Numerical Weather Prediction NWP and the WRF Model
- Slides: 45
Numerical Weather Prediction (NWP) and the WRF Model Jason Knievel Material contributed by: George Bryan, Jimy Dudhia, Dave Gill, Josh Hacker, Joe Klemp, Bill Skamarock, Wei Wang, and The COMET Program Jason Knievel ATEC Forecasters’ Conference, July and August 2006 1
Numerical weather prediction Q: What is NWP? A: A method of weather forecasting that employs: – – – Jason Knievel A set of equations that describe the flow of fluids, Which is translated into computer code, Combined with parameterizations of other processes, Then applied on a specific domain, And integrated, based on initial conditions and conditions at the domains’ boundaries ATEC Forecasters’ Conference, July and August 2006 2
Numerical weather prediction § Almost every step in NWP includes – – Jason Knievel Omissions Estimations Approximations Compromises ATEC Forecasters’ Conference, July and August 2006 3
Numerical weather prediction Q: What is NWP? A: A method of weather forecasting that employs: – – – Jason Knievel A set of equations that describe the flow of fluids, Which is translated into computer code, Combined with parameterizations of other processes, Then applied on a specific domain, And integrated, based on initial conditions and conditions at the domains’ boundaries ATEC Forecasters’ Conference, July and August 2006 4
Numerical weather prediction Q: What is NWP? A: A method of weather forecasting that employs: – – – Jason Knievel Governing equations Numerical methods Parameterizations Domains Initial and boundary conditions ATEC Forecasters’ Conference, July and August 2006 5
Numerical weather prediction Q: What is NWP? A: A method of weather forecasting that employs: – – – Jason Knievel Governing equations Numerical methods Parameterizations Domains Initial and boundary conditions ATEC Forecasters’ Conference, July and August 2006 6
Governing equations § Conservation of momentum (Newton’s laws) – 3 equations for accelerations of 3 -d wind (F = Ma) § Conservation of mass – 1 equation for conservation of air (mass continuity) – 1 equation for conservation of water § Conservation of energy – 1 equation for the first law of thermodynamics § Relationship among p, V, and T – 1 equation of state (ideal gas law) Jason Knievel ATEC Forecasters’ Conference, July and August 2006 7
Governing equations § Almost every model uses a slightly different set of equations. § Why? – – – Jason Knievel Application to different parts of the world Focus on different atmospheric processes Application to different time and spatial scales Ambiguity and uncertainty in formulations Tailoring to different uses ATEC Forecasters’ Conference, July and August 2006 8
Governing equations § The WRF Model is one of the first cloud-scale models designed to conserve mass, momentum, and energy. § But… – Water is not yet perfectly conserved – There is still debate about whether momentum is perfectly conserved – Internal energy is conserved for dry processes, not moist Jason Knievel ATEC Forecasters’ Conference, July and August 2006 9
Governing equations § An example of one momentum equation: 1 -d wind accelerated by only the pressure gradient force Computers cannot deal with even this very simple equation! Jason Knievel ATEC Forecasters’ Conference, July and August 2006 10
Governing equations § The problem: computers can perform arithmetic but not calculus § The solution: numerical methods Jason Knievel ATEC Forecasters’ Conference, July and August 2006 11
Numerical weather prediction Q: What is NWP? A: A method of weather forecasting that employs: – – – Jason Knievel Governing equations Numerical methods Parameterizations Domains Initial and boundary conditions ATEC Forecasters’ Conference, July and August 2006 12
Numerical methods § Goal: convert spatial and temporal derivatives into algebraic equations that computers can solve § Examples of methods: – Finite difference (based on Taylor series) – Finite volume (based on fluxes in and out of volume) – Spectral (calculated in Fourier space) Jason Knievel ATEC Forecasters’ Conference, July and August 2006 13
Numerical methods § WRF Model uses finite differences § Taylor series: Equality only true if series is infinite… an impossibility! Truncation is always necessary – What gets cut (truncation error) defines order of scheme Jason Knievel ATEC Forecasters’ Conference, July and August 2006 14
