Tropical cyclone forecast verification - southern hemisphere 2019-20

1. Introduction

A summary of tropical cyclone activity in the Southern Hemisphere for the 2019-20 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 2018-19 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

Although only slightly fewer storms, activity in this basin was much less than in 2018-19 due to shorter-lived storms. Track forecast errors were much lower than the five year mean at longer lead times. Skill scores against CLIPER were high. There was a slow bias in forecasts. 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

Storms were generally short-lived this season meaning activity was much lower than last season in this basin. Track forecast errors were well below the five year mean at longer lead times. Track biases were generally small. The central pressure bias became negative at longer lead times.

3.3 South Atlantic basin storms

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

There are no verification statistics since there was just one short-lived storm in this region.

3.4 Combined Statistics for whole Southern Hemisphere

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

Total storm numbers were similar to recent seasons in the southern hemisphere, but since many were short-lived overall activity saw a big drop compared to recent seasons. Only a small number of forecasts were verified at long lead times making these statistics unreliable. 

At lead times of 72 hours and longer track forecast errors were well 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

There was a continued downwards trend in the five-year running mean of track forecast errors.

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

The central pressure bias was similar to last season - close to zero initially, but becoming strongly negative at long lead times due to overdeepening of strong cyclones. The 10m wind bias was initially negative, but moved towards zero with increasing lead time. Mean absolute errors for central pressure grew with lead time, but for 10m winds remained fairly constant at all lead times.

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.