probabilistic forecasts of (severe) thunderstorms for the purpose of issuing a weather alarm maurice...
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Probabilistic forecasts of (severe) thunderstorms for the purpose of
issuing a weather alarm
Maurice Schmeits, Kees Kok, Daan Vogelezang and Rudolf van Westrhenen
KNMI
IUGG 2007
Outline
Introduction: Weather alarm for severe thunderstorms
Method: Model output statistics (MOS) Data used in MOS system for (severe) thunderstorms Illustration of statistical method Definitions of predictands Case (10 June 2007) Verification results Conclusions and outlook
IUGG 2007
Weather alarm for severe thunderstorms (I)
Weather alarm: if probability of ≥ 500 discharges/5 min./(50x50 km2) ≥ 90% in next 12 hours
One of the least predictable phenomena History (note: other criterion): many misses, only
a few hits and no false alarms Goal: decrease number of misses and increase
number of hits, while keeping number of false alarms low
Means: new objective probabilistic forecasting system
IUGG 2007
Model output statistics (MOS) Aim:
Features:
To determine a statistical relationship (mostly via regression) between a predictand (i.e. the occurrence of a thunderstorm in this case) and predictors from NWP model forecasts (and possibly from observations)
forecasts possible for predictands that are not available from direct model output
(reliable) probabilistic forecasts possible, even while using output from a single model run
separate regression equation for each forecast projection (correction of systematic model errors)
IUGG 2007
MOS system for (severe) thunderstorms
Ensemble of advectedradar data (0 to +6 h)
(Ensemble of advected)
lightning data (0 to +6 h)
Probability of thunderstorms (0 to +6 h/ +6 to +12 h)
In developing the LR model you need a 3/2-year long data archive
Archive:•2/3 part for development•1/3 part for verification
NWP model forecasts (0 to
+12 h)
Logistic regression (LR) model
Example of logistic regression equation
using only the first predictor (region M-MS; period: 15-21 UTC)
)....exp(1
1
110 ppXaXaaP
(event) :regression Logistic
Pro
bab
ility
of
thu
nd
ers
torm
s
Fraction of ensemble with no. of flashes ≥ 4 [SAFIR 1440 +0620]
binary predictandlogistic curve
IUGG 2007
Weather alarm for severe thunderstorms (II)
Weather alarm: if probability of ≥ 500 discharges/5 min./(50x50 km2) ≥ 90% in next 12 hours
2000-2005 ‘climatology’ on the basis of this criterion: only twice a year (between 30 April and 15 September)
Statistical methods are not capable of handling such rare events.
Therefore, other predictand definitions have been used.
IUGG 2007
Predictand definitions
Predictand for thunderstorms: Probability of > 1 lightning discharge in a 6h period (00-06, 03-09, 06-12, 09-15, 12-18, 15-21, 18-00 or 21-03 UTC) in a 90x80 km2 region.
Predictands for severe thunderstorms: Conditional probability of ≥ X, ≥ Y or ≥ Z discharges/ 5 min. in a 6h period in a 90x80 km2 region with condition > 1 discharge in the same 6h period in the same region. Here X =50 (all 6-h periods); Y = 100 and Z =200 (12-18, 15-21 and 18-00 UTC).
IUGG 2007
Case 10 June 2007 (15-21 UTC; +6 to +12 h)
09 UTC run (based on H 1006 and EC 0912)
‘Clim.’ prob. of thunderstorms: 5-19 % ‘Clim.’ cond. prob. of severe thunderstorms (≥ 200 discharges/5 min.): 5 % (abs. prob.: < 1 %)
Probability of thunderstorms
Cond. prob. of severe thunderstorms(≥ 50 discharges/ 5 min.) (≥ 200 discharges/
5 min.)
Maximum 5-min. lightning intensity
IUGG 2007
Verification results 2006 (Probability of > 1 discharge)
Bri
er
skill
sco
re
(%)
0 to +6 h +6 to +12 h
Bri
er
skill
sco
re (
%)
Time (UTC) Time (UTC)
Bri
er
skill
sco
re
(%)
IUGG 2007
Reliability diagrams (’05-’06;15-21 UTC; 0 to +6h)
Ob
serv
ed
fr
eq
uen
cy
Forecast probability
Ob
serv
ed
fr
eq
uen
cy
Forecast probability
≥ 50 discharges/ 5 min. ≥ 100 discharges/ 5 min.
IUGG 2007
Conclusions and outlook
Probabilistic forecasts for thunderstorms (> 1 discharge) are skilful with respect to the 2000-2004 climatology.
Probabilistic forecasts for severe thunderstorms (≥ 50/ ≥ 100 discharges per 5 min.) are reasonably skilful with respect to the 2000-2004 climatology.
The system has been pre-operational at KNMI since Spring of 2006 and will be fully operational later this year.
It is expected that this system will help the forecasters to decide whether a weather alarm for severe thunderstorms should be issued.