Numerical methods § Numerical methods directly affect model output, mostly at small scales § Some model features are real, but some are due to numerical techniques. In the WRF Model: – Larger than 6 x, it may be real – Smaller than 6 x, it’s not to be trusted Jason Knievel ATEC Forecasters’ Conference, July and August 2006 15
Numerical methods § MM 5: leapfrog (t) and 2 nd-order centered (x) From George Bryan Jason Knievel ATEC Forecasters’ Conference, July and August 2006 16
Numerical methods § WRF: Runge-Kutta (t) and 6 th-order centered (x) From George Bryan Jason Knievel ATEC Forecasters’ Conference, July and August 2006 17
Introduction to numerical weather prediction Q: What is NWP? A: A method of weather forecasting that employs: – – – Jason Knievel Governing equations Numerical methods Parameterizations Domains Initial and boundary conditions ATEC Forecasters’ Conference, July and August 2006 18
Parameterizations § Parameterizations approximate the bulk effects of physical processes too small, too brief, too complex, or too poorly understood to be explicitly represented Jason Knievel ATEC Forecasters’ Conference, July and August 2006 19
Parameterizations § In the WRF Model, parameterizations include: – – – Cumulus convection Microphysics of clouds and precipitation Radiation (short-wave and long-wave) Turbulence and diffusion Planetary boundary layer and surface layer Interaction with Earth’s surface § Some of the biggest future improvements in the WRF Model will be in parameterizations Jason Knievel ATEC Forecasters’ Conference, July and August 2006 20
Introduction to numerical weather prediction Q: What is NWP? A: A method of weather forecasting that employs: – – – Jason Knievel Governing equations Numerical methods Parameterizations Domains Initial and boundary conditions ATEC Forecasters’ Conference, July and August 2006 21
Domains § Number of dimensions § Degree and kind of structure § Shape § Vertical coordinate § Resolution Jason Knievel ATEC Forecasters’ Conference, July and August 2006 22
Domains § Number of dimensions 3 D: Simulation of thunderstorm 1 D: Single-column model From Joe Klemp 2 D: Simulation of density current From Joe Klemp From Josh Hacker Jason Knievel ATEC Forecasters’ Conference, July and August 2006 23
Domains § Degree and kind of structure MM 5 and others WRF and others From Randall (1994) Jason Knievel ATEC Forecasters’ Conference, July and August 2006 24
Domains § Degree and kind of structure Hexagonal Triangular From ccrma. standford. edu/~bilbao Jason Knievel ATEC Forecasters’ Conference, July and August 2006 25
Domains § Degree and kind of structure Unstructured: Omega Model From Boybeyi et al. (2001) Jason Knievel ATEC Forecasters’ Conference, July and August 2006 26
Domains § Shape Flat Spherical From mitgcm. org (2006) From Rife et al. (2004) Jason Knievel ATEC Forecasters’ Conference, July and August 2006 27
Key features of WRF Model § Nesting of domains – One-way and two-way communication Nested domain Information flows both directions between grids Information flows only to finer grid Parent domain Jason Knievel ATEC Forecasters’ Conference, July and August 2006 28
Domains § Vertical coordinate From Pielke (2002) Jason Knievel ATEC Forecasters’ Conference, July and August 2006 29
Domains § Vertical coordinate In WRF Model, vertical coordinate is normalized hydrostatic pressure, From Wei Wang Jason Knievel ATEC Forecasters’ Conference, July and August 2006 30
Domains § Resolution RTFDDA terrain elevation on different domains x = 30 km x = 3. 3 km From Rife and Davis (2005) Jason Knievel ATEC Forecasters’ Conference, July and August 2006 31
Introduction to numerical weather prediction Q: What is NWP? A: A method of weather forecasting that employs: – – – Jason Knievel Governing equations Numerical methods Parameterizations Domains Initial and boundary conditions ATEC Forecasters’ Conference, July and August 2006 32
Initial and boundary conditions § Initial conditions define the atmosphere’s current state…the starting point § Boundary conditions define the atmosphere’s state at domains’ edges Jason Knievel ATEC Forecasters’ Conference, July and August 2006 33
