Weather Forecasting. March 26, 2009

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Aldous Blair
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1 Weather Forecasting Chapter 13 March 26, 2009

2 Forecasting The process of inferring weather from a blend of data, understanding, climatology, and solutions of the governing equations Requires an analysis of the current conditions and then the formation of a hypothesis about how the current weather came to be

3 Forecasting Requires forming hypothesis of current weather Hypothesis based on conceptual models Includes atmospheric processes and how they look like in routine data Includes physical models based on theory, experience and climatology Goal is to maximize i our ability visualize processes, form realistic conceptual models and minimize incorrect judgment

4 Acquisition of Weather Information World-wide: 10, land-based d stations, ti hundreds d of ships and buoys Data from airports hourly Upper level: radiosonde, aircraft, satellites, profilers Organizations involved: United Nations World Meteorological Organization, 175 countries World Meteorological Centers: Melbourne, Moscow, Washington D.C. National Centers for Environmental Prediction (NCEP) US National Weather Service (NWS)

5 Weather Forecasting Tools Advanced Weather Interactive Processing System (AWIPS) High speed data modeling systems for communication, storage, processing, and display Doppler radar (NEXRAD, Terminal Doppler) Satellite imagery (GOES, MODIS, AMSR-E) Forecast charts Soundings Wind profiles

6 AWIPS workstation

7 Automated Surface Observations

8 Remote Automated Weather Stations (RAWS)

9 NEXRAD radar

11 Weather Forecasting Methods 1950s maps, charts plotted by hand Numerical weather prediction Solves equations using gridded data Final chart called analysis 24 hr forecast for the N. Hemisphere requires millions of calculations Resolution Guidance and rules of thumb

12 Process to Incorporate Models From a talk by Stephen Lord, Director, NCEP Environmental Modeling Center

13 Numerical Weather Prediction (NWP) Basic physical laws converted into a series of mathematical equations Physical laws of motion (e.g. Newtons 2 nd law) Conservation of energy (e.g. 1 st law thermo) Basic prediction If we know initial condition of the atmosphere, we can solve the equations to obtain new values of variables at a later time

14 Basic NWP A model in its simplest form

15 Weather Research & Forecasting (WRF) The current state of the art forecast modeling system 12 km grid spacing Terrain following vertical coordinate Ingests observational data Includes physical models for Land surface, snow cover and soil effects Cloud physics (cumulus) Precipitation Radiation, etc.

16 Global Forecasting System (GFS) A commonly used global forecasting model grid spacing (~60 km) Sigma vertical coordinate Ingests observational data Includes physical models for Land surface, snow cover and soil effects Cloud physics Radiation Oceans, etc.

17 Grid Resolution Various scales of physical processes

18 24km resolution Effects of Terrain in Models 12km Resolution In WRF-NMM

19 Effects of Ice/Snow Resolution

20 NCEP Supercomputing IBM Power6 p Teraflops Linpack #36 Top 500 Nov Power6 32-way Nodes 4,992 47GHz 4.7GHz 19,712 gigabytes memory 170 terabytes of disk space 100 terabyte tape archive Slide adapted from a talk by Ben Kyger, Director, NCEP Central Operations

21 NCEP Supercomputing Performance (Linpack) Gigaflops Years Slide from Ben Kyger, Director, NCEP Central Operations

25 Forecasting Rules of Thumb

26 Weather Forecasting Methods Thickness Charts Difference in height between two constant pressure surfaces (1000mb-500mb) Higher thickness equals warmer air Why Forecasts Go Awry Assumptions Models not global Regions with few observations Cannot model small-scale features All factors cannot be modeled Ensemble Forecasts: Spaghetti model, robust

28 Weather Forecasting Methods Other Forecasting Techniques Persistence Trend Analogue Statistical Weather type Climatological

29 Probability of White Christmas

30 Weather Forecasting Methods Types of Forecasts Nowcast < 6 hrs Short range 12 to 65 hrs Medium range 3 to 8.5 days Long Range > 8.5 days Accuracy and Skill hrs most accurate 2-5d days good Skill = more accurate than a forecast just utilizing i persistence of climatology l

31 Weather Forecasting Using Surface Charts Some tips on the movement of Systems 1. Mid-lat cyclones move in same direction and speed as previous 6 hrs 2. Lows move in direction parallel the isobars in the warm air ahead of the cold front 3. Lows move toward region of greatest pressure drop

32 Frontal Trajectories Movement in 6 hours

33 Isallobars-Lines of Equal 3hr Pressure Change

34 500mb chart Some Rules of Thumb 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 difference below 500 mb 700mb chart At 700mb level, RH>70%=clouds, RH>90%=precipitation it ti

WEATHER FORECASTING Acquisition of Weather Information WFO Regions Weather Forecasting Tools Weather Forecasting Tools Weather Forecasting Methods

WEATHER FORECASTING Acquisition of Weather Information WFO Regions Weather Forecasting Tools Weather Forecasting Tools Weather Forecasting Methods 1 2 3 4 5 6 7 8 WEATHER FORECASTING Chapter 13 Acquisition of Weather Information 10,000 land-based stations, hundreds of ships and buoys; four times a day, airports hourly Upper level: radiosonde, aircraft,