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THE NCEP ETA DATA ASSIMILATION SYSTEM (EDAS) : AN OVERVIEW
COMET Faculty Course on Numerical Weather Prediction
10 June 1999
Presented by Eric Rogers
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OUTLINE
• Philosophy
• General Description / History
• Current Operational 32-km EDAS
- Impact of 3DVAR analysis and cycled EDAS
- Observations
- Recent Performance : The Good, the Bad, and the Ugly
• Future Plans
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GOAL : PRODUCE BEST POSSIBLE INITIAL CONDITIONS FOR ETA MODEL FORECAST*
• KEY COMPONENTS
- State of the art analysis (variational)
- Consistency between assimilating and forecast model (resolution, physics, dynamics)
- Intelligent selection and use of observations
* NOT necessarily the same as fitting all the observations exactly (more to come on this)
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EDAS : PAST CONFIGURATION MILESTONES
• June 1993 : Eta-80 (Early Eta) replaces LFM; initialized with single Eta Optimum Interpolation (OI) analysis at 00/12Z using GDAS first guess
• March 1995 : Eta-29 (Meso Eta) ; 3-h EDAS with OI analysis used as first guess for 03/15Z forecasts
• October 1995 : Eta-48 replaces Eta-80 in Early Eta slot, initialized by a 12-h intermittent EDAS cycle with 3-h OI analysis updates
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EDAS Original Configuration : Eta-48 forecast 00Z/12Z, Eta-29 forecast from 03Z/15Z
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48KM EDAS : PROBLEMS AND CONCERNS• Forecasters wanted higher resolution run earlier • Starting the EDAS using a GDAS first guess still caused inconsistencies (e.g., soil moisture)
• Eta OI analysis not easily adaptable for use of non-traditional data sets (satellite radiances, NEXRAD radial velocity)
• Eta OI analyses heights, not temperatures, so it can’t use aircraft mass information
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Eta-48 Surface Meteogram at Riyadh, Saudi Arabia from 12Z 6/25/97 Solid = Observations Dashed = Forecast
2-m T
2-m Td
12Z 6/25 00Z 6/26 12Z 00Z 6/27 12Z
• Ran operational EDAS/Eta-48 system over Saudi Arabia - Forecast surface temperature 4-10oC too cold - 00-h analysis Td = 19oC, observed Td = -6oC!
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SOLUTION : ETA-32 UPGRADE
• Stage 1 : Implemented 9 February 1998*
- Resolution of 00Z/12Z EDAS and Early Eta changed from 48km/38levs to 32km/45levs
- Eta OI analysis replaced by Eta 3-d variational analysis (3DVAR)
- “Partially” cycled EDAS : Basic atmosphericvariables (T,q,u,v,sfc p) at the start of the 12-hEDAS obtained from GDAS; soil moisture /temperature, cloud water, turbulent kinetic energy from the previous EDAS cycle
* http://www.nws.noaa.gov/om/tpb/447.htm
- Meso Eta-29 unchanged
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ETA 3DVAR ANALYSIS (Parrish et.al1996 NWP Preprint Volume)
• Loosely patterned after NCEP global SSI analysis
• Analysis variables: - Stream function- Potential function
- Temperature- Specific humidity
- Surface pressure - Geopotential height
• More adaptable for use of new data types than OI (e.g., NEXRAD radial velocities used in Eta-10 runs during 1996 Olympics)
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3DVAR vs. OI : 00Z/12Z First Guess Temperature vs RAOBS
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3DVAR vs. OI : 00Z/12Z first guess vector wind RMS vs. RAOBS
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3DVAR vs. OI
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WHY DO CYCLING?
• Initial conditions more consistent with forecast model
• Less spinup of divergence, cloud, precipitation, and TKE
• More accurate representation of soil moisture
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IMPACT OF CYCLING ON SOIL MOISTURE
48km EDAS : 12-h cycle 32km EDAS : 3 week cycle
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48-h Eta forecast 2-m temperature - sfc T obs : Model shows no disquieting drift from cycled EDAS
Bia
s (d
eg
C)
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ETA-32 UPGRADE• Stage 2 : Implemented 3 June 1998
- EDAS converted to “full” cycling mode with atmospheric variables cycled from the previous EDAS as well as soil/cloud/TKE
- Meso Eta-29 converted to 32km
- Robust backup system in place to ensure that EDAS soil moisture is never reinitialized from the GDAS
1) 03Z Eta-29 ----> 03Z Eta-32 (33-h fcst)
2) 15Z Eta-29 ----> 18Z Eta-32 (30-h fcst)
3) Both off-time runs connected to fully cycled EDAS
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DATA DUMP TIMES
Cycle Dump Time
00Z 0111Z
03Z 0340Z
12Z 1313Z
18Z 1835Z
Cycle AnalysisTimes
Dump Time
00Z 18,21Z 2243Z
12Z 00,03,06,09Z
1002Z
18Z 12,15Z 1720Z
Early Eta EDAS
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OBSERVATIONS USED BY ETA 3DVAR • Upper air data
- Rawinsonde height/temperature/wind/moisture- Dropwindsondes- Wind Profilers- NESDIS thickness retrievals from polar orbiting satellites (oceans only)- VAD winds from NEXRAD- Aircraft (conventional and ACARS) winds/temps- Satellite cloud drift winds- SSM/I and GOES precipitable water retrievals- Synthetic tropical cyclone data
• Surface data- Surface land wind/temperature/moisture- Ships and buoys- SSM/I oceanic surface winds
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DATA QUALITY CONTROL• CQC: Complex QC of raob height/temps (baseline, hydrostatic, lapse rate, radiation correction, etc.)• ACQC : Quality control of conventional aircraft data (remove duplicates, track checks, create “superobs”)
• 3DVAR : Analysis performs gross check vs. first guess:
- Temperature : +/- 15oC- Wind :+/- 25 ms-1
- RH : +/- 90%- Precipitable water : +/- 12 g/kg- Height : +/- 100 m
• SDMEDIT : NCEP Senior Duty Meteorologist can flag all or parts of suspect raobs
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Use of Surface Data: Eta OI vs. Eta 3DVAR
Eta OI Analysis
Eta 3DVAR Analysis
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RECENT ETA/EDAS PERFORMANCE• Original 3DVAR in Eta-32 : degraded surface analysis (especially moisture)
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EXAMPLE : 00Z 7/13/98 at Rapid City, SD
Solid = RAOBDashed = 80 km OPNL 3DVAR
5oC analysis error in Td at 850 mb !
