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Composite Products And Nowcast Decision Support for the Beijing 2008
FDP
John BallyDavid Scurrah
Beth EbertDebin Su
Goals
Create consensus nowcast productsSummarise FDP output for forecastersDeliver guidance directly to VIPSTest ability to manually add skill
Rainfall products
Generate probability of exceeding rainfall R
Logistic regression against ‘07 trial dataP(exceeding R) = 1 / (1 + (ec1 * QPFc2)) Generate PoP 2 Probability of exceeding warning thresholdsCombine systems by simple averaging
Probability of Precipitation… 2mm
Example ensemble PoP2 including STEPS, GRAPES, MAPLE, SWIRLS and the BJANC
No calibrationPerformance still good Easily adapted to other thresholds
Rainstorm warnings
First... measure accumulation from STEPS
Use 3hr accum, or 2 hr accum + 1hr forecast
Aim for 1 hr leadtime Calculate R = threshold – 2hr
accumulation Again use P(exceeding R) = 1 / (1 + (ec1 *
QPFc2)) Thresholds for guidance were 40
(80mm) in 3hr for orange (red) warnings
Rainstorm warnings
Example “orange” rainstorm consensus product, using STEPS, GRAPES, MAPLE, SWIRLS, BJANC and CARDS.
Contours of 20 and 50% chance of exceeding the threshold delivered to VIPS
Thunderstorm track error stats
Histogram of storm detections wrt forecast (normalizedspeed and direction). after Dance, Ebert, Scurrah
Standard deviation of velocity and direction errors (wrt forecast) was reasonably constant over forecast time (unlike most other measures). after Dance, Ebert, Scurrah
Mathematics
where (x, θ is a point in polar coordinates, t is forecast time, V is speed, σv is speed standard deviation, σ θ direction standard deviation. after Dance, Ebert, Scurrah
=
Probability density function that thunderstorm centre is over a point, based on Gaussian distribution in polar coordinates.
To compute strike probability from probability density function:integrate along 'trailing edge' for each point in region. after Dance, Ebert, Scurrah
Strike Probability
Use the THESPA (Dance, Ebert, Scurrah) algorithm to calculate Strike probability for each cell as Pn
Within a track set, each cell is independent:
Total Strike Prob = 1 - Πn (1- Pn)
Strike Probability
Auto product combined strike prob from TITAN, SWIRLS, CARDS
Tracks used different reflectivity thresholds
Manual product had choice of TITAN thresholds (35, 40, 45dB) and could include tracks based on NIWOT & VDRAS advice
Strike Probability
Manual vs Auto product for 60 forecasts
Manual products shows increased skill for these more difficult cases