Tropical cyclone forecast verification - northern hemisphere 2007
Northern hemisphere 2007
1. Introduction
A summary of tropical cyclone activity in the Northern Hemisphere for the 2007 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. The global model produces a 6-day forecast every 12 hours. Verification is performed at 24-hour intervals up to forecast time T+120. The global model resolution in operation during most of the season was 0.5625°x 0.375°x 50 levels. This is equivalent to a horizontal resolution of 62km x 42km at the equator.
Advisory positions from RSMC Tokyo, JTWC Hawaii, NHC Miami and CPHC Honolulu are used as verifying observations of storm location when track verification is carried out immediately after a storm has ended. Best track data from these centres has been obtained for this season’s storms and the track forecasts verified again. Statistics for verification against best-track observations are presented in detail. Some statistics for verification against real-time observations are included for the purposes of a comparison and are denoted 'RT'. 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
| NWP | NEP | NAT | NI | TOTAL | |
|---|---|---|---|---|---|
| Tropical depressions (<34 knots) | 3(4) | 4(6) | 2(0) | 0(0) | 9(10) |
| Tropical storms (>34 knots, <64 knots) | 10(8) | 7(8) | 9(5) | 3(5) | 29(26) |
| Hurricanes/typhoons (>64 knots) | 14(15) | 4(10) | 6(5) | 3(1) | 27(31) |
| Total | 27*(27) | 15(24) | 17(10) | 6(6) | 65(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 2003 season.
* Includes tropical storm Haiyan added to the database at the end of the season.
3. Summary of all northern hemisphere storms
3.1 North-west Pacific basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | |
|---|---|---|---|---|---|---|
| Possibly verified | 210 | 162 | 125 | 91 | 63 | 42 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 98 |
| Mean DX | 3 | -16 | -29 | -48 | -127 | -136 |
| Mean DY | -7 | 4 | 1 | -35 | -64 | -69 |
| Mean AT | -14 | -30 | -24 | -48 | -131 | -207 |
| Mean CT | -4 | -2 | -29 | -53 | -59 | -40 |
| Mean skill (%) | ***** | 27 | 51 | 54 | ***** | ***** |
| 2006 skill (%) | ***** | 20 | 36 | 44 | ***** | ***** |
| Mean DPE | 70 | 136 | 201 | 276 | 404 | 517 |
| * 2006 DPE | 68 | 116 | 206 | 305 | 424 | 553 |
| RT DPE # | 63 | 133 | 199 | 279 | 407 | 513 |
| Intensity skill (%) | ***** | 31 | 21 | 19 | 14 | -2 |
* DPE for all north-west Pacific storms in 2006 season
# DPE in 2007 calculated using real time observations as opposed
to best track.
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
Although storm and typhoon counts were similar to 2006, the total number of forecasts verified was considerably lower indicating that storm tracks were shorter than in 2006. Track forecast errors were lower than last season's values at all lead times except T+24. Skill against CLIPER was higher than 2006 at all lead times. There was a small slow and left-of-track bias in forecasts overall. The intensity tendency skill was 20% overall.
3.2 North-east Pacific basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | |
|---|---|---|---|---|---|---|
| Possibly verified | 82 | 60 | 41 | 27 | 16 | 9 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 94 | 67 |
| Mean DX | 5 | -27 | -64 | -104 | -168 | -231 |
| Mean DY | -7 | 27 | 13 | -48 | -200 | -161 |
| Mean AT | -9 | -10 | -16 | 26 | -76 | 114 |
| Mean CT | 6 | 5 | -40 | -85 | -226 | -180 |
| Mean skill (%) | ***** | -4 | 23 | 41 | ***** | ***** |
| 2006 skill (%) | ***** | 17 | 28 | 26 | ***** | ***** |
| Mean DPE | 56 | 112 | 192 | 246 | 453 | 397 |
| * 2006 DPE | 53 | 101 | 188 | 264 | 365 | 428 |
| RT DPE # | 52 | 111 | 191 | 248 | 453 | 399 |
| Intensity skill (%) | ***** | 57 | 41 | 41 | 73 | 0 |
* DPE for all north-east Pacific storms in 2006 season
# DPE in 2007 calculated using real time observations as opposed
to best track.
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 2007 season in this basin was the quietest for 30 years. The total of just four hurricanes was the lowest since 1977, which also saw four. Track forecast errors were near to last season’s values at most lead times. Skill scores against CLIPER were low at short lead times, but high at longer lead times. There was a left-of-track bias in forecasts at longer lead times. The intensity tendency skill was 49% overall.
