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JP1.29 EFFECTIVE USE OF
REGIONAL ENSEMBLE DATA IN FORECASTING
ByRichard H. Grumm
National Weather Service State College PA 16803
and Robert Hart
The Pennsylvania State University
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Introduction• Two winter storms
– one well forecast– one poorly forecast
• Why we need ensembles– one model many potential initial states!– Physics differences and potential impact on forecasts– avoid model of the day problem: pick a single solution
which may or may not be correct (binary YES/NO)– deal with uncertainty and probabilities– Mesoscale ensembles help to deal with:
• Uncertainties in data• Uncertainties in data verse resolution• Uncertainties in physics require display strategies
• Need to consider how to display these data
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Ensemble Strategies• Lagged Average Forecasts (LAF)
– dProg/dt feature in AWIPS– simple uses 1 model and different runs– last member considered most skillful
• Single model with perturbed members– all members should be of similar skill– all initialized at same time (better than LAF)– NCEP MRF-ENS system
• Ensemble of many models– NCEP SREF System
• Eta in SREF not current operational Eta• RSM in SREF physics are older than AVN/MRF
– “super ensemble”
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Ensembles with different initial conditions
Forecast LengthForecasts Initialized at most recent data time
Envelope of solutions
at single time
Solution
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Case 1: 3-4 Dec 2000 Winter Storm Case
• 22-km Eta had more QPF and farther west then operational AVN
• There was a trend in the models (dProg/dt)
• SREF data – showed uncertainty– Eta members clustered different than RSM
members– allowed forecasters to view uncertainty– provided visual cues on high and low probability
areas for Winter Storm Warning (WSW) products.
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0000 UTC 2 December
• Cyclone Track difference• 0.20 QPF forecasts
– advisory snow over most of region
• 0.50 QPF forecasts– warning snow over most of region
• T850 – will it snow or rain?
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1200 UTC 2 December
• Cyclone Track difference• 0.20 QPF forecasts
– advisory snow over most of region
• 0.50 QPF forecasts– warning snow over most of region
• T850 – will it snow or rain?
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3-4 Dec 2000 summary
• SREFs – provided useful information– were better than any single model when
considering high probability outcomes– QPF
• good at low amounts• not so good high amounts (>0.70)
– Thermal character• good for probabilities snow/ZR/Rain
– Not a fluke as SREF’s did very well on 30 Dec 2000 Storm
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SREFs Ain’t Perfect
• Recent event 06-07 January failure– in defense AVN/Eta were slow to adjust– SREF never did adjust– heavy snow low probability in PA
• Out of the “envelope solution”– the system is not perfect– NCEP aware and working on problems
• Never rely on SREF alone– blend more skillful AVN/Eta– use AVN/Eta to evaluate strength of system– ensemble consensus waters down anomalous aspects of
systems.
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First cut at the snowfallcourtesy of the Pennsylvania State
University
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What verifiedan anomalously deep cyclone and easterly jet
Figure 2: Eta 00-hour forecast of MSLP and 700 hPa heights initialized at 0000 UTC 7 January 2002. Anomalies are shown in the color bar to the left.
Figure 3: As in Figure 2 except 850 hPa winds valid at 0000 UTC 7 January 2002.
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SREF-enhanced by Eta/AVN
Figure 8: SREF and operational Eta and AVN MSLP forecasts. Eta is in Blue and AVN is in red. Forecasts from 2100 UTC 5 January SREF and 0000 UTC Eta and AVN.
Figure 7: 2100 UTC 5 January 2002 SREF and operational Eta (BLUE) and AVN (RED) initialized at 0000 UTC 6 January 2002. Forecast show probability and consensus and spaghetti plots
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More QPF
Figure 9. As in Figure 7 except 0900 UTC SREF and 1200 UTC 6 January 2002 operational Eta and AVN forecasts of 0.40 and 0.60 inches of QPF for the 24-hour period ending 09Z 07 January 2002. Note Eta is blue and AVN is red.
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SREF Conclusions
– show great promise on some events– did well 3-4 Dec and 30 Dec 2000 indicating
• areas of heavy snow• areas rain/snow• cyclone track, out performing 22-km Eta
– did poorly 6-7 Jan 2002 • QPF too far east, not enough to west• model divergence smoothed out how anomalous system was• operational models caught on and showed potential faster
– SREF system is not perfect (but getting better!)• RSM is older code will be updated Spring 02• Eta is older version than 12km Eta to be updated Jan 02
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Consider this• NCEP SREF data
– a valuable forecast tool and resource– can be used to exploit all visual concepts with ensembles
• Consider this: One detailed mesoscale model (the model of the day) will allow the user to make highly specific and detailed inaccurate forecast.