Tropical cyclone forecast verification - northern hemisphere 2011

A summary of tropical cyclone activity in the Northern Hemisphere for the 2011 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 2011 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° x 0.234375° x 70 levels. This is equivalent to a horizontal resolution of 39km x 26km 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)5(5)2(5)1(2)0(0)8(12)
Tropical storms (>34 knots, <64 knots)9(6)1(5)12(7)5(1)27(19)
Hurricanes/typhoons (>64 knots)10(8)10(3)7(12)1(4)28(23)
Total24(19)13(13)20(21)6(5)63(58)

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 2010 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 verified241201166133103745436
Detection rate (%)10010010097979798100
Mean AT-1-11-36-82-180-294-401-331
Mean CT0-10-51-112-136-10542184
Mean skill (%)*****394748********************
2010 skill (%)*****315253********************
Mean DPE34113208302405490650763
* 2010 DPE 43115201311363414513*****
Intensity skill (%)*****42312734532822

* DPE for all north-west Pacific storms in 2010 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

2011 was more active than 2010 in this region, but still below average. Track forecast errors were very similar to those seen in the last few years. Skill against CLIPER was also similar to last season. There was a slow bias in forecasts overall. The intensity tendency skill was 35% overall.

3.2 North-east Pacific basin storms

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144T+168
Possibly verified1139169503422128
Detection rate (%)1001009998949510088
Mean AT-6-21-12-24-29-1264-61
Mean CT20-16-24-29-88-213-108
Mean skill (%)*****426171********************
2010 skill (%)*****113755********************
Mean DPE2184124161213325341382
* 2010 DPE 2983128145232390380*****
Intensity skill (%)*****49564750243371

* DPE for all north-east Pacific storms in 2010 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

The 2011 season in this basin saw low levels of storm numbers as in 2011, but 10 of the 11 storms which formed reached hurricane status. Track forecast errors were the lowest ever recorded at nearly all lead times. Skill scores against CLIPER were the highest ever recorded. There was a very slight slow bias in forecasts at some lead times. The intensity tendency skill was 49% overall.

3.3 North Atlantic basin storms

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144T+168
Possibly verified1831451098567554637
Detection rate (%)10098989699969686
Mean AT-5-9-6-23-10-75-228
Mean CT-4-12-25-51-34-228-63
Mean skill (%)*****496156********************
2010 skill (%)*****385558********************
Mean DPE32921582634486849251070
* 2010 DPE 37111184246283347475*****
Intensity skill (%)*****5038322743326

* DPE for all North Atlantic storms in 2010 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

2011 was another active Atlantic season, but unlike 2010 many of the storms were weak and short-lived. Track forecast errors were low at short lead times, but grew at longer lead times to be above the recent average. Skill scores against CLIPER were higher than the last couple of seasons. There was a left-of-track bias in forecast tracks. The intensity tendency skill score was 38% overall.

3.4 North Indian Basin Storms

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144T+168
Possibly verified26151062000
Detection rate (%)100100100100100***************
Mean AT-17-30441131***************
Mean CT8-24-44-53-7***************
Mean skill (%)*****286987********************
2010 skill (%)*****144865********************
Mean DPE55136122129181***************
* 2010 DPE 57128245480525939**********
Intensity skill (%)*****606067100***************

* DPE for all North Indian storms in 2010 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 average in 2010 in terms of number of storms, but just two of these persisted for any length of time. Track forecast errors were low and skill scores against CLIPER high for these storms. The intensity tendency skill was 69% overall.

3.5 Combined statistics for whole northern hemisphere

Table of mean error statistics
 T+0T+24T+48T+72T+96T+120T+144T+168
Possibly verified56345035427420615111281
Detection rate (%)10099999797979793
Mean AT-4-13-21-50-94-147-218-262
Mean CT0-9-36-75-84-73051
Mean skill (%)*****425455********************
2010 skill (%)*****325256********************
Mean DPE32101174260386536727859
* 2010 DPE40110186264307375479*****
Intensity skill (%)*****47393335453020

* DPE for all northern hemisphere storms in 2010 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 2011 was higher than last year, but still relatively low despite the active Atlantic season. Track forecast errors were the lowest ever recorded at lead times up to 72 hours. At longer lead times errors were larger than last year, but still near the average for the last few seasons. Skill scores against CLIPER were the highest ever achieved at nearly all lead times. There was a left-of-track and slow bias in forecast tracks. Detection percentages were fairly high. The intensity tendency skill was 39% overall, which is near to the average of the last few years.

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 the "Weather/World" 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.

5. Acknowledgements

Mr.S.Lord, NMC, 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.

Last updated: 17 December 2013