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Tropical cyclone forecast verification - southern hemisphere 2015-16

1. Introduction

A summary of tropical cyclone activity in the Southern Hemisphere for the 2015-16 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

Tropical cyclone activity
SWI AUS SAT TOTAL
Tropical depressions (<35 knots) 1 (0) 3 (0) 0 (0) 4 (0)
Tropical storms (35-63 knots) 4 (8) 4 (5) 0 (2) 8 (15)
Hurricanes/typhoons (>63 knots) 2 (4) 6 (8) 0 (0) 8 (12)
Total 7 (12) 13 (13) 0 (0) 20 (27)

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 2014-15 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+0 T+24 T+48 T+72 T+96 T+120 T+144 T+168
Cases verified 82 66 51 40 30 22 16 12
Detection rate (%) 100 100 100 100 100 100 100 100
AT error (km) -4 -40 -80 -81 -12 174 281 555
CT error (km) -4 -18 -39 -35 -16 -57 -109 -213
Track skill (%) ***** 47 55 55 ***** ***** ***** *****
* 2014-15 skill (%) ***** 69 72 80 ***** ***** ***** *****
DPE (km) 44 104 223 345 495 667 990 1565
* 2014-15 DPE (km) 38 78 155 195 233 306 522 934
Central pressure bias (mb) 6.0 5.9 5.9 8.4 10.5 6.2 -1.4 -9.3

* DPE and skill for all south-west Indian storms in 2014-15 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 lower than last season. Track forecast errors were higher than the exceptionally low values of last season, although close to values for the season before at most lead times. Skill scores against CLIPER were also lower than last season, but close to values from the season before. There was a slight fast bias at longer lead times. There was a weak bias in the analysis which initially increased with forecast lead time.

3.2 Australian basin storms

Table of mean error statistics
T+0 T+24 T+48 T+72 T+96 T+120 T+144 T+168
Cases verified 118 91 69 57 49 42 35 28
Detection rate (%) 100 100 100 98 100 100 97 93
AT error (km) -8 -26 -51 -112 -270 -314 -167 -110
CT error (km) -8 -6 18 17 56 175 386 639
Track skill (%) ***** 78 71 64 ***** ***** ***** *****
* 2014-15 skill (%) ***** 70 70 73 ***** ***** ***** *****
DPE (km) 41 85 146 235 400 611 865 1262
* 2014-15 DPE (km) 43 98 166 235 314 464 655 1026
Central pressure bias (mb) 4.7 7.3 8.4 9.0 11.8 14.2 19.5 23.5

* DPE and skill for all Australian storms in 2014-15 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 slightly lower than last season with fewer strong tropical cyclones. Track forecast errors were lower than last season at short lead times. However, longer lead time forecast errors were high mostly due to poor long lead time forecasts for Cyclone Winston. Track forecast skill against CLIPER was similar to last season. There was a slow bias and a polewards bias at longer lead times. A small weak bias grew larger during the forecast.

3.3 South Atlantic basin storms

There were no South Atlantic tropical cyclones this season.

3.4 Combined Statistics for whole Southern Hemisphere

Table of mean error statistics
T+0 T+24 T+48 T+72 T+96 T+120 T+144 T+168
Cases verified 200 157 120 97 79 64 51 40
Detection rate (%) 100 100 100 99 100 100 98 95
AT error (km) -6 -32 -63 -99 -172 -146 -26 89
CT error (km) -7 -11 -6 -5 28 95 231 383
Track skill (%) ***** 62 59 57 ***** ***** ***** *****
* 2014-15 skill (%) ***** 70 71 76 ***** ***** ***** *****
DPE (km) 42 93 179 281 436 630 904 1353
* 2014-15 DPE (km) 42 89 161 218 284 413 623 1014
Central pressure bias (mb) 5.3 6.7 7.3 8.8 11.3 11.5 12.9 13.6

* DPE and skill for all southern hemisphere storms in 2014-15 season

The southern hemisphere saw fewer tropical cyclones than last season, although they lasted longer. Thus there were more verifiable forecasts.

Track forecast errors were higher than last season, but at most forecast times on a par with the previous few seasons' average. However, long lead time errors were very large. This was mostly due to poor forecasts for the two longest lived tropical cyclones of the season, Winston and Fantala. Skill scores against CLIPER were the lower than last season, but still above values for the previous few seasons. Track forecast biases were mixed, although there was a polewards bias at longer lead times.

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

There was a small weak bias in central pressure in the analysis. However, unlike last season, this weak bias grew during the forecast. Similarly, the weak bias in 10m wind in the analysis grew with forecast lead time. This was primarily due to a failure to predict the intensification of Cyclone Winston.

Central pressure forecast bias for the whole southern hemisphere
Peak 10m wind forecast bias for the whole southern hemisphere

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.

Tropical Cyclone information can be found on the home page under the "World weather" 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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