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The Met Office verification scheme identifies the centre
of a tropical cyclone in the model by locating a local maximum
of relative vorticity at 850hPa. This local maximum is also
used in the verification of intensity. A fourth order interpolation
is used to locate the centre accurately. Advisory data received
in real time from JTWC Pearl Harbour, NHC Miami, CPHC Honolulu,
RSMC La Réunion, TCWC Darwin and RSMC Fiji are used as
verifying observations of storm location.
The terms used in the statistics tables are explained below.
- Possibly verified - Number of forecasts fulfilling requirements
1, 2 and4 below
- Detection Rate - Percentage of possibly-verified forecasts
which also fulfil requirement 3 below
- Mean DX - Mean of positional errors in the East-West direction
- Mean DY - Mean of positional errors in the North-South direction
- Mean AT - Mean of positional errors in the Along Track direction
- Mean CT - Mean of positional errors in the Cross Track direction
- Track skill - Percentage skill of model forecast position
against CLIPER
- Mean DPE - Mean of direct positional errors
- Intensity skill - Percentage skill of model in predicting
strengthening or weakening (positive if better than chance).
All errors are measured in kilometres except where indicated.
- Observed maximum wind at least 31 knots at the verifying
time.
- Forecast tropical cyclone centre equatorwards of 45 degrees.
- Forecast tropical cyclone centre relative vorticity above
a critical limit for verification (0.70x10-4 per
second).
- Observation within 6 hours of verifying time present for
use in verification.
- Track skill is defined as:- (CLIPER DPE - Model DPE) /
CLIPER DPE x 100%
- Positive skill indicates the model forecast is better than
CLIPER
- Negative skill indicates the CLIPER forecast is better than
the model
See below for a diagrammatic explanation of other error statistics
and their sign conventions, for full details see the forecast
tracks verification method.
Diagrammatic explanation of forecast errors
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OB1-3
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Observed positions |
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FC
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: |
Forecast position: verifying against observation OB2 |
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DPE
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: |
Direct positional error |
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DX
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: |
Error in the East-West direction |
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DY
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: |
Error in the North-South direction |
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AT
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: |
Error in the Along Track direction |
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CT
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: |
Error in the Cross Track direction |
Sign conventions
- DPE values are always positive
- DX errors are positive if the forecast position lies eastwards
of the observed position
- DY errors are positive if the forecast position lies polewards
of the observed position
- AT errors are positive if the forecast position lies ahead
of the observed position along the tropical cyclone track
- CT errors are positive if the forecast position lies right
of the observed track in the northern hemisphere and left
of the observed track in the southern hemisphere.
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