Atmospheric flow modeling II

WIND ENERGY METEOROLOGY

UNIT 6
ATMOSPHERIC FLOW MODELING II:
MESO-SCALE MODELING

Detlev Heinemann

ENERGY METEOROLOGY GROUP
INSTITUTE OF PHYSICS
OLDENBURG UNIVERSITY
FORWIND – CENTER FOR WIND ENERGY RESEARCH

Mittwoch, 15. Juni 2011

ATMOSPHERIC FLOW MODELING II

SCALES OF ATMOSPHERIC MOTION

‣ The atmosphere features a wide range of circulation types, with a
wide variety of different behaviors.
‣ Typically, these circulations are classified according to their size
(spatial scale) and/or their oscillation period or duration (time
scale)

2



Scale Category Time Scale Spatial Scale Examples

seconds to turbulence, small
microscale meters to 1 km
minutes cumulus clouds

thunderstorms, sea
minutes to hours kilometers to
mesoscale breezes, mountain
to 1 day hundreds of km
circulations

fronts, cyclones,
synoptic scale days to weeks thousands of km
anticyclones

planetary waves,
planetary scale weeks to months global
el niño

3


EXAMPLE: MICROSCALE

boundary layer turbulence small cumulus clouds /
turbulent eddies

4


EXAMPLE: MESOSCALE

thunderstorms/collections of individual storms and their
thunderstorms component parts

5


EXAMPLE: MESOSCALE

mountain circulations (lee vortices in this case)

6


EXAMPLE: MESOSCALE

sea breeze circulations

7


EXAMPLE: SYNOPTIC SCALE

high and low pressure most of what we consider day-to-
systems, warm and cold fronts day weather

8


EXAMPLE: GLOBAL SCALE

planetary-scale waves climate patterns (e.g., el niño /
la niña)

9


MESOSCALE MODELING
EXAMPLE: WEATHER RESEARCH &
FORECASTING MODEL (WRF)

http://www.wrf-model.org

10


WHAT IS WRF...?

‣ Weather Research and Forecasting Model
‣ Operational forecasting and atmospheric research
‣ 'Community Model'
‣ Developed by NCAR and NOAA
‣ New Version – 3.3: released April 2011

11


WHAT IS WRF...?

WRF system components

12


WHAT IS WRF...?

‣ Non-hydrostatic model
‣ Terrain-following hydrostatic pressure coordinate
‣ Arakawa C-grid staggering
‣ Runge-Kutta 2nd and 3rd order time integration
schemes
‣ 2nd to 6th order advection schemes
‣ Semi-implicit acoustic step off-centering
‣ ARW and NMM dynamical cores.

13


WHAT IS WRF...?

‣ Advanced Research WRF (ARW) and Nonhydrostatic
Mesoscale Model (NMM) are both dynamical cores
‣ Dynamical core includes advection, pressure-
gradients, Coriolis, buoyancy, filters, diffusion
and time-stepping.
‣ Both use Eulerian mass dynamical cores with
terrain-following vertical coordinates
‣ Both share physics, software framework, and parts
of the pre- and post-processing systems.

14


ARW OR NMM...?

‣ ARW and NMM
‣ Atmospheric physics research
‣ Case-study research
‣ Real-time NWP and forecast system research
‣ Data assimilation research
‣ Teaching dynamics and NWP

‣ ARW only
‣ Regional climate and seasonal time-scale research
‣ Coupled-chemistry applications
‣ Global simulations
‣ Idealized simulations at many scales (e.g. convection,
baroclinic waves, large eddy simulations)

15


ARW DYNAMICS: KEY FEATURES

‣ Equations
‣ Fully compressible
‣ Non-hydrostatic
‣ Scalar conservative

‣ Vertical Coordinate
‣ Mass-based terrain
following coordinate
‣ Top of model is a constant
pressure surface

16


GOVERNING EQUATIONS

compressible,
nonhydrostatic, flux-form
Euler equations
(using a terrain-following
mass vertical coordinate)

plus

diagnostic relation for
the inverse density

equation of state

(plus inclusion of moisture...)
17


VERTICAL COORDINATE

The ARW equations are formulated using a terrain-following
hydrostatic-pressure vertical coordinate denoted by η and
defined as

where where

ph: hydrostatic component of the pressure
phs and pht: values along the surface and top boundaries,
respectively.

