Transcript S E F R
Description and Preliminary Evaluation of the Expanded UW
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hort
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ange
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nsemble
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orecast System
Maj. Tony Eckel, USAF University of Washington Atmospheric Sciences Department Advisor: Prof. Cliff Mass March 2002
Overview
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UW SREF Methodology
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Generating New Initial Conditions
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Case Study
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Error Statistics
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Individual SREF Members
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Ensemble-Based Probability
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Summary
Ensemble Forecasting Theory
- Construct the initial state of the atmosphere with multiple, equally likely analyses, or initial conditions (ICs)
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From our point of view, truth is random sample from the pdf - Let all ICs evolve to build PDF at future time (i.e., a forecast pdf) - Error growth spreads out PDF as forecast lead time increases
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0.4
dnorm( , , ) 7 6 5 3 2 1 7.6946e-023 0 0 1 0 3 5 5 7 9 10 11 x 13 15 15 dnorm( , ) 5 0.2
4 3 2 1 17 7.4336e-007 19 20 20 0 0 1 0 3 5 5 7 9 10 11 13 .5
0.4
dnorm( , ) 0.2
17 1 0.000514093
19 20 0 20 4 3 2 0 1 0 3 5 5 7 9 10 11 13 15 15 17 48hr Forecast State 19 20 20
Limitations of EF
Difficult to consistently construct the “correct” analysis/forecast pdf.
Errors in mean and spread result from: 1) Model error 2) Choice of ICs 3) Under sampling due to limits of computer processing Result: EF products don’t always perform the way they should.
(especially a problem for SREF)
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0.4
7 dnorm( , , ) 6 5 3 2 1 7.6946e-023 0 0 0 1 3 5 5 7 9 10 x 11 13 Initial State 15 15 dnorm( , ) 7.4336e-007 17 20 19 20 0.2
5 4 3 2 1 0 0 0 1 3 5 5 7 .5
0.4
dnorm( , ) 0.2
11 13 15 15 0.000514093
17 20 19 20 0 4 3 2 1 0 0 1 3 5 5 7 10 9 11 15 17 20 19 20
gsp avn ngp uk cwb cmc eta cmc T cwb C eta ngp uk gsp UW SREF Methodology Overview avn Analysis pdf : Forecast pdf :
7 “independent” atmospheric analyses, the centroid, plus 7 “mirrored” ICs 15 divergent, “equally likely” solutions using the same primitive equation model, MM5
A point in phase space completely describes an instantaneous state of the atmosphere.
For a model, a point is the vector of values for all parameters (pres, temp, etc.) at all grid points at one time.
48hr true state 48hr forecast state (core) 48hr forecast state (perturbation)
Filling in the Holes of the IC Cloud
STEP 1: Calculate best guess for truth (the centroid) by averaging all analyses. STEP 2: Find error vector in model phase space between one analysis and the centroid by differencing all state variables over all grid points. STEP 3: Make a new IC by mirroring that error about the centroid.
cmcg C
1006
cmcg*
1004 1002 1000 998 996 ~1000 km 994 170°W 165°W 160°W 155°W 150°W 145°W 140°W 135°W
ICs: Analyses, Centroid, and Mirrors Strengths
• Good representation of analysis error • Perturbations to synoptic scale disturbances • Magnitude of perturbation(s) set by spread among analyses • Bigger spread Bigger perturbations • Dynamically conditioned ICs • Computationally affordable
Weaknesses
• Limited by number and quality of available analyses • May miss key features of analysis error • Analyses must be independent (i.e., dissimilar biases) • Calibration difficult; no stability since analyses may change techniques
CASE STUDY: Thanksgiving Day Non-Wind Event eta-MM5 Initialized: 00z, 21 Nov 2001
(Tuesday evening)
39h, valid 22 Nov 15z
(Thursday 7AM)
42h, valid 22 Nov 18z
(Thursday 10AM) Note:
This study used only 13 ensemble members since missing gasp grids.
ZCZC SEANPWSEA WWUS45 KSEA 212348 URGENT - WEATHER MESSAGE NATIONAL WEATHER SERVICE SEATTLE WA 344 PM PST WED NOV 21 2001 AN INTENSE LOW PRESSURE SYSTEM WILL MOVE ALONG THE NORTH WASHINGTON COAST EARLY MORNING THANKSGIVING DAY...AND MOVE INLAND OVER THE NORTH INTERIOR OF WESTERN WASHINGTON BY MIDDAY. STRONG SOUTH WINDS WILL DEVELOP ALONG THE COAST AFTER MIDNIGHT. AS THIS SYSTEM MOVES INLAND THANKSGIVING MORNING IT HAS THE
POTENTIAL
TO CAUSE HIGH WINDS ACROSS THE INTERIOR OF WESTERN WASHINGTON AFTER ABOUT 8 AM. ...HIGH WIND WATCH FOR THURSDAY MORNING THROUGH THURSDAY EVENING REMAINS IN EFFECT...
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36km Outer Domain 5.0
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48h 24h
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12h
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12km Inner Domain 5.0
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___RMSE of 36km Domain___
Initialization 00h cmcg 21 Nov 00z 00h cmcg* 21 Nov 00z 00h cent 21 Nov 00z
36 Hour Forecast 36h cmcg 22 Nov 12z 36h cent 22 Nov 12z 36h cmcg* 22 Nov 12z “Verification” 00h cent 22 Nov 12z
42h (18z, 22 Nov) mslp and sfc wind. eta ukmo tcwb ngps cmcg avn cent eta* ukmo* tcwb* “Verification” ngps* cmcg* avn*
10% 30% 50% 10% 90% Ensemble-Based Probability of Wind Speed Prob. of (sustained) Winds > 21 kt 39h, 22 Nov 15z
(Thursday 7AM)
42h, 22 Nov 18z
(Thursday 10AM)
39h, 22 Nov 21z
(Thursday 1PM)
Summary
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Set of 15 ICs for UW SREF are not optimal, but may be good enough to represent important features of analysis error
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The centroid may be the best bet deterministic model run, in the big picture
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Need further evaluation
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How often does the ensemble fail to capture the truth?
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How reliable are the probabilities?
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Does the ensemble dispersion represent forecast uncertainty?