Tropical cyclone forecast verification - northern hemisphere 2011

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.

Tropical cyclone forecast verification - measures of error

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
	NWP	NEP	NAT	NI	TOTAL
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)
Total	24(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+0	T+24	T+48	T+72	T+96	T+120	T+144	T+168
Possibly verified	241	201	166	133	103	74	54	36
Detection rate (%)	100	100	100	97	97	97	98	100
Mean AT	-1	-11	-36	-82	-180	-294	-401	-331
Mean CT	0	-10	-51	-112	-136	-105	42	184
Mean skill (%)	*****	39	47	48	*****	*****	*****	*****
2010 skill (%)	*****	31	52	53	*****	*****	*****	*****
Mean DPE	34	113	208	302	405	490	650	763
* 2010 DPE	43	115	201	311	363	414	513	*****
Intensity skill (%)	*****	42	31	27	34	53	28	22

* 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+0	T+24	T+48	T+72	T+96	T+120	T+144	T+168
Possibly verified	113	91	69	50	34	22	12	8
Detection rate (%)	100	100	99	98	94	95	100	88
Mean AT	-6	-21	-12	-24	-29	-12	64	-61
Mean CT	2	0	-16	-24	-29	-88	-213	-108
Mean skill (%)	*****	42	61	71	*****	*****	*****	*****
2010 skill (%)	*****	11	37	55	*****	*****	*****	*****
Mean DPE	21	84	124	161	213	325	341	382
* 2010 DPE	29	83	128	145	232	390	380	*****
Intensity skill (%)	*****	49	56	47	50	24	33	71

* 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+0	T+24	T+48	T+72	T+96	T+120	T+144	T+168
Possibly verified	183	145	109	85	67	55	46	37
Detection rate (%)	100	98	98	96	99	96	96	86
Mean AT	-5	-9	-6	-23	-1	0	-75	-228
Mean CT	-4	-12	-25	-51	-34	-22	8	-63
Mean skill (%)	*****	49	61	56	*****	*****	*****	*****
2010 skill (%)	*****	38	55	58	*****	*****	*****	*****
Mean DPE	32	92	158	263	448	684	925	1070
* 2010 DPE	37	111	184	246	283	347	475	*****
Intensity skill (%)	*****	50	38	32	27	43	32	6

* 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+0	T+24	T+48	T+72	T+96	T+120	T+144	T+168
Possibly verified	26	15	10	6	2	0	0	0
Detection rate (%)	100	100	100	100	100	*****	*****	*****
Mean AT	-17	-30	4	41	131	*****	*****	*****
Mean CT	8	-24	-44	-53	-7	*****	*****	*****
Mean skill (%)	*****	28	69	87	*****	*****	*****	*****
2010 skill (%)	*****	14	48	65	*****	*****	*****	*****
Mean DPE	55	136	122	129	181	*****	*****	*****
* 2010 DPE	57	128	245	480	525	939	*****	*****
Intensity skill (%)	*****	60	60	67	100	*****	*****	*****

* 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+0	T+24	T+48	T+72	T+96	T+120	T+144	T+168
Possibly verified	563	450	354	274	206	151	112	81
Detection rate (%)	100	99	99	97	97	97	97	93
Mean AT	-4	-13	-21	-50	-94	-147	-218	-262
Mean CT	0	-9	-36	-75	-84	-73	0	51
Mean skill (%)	*****	42	54	55	*****	*****	*****	*****
2010 skill (%)	*****	32	52	56	*****	*****	*****	*****
Mean DPE	32	101	174	260	386	536	727	859
* 2010 DPE	40	110	186	264	307	375	479	*****
Intensity skill (%)	*****	47	39	33	35	45	30	20

* 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 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.

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.