Tropical cyclone forecast verification - northern hemisphere 2012

A summary of tropical cyclone activity in the Northern Hemisphere for the 2012 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 Northern Hemisphere for the 2012 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 North Pacific, North Atlantic and North Indian Oceans and nearby tropical seas. For the purpose of tropical cyclone verification the northern hemisphere is divided in to four basins; the North-West Pacific (west of 180° E to the Malay Peninsula), the North-East Pacific (east of 180° E), the North Atlantic and the North Indian Ocean (west of the Malay Peninsula). Mean error statistics for each basin are presented together with a table of statistics for the whole Northern Hemisphere. 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 0.3515625° × 0.234375° × 70 levels. This is equivalent to a horizontal resolution of 39 km × 26 km at the equator.

An explanation of error statistics and their sign conventions.

Advisory positions received in real time from RSMC Tokyo, JTWC Hawaii, NHC Miami and CPHC Honolulu 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 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
 NWPNEPNATNITOTAL
Tropical depressions (<34 knots)2(5)0(2)0(1)0(0)2(8)
Tropical storms (34-63 knots)9(9)7(1)9(12)4(5)29(27)
Hurricanes/typhoons (>63 knots)16(10)10(10)10(7)0(1)36(28)
Total24(24)17(13)19(20)4(6)67(63)

Categories assigned based on 1-minute averaged winds

Basin name abbreviations:
NWP : North-west Pacific (west of 180° E)
NEP : North-east Pacific (east of 180° E)
NAT : North Atlantic
NI : North Indian (west of Malay Peninsula)

The number in brackets indicates the figure for the 2011 season.

3. Summary of all northern hemisphere storms

3.1 North-west Pacific basin storms

Mean Error Statistics
 T+0T+24T+48T+72T+96T+120T+144T+168
Possibly verified296246196150111755339
Detection rate (%)100100989896959187
AT error (km)7-19-55-67-83-54-10-34
CT error (km)-2-18141452175276
Track skill (%)*****495754********************
* 2011 skill (%)*****394748********************
DPE (km)5299182288409475610750
* 2011 DPE (km) 34113208302405490650763
Intensity skill (%)*****432527715-453

* DPE and skill for all north-west Pacific storms in 2011 season

Plot of the observed tracks of all storms in the north-west Pacific basin
Forecast positional errors in the north-west Pacific basin
Forecast skill in the north-west Pacific basin

2012 was more active than 2011 in this region due to a greater number of typhoons. Track forecast errors were slightly lower than last season at most lead times and the best ever recorded at short lead times. Skill against CLIPER was better than last season. At T+24 and T+48 skill scores were the best ever recorded. There was a small slow bias in forecasts overall. The intensity tendency skill was 29% overall.

3.2 North-east Pacific basin storms

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144T+168
Possibly verified131996944221141
Detection rate (%)100999998866475100
AT error (km)-5810-16-29-144-177-145
CT error (km)-6-18-43-80-86-182-190-141
Track skill (%)*****274753********************
* 2011 skill (%)*****426171********************
DPE (km)4699158198211343301203
* 2011 DPE(km)2184124161213325341382
Intensity skill (%)*****6359587943100100

* DPE and skill for all north-east Pacific storms in 2011 season

Plot of the observed tracks of all storms in the north-east Pacific basin
Forecast positional errors in the north-east Pacific basin
Forecast skill in the north-east Pacific basin

There were a greater number of storms in this region in 2012, but the many of these were shorter lived than in 2011. Track forecast errors were a little higher than the last couple of seasons at short lead times, but similar or lower at longer lead times. Skill scores against CLIPER were lower than last season's record values. There was a very slight left-of-track bias in forecasts and a slow bias at longer lead times. The intensity tendency skill was 63% overall.

3.3 North Atlantic basin storms

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144T+168
Possibly verified19916313210991745946
Detection rate (%)1009910010010010010098
AT error (km)-4-25-56-83-103-7828-160
CT error (km)-2-6-16-13-13-68-1219
Track skill (%)*****646866********************
* 2011 skill (%)*****496156********************
DPE (km)3687159241336507644828
* 2011 DPE (km) 32921582634486849251070
Intensity skill (%)*****54554763595338

* DPE and skill for all North Atlantic storms in 2011 season

Plot of the observed tracks of all storms in the North Atlantic basin
Forecast positional errors in the North Atlantic basin
Forecast skill in the North Atlantic basin

2012 was another active Atlantic season and had more verifiable forecasts than 2011. Track forecast errors were the lowest ever at T+24 and amongst the lowest at other lead times. Skill scores against CLIPER were the highest ever at T+24 and T+48 and the second highest at T+72. There was a slight slow and left-of-track bias in some forecast tracks. The intensity tendency skill score was 55% overall.

3.4 North Indian Basin Storms

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144T+168
Possibly verified146100000
Detection rate (%)100100100*************************
AT error (km)-11-164-87*************************
CT error (km)-10-17207*************************
Track skill (%)*****22******************************
* 2011 skill (%)*****286987********************
DPE (km)114220225*************************
* 2011 DPE (km) 55136122129181***************
Intensity skill (%)*****0100*************************

* DPE and skill for all North Indian storms in 2011 season

Plot of the observed tracks of all storms in the North Indian basin
Forecast positional errors in the North Indian basin
Forecast skill in the North Indian basin

Cyclone activity was low in 2012 and there were very few forecasts to verify. Track forecast errors were above last season's values. The intensity tendency skill was 40% overall.

3.5 Combined statistics for whole northern hemisphere

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144T+168
Possibly verified64051439830322416011686
Detection rate (%)100100999997959593
AT error (km)1-17-44-65-87-706-106
CT error (km)-3-6-8-10-6-1765126
Track skill (%)*****526059********************
* 2011 skill (%)*****425455********************
DPE (km)4796170258361485620787
* 2011 DPE (km)32101174260386536727859
Intensity skill (%)*****50413836382945

* DPE and skill for all northern hemisphere storms in 2011 season

Forecast positional errors for the whole northern hemisphere
Forecast skill for the whole northern hemisphere
Along-track errors for the whole northern hemisphere
Cross-track errors for the whole northern hemisphere
Intensity skill for the whole northern hemisphere.

Overall, activity in the northern hemisphere in 2012 was higher than last year. Track forecast errors were the lowest or second lowest ever recorded at most lead times. The mean T+24 error fell below 100 km for the first time. Skill scores against CLIPER were the highest ever achieved at all lead times. Forecast biases were small with only a slight slow bias at some lead times of any note. Detection percentages were fairly high. The intensity tendency skill was 44% overall, which is the highest ever achieved.

It is notable that analysis errors were higher this season than for the last few seasons. This was due to the Met Office tropical cyclone initialisation scheme being switched off in July 2012. Trial results suggested that analysis errors would be larger, but forecast errors smaller as a result of this change and this appears to be reflected in the season's verification figures which show a very good track forecasting performance.

4. Further tropical cyclone information

The Met Office web pages 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 world weather.

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.

Last updated: 17 December 2013