3.3 North Atlantic basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | |
|---|---|---|---|---|---|---|
| Possibly verified | 69 | 41 | 27 | 18 | 10 | 6 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 100 |
| Mean DX | 12 | 26 | 20 | -47 | -124 | -64 |
| Mean DY | 17 | 31 | 81 | 112 | 48 | 209 |
| Mean AT | -6 | -32 | -13 | 47 | 8 | 105 |
| Mean CT | 16 | 38 | 86 | 58 | 79 | 201 |
| Mean skill (%) | ***** | 35 | 54 | 68 | ***** | ***** |
| 2006 skill (%) | ***** | 30 | 48 | 48 | ***** | ***** |
| Mean DPE | 63 | 102 | 151 | 179 | 228 | 265 |
| * 2006 DPE | 50 | 112 | 213 | 322 | 428 | 529 |
| RT DPE # | 58 | 104 | 155 | 184 | 230 | 264 |
| Intensity skill (%) | ***** | 2 | -19 | -33 | 40 | -33 |
* DPE for all North Atlantic storms in 2006 season
# DPE in 2007 calculated using real time observations as opposed
to best track.
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
Despite the above average number of storms and two category 5 hurricanes, activity in 2007 was low by other measures due to the short-lived nature of the majority of storms. Track forecast errors were the lowest ever achieved in this basin. Skill against CLIPER was a very high. Track biases were generally small. Despite excellent track forecasts, intensity tendency skill was a poor
-8% overall.
3.4 North Indian Basin Storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | |
|---|---|---|---|---|---|---|
| Possibly verified | 40 | 26 | 17 | 11 | 7 | 2 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 100 |
| Mean DX | -8 | -26 | -30 | -50 | -106 | 64 |
| Mean DY | -3 | -20 | -98 | -254 | -351 | -145 |
| Mean AT | 8 | -11 | -38 | -185 | -317 | -106 |
| Mean CT | 5 | 26 | -29 | -91 | -119 | 3 |
| Mean skill (%) | ***** | 7 | 38 | 43 | ***** | ***** |
| 2006 skill (%) | ***** | 15 | 29 | 20 | ***** | ***** |
| Mean DPE | 77 | 147 | 203 | 319 | 509 | 387 |
| * 2006 DPE | 85 | 154 | 245 | 435 | 944 | 897 |
| RT DPE # | 80 | 144 | 207 | 334 | 525 | 393 |
| Intensity skill (%) | ***** | 38 | 41 | 9 | 60 | -100 |
* DPE for all North Indian storms in 2006 season
# DPE in 2007 calculated using real time observations as opposed
to best track.
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
The North Indian basin was the only northern hemisphere basin to see higher than usual activity in 2007. There were three significant cyclones – Gonu, Yemyin and Sidr. Track forecast errors were generally low. Only at T+96 were errors on the high side. Skill scores against CLIPER were positive. There was a slow bias in forecasts. The intensity tendency skill was 31% overall.
3.5 Combined statistics for whole northern hemisphere
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | |
|---|---|---|---|---|---|---|
| Possibly verified | 401 | 289 | 210 | 147 | 96 | 59 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 99 | 93 |
| Mean DX | 4 | -13 | -30 | -58 | -132 | -131 |
| Mean DY | -2 | 11 | 5 | -36 | -95 | -52 |
| Mean AT | -10 | -24 | -22 | -33 | -121 | -135 |
| Mean CT | 2 | 8 | -16 | -48 | -75 | -27 |
| Mean skill (%) | ***** | 22 | 46 | 54 | ***** | ***** |
| 2006 skill (%) | ***** | 21 | 37 | 43 | ***** | ***** |
| Mean DPE | 66 | 127 | 193 | 262 | 401 | 472 |
| * 2006 DPE | 62 | 113 | 204 | 304 | 424 | 538 |
| RT DPE # | 62 | 125 | 193 | 266 | 404 | 469 |
| Intensity skill (%) | ***** | 33 | 18 | 16 | 29 | -9 |
* DPE for all northern hemisphere storms in 2006 season
# DPE in 2007 calculated using real time observations as opposed
to best track.
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
By several measures activity in the northern hemisphere in 2007 was the lowest for 30 years. Track forecast errors were the lowest ever achieved at all lead times except T+24. Skill scores against CLIPER were the highest ever achieved. There was a small slow bias in forecasts. Cross-track errors were generally small. Detection percentages were fairly high with only a few T+96 and T+120 forecasts being missed. Differences between forecast positional errors derived using real time observations and those derived using best track observations were generally small. The intensity tendency skill was 22% overall. This compares to a value of 24% for the Northern Hemisphere 2006 season.
Further tropical cyclone information
The Met Office pages contain information on tropical cyclone forecasting at the Met Office. Monthly bulletins of tropical cyclone activity and forecasts are held together with observed and forecast track information of recent storms, track prediction error statistics, lists of names, images, movies and photographs and details of advances made in tropical cyclone track prediction at the Met Office. Real time tropical cyclone forecast guidance is also now available.
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 e-mail the Met Office. Past seasonal reports can also be found on-line under the Forecast Verification section of the tropical cyclone pages on the Met Office web site.
________________________
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.
Best track observations for the northern hemisphere were obtained from the National Hurricane Center, Miami, RSMC Tokyo – Typhoon Center and the Joint Typhoon Warning Center, Hawaii.
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