Tropical cyclone forecast verification - northern hemisphere 2013

1. Introduction

A summary of tropical cyclone activity in the Northern Hemisphere for the 2013 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.

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) 7(2) 1(0) 1(0) 0(0) 9(2)
Tropical storms (34-63 knots) 12(9) 11(7) 12(9) 4(4) 39(29)
Hurricanes/typhoons (>63 knots) 15(16) 9(10) 2(10) 3(0) 29(36)
Total 34(24) 21(17) 15(19) 7(4) 77(67)

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 257 196 142 94 60 33 15 7
Detection rate (%) 100 95 94 91 88 91 80 71
AT error (km) 5 -28 -46 -54 -121 -115 109 712
CT error (km) -8 -8 -27 -26 -13 49 130 527
Track skill (%) ***** 50 62 68 ***** ***** ***** *****
* 2012 skill (%) ***** 54 60 56 ***** ***** ***** *****
DPE (km) 46 86 157 216 332 540 909 1149
* 2012 DPE (km) 51 98 181 291 409 477 609 752
Intensity skill (%) ***** 36 16 9 17 -7 50 -60

* DPE and skill for all north-west Pacific storms in 2012 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

2013 saw more storms than 2012, but fewer verifiable forecasts. Track forecast errors were lower than last season at short lead times, but higher at longer lead times. Skill against CLIPER was the highest ever recorded. The intensity tendency skill was 24% 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 115 75 50 37 25 16 10 6
Detection rate (%) 100 85 84 62 60 63 70 100
AT error (km) -8 2 32 65 77 65 50 51
CT error (km) -15 2 1 -41 -73 -175 -241 -446
Track skill (%) ***** 27 45 41 ***** ***** ***** *****
* 2012 skill (%) ***** 14 42 47 ***** ***** ***** *****
DPE (km) 63 112 171 269 305 418 502 559
* 2012 DPE (km) 49 98 158 198 218 341 305 201
Intensity skill (%) ***** 47 38 22 73 20 71 33

* DPE and skill for all north-east Pacific storms in 2012 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 2013, but the many of these were shorter lived than in 2012. Track forecast errors were higher than the last two seasons at almost all lead times. However, skill scores against CLIPER were little different to last season. There was a very slight fast bias in forecasts and a left-of-track bias at longer lead times. The intensity tendency skill was 40% 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 92 63 39 18 12 10 9 6
Detection rate (%) 100 90 85 94 100 100 89 100
AT error (km) 2 5 33 21 125 220 239 212
CT error (km) 0 -9 -26 -19 75 140 266 322
Track skill (%) ***** 63 74 76 ***** ***** ***** *****
* 2012 skill (%) ***** 60 67 65 ***** ***** ***** *****
DPE (km) 45 87 145 209 221 340 514 579
* 2012 DPE (km) 36 85 158 240 334 507 592 827
Intensity skill (%) ***** 75 70 76 83 80 50 0

* DPE and skill for all North Atlantic storms in 2012 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

2013 had many storm lived storms, but activity was very low in terms of verifiable forecasts. Track forecast errors were the lowest ever achieved at most lead times. Skill scores against CLIPER were also the highest ever achieved. There was a fast bias at longer lead times. The intensity tendency skill score was 65% 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 55 43 31 20 12 5 1 0
Detection rate (%) 100 100 100 100 100 100 100 *****
AT error (km) -29 -62 -57 1 185 299 -429 *****
CT error (km) 11 10 19 93 70 -71 -443 *****
Track skill (%) ***** 17 26 14 ***** ***** ***** *****
* 2012 skill (%) ***** 31 ***** ***** ***** ***** ***** *****
DPE (km) 70 138 221 323 440 501 617 *****
* 2012 DPE (km) 121 215 243 ***** ***** ***** ***** *****
Intensity skill (%) ***** 63 48 50 33 -20 100 *****

* DPE and skill for all North Indian storms in 2012 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 picked up in 2013 after a few inactive seasons. Track forecast errors were close to long term averages. The intensity tendency skill was 46% 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 519 377 262 169 109 64 35 19
Detection rate (%) 100 93 91 86 84 86 80 89
AT error (km) -2 -21 -23 -19 -17 16 112 302
CT error (km) -6 -4 -16 -11 0 14 56 111
Track skill (%) ***** 46 59 62 ***** ***** ***** *****
* 2012 skill (%) ***** 52 61 59 ***** ***** ***** *****
DPE (km) 52 97 166 238 327 478 684 739
* 2012 DPE (km) 46 94 170 259 361 485 592 787
Intensity skill (%) ***** 48 31 25 37 13 57 -6

* DPE and skill for all northern hemisphere storms in 2012 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 2013 was lower than last year despite a greater number of storms. many of these were short-lived. Track forecast errors were the lowest ever at many lead times. Skill scores against CLIPER were slightly below large season's highest ever values at most lead times. Forecast biases were small at all but the longest lead times. Detection percentages were as low as 80% at longer times, but this may in part be due to a change to more stringent measures of storm detection during the season. The intensity tendency skill was 36% overall, which is lower than last season.

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.

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