Weather Forecasting

Chapter 13Chapter 13March 26, 2009

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

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

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)

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

AWIPS workstation

Automated Surface Observations

Remote Automated Weather Stations (RAWS)

NEXRAD radar

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

Process to Incorporate ModelsProcess to Incorporate Models

From a talk by Stephen Lord, Director, NCEP Environmental Modeling Center

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

Basic NWPBasic NWP

• A model in its simplest form

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.

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.

Grid ResolutionGrid Resolution

• Various scales of physical processes

24km

Eff t f

resolution

Effects of Terrain inTerrain in Models

12km12kmResolutionIn WRF-NMM

Effects of Ice/Snow ResolutionEffects of Ice/Snow Resolution

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

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

Forecasting Rules of ThumbForecasting Rules of Thumb

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 ,

Weather Forecasting MethodsWeather Forecasting Methods

• Other Forecasting Techniques• Persistence• Trend• Analogue• Analogue• Statistical• Weather type• Climatologicalg

Probability of White ChristmasProbability of White Christmas

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

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

Frontal TrajectoriesFrontal Trajectories

Movement in 6 hours

Isallobars-Lines of Equal 3hr Pressure Change

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