Tropical cyclone forecast verification - northern hemisphere 2008
Northern hemisphere 2008
1. Introduction
A summary of tropical cyclone activity in the Northern Hemisphere for the 2008 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+144, although statistics are only presented at 24-hour intervals in this report. The global model resolution in operation during 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.
An explanation of error statistics and their sign conventions.
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
| NWP | NEP | NAT | NI | TOTAL | |
|---|---|---|---|---|---|
| Tropical depressions (<34 knots) | 3(3) | 2(4) | 1(2) | 0(0) | 6(9) |
| Tropical storms (>34 knots, <64 knots) | 13(10) | 10(7) | 8(9) | 6(3) | 37(29) |
| Hurricanes/typhoons (>64 knots) | 12(14) | 7(4) | 8(6) | 1(3) | 28(27) |
| Total | 28(27) | 19(15) | 17(17) | 6(6) | 71(66) |
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 2007 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 | T+144 | |
|---|---|---|---|---|---|---|---|
| Possibly verified | 197 | 146 | 112 | 81 | 55 | 34 | 19 |
| Detection rate (%) | 100 | 100 | 100 | 99 | 96 | 97 | 100 |
| Mean DX | 7 | -4 | -25 | -3 | 23 | 124 | 168 |
| Mean DY | 2 | -12 | -36 | -58 | -45 | -5 | 70 |
| Mean AT | -4 | -29 | -83 | -144 | -185 | -122 | -53 |
| Mean CT | -1 | -2 | -2 | -5 | 46 | 146 | 147 |
| Mean skill (%) | ***** | 33 | 47 | 43 | ***** | ***** | ***** |
| 2007 skill (%) | ***** | 27 | 51 | 54 | ***** | ***** | ***** |
| Mean DPE | 62 | 110 | 190 | 313 | 473 | 689 | 589 |
| * 2007 DPE | 70 | 136 | 201 | 276 | 404 | 517 | ***** |
| Intensity skill (%) | ***** | 34 | 43 | 40 | 40 | -9 | 26 |
* DPE for all north-west Pacific storms in 2007 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
There were slightly fewer typhoons, but more tropical storms than in 2007 in this region. Total verifiable forecasts were slightly lower than 2007. Track forecast errors were the lowest ever recorded at T+24 and T+48, but at longer lead times were higher than last year and, in the case of T+120, the highest since 2001. Skill against CLIPER was higher than 2007 at T+24, but lower at other lead times. However, values were still high when set against the longer term record. There was a small slow bias in forecasts overall. The intensity tendency skill was 32% overall.
3.2 North-east Pacific basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | |
|---|---|---|---|---|---|---|---|
| Possibly verified | 129 | 95 | 68 | 47 | 32 | 18 | 7 |
| Detection rate (%) | 100 | 100 | 99 | 94 | 84 | 61 | 43 |
| Mean DX | 3 | -47 | -92 | -114 | -120 | -55 | -128 |
| Mean DY | -14 | 8 | 2 | -38 | -40 | -17 | 333 |
| Mean AT | -8 | 27 | 54 | 46 | 111 | 98 | 265 |
| Mean CT | -8 | -6 | -34 | -68 | -56 | -42 | -191 |
| Mean skill (%) | ***** | 21 | 31 | 23 | ***** | ***** | ***** |
| 2007 skill (%) | ***** | -4 | 23 | 41 | ***** | ***** | ***** |
| Mean DPE | 46 | 107 | 186 | 267 | 314 | 318 | 370 |
| * 2007 DPE | 56 | 112 | 192 | 246 | 453 | 397 | ***** |
| Intensity skill (%) | ***** | 37 | 40 | 36 | 26 | 64 | 33 |
* DPE for all north-east Pacific storms in 2006 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 2008 season in this basin was considerably more active than 2007, which saw record low activity. Track forecast errors were near to last season's values at lead times up to T+72, but were the lowest ever recorded at T+96 and T+120. Skill scores against CLIPER were variable with improvements at T+24 and T+48, but a worsening at T+72 compared to 2007. There was a slight fast and left-of-track bias in forecasts. The intensity tendency skill was 40% overall.
