Tropical cyclone forecast verification - southern hemisphere 2020-21

1. Introduction

A summary of tropical cyclone activity in the Southern Hemisphere for the 2020-21 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

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 2019-20 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

* 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

The level of activity was similar to 2019-20. Track forecast errors were similar to the five year mean at short lead times, but much lower at longer lead times. Skill scores against CLIPER were similar to recent seasons. There was a slow and left-of-track bias in forecasts at several lead times. The central pressure bias was mostly quite small.

3.2 Australian basin storms

* 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 higher than last season in this basin. Track forecast errors were below the five year mean at all lead times. There was a slight slow bias at shorter lead times. The negative central pressure bias became larger at longer lead times.

3.3 South Atlantic basin storms

* DPE and skill for all Australian storms in the previous five seasons.

Plot of the observed tracks of all storms in the south Atlantic basin

There was a high number of tracked storms this season in this basin. Track forecast errors were low at very short lead times. Few forecasts were verified at T+48 and beyond.

3.4 Combined Statistics for whole Southern Hemisphere

* DPE and skill for all southern hemisphere storms in the previous five seasons.

Activity in the southern hemisphere was higher than last season and similar to the couple of seasons prior to that. 

At lead times of 96 hours and longer track forecast errors were below the five year mean. Skill scores against CLIPER were high. There was a slow bias in forecastsat some 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

The five-year running mean of track forecast errors dropped sharply at longer lead times. This was mostly due to the 2015-16 season, with very high errors, dropping out of the mean value.

Five-year running mean track forecast errors for the whole southern hemisphere

The central pressure bias was very low at all lead times. The 10m wind bias was negative, but mostly with small values. Mean absolute errors for central pressure were similar to recent years. For 10m winds the mean absolute errors were low for short lead times, but large at longer lead tims.

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.