η varies from a value of 1 at the surface to 0 at the upper
boundary of the model domain. This vertical coordinate is also
called a mass vertical coordinate.

18



Horizontal grid

Arakawa C-grid staggering

19



‣ Prognostic Variables
‣ Velocity componets u, v
‣ Vertical velocity w, potential temp, geopotential, surface
pressure

‣ Time Integration
‣ 3rd order Runge-Kutta scheme
(Wicker and Skamarock, 2002)

Defining the prognostic variables in the ARW
solver as Φ = (U, V, W, Θ, φ′, µ′, Qm) and the
model equations as Φt = R(Φ), the RK3
integration takes the form of 3 steps to
advance a solution Φ(t) to Φ(t + ∆t):

20



‣ Spatial Discretization
‣ 2nd to 6th order advection options in horizontal and vertical

‣ Turbulent Mixing and Model Filters
‣ Divergence damping, sub-grid scale turbulence formulation

‣ Initial Conditions
‣ 3 dimensional for real cases
‣ Digital filtering initialization (DFI)

21



‣ Lateral Boundary Conditions
‣ Periodic, open, symmetric

‣ Top Boundary Conditions
‣ Gravity wave absorbing
‣ Constant pressure level
‣ Rigid lid option

‣ Bottom Boundary Conditions
‣ Physical or free-slip

22



‣ Earth's rotation
‣ Full Coriolis terms included

‣ Mapping to Sphere
‣ Four supported map projections

‣ Nesting
‣ One-way and two-way nesting
‣ Static or moving grids

‣ Nudging
‣ Grid analysis and observation nudging capabilities

23


WRF MODEL PHYSICS

‣ Microphysics: Bulk schemes ranging from simplified physics
suitable for mesoscale modeling to sophisticated mixed-phase
physics suitable for cloud-resolving modeling.
‣ Cumulus parameterizations: Adjustment and mass-flux
schemes for mesoscale modeling including NWP.
‣ Surface physics: Multi-layer land surface models ranging from a
simple thermal model to full vegetation and soil moisture
models, including snow cover and sea ice.
‣ Planetary boundary layer physics: Turbulent kinetic energy
prediction or non-local K schemes.
‣ Atmospheric radiation physics: Longwave and shortwave
schemes with multiple spectral bands and a simple shortwave
scheme. Cloud effects and surface fluxes are included.

24


WRF MODELING SYSTEM

25


WRF PRE-PROCESSING SYSTEM

‣ The purpose of the WPS is to prepare input to WRF for real-
data simulations. It:

‣ defines simulation domain and ARW nested domains
‣ computes latitude, longitude, map scale factor and Coriolis
parameters at every grid point
‣ interpolates time invariant terrestrial data to simulation grids
(e.g. terrain height and soil type)
‣ interpolates time-varying meteorological fields from another
model onto simulation domains

26



27


THE GEOGRID PROGRAM

‣ Geogrid defines
‣ Map projection
‣ Geographic location of domains
‣ Dimensions of domain

‣ Geogrid provides
‣ Values for static fields at each model grid point
‣ Computes latitude, longitude, map scale factor and Coriolis
parameters at each grid point
‣ Horizontally interpolates static terrestrial data (e.g.
topography, height, land use category, soil type, vegetation
fraction, monthly surface albedo)

28


THE GEOGRID PROGRAM

‣ First, choose a map projection
‣ Why? - The Earth is roughly ellipsoidal, but WRF
computational domains are defined by rectangles on a plane

