1. Introduction
1. Introduction
A summary of tropical cyclone activity in the northern hemisphere for the 2001 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 present global model has a resolution of 0.83°x 0.55°x 30 levels. This is equivalent to a horizontal resolution of 93km x 62km at the equator.
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) 4(8) 4(2) 2(4) 0(0) 10(14) Tropical Storms (>34 knots, <64 knots) 8(10) 7(13) 6(6) 4(2) 25(31) Hurricanes/typhoons (>64 knots) 20(15) 8(6) 9(8) 1(2) 38(31) Total 32(33) 17(21) 17(18) 5(4) 71(76)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 2000 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 287 230 183 141 106 74 Detection Rate (%) 100 100 100 95 92 80 Mean DX -4 -21 -82 -159 -234 -397 Mean DY 2 23 33 4 -29 -135 Mean AT -3 -8 -14 -52 -117 -217 Mean CT -4 -16 -72 -140 -204 -255 Mean Skill (%) ***** 15 29 38 ***** ***** 2000 Skill (%) ***** 14 29 33 ***** ***** Mean DPE 41 132 253 357 523 763 * 2000 DPE 43 141 267 401 496 623 RT DPE # 37 132 253 359 528 768* DPE for all North-West Pacific storms in 2000 season
# DPE in 2001 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 greater than last year in terms of verifiable
forecasts. There were five more typhoons than in the 2000 season. Forecast
errors were slightly lower than in 2000 up to T+72, but slightly higher at
longer ranges. Skill against CLIPER was similar to 2000 at T+24 and T+48, but
higher at T+72. There were slight slow and left-of-track biases 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 117 87 60 40 28 17 Detection Rate (%) 100 100 98 88 75 71 Mean DX 6 -3 0 52 170 254 Mean DY -4 3 -44 -114 -158 -122 Mean AT -8 -18 -64 -145 -260 -275 Mean CT 3 0 -51 -79 -23 48 Mean Skill (%) ***** 14 22 15 ***** ***** 2000 Skill (%) ***** 25 33 39 ***** ***** Mean DPE 35 108 190 290 386 360 * 2000 DPE 31 107 184 250 342 420 RT DPE # 31 110 199 297 389 361* DPE for all North-East Pacific storms in 2000 season
# DPE in 2001 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.
There was less activity in this basin in 2001 than 2000, although there were more hurricanes. Forecast errors were near to or below last season's values at all times exceptT+72 and T+96. The model's skill over CLIPER was lower than in 2000 and 1999. There was a slight slow bias in forecasts and the detection rate was relatively poor at longer forecast ranges after very good performance in 2000.
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 132 99 71 48 36 26 Detection Rate (%) 100 100 99 94 86 88 Mean DX -3 -20 -33 -83 -59 -40 Mean DY 5 41 84 75 78 180 Mean AT -11 -41 -56 -48 15 -81 Mean CT -5 3 -11 -64 -35 -42 Mean Skill (%) ***** 37 46 61 ***** ***** 2000 Skill (%) ***** 18 38 43 ***** ***** Mean DPE 34 134 256 352 366 473 * 2000 DPE 22 144 247 349 412 520 RT DPE # 35 134 256 351 366 474* DPE for all North Atlantic storms in 2000 season
# DPE in 2001 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.
Activity was more or less the same in 2001 as in 2000. Forecast errors were fairly similar as well, with slightly lower errors at T+96 and T+120. However, skill against CLIPER was higher than in any previous season. There were no significant biases in forecasts.
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 10 8 6 4 Detection Rate (%) 100 100 100 75 83 75 Mean DX -5 27 -24 -85 -177 -169 Mean DY 8 44 132 156 145 -7 Mean AT -18 -23 5 23 8 -91 Mean CT 22 -21 -81 -89 -255 -155 Mean Skill (%) ***** -21 -31 -26 ***** ***** 2000 Skill (%) ***** -14 5 ***** ***** ***** Mean DPE 48 142 275 351 358 265 * 2000 DPE 61 145 251 498 ***** ***** RT DPE # 30 144 270 343 358 259* DPE for all North Indian storms in 2000 season
# DPE in 2001 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.
There was slightly more activity in 2001 than 2000 and some longer range forecasts were verified. Only at T+48 were forecast errors slightly higher than last season. However, skill scores against CLIPER were poorer than for many years. There was a left-of-track bias in forecasts at longer ranges.
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 560 430 324 237 176 121 Detection Rate (%) 100 100 99 93 88 80 Mean DX -2 -15 -54 -108 -143 -225 Mean DY 2 24 33 4 -19 -55 Mean AT -7 -18 -32 -64 -106 -188 Mean CT -2 -9 -55 -114 -147 -164 Mean Skill (%) ***** 21 32 42 ***** ***** 2000 Skill (%) ***** 17 32 37 ***** ***** Mean DPE 38 128 243 345 468 629 * 2000 DPE 38 134 244 357 440 551 RT DPE # 35 128 244 347 471 632* DPE for all Northern hemisphere storms in 2000 season
# DPE in 2001 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.
Forecast errors for the whole northern hemisphere in 2001 were similar to those for 2000 up to T+72 and slightly higher at T+96 and T+120. Overall, errors were lower than in any year prior to 1999. Skill scores against CLIPER were slightly higher than in 2000 at T+24 and T+72. There was a slight slow and left-of-track bias in forecasts overall. Detection percentages were not so good as in 2000, with only 80% of T+120 forecasts being verifiable.
There was very little difference between forecast positional errors derived using real time observations and those derived using best track observations.
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 News" 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
CLIPER models for various basins were supplied by Mr.S.Lord, NMC, Washington,
USA and Mr.C.Mauck, FNOC, Monterey, USA.
GrADS software used to produce track plotting charts was supplied by Mr.S.Lord
and Dr.M.Fiorino.
Best track observations for the northern hemisphere were obtained from the
National Hurricane Center, Miami and the Joint Typhoon Warning Center,
Hawaii.