Initial and boundary conditions § Idealized lateral boundary conditions – Open – Rigid – Periodic § Operational lateral boundary conditions – Generally updated during simulations – Not needed for global models, only for limited-area models (LAMs), such as RTFDDA – Can come from larger domains of same/different model or from global model • For RTFDDA, source is NAM (was Eta, now NMM-WRF) Jason Knievel ATEC Forecasters’ Conference, July and August 2006 34
Introduction to WRF Model § Weather Research and Forecasting Model § The term WRF Model does not mean the same thing to all people § Different WRF Models with same architecture but different core codes – ARW (Advanced Research WRF) at NCAR – NMM (Non-Hydrostatic Mesoscale Model) at NCEP • Based on Eta Model’s code • Is now the source of NAM simulations – Other cores may be coming soon Jason Knievel ATEC Forecasters’ Conference, July and August 2006 35
Architecture of WRF Model § Based on an innovative software architecture that makes it easy for users to contribute and modify code Jason Knievel ATEC Forecasters’ Conference, July and August 2006 36
WRF Model in RTFDDA § The WRF Model is replacing MM 5 as the forecast engine in RT-FDDA – MM 5 -RTFDDA will be run in parallel as back-up § MM 5 will not be turned off until ATEC is ready, or until maintenance becomes impossible Jason Knievel ATEC Forecasters’ Conference, July and August 2006 37
History of WRF Model § WRF Model is young § Releases – 2000: V 1. 0 (beta release of EH core) – 2002: V 1. 2 (beta release of EM core) – 2004: V 2. 0 (first official release) § Current version: 2. 1 (released in August 2005) § Version 2. 2 is scheduled for later this summer Jason Knievel ATEC Forecasters’ Conference, July and August 2006 38
Importance of age § WRF Model is based on more recent technology and techniques § But… The WRF Model has not benefited from many years of trouble-shooting and input from users Jason Knievel ATEC Forecasters’ Conference, July and August 2006 39
Grand vision for WRF Model § From the start, WRF was intended to be used for both research and operations – Shorten time between research developments in NWP and application to operations – Increase communication and understanding between research and operational communities § MM 5 started as a research model and was later adopted by some operational forecasters Jason Knievel ATEC Forecasters’ Conference, July and August 2006 40
Platforms for WRF Model § Can be run on a variety of platforms on single processor or with shared or distributed memory NCAR’s Bluesky Courtesy of Dell Courtesy of NCAR Jason Knievel ATEC Forecasters’ Conference, July and August 2006 41
Numerics in WRF Model § The WRF Model’s numerics are higher order than MM 5’s, so they contain more terms and better approximate the governing equations – Horizontal advection: 5 th order – Vertical advection: 3 rd order – Temporal integration: 3 rd order Jason Knievel ATEC Forecasters’ Conference, July and August 2006 42
Numerics in WRF Model wavelength (km) Energy Spectra grid interval: 10 km § Higher order advection schemes, which lead to a higher effective resolution than in many other NWP models After Skamarock (2004) wavenumber (km-1) Jason Knievel ATEC Forecasters’ Conference, July and August 2006 43
Closing comments § Numerical weather prediction models are: – – Powerful and useful Founded on basic physics The result of many compromises and approximations Always wrong — at least a little…this includes the WRF Model § The WRF Model is state-of-the-art in operational mesoscale NWP Jason Knievel ATEC Forecasters’ Conference, July and August 2006 44
Additional reading § Kalnay, E. , 2003: Atmospheric Modeling, Data Assimilation, and Predictability. Cambridge University Press, 341 pp. § Klemp, J. B. , and R. B. Wilhelmson, 1978: The simulation of three-dimensional convective storm dynamics. J. Atmos. Sci. , 35, 1070– 1096. § Pielke, R. A. , Sr. , 2002: Mesoscale Meteorological Modeling, 2 nd edition. Academic Press, 676 pp. § Skamarock, W. C. , 2004: Evaluating Mesoscale NWP Models Using Kinetic Energy Spectra. Monthly Weather Review: 132, 3019– 3032. § WRF Tutorial presentations in PPT and PDF http: //www. mmm. ucar. edu/wrf/users/supports/tutorial. html § WRF technical paper http: //www. mmm. ucar. edu/wrf/users/pub-doc. html Jason Knievel ATEC Forecasters’ Conference, July and August 2006 45
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