Tropical cyclone forecast verification - southern hemisphere 2014-15
A summary of tropical cyclone activity in the southern hemisphere for the 2014-15 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 2014-15 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 0.234375° x 0.15625° x 70 levels. This is equivalent to a horizontal resolution of 26 km × 17 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 (1) | 0 (0) | 0 (0) | 0 (1) |
| Tropical storms (35-63 knots) | 8 (5) | 5 (8) | 2 (0) | 15 (13) |
| Hurricanes/typhoons (>63 knots) | 4 (4) | 8 (7) | 0 (0) | 12 (11) |
| Total | 12 (10) | 13 (15) | 0 (0) | 27 (25) |
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 2013-14 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 | T+168 | |
|---|---|---|---|---|---|---|---|---|
| Cases verified | 88 | 65 | 44 | 29 | 18 | 12 | 6 | 2 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| AT error (km) | 0 | -8 | -22 | -42 | -89 | -153 | 157 | 588 |
| CT error (km) | 0 | 22 | 53 | 50 | 31 | -39 | -336 | -687 |
| Track skill (%) | ***** | 69 | 72 | 80 | ***** | ***** | ***** | ***** |
| * 2013-14 skill (%) | ***** | 55 | 54 | 49 | ***** | ***** | ***** | ***** |
| DPE (km) | 38 | 78 | 155 | 195 | 233 | 306 | 522 | 934 |
| * 2013-14 DPE (km) | 40 | 115 | 239 | 343 | 502 | 678 | 825 | 1586 |
| Intensity skill (%) | ***** | 35 | 50 | 45 | 44 | 67 | 67 | 0 |
* DPE and skill for all south-west Indian storms in 2013-14 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
Activity was very similar to last season. Track forecast errors were the lowest ever achieved at all lead times except T+168. Skill scores against CLIPER were the highest ever achieved. Track forecast biases were mostly small. 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
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
|---|---|---|---|---|---|---|---|---|
| Cases verified | 111 | 85 | 60 | 40 | 31 | 25 | 19 | 14 |
| Detection rate (%) | 100 | 100 | 100 | 98 | 100 | 100 | 100 | 100 |
| AT error (km) | -11 | -45 | -60 | -57 | -5 | -32 | -311 | -906 |
| CT error (km) | 0 | 6 | 22 | 60 | 149 | 248 | 213 | 111 |
| Track skill (%) | ***** | 70 | 70 | 73 | ***** | ***** | ***** | ***** |
| * 2013-14 skill (%) | ***** | 55 | 61 | 73 | ***** | ***** | ***** | ***** |
| DPE (km) | 43 | 98 | 166 | 235 | 314 | 464 | 655 | 1026 |
| * 2013-14 DPE (km) | 59 | 118 | 212 | 281 | 286 | 388 | 688 | 707 |
| Intensity skill (%) | ***** | 39 | 33 | 30 | 35 | 52 | 26 | 14 |
* DPE and skill for all Australian storms in 2013-14 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
Activity this season was similar to last season. Track forecast errors were the lowest or second lowest ever achieved at short lead times. At longer lead times errors were near the average for the last few seasons, although were large at long lead times. This was mostly due to a few very poor forecasts for Cyclone Nathan. Track forecast skill against CLIPER was the highest ever achieved. Track forecast biases were small at most lead times. The intensity tendency skill was 39% overall. The charts show the long term trend in forecast errors and skill.
3.3 South Atlantic basin storms
Plot of the observed track of the two storms in the South Atlantic basin
The South Atlantic saw two storms named Bapo and Cari this season. They were both only briefly classified as tropical storms and few forecasts were verified.
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 | 203 | 151 | 104 | 69 | 49 | 37 | 25 | 16 |
| Detection rate (%) | 100 | 100 | 100 | 99 | 100 | 100 | 100 | 100 |
| AT error (km) | -6 | -29 | -44 | -51 | -36 | -71 | -199 | -720 |
| CT error (km) | -1 | 12 | 35 | 55 | 105 | 155 | 81 | 11 |
| Track skill (%) | ***** | 70 | 71 | 76 | ***** | ***** | ***** | ***** |
| * 2013-14 skill (%) | ***** | 55 | 57 | 61 | ***** | ***** | ***** | ***** |
| DPE (km) | 42 | 89 | 161 | 218 | 284 | 413 | 623 | 1014 |
| * 2013-14 DPE (km) | 51 | 117 | 224 | 310 | 382 | 512 | 734 | 817 |
| Intensity skill (%) | ***** | 38 | 40 | 36 | 39 | 57 | 36 | 12 |
* DPE and skill for all southern hemisphere storms in 2013-14 season
Activity for the southern hemisphere as a whole was similar to last season.
Track forecast errors were the lowest ever achieved at all but the longest lead times. Skill scores against CLIPER were the highest ever achieved. Track forecast biases were low except at the longest lead times. Detection percentages were high.
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 intensity tendency skill was 39% overall, which is lower than last season. However, the mean bias in central pressure and 10m wind shows a large reduction, particularly at shorter lead times, compared to the previous few seasons as seen in the charts below.
Central pressure forecast bias for the whole southern hemisphere
Peak 10m wind forecast bias for the whole southern hemisphere
Intensity skill for the whole southern hemisphere
Following on from similar results for the 2014 northern hemisphere season, the 2014-15 southern hemisphere season saw a large drop in track forecast errors and intensity forecast bias to the lowest ever values at all lead times. This can be attributed to the major model change implemented on 15th July 2014 which primarily included a new dynamic core, physics upgrade and an increase in horizontal resolution. Reduction in the analysis and short lead-time bias in intensity can also be attributed to the assimilation of central pressure estimates which was introduced from 3rd February 2015.
Further tropical cyclone information
The Weather and climate change 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.
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.
