Tropical cyclone forecast verification - southern hemisphere 2022-23
A summary of tropical cyclone activity in the southern hemisphere for the 2022-23 season together with an assessment of the performance of the Met Office global model in predicting the tracks of these tropical cyclones.
1. Introduction
A summary of tropical cyclone activity in the Southern Hemisphere for the 2022-23 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. There are also occasional tropical or subtropical storms in the South Atlantic Ocean. For the purpose of tropical cyclone verification the Southern Hemisphere is divided in to three basins; the South-West Indian (west of 90° E), Australian (east of 90° E) and South Atlantic. Mean error statistics for each basin are presented together with a table of statistics for the whole Southern Hemisphere. The global model produces a seven-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 0.140625° x 0.09375° x 70 levels. This is equivalent to a horizontal resolution of 16 km × 10 km at the equator.
Tropical cyclone forecast verification - measures of error
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 | SAT | TOTAL | |
|---|---|---|---|---|
| Tropical depressions (<35 knots) | 0 | 0 | 0 | 0 |
| Tropical storms (35-63 knots) | 2 | 6 | 0 | 8 |
| Hurricanes/typhoons (>63 knots) | 6 | 5 | 0 | 11 |
| Total | 8 | 11 | 0 | 19 |
Basin name abbreviations:
SWI : South-West Indian (west of 90° E)
AUS : Australian (east of 90° E)
SAT : South Atlantic
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 | T+168 | |
|---|---|---|---|---|---|---|---|---|
| Cases verified | 149 | 128 | 111 | 95 | 78 | 67 | 55 | 43 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 99 | 100 | 100 | 100 |
| AT error (km) | 1.6 | -12.8 | -25.5 | -61.6 | -125.0 | -170.6 | -207.3 | -271.0 |
| CT error (km) | -2.2 | 6.7 | 1.0 | -22.5 | -24.4 | -51.2 | -53.7 | -64.4 |
| Track skill (%) | ***** | 63.6 | 75.2 | 78.0 | ***** | ***** | ***** | ***** |
| DPE (km) | 28.6 | 60.3 | 98.6 | 150.9 | 218.7 | 292.5 | 338.9 | 371.3 |
| * 2017-22 DPE (km) | 36.3 | 73.1 | 132.4 | 190.4 | 255.9 | 338.7 | 437.1 | 523.8 |
| Central pressure bias (mb) | -2.2 | 4.2 | 8.3 | 10.4 | 12.1 | 14.5 | 15.6 | 17.9 |
* DPE and skill for all south-west Indian storms for the previous five seasons.
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 less tropical cyclones in total than recent seasons, but a higher proportion of strong cyclones. Track forecast errors were near to or below the five year mean. Skill scores against CLIPER were similar to recent seasons. There was a small slow bias, particularly at longer lead times. The central pressure bias was much larger than recent seasons.
3.2 Australian basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
|---|---|---|---|---|---|---|---|---|
| Cases verified | 65 | 45 | 30 | 19 | 9 | 3 | 0 | 0 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 100 | 0 | 0 |
| AT error (km) | 0.0 | -23.4 | -62.8 | -163.0 | -270.3 | -242.3 | ***** | ***** |
| CT error (km) | -2.2 | 1.9 | -21.4 | -58.7 | -71.7 | -269.3 | ***** | ***** |
| Track skill (%) | ***** | 71.0 | 77.1 | 78.4 | ***** | ***** | ***** | ***** |
| DPE (km) | 36.6 | 66.8 | 110.5 | 208.9 | 404.4 | 706.3 | ***** | ***** |
| * 2017-22 DPE (km) | 38.4 | 80.3 | 133.9 | 189.4 | 219.9 | 278.7 | 368.8 | 375.8 |
| Central pressure bias (mb) | 1.9 | 10.4 | 19.2 | 17.0 | 17.6 | 3.7 | ***** | ***** |
* DPE and skill for all Australian storms in the previous five seasons.
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
Activity was very low this season with no forecasts verified beyond T+120. Track forecast errors at short lead times were low, but higher at longer lead times. There was a slow bias in forecsts. The central pressure bias grew quickly with lead time.
3.3 South Atlantic basin storms
No storms this season.
3.4 Combined Statistics for whole Southern Hemisphere
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
|---|---|---|---|---|---|---|---|---|
| Cases verified | 215 | 173 | 141 | 114 | 87 | 70 | 55 | 43 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 99 | 100 | 100 | 100 |
| AT error (km) | 1.2 | -15.5 | -33.5 | -78.5 | -140.1 | -173.6 | -207.3 | -271.0 |
| CT error (km) | -2.2 | 5.5 | -3.8 | -28.6 | -29.3 | -60.5 | -53.7 | -64.4 |
| Track skill (%) | ***** | 64.9 | 75.4 | 78.0 | ***** | ***** | ***** | ***** |
| DPE (km) | 31.0 | 62.0 | 101.1 | 160.6 | 237.9 | 310.2 | 338.9 | 371.3 |
| * 2017-22 DPE (km) | 38.6 | 76.9 | 133.5 | 191.0 | 241.2 | 316.4 | 412.0 | 470.7 |
| Central pressure bias (mb) | -1.0 | 5.8 | 10.7 | 11.6 | 12.6 | 14.0 | 15.6 | 17.9 |
* DPE and skill for all southern hemisphere storms in the previous five seasons.
Activity in the southern hemisphere was the lowest since the 1990-1 season by storm numbers.
Track forecast errors were lower than the five year mean and the lowest ever at short lead times. Skill scores against CLIPER were high. There was a marked slow bias in forecasts.
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
The five-year running mean of track forecast errors shows a slight reduction on last year.
Five-year running mean track forecast errors for the whole southern hemisphere
The central pressure bias grew quickly during early lead times. This was indicative of a weak bias in forecasts which has been apparent since a major model change in May 2022. The 10m wind bias was negative also indicating a weak bias in forecast intensity. Mean absolute errors for central pressure were similar to recent years at shorter lead times. For 10m winds the mean absolute errors were larger than all recent years from lead times of 24 hours onwards.
Central pressure forecast bias for the whole southern hemisphere
Peak 10m wind forecast bias for the whole southern hemisphere
Central pressure forecast mean abosolute error for the whole southern hemisphere
Peak 10m wind forecast mean absolute error for the whole southern hemisphere
Further tropical cyclone information
The Met Office tropical cyclone web pages contain information on tropical cyclone forecasting at the Met Office. Updates of tropical cyclone activity and forecasts are made monthly, together with observed and forecast track information of recent storms, track and intensity prediction error statistics, lists of names and real-time tropical cyclone forecast guidance.
For further information on tropical cyclone forecasting at the Met Office please email us.
