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Tropical cyclone forecast verification - southern hemisphere 2016-17

1. Introduction

A summary of tropical cyclone activity in the Southern Hemisphere for the 2016-17 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) 0 (1) 2 (3) 0 (0) 2 (4)
Tropical storms (35-63 knots) 3 (4) 8 (4) 2 (0) 13 (8)
Hurricanes/typhoons (>63 knots) 2 (2) 5 (6) 0 (0) 7 (8)
Total 5 (7) 15 (13) 0 (0) 22 (20)

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 2015-16 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 45 32 22 14 9 6 3 1
Detection rate (%) 100 100 100 100 100 100 100 100
AT error (km) 3 -15 -35 -34 -61 -89 -86 -635
CT error (km) -5 13 51 94 110 -208 -274 -484
Track skill (%) ***** 48 69 74 ***** ***** ***** *****
* 2015-16 skill (%) ***** 47 55 55 ***** ***** ***** *****
DPE (km) 37 90 153 201 276 411 366 799
* 2015-16 DPE (km) 44 104 223 345 495 667 990 1565
Central pressure bias (mb) -1.0 -0.1 0.6 0.6 -6.3 -13.3 -20.7 -11.0

* DPE and skill for all south-west Indian storms in 2015-16 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 low with few long lead time forecasts verified. Track forecast errors were lower than last season. Skill scores against CLIPER were high. Track biases were small. Central pressure biases were small at short lead times, which comprised most of the sample of forecasts.

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 86 54 36 22 9 6 3 1
Detection rate (%) 100 93 92 96 90 100 100 100
AT error (km) 5 -21 -10 77 246 185 88 -102
CT error (km) -11 -13 7 59 276 430 427 421
Track skill (%) ***** 59 58 55 ***** ***** ***** *****
* 2015-16 skill (%) ***** 78 71 64 ***** ***** ***** *****
DPE (km) 61 124 209 308 468 538 442 434
* 2015-16 DPE (km) 41 85 146 235 400 611 865 1262
Central pressure bias (mb) 10.0 10.9 10.2 10.8 15.3 24.7 17.3 0.0

* DPE and skill for all Australian storms in 2015-16 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

Despite more storms than last season activity this season was lower than last season due to storms being weaker and shorter-lived. Track forecast errors were higher than last season at most lead times. Track forecast skill against CLIPER was lower than last season. At short lead times biases were small. There was a weak bias in forecasts as measured by central pressure.

3.3 South Atlantic basin storms

There were two South Atlantic subtropical storms this season, but both were very short-lived such that statistics will not be presented here.

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 135 91 61 37 19 12 6 2
Detection rate (%) 100 96 95 97 95 100 100 100
AT error (km) 3 -19 -20 34 93 48 1 -369
CT error (km) -9 -4 24 72 193 111 77 -32
Track skill (%) ***** 55 63 66 ***** ***** ***** *****
* 2015-16 skill (%) ***** 62 59 57 ***** ***** ***** *****
DPE (km) 53 111 188 266 372 474 404 617
* 2015-16 DPE (km) 42 93 179 281 436 630 904 1353
Central pressure bias (mb) 6.1 6.8 6.6 6.8 4.5 5.7 -1.7 -5.5

* DPE and skill for all southern hemisphere storms in 2015-16 season

The southern hemisphere saw very low activity this season. There were very few verifiable forecasts at longer lead times reducing the significance of these results.

Track forecast errors were a little higher than last season at short lead times, but lower at longer lead times. Skill scores against CLIPER were higher than last season. Track forecast biases were low at short 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 weak bias in central pressure in the analysis. This bias remained at short lead times, but reduced at longer lead times. The 10m wind biases were similar to those for central pressure.

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.

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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.

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