3.3 North Atlantic basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | |
|---|---|---|---|---|---|---|---|
| Possibly verified | 169 | 135 | 110 | 88 | 71 | 60 | 47 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 97 | 89 |
| Mean DX | 10 | -9 | -51 | -92 | -104 | -163 | -137 |
| Mean DY | 12 | -2 | -16 | -40 | -48 | -73 | -84 |
| Mean AT | -3 | -33 | -66 | -109 | -120 | -153 | -229 |
| Mean CT | 11 | 9 | 4 | -17 | -39 | -123 | -69 |
| Mean skill (%) | ***** | 53 | 61 | 60 | ***** | ***** | ***** |
| 2007 skill (%) | ***** | 35 | 54 | 68 | ***** | ***** | ***** |
| Mean DPE | 50 | 95 | 186 | 278 | 374 | 511 | 619 |
| * 2007 DPE | 63 | 102 | 151 | 179 | 228 | 265 | ***** |
| Intensity skill (%) | ***** | 45 | 42 | 43 | 35 | 48 | 19 |
* DPE for all North Atlantic storms in 2007 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
2008 saw much greater activity than both 2006 and 2007. If the exceptionally low tracks errors of 2007 are discounted due to the short-lived nature of the storms, the 2008 errors were the lowest recorded in this region at lead times up to T+72 and close to the lowest at longer lead times. Skill against CLIPER was the highest ever recorded at T+24 and T+48, even with 2007 included. There was a slow bias in forecast tracks. The intensity tendency skill score was 42% overall.
3.4 North Indian Basin Storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | |
|---|---|---|---|---|---|---|---|
| Possibly verified | 30 | 17 | 10 | 7 | 5 | 3 | 1 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| Mean DX | 0 | 10 | -26 | -77 | -190 | -479 | 85 |
| Mean DY | -8 | -61 | -158 | -195 | -178 | -363 | -334 |
| Mean AT | -7 | -70 | -91 | -167 | -237 | -585 | -196 |
| Mean CT | 16 | 9 | 66 | 151 | 197 | -57 | 287 |
| Mean skill (%) | ***** | 39 | 52 | 66 | ***** | ***** | ***** |
| 2007 skill (%) | ***** | 7 | 38 | 43 | ***** | ***** | ***** |
| Mean DPE | 67 | 116 | 227 | 336 | 419 | 708 | 348 |
| * 2007 DPE | 77 | 147 | 203 | 319 | 509 | 387 | ***** |
| Intensity skill (%) | ***** | 18 | 20 | 14 | 20 | 33 | 100 |
* DPE for all North Indian storms in 2007 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 slightly lower than in 2007 with less major cyclones, but more tropical storms. Track forecast errors remained low at short lead times. The T+24 errors were almost the lowest ever (just beaten by 1999). Skill scores against CLIPER were the highest or very nearly the highest ever recorded. There was a slow bias in forecasts. The intensity tendency skill was 29% overall.
3.5 Combined statistics for whole northern hemisphere
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | |
|---|---|---|---|---|---|---|---|
| Possibly verified | 525 | 393 | 300 | 223 | 163 | 115 | 74 |
| Detection rate (%) | 100 | 100 | 100 | 98 | 96 | 91 | 88 |
| Mean DX | 7 | -16 | -49 | -63 | -67 | -71 | -44 |
| Mean DY | 1 | -6 | -24 | -51 | -50 | -54 | -23 |
| Mean AT | -5 | -19 | -46 | -93 | -106 | -129 | -155 |
| Mean CT | 2 | 1 | -5 | -17 | -5 | -28 | -6 |
| Mean skill (%) | ***** | 39 | 51 | 50 | ***** | ***** | ***** |
| 2007 skill (%) | ***** | 22 | 46 | 54 | ***** | ***** | ***** |
| Mean DPE | 55 | 104 | 189 | 291 | 399 | 552 | 594 |
| * 2007 DPE | 66 | 127 | 193 | 262 | 401 | 472 | ***** |
| Intensity skill (%) | ***** | 38 | 41 | 40 | 35 | 31 | 23 |
* DPE for all northern hemisphere storms in 2007 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.
Activity in the northern hemisphere in 2008 was considerably higher than 2007, which saw record low activity. Track forecast errors at T+24, T+48 and T+96 were the lowest ever achieved and at T+72 they were the second lowest (2007 was the lowest). Skill scores against CLIPER were the highest ever achieved at T+24 and T+48. The results continue the recent trend in a steady reduction in errors and increase in skill scores. There was a small slow bias in forecasts. Cross-track errors were generally small and lower than last season. Detection percentages were a little lower than last season with a small percentage of forecasts being missed from T+72 onwards. The intensity tendency skill was 37% overall, which is the highest value recorded since intensity verification started in 2001.
Further tropical cyclone information
The Met Office pages on the World Wide Web contain information on tropical cyclone forecasting at the Met Office. The Met Office pages on the World Wide Web 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 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 email the Met Office.
________________________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.
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