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On Methods of Precipitation Efficiency Estimation
Brian Pettegrew
Dept. of SEAS
Seminar – NATR 410
April 5, 2004
Outline
• Introduction• Why use PE?
• Motivation
• Method/Data
• Synoptic Case Study
• Results and Conclusions
Introduction
• What is Precipitation Efficiency?
• mp is moisture released via precipitation in the form of rain
• mi is ingested moisture via a warm, moist updraft
i
p
m
mPE
Introduction
• Why are we concerned about PE?• Flash flood forecasting• More deaths cause annually from flashfloods
than any other weather phenomena
• Which pre-storm environment is more threatening?• PW of 2.0 and PE of 20%• PW of 2.0 and PE of 80%
Introduction
• Instantaneous PE can very greatly
t=0hE~0
t=1hE~max
t=2hE
mimpInstantaneou
s PE
Introduction• Lag in water cycling
• Doswell (1996)…
Introduction
• Although instantaneous PE can vary, PE over the lifetime of an MCS is constant.
- Lifetime PE a posteriori cannot vary and must stay between 0 and 1
Methods
• How is PE calculated?• Current Operational Method
• RH is a mean value taken between 1000-700 mb
• Value derived in inches
RHPWPE
Methods
• Scofield (1987)• NESDIS – National Environmental Satellite,
Data, and Information Services• Geostationary Operational Environmental
Satellite (GOES)• Operational convective precipitation estimation
technique• Developed for estimating convective precipitation
every half hour
Methods
• Moisture Correction Scheme• Take into account for long-living storms with
strong, steady-state updrafts and outflows, very saturated column, and no dry air entrainment
• Estimates can be amplified
• Scheme adjusts estimates for air that is too moist or too dry
Methods
• Operational use• Hydrometeorological applications
• National Weather Service (NWS) and River Forecast Centers (RFC’s)
• Forecasting QPF
Methods
• Noel and Dobur (2003)• PW/RH method useful in identifying axis of
precipitation, but not stand alone indicator.
• Note: This relationship indicates a potential for the environment to produce precipitation, not an actual efficiency
Methods
• Sellers (1965)
• Climatological mean
• Has seasonal and latitudinal variations
WaterlePrecipitab Average
Depthion Precipitat AveragePE
Methods
• Market et al. (2003)• Used Seller’s to predict PE using GOES
sounder derived values in a pre-convective environment
• Known for its spatial and temporal density
• Initial PE values calculated using gauge network rainfall totals and Rapid Update Cycle (RUC) derived PW
Methods
• Correlated bulk environmental parameters from GOES soundings to calculated PE values
• Best correlations were made to CIN, RH, LCL height, and cloud shear
Methods
• Modified version
• PW difference over a period • Accounts for moisture fluxing out of the storm over
that same time• First experiment using this method
totalRainfall
PWPWPE
earlierlater
Methods
• Moisture Budget• Chester and Newton (1969)
0
0
0
0
11 pp
qVdpg
dpt
q
gEP
Methods
• P is total precipitation
• E is evaporation at the SFC
• g is acceleration due to gravity
• dq/dt is change in specific humidity
• Represents advection of moisture
qV
Methods
• Computes horizontal moisture flux and divergence
• True efficiency• Accounts for all moisture in and out of a system
Data
• Rapid Update Cycle (RUC)• RUC-2
• 40-km gridspacing
• Smoothed to 80-km
• Released in 1998 as update to RUC-1
Data
• Data Assimilation• Initialized from ETA derived model fields and
previous hour’s RUC forecast
• 12-hr output every hour
Data
• Horizontal Resolution• “slope envelope” topography
• Terrain calculated with respect to a plane fit to the high-resolution topography in each grid space (Benjamin et al. 2002)
Data• Horizontal Resolution
40-km RUC Terrain *Benjamin et al. 2002
Data
• Vertical Resolution• All RUC models use hybrid isentropic-sigma
coordinates in forecast and analysis (Bleck and Benjamin 1993)
• RUC-2 fit with 40 vertical levels
Data• Vertical Resolution
40km RUC 40 levels
S-N vertical cross-section - Miss – Wisc – Lake Superior - w. Ontario 12h fcsts valid 1200 UTC 2 Apr 2002
*Benjamin et al. 2002
Data
• Vertical Resolution• Higher resolution leads to better handling of:
• Vertical advection processes
• Improved conservation of potential vorticity
• Improved air-mass integrity and frontal structure with observation influence (Benjamin et al. 2002)
Data
• Convective Parameterization• Model generated convection!!!
