1. Introduction
1. Introduction
A summary of tropical cyclone activity in the Northern Hemisphere for the 2002 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 the first part of the season was 0.83°x 0.55°x 30 levels. This is equivalent to a horizontal resolution of 93km x 62km at the equator. The number of levels was increased to 38 from early August 2002 when the 'New Dynamics' package of changes was introduced.
Click here for details of the TC verification technique and an explanation of error statistics and their sign conventions.
Advisory positions from 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) 6(4) 4(4) 2(2) 0(0) 12(10) Tropical Storms (>34 knots, <64 knots) 11(8) 7(7) 8(6) 5(4) 31(25) Hurricanes/typhoons (>64 knots) 16(20) 7(8) 4(9) 0(1) 27(38) Total 33(32) 18(19) 14(17) 5(5) 70(73)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
The number in brackets indicates the figure for the 2001 season.
3. SUMMARY OF ALL NORTHERN HEMISPHERE STORMS
3.1 North-West Pacific Basin Storms
Table of Mean Error Statistics
T+0 T+24 T+48 T+72 T+96 T+120 Possibly Verified 299 245 203 165 134 107 Detection Rate (%) 100 100 99 99 96 93 Mean DX -2 -3 -40 -92 -128 -149 Mean DY -1 26 20 -9 -46 -30 Mean AT -8 -18 -17 -38 -47 -46 Mean CT -6 10 9 21 14 11 Mean Skill (%) ***** 17 43 45 ***** ***** 2001 Skill (%) ***** 15 29 38 ***** ***** Mean DPE 49 139 231 354 453 560 * 2001 DPE 41 132 253 357 523 763 RT DPE # 43 141 234 355 455 561 Intensity skill (%) ***** 13 6 21 16 22
* DPE for all North-West Pacific storms in 2001 season
# DPE in 2002 calculated using real time observations as opposed to best track.
A 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.
Activity in this basin was slightly greater than last year in terms of
verifiable forecasts. There were four less typhoons than in the 2001 season,
but three more tropical storms. Track forecast errors were near to last
season's values up to T+72, but lower at longer forecast ranges. Skill against
CLIPER was higher than in 2001. There were very slight slow and right-of-track
biases overall. The intensity tendency skill was 15% 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 Possibly Verified 123 94 69 49 34 22 Detection Rate (%) 100 100 99 96 85 82 Mean DX 13 56 92 77 135 102 Mean DY 0 26 43 5 -14 -44 Mean AT -18 -48 -34 -41 -101 -176 Mean CT 3 26 55 66 69 63 Mean Skill (%) ***** 19 49 64 ***** ***** 2001 Skill (%) ***** 14 22 15 ***** ***** Mean DPE 45 132 212 261 314 471 * 2001 DPE 35 108 190 290 386 360 RT DPE # 39 131 211 264 312 466 Intensity skill (%) ***** 36 41 45 36 25* DPE for all North-East Pacific storms in 2001 season
# DPE in 2002 calculated using real time observations as opposed to best track.
A 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.
2002 saw very similar levels of activity to 2001 in this basin. Track forecast
errors were higher than last season at T+24, 48 and 120, but lower at T+72 and
96. The model's skill over CLIPER was much higher this season than in 2001.
There was a slight slow and right-of-track bias in forecasts and the detection
rate was better than in 2001, although still below 90% at longer forecast
ranges. The intensity tendency skill was 38% overall.
3.3 North Atlantic Basin Storms
Table of Mean Error Statistics
T+0 T+24 T+48 T+72 T+96 T+120 Possibly Verified 122 99 81 70 61 52 Detection Rate (%) 100 100 93 87 74 69 Mean DX 0 0 25 68 156 218 Mean DY 2 17 30 38 109 116 Mean AT -11 -29 -45 -69 -179 -253 Mean CT -3 9 51 45 60 130 Mean Skill (%) ***** 39 51 55 ***** ***** 2001 Skill (%) ***** 37 46 61 ***** ***** Mean DPE 41 127 226 314 475 703 * 2001 DPE 34 134 256 352 366 473 RT DPE # 38 129 230 318 477 705 Intensity skill (%) ***** 17 12 34 38 37* DPE for all North Atlantic storms in 2001 season
# DPE in 2002 calculated using real time observations as opposed to best track.
A 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.
