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Tropical cyclone forecast verification - northern hemisphere 2014

1. Introduction

A summary of tropical cyclone activity in the Northern Hemisphere for the 2014 season is presented below together with an assessment of the performance of the Met Office global model in predicting the track and intensity 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 up to 15th July 2014 was 0.3515625° × 0.234375° × 70 levels. This is equivalent to a horizontal resolution of 39 km × 26 km at the equator. From 15th July 2014 onwards the global model resolution in operation was 0.234375° x 0.15625° x 70 levels. This is equivalent to a horizontal resolution of 26 km × 17 km 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) 3(7) 1(1) 1(1) 0(0) 5(9)
Tropical storms (34-63 knots) 10(12) 6(11) 2(12) 3(4) 21(39)
Hurricanes/typhoons (>63 knots) 11(15) 16(9) 6(2) 2(3) 35(29)
Total 24(34) 23(21) 9(15) 5(7) 60(77)

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 2012 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 223 179 145 114 87 63 45 30
Detection rate (%) 100 99 97 96 97 97 100 93
AT error (km) 11 5 -14 -32 -50 -142 -161 -5
CT error (km) 1 0 -6 -3 2 29 57 -33
Track skill (%) ***** 55 66 63 ***** ***** ***** *****
* 2013 skill (%) ***** 50 62 68 ***** ***** ***** *****
DPE (km) 57 85 124 193 265 354 426 395
* 2013 DPE (km) 46 86 157 216 332 540 909 1149
Intensity skill (%) ***** 34 39 33 33 15 11 7

* DPE and skill for all north-west Pacific storms in 2013 season

Plot of the observed tracks of all storms in the north-west Pacific basin
Track forecast errors in the north-west Pacific basin
Track forecast skill in the north-west Pacific basin

2014 saw lower activity than in 2013. Track forecast errors were the lowest ever recorded. Skill against CLIPER was the highest ever recorded at 24 and 48 hours. 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 248 203 167 134 102 70 46 34
Detection rate (%) 100 100 100 100 97 99 98 94
AT error (km) 0 14 29 31 27 -3 -60 -106
CT error (km) -8 -17 -40 -75 -92 -158 -227 -265
Track skill (%) ***** 52 65 70 ***** ***** ***** *****
* 2013 skill (%) ***** 27 45 41 ***** ***** ***** *****
DPE (km) 40 76 129 198 275 386 524 642
* 2013 DPE (km) 63 112 171 269 305 418 502 559
Intensity skill (%) ***** 48 54 48 64 59 47 50

* DPE and skill for all north-east Pacific storms in 2013 season

Plot of the observed tracks of all storms in the north-east Pacific basin
Track forecast errors in the north-east Pacific basin
Track forecast skill in the north-east Pacific basin

2014 was the most active season in this region since 1992. Track forecast errors were lower than last season at most lead times and amongst the three lowest years ever. Skill scores against CLIPER were the highest ever achieved. There was a very slight slow bias in forecasts.  The intensity tendency skill was 53% 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 70 55 43 31 21 13 6 2
Detection rate (%) 100 100 98 100 100 100 100 100
AT error (km) 9 8 18 22 19 -149 -157 -175
CT error (km) -2 -5 -7 -21 -55 -28 -111 38
Track skill (%) ***** 70 80 78 ***** ***** ***** *****
* 2013 skill (%) ***** 63 74 76 ***** ***** ***** *****
DPE (km) 41 81 114 180 199 207 319 381
* 2013 DPE (km) 45 87 145 209 221 340 514 579
Intensity skill (%) ***** 45 43 42 43 38 33 100

* DPE and skill for all North Atlantic storms in 2013 season

Plot of the observed tracks of all storms in the North Atlantic basin
Track forecast errors in the North Atlantic basin
Track forecast skill in the North Atlantic basin

2014 saw a low number of storms, but a high proportion reached hurricane strength and there were more verifiable forecasts than in 2013 at most lead times. Track forecast errors were the lowest ever achieved at all lead times except one. Skill scores against CLIPER were the highest ever achieved. The intensity tendency skill score was 48% 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 35 25 16 10 5 1 0 0
Detection rate (%) 100 100 100 100 100 100 ***** *****
AT error (km) -30 -44 12 103 294 417 ***** *****
CT error (km) 12 42 83 90 -53 -79 ***** *****
Track skill (%) ***** 5 38 52 ***** ***** ***** *****
* 2013 skill (%) ***** 17 26 14 ***** ***** ***** *****
DPE (km) 61 97 137 203 323 425 ***** *****
* 2013 DPE (km) 70 138 221 323 440 501 617 *****
Intensity skill (%) ***** 52 63 40 20 -100 ***** *****

* DPE and skill for all North Indian storms in 2013 season

Plot of the observed tracks of all storms in the North Indian basin
Track forecast errors in the North Indian basin
Track forecast skill in the North Indian basin

Cyclone activity was a little lower than 2013. Track forecast errors were lower than last season and skill scores against CLIPER mostly higher. The intensity tendency skill was 52% 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 576 462 371 289 215 147 97 66
Detection rate (%) 100 100 99 99 97 98 99 94
AT error (km) 4 7 10 8 2 -72 -113 -63
CT error (km) -3 -5 -18 -35 -50 -66 -87 -151
Track skill (%) ***** 55 68 68 ***** ***** ***** *****
* 2013 skill (%) ***** 46 59 62 ***** ***** ***** *****
DPE (km) 48 81 126 194 265 357 465 522
* 2013 DPE (km) 52 97 166 238 327 478 684 739
Intensity skill (%) ***** 42 48 41 48 38 29 32

* DPE and skill for all northern hemisphere storms in 2013 season

Overall, activity in the northern hemisphere in 2014 was higher than last year mostly due to high activity in the north-east Pacific.

Track forecast errors were the lowest ever at all lead times. Track forecast errors were on average 25% lower than the mean for the last five seasons. Skill scores against CLIPER were the highest ever at all lead times. Forecast biases were generally small.  Detection percentages were high.

Track forecast errors for the whole northern hemisphere
Track forecast skill for the whole northern hemisphere
Along-track errors for the whole northern hemisphere
Cross-track errors for the whole northern hemisphere

The intensity tendency skill was 45% overall, which is higher than last season. The mean bias in central pressure and 10m wind show a large reduction, particularly at longer lead times, compared to the previous few seasons as seen in the charts below.

Central pressure forecast bias for the whole northern hemisphere
Peak 10m wind forecast bias for the whole northern hemisphere
Intensity forecast skill for the whole northern hemisphere

2014 saw a large drop in track forecast errors and intensity forecast bias to the lowest ever values at all lead times. This can be attributed to the major model change implemented on 15th July 2014 which primarily included a new dynamic core, physics upgrade and an increase in horizontal resolution.

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.

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.

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