‣ ARW can use the following projections
‣ Lambert conformal
‣ Mercator
‣ Polar stereographic
‣ Latitude-longitude

29


ARW PROJECTIONS

30


ARW DYNAMICS: NESTING

‣ A nested domain is wholly contained within its parent domain
and receives information from its parent.
‣ It may also feed information back to its parent [2-way nesting]
‣ A nested domain has exactly one parent
‣ A domain may have one or more children
‣ 2-way nests on the same nesting level must not overlap in
coverage

31


ARW DYNAMICS: NESTING

Nesting structure as a tree
for the domains at left

Example configuration: 4 domains Each domain is assigned
a domain ID #

32


GEOGRID: INTERPOLATING STATIC FIELDS

‣ Geogrid interpolates terrestrial, time-invariant fields
‣ Topography height
‣ Land use categories
‣ Soil type (top layer and bottom layer)
‣ Annual mean soil temperature
‣ Monthly vegetation fraction
‣ Monthly surface albedo

33



Normally, source data are given on a different projection from the
model grid

34



‣ Interpolation options

‣ 4-point bilinear
‣ 16-point overlapping parabolic
‣ 4-point average (simple or weighted)
‣ 16-point average (simple or weighted)
‣ Grid cell average
‣ Nearest neighbour
‣ Breadth-first search

35



36


THE UNGRIB PROGRAM

‣ What is a GRIB file?
‣ WMO standard file format for storing regulary distributed
fields
‣ General Regularly-distributed Information in Binary
‣ Fields are compressed with a lossy compression
[Think of truncating numbers to a fixed number of digits]

‣ Fields in file are identified by code number
‣ These numbers are referenced against an external table to
determing the corresponding field

37


THE UNGRIB PROGRAM

‣ What does it do?
‣ reads in GRIB data
‣ extracts meteorological fields
‣ derives required fields if necessary
‣ e.g. Computes RH from T, P and Q
‣ writes requested fields to an intermediate file format

‣ How does ungrib know which fields to extract?
‣ from vtables ...

38


THE UNGRIB PROGRAM: VTABLES

39



40


THE METGRID PROGRAM

‣ Horizontally interpolates meteorological data, extracted by
ungrib, to simulation domains, defined by geogrid
‣ Rotates winds to WRF grid
‣ i.e. rotates so that U-component is parallel to x-axis, V-
component parallel to y-axis

41


WPS SUMMARY

42


INITIALIZATION

‣ ideal.exe
‣ Program for controlled (idealized) scenarios
‣ Examples include 2-D and 3-D idealized cases, with or
without topography, with or without an initial thermal
perturbation.
‣ real.exe
‣ Program for real data cases
‣ Interpolates the intermediate files generated by metgrid.exe
in the vertical, creates boundary and initial condition files and
does some consistency checks.

43


RUNNING WRF

‣ ideal.exe
‣ Program for controlled (idealized) scenarios
‣ Examples include 2-D and 3-D idealized cases, with or
without topography, with or without an initial thermal
perturbation.
‣ real.exe
‣ Program for real data cases
‣ Interpolates the intermediate files generated by metgrid.exe
in the vertical, creates boundary and initial condition files and
does some consistency checks.

44


POST-PROCESSING: GRAPHICS

‣ Several graphical packages available
‣ NCL
‣ ARWpost
‣ RIP4
‣ VAPOR
‣ IDV

45


POST-PROCESSING: GRAPHICS

NCL graphis
‣ NCAR Command Language
‣ reads in WRF-ARW data
directly
‣ generates a number of
graphical plots using scripts
‣ e.g. Horizontal, cross-
section, skewT,
meteogram, panel

46


POST-PROCESSING: VERIFICATION

MET verification software

‣ Model Evaluation Tools
‣ All the basics – RMSE, bias, skill scores
‣ Advanced spatial methods (wavelets, objects)
‣ Confidence intervals

47

Atmospheric flow modeling II

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Atmospheric flow modeling II