There were two more tropical storms in 2002 than in 2001, but five less
hurricanes. Tropical cyclones tended to be longer lived resulting in a higher
number of verifiable longer range forecasts. Track forecast errors were lower
than last season at shorter forecast ranges, but not at T+96 and T+120. Skill
against CLIPER was at similar levels to last season. There was a slight slow
and right-of-track bias in forecasts. The intensity tendency skill was 24%
overall.
3.4 North Indian Basin Storms
Table of Mean Error Statistics
T+0 T+24 T+48 T+72 T+96 T+120 Possibly Verified 24 14 6 2 0 0 Detection Rate (%) 100 100 100 100 ***** ***** Mean DX 22 98 288 619 ***** ***** Mean DY -5 -53 -143 67 ***** ***** Mean AT -18 -131 -325 -501 ***** ***** Mean CT 21 84 94 339 ***** ***** Mean Skill (%) ***** -70 -54 ***** ***** ***** 2001 Skill (%) ***** -21 -31 -26 ***** ***** Mean DPE 83 215 376 625 ***** ***** * 2001 DPE 48 142 275 351 358 265 RT DPE # 81 208 381 636 ***** ***** Intensity skill (%) ***** 14 67 0 ***** ****** DPE for all North Indian storms in 2001 season
# DPE in 2002 calculated using real time observations as opposed to best track.
A 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.
Total cyclone numbers were the same this season as in 2001, but there was none
of hurricane strength and none persisted long enough to produce verifiable T+96
and T+120 forecasts. Track forecast errors were high and skill scores low
compared to last season. There was a slow and right-of-track bias in forecasts.
The intensity tendency skill was 27% overall. The number of verifiable
forecasts was low in this basin which will reduce the statistical significance
of these results.
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 Possibly Verified 568 452 359 286 229 181 Detection Rate (%) 100 100 97 95 89 85 Mean DX 3 13 5 -22 -27 -34 Mean DY -1 22 24 5 -7 2 Mean AT -11 -30 -32 -49 -84 -110 Mean CT -2 16 28 37 32 45 Mean Skill (%) ***** 22 46 50 ***** ***** 2001 Skill (%) ***** 21 32 42 ***** ***** Mean DPE 47 137 228 331 438 583 * 2001 DPE 38 128 243 345 468 629 RT DPE # 43 139 231 333 440 584 Intensity skill (%) ***** 19 15 28 24 26
* DPE for all Northern Hemisphere storms in 2001 season
# DPE in 2002 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.
Cross track errors for the whole northern hemisphere.
Along track errors for the whole northern hemisphere.
Track forecast errors for the whole northern hemisphere in 2002 were lower than
those in 2001 except at T+24. Overall, errors were lower than in any year
except 1999. Skill scores against CLIPER were higher than in 2001 and at T+48
and T+72 were the highest ever recorded. There was a slow bias of magnitude
less than in 2001 and last year's left-of-track bias was reversed into a small
right-of-track bias. Detection percentages were better than in 2001, but were
below 90% at T+96 and T+120. There was very little difference between forecast
positional errors derived using real time observations and those derived using
best track observations. The intensity tendency skill was 21% overall. This
compares to a value of 11% for the Northern Hemisphere 2001 season from
September (when intensity tendency was first verified) to December.
Further Tropical Cyclone information
The Met Office pages on the World Wide Web 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.
The Met Office web site is:-
http://www.metoffice.gov.uk
Tropical Cyclone information can be found on the home page under the "World
Weather" 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 at the Met Office email tropical_cyclones@metoffice.gov.uk.
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 and the Joint Typhoon Warning Center, Hawaii.
