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Tropical cyclone forecast verification - northern hemisphere 2015

1. Introduction

A summary of tropical cyclone activity in the Northern Hemisphere for the 2015 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 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 (3) 5 (1) 1 (1) 0 (0) 9 (5)
Tropical storms (34-63 knots) 7 (10) 9 (6) 7 (2) 3 (3) 26 (21)
Hurricanes/typhoons (>63 knots) 19 (11) 17 (16) 4 (6) 2 (2) 42 (35)
Total 29 (24) 31 (23) 12 (9) 5 (5) 77 (60)

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 2014 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 380 327 277 231 190 152 120 93
Detection rate (%) 100 100 100 100 99 100 99 100
AT error (km) 3 -4 -14 -20 -20 -31 -49 -50
CT error (km) 0 9 9 14 20 35 72 109
Track skill (%) ***** 56 64 66 ***** ***** ***** *****
* 2014 skill (%) ***** 55 66 63 ***** ***** ***** *****
DPE (km) 38 77 125 183 246 326 422 564
* 2014 DPE (km) 57 85 124 193 265 354 426 395
Central pressure bias (mb) 6.6 8.0 8.3 7.5 5.5 3.5 2.3 0.4

* DPE and skill for all north-west Pacific storms in 2014 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

2015 saw higher activity than in 2014. Track forecast errors were mostly slightly below last season's record low values. Skill against CLIPER was close to last season's values. There was a weak bias at short lead times, but this was almost eliminated in long lead time forecasts.

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 279 231 188 148 113 86 65 47
Detection rate (%) 100 98 98 98 100 100 100 100
AT error (km) 0 -5 -11 -29 3 -23 -95 -207
CT error (km) -9 -19 -24 -18 -28 -51 -76 -133
Track skill (%) ***** 47 60 62 ***** ***** ***** *****
* 2014 skill (%) ***** 52 65 70 ***** ***** ***** *****
DPE (km) 39 80 134 206 244 301 383 444
* 2014 DPE (km) 40 76 129 198 275 386 524 642
Central pressure bias (mb) 4.3 5.4 6.2 3.3 0.5 -1.9 -4.7 -5.7

* DPE and skill for all north-east Pacific storms in 2014 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

2015 was an exceptionally active season. Track forecast errors were slightly higher than last season at short lead times, but at record low values at longer lead times. Skill scores against CLIPER were fractionally lower than last season. There was a slight weak bias at short lead times which reversed and became a slight strong bias at long lead times.

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 90 57 48 32 22 13 7 3
Detection rate (%) 100 100 100 100 100 100 100 100
AT error (km) 3 -14 -30 -25 -64 -110 -383 *****
CT error (km) -1 4 30 25 -61 -273 -567 -794
Track skill (%) ***** 65 71 69 ***** ***** ***** *****
* 2014 skill (%) ***** 70 80 78 ***** ***** ***** *****
DPE (km) 30 71 133 208 386 673 921 1548
* 2014 DPE (km) 41 81 114 180 199 207 319 381
Central pressure bias (mb) 2.2 3.0 2.6 -1.0 -2.1 -0.7 8.0 19.3

* DPE and skill for all North Atlantic storms in 2014 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 below average activity with just four of the 11 tropical storms becoming hurricanes. Track forecast errors were above last year's values from T+48 and very high at long lead times. This was primarily caused by the very large errors in forecast tracks for Hurricane Joaquin early in its lifetime. Skill scores against CLIPER took a slight dip compared to last season's high values. The central pressure bias was small all but the longest lead times.

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 36 26 19 13 7 2 0 0
Detection rate (%) 100 100 100 100 86 100 ***** *****
AT error (km) -13 -32 -64 -113 -193 -125 ***** *****
CT error (km) -8 -18 -7 3 82 55 ***** *****
Track skill (%) ***** 35 61 63 ***** ***** ***** *****
* 2014 skill (%) ***** 5 38 52 ***** ***** ***** *****
DPE (km) 42 77 114 165 218 138 ***** *****
* 2014 DPE (km) 61 97 137 203 323 425 ***** *****
Central pressure bias (mb) 14.7 16.7 18.4 19.2 19.0 16.0 ***** *****

* DPE and skill for all North Indian storms in 2014 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 similar to 2014 in terms of storm numbers, but the storms were more intense. Track forecast errors were amongst the lowest ever achieved and skill scores against CLIPER higher than last season. There was a weak bias in forecasts.

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 785 652 532 424 332 253 192 143
Detection rate (%) 100 99 99 99 99 100 99 100
AT error (km) 1 -6 -16 -26 -18 -33 -77 -124
CT error (km) -3 -2 -1 3 -1 -10 -2 11
Track skill (%) ***** 54 63 65 ***** ***** ***** *****
* 2014 skill (%) ***** 55 68 68 ***** ***** ***** *****
DPE (km) 38 77 129 192 254 334 427 545
* 2014 DPE (km) 48 81 126 194 265 357 465 522
Central pressure bias (mb) 5.6 6.9 7.4 5.8 3.5 1.6 0.1 -1.2

* DPE and skill for all northern hemisphere storms in 2014 season

Northern hemisphere activity was at records levels due to high activity across the Pacific.

Track forecast errors were the lower than the record low values of 2014 at all lead times apart from T+48 and T+168. Skill scores against CLIPER were fractionally lower than last season's record values. The negative cross-track bias seen last season was mostly eradicated this season.  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 mean bias in central pressure and 10m wind show further reductions following the large improvements seen last season as seen in the charts below. In particular biases at short lead times were improved considerably.

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

The big drop in track errors and intensity biases in 2014 was 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. In 2015 the further reductions in errors and biases was at least in part due to central pressure data assimilation which was implemented on 3rd February 2015. The five-year running mean of track forecast errors shows a long term downwards trend which has accelerated in the last two years.

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

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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