Tropical cyclone forecast verification - southern hemisphere 2007-8
Southern hemisphere 2007-8
1. Introduction
A summary of tropical cyclone activity in the Southern Hemisphere for the 2007-8 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 6-day forecast every 12 hours. Verification is performed at 24 hour intervals up to forecast time T+120. 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 were used as verifying observations of storm location when track verification was carried out immediately after a storm had ended. Best track data from RSMCs La Réunion and Nadi, Fiji and Bureau of Meteorology, Australia have been obtained for this season’s storms and the track forecasts verified again. Statistics for verification against best-track observations are presented in detail. Some statistics for verification against real-time observations are included for the purposes of a comparison and are denoted 'RT'. 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) | 0(0) | 5(2) | 5(2) |
| Tropical storms (>34 knots, <64 knots) | 7(3) | 6(8) | 13(11) |
| Hurricanes/typhoons (>64 knots) | 5(7) | 6(5) | 11(12) |
| Total | 12(10) | 17(15) | 29(25) |
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 2006-7 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 | |
|---|---|---|---|---|---|---|
| Possibly verified | 100 | 77 | 60 | 45 | 34 | 25 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 92 |
| Mean DX | -8 | -57 | -172 | -261 | -320 | -296 |
| Mean DY | 3 | 23 | -11 | -72 | -149 | -143 |
| Mean AT | -1 | -2 | 17 | 18 | 23 | -4 |
| Mean CT | 2 | 6 | -28 | -33 | 13 | 202 |
| Mean skill (%) | ***** | 19 | 23 | 21 | ***** | ***** |
| 2006-7 skill (%) | ***** | 11 | 16 | 16 | ***** | ***** |
| Mean DPE | 57 | 125 | 261 | 400 | 514 | 665 |
| * 2006-7 DPE | 54 | 130 | 283 | 458 | 665 | 802 |
| RT DPE # | 51 | 123 | 257 | 397 | 515 | 665 |
| Intensity skill (%) | ***** | 51 | 53 | 42 | 35 | -13 |
* DPE for all south-west Indian storms in 2006-7 season
# DPE in 2007-8 calculated using real time observations as opposed to best track.
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
There were a larger number of Tropical storms this season compared to last, but fewer strong cyclones. Track forecast errors were lower at all lead times than last season. At T+24 errors were the lowest ever recorded, although errors at longer lead times were higher than several previous seasons.
Skill scores were slightly higher than last season, although below levels seen in seasons prior to that. Cross track and along track biases were generally fairly small. The intensity tendency skill had a value of 36% overall.
3.2 Australian basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | |
|---|---|---|---|---|---|---|
| Possibly verified | 103 | 77 | 58 | 42 | 26 | 16 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 100 |
| Mean DX | 0 | -18 | -45 | -67 | -177 | -470 |
| Mean DY | 11 | 17 | -19 | -54 | -57 | -1 |
| Mean AT | -12 | -44 | -71 | -87 | -160 | -392 |
| Mean CT | -4 | 6 | 1 | -73 | -174 | -208 |
| Mean skill (%) | ***** | 24 | 32 | 17 | ***** | ***** |
| 2006-7 skill (%) | ***** | 42 | 15 | -4 | ***** | ***** |
| Mean DPE | 70 | 126 | 225 | 373 | 570 | 907 |
| * 2006-7 DPE | 72 | 153 | 355 | 665 | 1075 | 1537 |
| RT DPE # | 61 | 120 | 223 | 373 | 567 | 902 |
| Intensity skill (%) | ***** | 45 | 45 | 38 | 8 | 13 |
* DPE for all Australian storms in 2006-7 season
# DPE in 2007-8 calculated using real time observations
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
Cyclone numbers were similar this season to last. Track forecast errors returned to more usual values after last season's high values, although errors were still quite high at long lead times. However, errors at T+24 and T+48 were the lowest ever recorded. Skill scores were positive, but lower than seen in some previous seasons. There was a right-of-track and slow bias in forecasts. The intensity tendency skill was 42% overall.
3.3 Combined Statistics for whole Southern Hemisphere
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | |
|---|---|---|---|---|---|---|
| Possibly verified | 203 | 154 | 118 | 87 | 60 | 41 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 95 |
| Mean DX | -4 | -38 | -110 | -168 | -258 | -368 |
| Mean DY | 7 | 20 | -15 | -64 | -109 | -84 |
| Mean AT | -6 | -23 | -26 | -33 | -57 | -163 |
| Mean CT | -1 | 6 | -14 | -53 | -68 | 34 |
| Mean skill (%) | ***** | 21 | 26 | 20 | ***** | ***** |
| 2006-7 skill (%) | ***** | 23 | 16 | 13 | ***** | ***** |
| Mean DPE | 64 | 125 | 243 | 387 | 538 | 764 |
| * 2006-7 DPE | 62 | 138 | 302 | 498 | 732 | 873 |
| RT DPE # | 56 | 122 | 240 | 385 | 537 | 762 |
| Intensity skill (%) | ***** | 48 | 49 | 40 | 23 | -3 |
* DPE for all southern hemisphere storms in 2006-7 season
# DPE in 2007-8 calculated using real time observations
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 slightly higher than last season across the southern hemisphere as a whole. Track forecast errors were lower than last season at all lead times and the lowest and second lowest ever at T+24 and T+48 respectively. Track skill scores against CLIPER were positive, but not as high as in several previous seasons. The right-of-track bias seen in previous seasons was considerably reduced. There was a slight slow bias again in forecasts after last season, which saw a very low bias. Forecast errors derived using best track data showed some small differences to errors derived using real-time data. The overall intensity tendency skill score was 39%, which is higher than the last four seasons and only fractionally lower than the 2002-3 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.
________________________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.
RSMC Météo-France, La Réunion, Bureau of Meteorology, Australia and RSMC Nadi, Fiji supplied best track observations for the Southern Hemisphere.
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