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Weather Forecasting
Chapter 13Chapter 13March 26, 2009
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ForecastingForecasting
• The process of inferring weather from a blend of data, understanding, climatology, , g, gy,and solutions of the governing equations
• Requires an analysis of the current• Requires an analysis of the current conditions and then the formation of a hypothesis about how the current weather came to be
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ForecastingForecasting• Requires forming hypothesis of current• Requires forming hypothesis of current
weather• Hypothesis based on conceptual models
– Includes atmospheric processes and howIncludes atmospheric processes and how they look like in routine dataIncludes physical models based on theory– Includes physical models based on theory, experience and climatology
G l i t i i bilit i li• Goal is to maximize our ability visualize processes, form realistic conceptual models and minimize incorrect judgment
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Acquisition of Weather InformationAcquisition of Weather Information
W ld id 10 000 l d b d t ti h d d• World-wide: 10,000 land-based stations, hundreds of ships and buoys
• Data from airports hourly• Data from airports hourly• Upper level: radiosonde, aircraft, satellites, profilers
Organizations involved:• Organizations involved:• United Nations World Meteorological Organization, 175
countries• World Meteorological Centers: Melbourne, Moscow,
Washington D.C.N ti l C t f E i t l P di ti (NCEP)• National Centers for Environmental Prediction (NCEP)
• US National Weather Service (NWS)
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Weather Forecasting ToolsWeather Forecasting Tools• Advanced Weather Interactive Processing• Advanced Weather Interactive Processing
System (AWIPS)Hi h d d d li f– High speed data modeling systems for communication, storage, processing, and di ldisplay
– Doppler radar (NEXRAD, Terminal Doppler)– Satellite imagery (GOES, MODIS, AMSR-E)– Forecast chartsForecast charts– Soundings
Wi d fil– Wind profiles
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AWIPS workstation
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Automated Surface Observations
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Remote Automated Weather Stations (RAWS)
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NEXRAD radar
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Weather Forecasting MethodsWeather Forecasting Methods
• 1950s maps, charts plotted by hand• Numerical weather predictionNumerical weather prediction
– Solves equations using gridded dataFi l h t ll d l i– Final chart called analysis
– 24 hr forecast for the N. Hemisphere requires millions of calculations
– Resolution– Guidance and rules of thumb
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Process to Incorporate ModelsProcess to Incorporate Models
From a talk by Stephen Lord, Director, NCEP Environmental Modeling Center
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Numerical Weather Prediction (NWP)Numerical Weather Prediction (NWP)
• Basic physical laws converted into a series of mathematical equationsq– Physical laws of motion (e.g. Newtons 2nd law)
Conservation of energy (e g 1st law thermo)– Conservation of energy (e.g. 1st law thermo)• Basic prediction
– If we know initial condition of the atmosphere, we can solve the equations to obtain new qvalues of variables at a later time
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Basic NWPBasic NWP
• A model in its simplest form
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Weather Research & Forecasting (WRF)Weather Research & Forecasting (WRF)• The current state of the art forecastThe current state of the art forecast
modeling system12 k id i• 12 km grid spacing
• Terrain following vertical coordinateg• Ingests observational data
I l d h i l d l f• Includes physical models for– Land surface, snow cover and soil effects– Cloud physics (cumulus)– Precipitation– Radiation, etc.
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Global Forecasting System (GFS)Global Forecasting System (GFS)• A commonly used global forecastingA commonly used global forecasting
model0 5° id i ( 60 k )• 0.5° grid spacing (~60 km)
• Sigma vertical coordinateg• Ingests observational data
I l d h i l d l f• Includes physical models for– Land surface, snow cover and soil effects– Cloud physics– Radiation– Oceans, etc.
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Grid ResolutionGrid Resolution
• Various scales of physical processes
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24km
Eff t f
resolution
Effects of Terrain inTerrain in Models
12km12kmResolutionIn WRF-NMM
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Effects of Ice/Snow ResolutionEffects of Ice/Snow Resolution
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NCEP SupercomputingNCEP SupercomputingIBM P 6 575• IBM Power6 p575– 69.7 Teraflops Linpack
#36 T 500 N 2008• #36 Top 500 Nov 2008
– 156 Power6 32-way NodesNodes
– 4,992 processors @
4 7GHz4.7GHz– 19,712 gigabytes
memorymemory – 170 terabytes of disk
spacep– 100 terabyte tape
archive Slide adapted from a talk by Ben Kyger, Director, NCEP Central Operations
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NCEP Supercomputing Performance
80000
69735
60000
70000
40000
50000
(Lin
pack
)
30000
40000
Gig
aflo
ps
13990 1547010000
20000
G
350 1179 1179 1849 18494379 4379
0
10000
1999 2000 2001 2002 2003 2004 2005 2006 2007 20081999 2000 2001 2002 2003 2004 2005 2006 2007 2008
YearsSlide from Ben Kyger, Director, NCEP Central Operations
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Forecasting Rules of ThumbForecasting Rules of Thumb
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Weather Forecasting MethodsWeather Forecasting MethodsThi k Ch t• Thickness Charts– Difference in height between two constant pressure
surfaces (1000mb-500mb)surfaces (1000mb 500mb)– Higher thickness equals warmer air
• Why Forecasts Go Awryy y– Assumptions– Models not global
R i ith f b ti– Regions with few observations– Cannot model small-scale features– All factors cannot be modeledAll factors cannot be modeled
• Ensemble Forecasts:– Spaghetti model, robustp g ,
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Weather Forecasting MethodsWeather Forecasting Methods
• Other Forecasting Techniques• Persistence• Trend• Analogue• Analogue• Statistical• Weather type• Climatologicalg
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Probability of White ChristmasProbability of White Christmas
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Weather Forecasting MethodsWeather Forecasting MethodsT f F t• Types of Forecasts• Nowcast < 6 hrs• Short range 12 to 65 hrs• Medium range 3 to 8.5 daysg y• Long Range > 8.5 days
• Accuracy and Skill• Accuracy and Skill• 12 - 24 hrs most accurate
2 5 d d• 2 - 5 days good• Skill = more accurate than a forecast
j t tili i i t f li t ljust utilizing persistence of climatology
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Weather Forecasting Using Surface Charts
S ti th t f S tSome tips on the movement of Systems1. Mid-lat cyclones move in same direction and
d i 6 hspeed as previous 6 hrs2. Lows move in direction parallel the isobars in
th i h d f th ld f tthe warm air ahead of the cold front3. Lows move toward region of greatest pressure
ddrop
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Frontal TrajectoriesFrontal Trajectories
Movement in 6 hours
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Isallobars-Lines of Equal 3hr Pressure Change
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Some Rules of Thumb• 500mb chart
5640m heights over NoCA rain over central CA– 5640m heights over NoCA – rain over central CA– Eastern US, < 5400m – snow rather than rain– Blocking or Omega high – persists in same
location, keeps trofs in their positions– The tighter the height contours, the higher the
wind speed, the stronger the temperature p , g pdifference below 500 mb
• 700mb chart• 700mb chart– At 700mb level, RH>70%=clouds,
RH 90% i it tiRH>90%=precipitation