Problems : 1) 3DVAR tuned to give too much weight to first guess; 2) code error excluded all surface data!
Solid = RAOBDashed = 80 km Modified 3DVAR
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• New 3DVAR tested in July 1998 and showed improved fit to surface and raobs (especially moisture)
• Re-tuned 3DVAR implemented on 3 November 1998
• We thought everything was OK…..
BUT……..
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Solid = Eta Short Dash = NGM Long Dash = AVN/MRF
24-H ACCUMULATED PRECIPITATION EQUITABLE THREAT SCORES: ALL FCSTS
12/1/97 - 2/28/98 12/1/98 - 2/28/99
10-15% drop in Eta skill between 1997-98 and 1998-99
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Persistent synoptic error in Eta-32 during winter of 98-99: weaker and faster Eastern Pacific troughs/cyclones than observed
Example : 48-h Forecasts valid 1200 UTC 17 March 1999
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AVN had much better SLP forecast then Eta :
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PROBLEM 1: November 98 change degraded mass/wind balance in 3DVAR
• If mass / wind balance well-behaved, positive height correction is coincident with center of anticyclonic wind correction 850 mb ANL-GUESS height/wind
80KM EDAS valid 00Z 3/15/99• Note 10 degree longitude displacement between centers of wind and height correction
• Problem is most severe in regions and at analysis times without widespread raob data but with large amounts of wind or mass only data (e.g., satellite winds)
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SOLUTION : Improve geostrophic coupling of mass/wind analysis corrections in 3DVAR Operational 3DVAR analysis Modified 3DVAR analysis
Note: Improved height/wind coupling near Aleutians
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PROBLEM 2: Horizontal/vertical correlations too narrow : observation had VERY limited impact on analysis away from its level
• One observation test : Insert one height observation 10 m greater than first guess at 200, 500, 900 mb and measure impact in horizontal/vertical
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Operational 32-km 3DVAR
Modified 32-km 3DVAR
200 mb
900mb
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Performance of new 3DVAR : 3 December 1998 to 16 January 1999 test at 80 km resolution
24-h accumulated precipitation threat scores: All forecasts Dashed = Modified 3DVAR Solid = Operational 3DVAR
Equ
itabl
e T
hrea
t Sco
re
Threshold (in)
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00-h
24-h
48-h
00,24,48-h forecast vector RMS wind error vs RAOBSDashed = Modified 3DVAR Solid = Operational 3DVAR
Vec
tor
RM
S e
rror
(m
/s)
Pressure (mb)
NOTE = 200 mb 24-h modified 3DVAR error = 00-h error in operational 3DVAR!
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NEW 3DVAR IMPLEMENTED 13 MAY 1999* : IMPACT
* Further documentation at http://www.nws.noaa.gov/om/tpb/3d-eta.htm
200 mb Vector Wind RMS error vs RAOBS : 3/1/99-5/31/99 Blue = 48-h Eta-32 fcst Purple = 48-h AVN fcst
Old 3DVAR New 3DVAR
RM
S V
ecto
r W
ind
Err
or (
m/s
)
Rog
ers
on le
ave;
sof
twar
e br
eaks
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FUTURE EDAS DEVELOPMENT• 0-6 months (Day 1 on IBM)
- Move 03Z Eta to 06Z; run 12-h EDAS for 00/12Z cycle; 06-h EDAS for 06/18Z cycle- Use of GOES/TOVS radiances in Eta 3DVAR
• 6-12 months - Increase resolution to 22km/50levs- Assimilation of observed precipitation (Lin et.al, 1998 NWP Conference, 1999 Mesoscale Conference)
• 12-18 Months - Initialization of cloud water (Zhao et.al 1998 NWP Conference)- Use of NEXRAD radial velocity and surface mesonet data in Eta 3DVAR; switch from 3-h to 1-h analysis cycle in EDAS
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Impact of precip assimilation on Eta-80 24-h forecast valid 12Z 7/5/98
Control forecast Forecast w/precip assimilation
24-h observed precip (RFC analysis)
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Rainfall Data Assimilation (Lin et.al, 1998 NWP Conference, Phoenix, AZ)
• During the 12h pre-forecast assimilation period at each timestep compare the model predicted rainfall to observed
• Adjust the model’s latent heating profile accordingly (Carr and Baldwin, 1991)
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Cloud Data Assimilation(Zhao et al, 1998, 12th NWP, Phoenix, AZ)
• Data sources– real-time Neph Analyses (USAFGWC)– hourly radar/gauge observations