• Grell Scheme• Three phases
• 1) Dynamic Control
• 2) Feedback
• 3) Static Control
Data
• Dynamic Control• Determines modulation of convection by the
environment
• Based on environmental stability
• Observed change of available buoyant energy is known
Data
• Dynamic Control (cont’d)• Purely Predictive
• Amount and size of convective elements
• Convective activity related to total moisture convergence
• Function of time and space, not cloud type (Grell 1993)
Data
• Feedback• Modification of the environment by the
convection
• Distributes total integrated heating and drying in the vertical
• Adjusts the atmosphere to a moist neutral state
• Dependent on temperature and moisture differences between cloud and environment
Data
• Static Control• Determines updraft or downdraft properties
• Feedback dependent on static control• Entrainment, detrainment, downdraft properties, and
microphysics of cloud model
• (Grell 1993)
Data
• RUC data used• Initial and forecast fields from RUC output
obtained every six-hours• Initial times 00, 06, 12, 18
– Forecast hours 03, 06, and 09 used
• All times in Zulu
Data
• Precipitable Water• PW of a column calculated via trapezoidal
integration from model derived variables
• Precipitation• 3-hr precipitation accumulation parameters
from RUC output were used• (all calculations performed via RUC data for
internal consistency)
Data
• GEMPAK (GEneral Meteorological PAcKage)• Scripts constructed to calculate PE
• Mapped out in grid format over desired region
Synoptic Analysis
May 6, 2003
• Huntsville, AL
• Several F1 and F0 Tornado Damage
• Flooding of Tennessee River• Huntsville metropolitan area severe flashfloods
Rainfall totals
•
SFC 00Z
850 mb 00Z
700 mb 00Z
500 mb 00Z
300 mb 00Z
SFC 12Z
850 mb 12Z
700 mb 12Z
500 mb 12Z
300 mb 12Z
Results
• Area MCS average• Precipitation
• Ingested Moisture
• PE• Scofield
• Seller’s
• Modified
• Moisture budget
Results
• A ratio of ingested moisture and total accumulated precipitation taken to have a total PE for the area to correlate too
Correlations
• Point correlation between four methods
Correlations
0900Z
Sellers Scofield Modified
Scofield 0.2257
Modified 0.2196 0.2679
Moisture
Budget
0.7108 0.0851 -0.2434
Correlations
1200 Z
Sellers Scofield Modified
Scofield 0.2568
Modified 0.4807 -0.1629
Moisture Budget
-0.1298 -0.1436 -0.0897
Correlations
1500 Z
Sellers Scofield Modified
Scofield 0.1695
Modified -0.1007 0.3625
Moisture Budget
-0.1063 -0.5535 -0.2439
Correlations
1800 Z
Sellers Scofield Modified
Scofield -0.1699
Modified -0.1689 0.5268
Moisture Budget
0.1534 -0.1141 -0.0159
Correlations
2100 Z
Sellers Scofield Modified
Scofield 0.2520
Modified 0.4504 0.0718
Moisture Budget
0.3986 -0.1485 0.1355
Correlations
• Values correlated over lifetime of storm
Correlations
Sellers Scofield Modified
Scofield -0.3825
Modified -0.3771 0.1102
Best PE 0.2535 -0.8685 0.3817
Conclusions
• Area average of efficiency calculations correlated well for a short-lived MCS
• Operational method showed strong negative correlation.
Future Work
• San Antonio, TX• Long-lived rain event
• Up to 33” of rain fell over a seven-day span
• Similar correlations in progress
Acknowledgements
• COMET/UCAR
• National Weather Service
• Dr. Patrick Market - UMC
• Dr. Neil Fox – UMC
• Chris Schultz – UMC
• Dave Jankowski – UMC
Works Cited
• Benjamin, S.G., J.M. Brown, K.J. Brundage, D. Devenyi, G.A. Grell, D. Kim, B.E. Schwartz, T.G. Smirnova, T.L. Smith, S. Weygandt, and G.S. Manikin, 2002: RUC20 – The 20-km version of the Rapid Update Cycle. NWS Technical Procedures Bulletin No. 490.
• Bleck, R., and S.G. Benjamin, 1993: Regional weather prediction with a model combining terrain-following and isentropic coordinates. Part I: model description. Mon. Wea. Rev.,121, 1770-1785.
• Doswell, C.A., III, H.E. Brooks, and R.A. Maddox, 1996: Flash flood forecasting: An ingredients-based methodology. Wea. Forecasting, 11, 560-581.
• Grell, G., 1993: Prognostic evaluation of assumptions used by cumulus parameterizations. Mon. Wea. Rev., 121, 764-787.
• Market, P.S., S. Allen, R.A. Scofield, R. Kuligowski, A. Gruber, 2003: Precipitation Efficiency of warm-season midwestern mesoscale convective systems. Wea. Forecasting, 18, 1273-1285.
Works Cited
• Newton, C.W. and E. Palmen, 1969: Atmospheric Circulation Systems: Their Structure and Physical Interpretation. Academic Press, 603 pp.
• Noel, J. and J.C. Dobur, 2003: A pilot study examining model derived precipitation efficiency for use in precipitation forecasting in the eastern united states. Nat. Wea. Dig., 26, 3-8.
• Scofield, R.A., 1987: The NESDIS Operational Convective Precipitation Estimation Technique. Mon. Wea. Rev., 115, 1773-1792.
• Sellers, W.D., 1965: Physical Climatology. The University of Chicago Press, 272 pp.
• Thank you for your time!
• Questions?
• Comments?
• Email: bpp4y8@mizzou.edu
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