 50 levels. This is equivalent to a horizontal resolution of 62km x 42km 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
| SWI | AUS | TOTAL | |
|---|---|---|---|
| Tropical depressions (<34 knots) | 1(0) | 0(5) | 1(5) |
| Tropical storms (>34 knots, <64 knots) | 8(7) | 13(6) | 21(13) |
| Hurricanes/typhoons (>64 knots) | 2(5) | 3(6) | 5(11) |
| Total | 11(12) | 16(17) | 27(29) |
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)
The number in brackets indicates the figure for the 2007-8 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
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | |
|---|---|---|---|---|---|---|---|
| Possibly verified | 60 | 41 | 24 | 14 | 8 | 4 | 1 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| Mean DX | 0 | -50 | -112 | -207 | -328 | -492 | -649 |
| Mean DY | -1 | -37 | -141 | -203 | -225 | -289 | -456 |
| Mean AT | -11 | -50 | -156 | -256 | -380 | -537 | -793 |
| Mean CT | 9 | 2 | -23 | -12 | -93 | -182 | -7 |
| Mean skill (%) | ***** | 49 | 57 | 44 | ***** | ***** | ***** |
| 2007-8 skill (%) | ***** | 19 | 23 | 21 | ***** | ***** | ***** |
| Mean DPE | 43 | 116 | 214 | 310 | 418 | 573 | 794 |
| * 2007-8 DPE | 57 | 125 | 261 | 400 | 514 | 665 | 643 |
| Intensity skill (%) | ***** | 56 | 67 | 43 | 50 | 100 | 100 |
* DPE for all south-west Indian storms in 2007-8 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 fewer strong cyclones. Track forecast errors were lower at all lead times than last season and at several lead times were the lowest or second-lowest ever recorded (since 1988-9).
Skill scores were significantly higher than last season and at the 24- and 48-hour lead times were the highest ever recorded. Cross track biases were generally fairly small, but the along-track errors showed a slow bias in forecasts. The intensity tendency skill had a value of 53% overall. Figs. 2 and 3 show the long term trend in forecast errors and skill.
3.2 Australian basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | |
|---|---|---|---|---|---|---|---|
| Possibly verified | 89 | 57 | 37 | 27 | 20 | 14 | 9 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 70 | 50 | 33 |
| Mean DX | -7 | -28 | -45 | -84 | -144 | -297 | -383 |
| Mean DY | -3 | -33 | -55 | -83 | -87 | -121 | 18 |
| Mean AT | -5 | -39 | -34 | -40 | -109 | -6 | 81 |
| Mean CT | -11 | -12 | -11 | -4 | 26 | 80 | 268 |
| Mean skill (%) | ***** | 61 | 57 | 71 | ***** | ***** | ***** |
| 2007-8 skill (%) | ***** | 24 | 32 | 17 | ***** | ***** | ***** |
| Mean DPE | 41 | 104 | 150 | 199 | 279 | 429 | 493 |
| * 2007-8 DPE | 70 | 126 | 225 | 373 | 570 | 907 | 928 |
| Intensity skill (%) | ***** | 51 | 78 | 56 | 57 | 43 | 100 |
* DPE for all Australian storms in 2007-8 season
Plot
of the observed tracks of all storms in the eastern Australian
basin
Plot
of the observed tracks of all storms in the wastern Australian
basin
Forecast
positional errors in the Australian basin
Forecast
skill in the Australian basin
There were a greater number of Tropical storms this season compared to last, but the number of more-intense cyclones was less. Track forecast errors were lower than last season and at most lead times were the lowest ever recorded. Skill scores were very high – the highest ever recorded at the 24- and 72-hour lead times. Cross track and along track biases were generally small. The intensity tendency skill was 60% overall. Figs. 6 and 7 show the long-term trend in forecast errors and skill scores.
3.3 Combined Statistics for whole Southern Hemisphere
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | |
|---|---|---|---|---|---|---|---|
| Possibly verified | 149 | 98 | 61 | 41 | 28 | 18 | 10 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 79 | 61 | 40 |
| Mean DX | -4 | -37 | -71 | -126 | -211 | -368 | -450 |
| Mean DY | -2 | -35 | -89 | -124 | -138 | -182 | -100 |
| Mean AT | -7 | -43 | -82 | -113 | -207 | -199 | -138 |
| Mean CT | -3 | -6 | -15 | -7 | -17 | -15 | 199 |
| Mean skill (%) | ***** | 55 | 57 | 61 | ***** | ***** | ***** |
| 2007-8 skill (%) | ***** | 21 | 26 | 20 | ***** | ***** | ***** |
| Mean DPE | 42 | 109 | 175 | 237 | 329 | 482 | 568 |
| * 2007-8 DPE | 64 | 125 | 243 | 387 | 538 | 764 | 767 |
| Intensity skill (%) | ***** | 53 | 74 | 51 | 55 | 64 | 100 |
* DPE for all southern hemisphere storms in 2007-8 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 lower than last season in terms of storm numbers and longevity of storms. There were also far fewer strong storms than last season. Track forecast errors were the lowest achieved on record (since 1988-9) at all lead times. Track skill scores against CLIPER were also the highest ever achieved. Cross-track errors were reduced to near zero. However, a slow bias again developed in forecasts as seen in several previous seasons. The overall intensity tendency skill score was 57%, 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.
________________________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.
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