Atmospheric Modeling in an Arctic System Model

John J. Cassano

Cooperative Institute for Research in Environmental Sciences

and Department of Atmospheric and Oceanic Sciences

University of Colorado

Proposed Arctic System Model Domain

Why develop an ASM?

• The Arctic is a unique region that presents unique challenges for climate modeling– An Arctic specific model can use model components that are developed

specifically for polar regions and polar processes • Polar clouds and radiative fluxes• Boundary layer and surface flux processes

• Existing regional Arctic climate models do not account for important feedbacks between Arctic climate system components– Current regional Arctic models simulate atmospheric state but have

specified ocean and ice properties

• Arctic regional models are not subject to low latitude errors present in global climate models– Simulations with an ASM can use “perfect” lateral boundary conditions– Can explore polar processes without feedbacks to lower latitudes

Why develop an ASM?

• Increased horizontal resolution compared to global climate models– Approx. 1 order of magnitude increase in horizontal

resolution compared to global models– Increased horizontal resolution allows for:

• Improved representation of topography and coastlines• Improved representation of small-scale processes

– Extreme events and storms

• More realistic representation of interactions and feedback processes among climate model components

• Better match between model resolution and:– end user needs (e.g. policy decisions)– resolution of other climate system component models– field studies

What Scientific Questions can be Addressed with an ASM?

• Feedback studies– Atmosphere / land hydrology

• Changes in permafrost, soil hydrology, and atmospheric circulation– How will degrading permafrost alter near surface soil moisture?– What impact will changes in soil moisture have on the atmosphere?– Will altered atmospheric state intensify or dampen initial soil moisture

changes due to permafrost degradation?

• Impact of extreme storm events on land hydrology

– Atmosphere / ice / ocean• Role of small scale processes such as polar lows

– How does ice / ocean state impact polar low development?– How do polar lows alter the ice / ocean system?– Do these small scale processes impact larger scales?

What Scientific Questions can be Addressed with an ASM?

• How do high-resolution simulations of the Arctic climate system differ from global climate model simulations?– Consider observed changes in Arctic sea ice

• How do ASM simulations differ from GCSM simulations?• Are different feedback processes acting in the ASM and GCSM?

• What is the role of lower latitude variability vs internal Arctic system processes on observed Arctic change?– Experiments using lateral boundary forcing from multiple GCSMs

and from global reanalyses

Atmospheric Model Requirements

• Community model with active research and development

• Suitable for high resolution (O 1-10 km) simulations

• Capable of long duration climate simulations• Model capable of being run on many different

computer platforms• Optimized for polar applications

ASM Atmospheric Model: WRF

• Suitable for high resolution (O 1-10 km) simulations – Fully compressible, nonhydrostatic dynamics– Significantly improved numerics and dynamics

compared to MM5– Designed for high-resolution applications

• Large eddy simulation• Cloud resolving model• Mesoscale applications

ASM Atmospheric Model: WRF

• Capable of long duration climate simulations– WRF is formulated for mass and scalar conservation

• No long term drift due to model numerics

– Complete atmospheric physics• Radiation• Surface fluxes and land surface• Planetary boundary layer (PBL)• Cloud microphysics• Cumulus parameterization• Multiple options are available for all physical processes

– WRF - Chem is currently under development• Will allow coupled atmosphere - chemistry simulations

WRF Physics Interactions

From NCAR WRF tutorial

ASM Atmospheric Model: WRF

• Model capable of being run on many different computer platforms – WRF runs on Unix single, OpenMP, and MPI platforms

• IBM• Linux (PGI and Intel compilers)• SGI Origin and Altix• HP / Compaq / DEC• Cray• Sun• Mac (xlf compiler)

ASM Atmospheric Model: WRF

• Optimized for polar applications– Prior atmospheric model development effort led to the widely

used Polar MM5• Focus on:

– Cloud and radiation processes– PBL and surface fluxes – Treatment of ice covered land– Sea ice

– On-going development of Polar WRF• Cassano research group at CU• Polar Meteorology Group - BPRC / OSU• NOAA ESRL - Boulder• NCAR - AMPS

Fairbanks - July 2006

WRF - RRTM

Bias: -0.1 mb

Corr: 0.98

WRF - CAM

Bias: 0.8 mb

Corr: 0.98

Barrow - January 2006

WRF - RRTM

Bias: 10.6 deg

WRF - CAM

Bias: 2.7 deg

SHEBA Site - January 1998

Courtesy of Keith HinesBPRC / OSU

Questions or comments?

WRF Model Details

• Mass basedterrain followingvertical coordinate

From NCAR WRF Tutorial

WRF Model Details

• Uses Arakawa C-grid staggering

From NCAR WRF Tutorial

WRF Model Details

• Lateral boundary conditions– Specified from reanalyses, global, or regional models– Open, symmetric, or periodic for idealized simulations

• Top boundary conditions– Constant pressure– Rayleigh damping– Absorbing upper layer– Gravity wave radiation condition (planned)

• Map projections– Polar stereographic– Lambert conformal– Mercator– Cartesian geometry (idealized only)

WRF Model Details

• 3rd order Runge-Kutte time integration• High-order advection scheme• Mass and scalar conserving numerics• One and two-way nesting options• Four dimensional data assimilation (FDDA)• Model physics

– Radiation– Surface– Planetary boundary layer (PBL)– Cloud microphysics– Cumulus

Radiation

• Provides:– Atmospheric temperature tendency– Surface radiative fluxes (SW and LW)

• Options:– Longwave

• RRTM• CAM3• GFDL

– Shortwave• MM5• Goddard• CAM3• GFDL

From NCAR WRF Tutorial

Surface

• Provides:– Surface turbulent fluxes– Soil temperature and moisture– Snow cover– Sea ice temperature

• Options:– Fluxes: Monin-Obukhov similarity theory– Noah LSM– NCEP Noah LSM– RUC LSM

From NCAR WRF Tutorial

Planetary Boundary Layer

• Provides:– Boundary layer fluxes– Vertical diffusion / mixing

• Options:– YSU PBL– Eta PBL– GFS PBL– MRF PBL

From NCAR WRF Tutorial

Cloud Microphysics

• Provides:– Atmospheric heat and moisture tendencies– Cloud and precipitation amount– Surface rainfall

• Options:– Kessler warm rain– Purdue - Lin– WSM 3-class– WSM 5-class– WSM 6-class– Ferrier (NAM)– Thompson

From NCAR WRF Tutorial

Cumulus

• Provides:– Atmospheric heat and moisture tendencies– Surface rainfall

• Options:– Kain-Fritsch– Betts-Miller-Janjic– Grell-Devenyi Ensemble– Simplified Arakawa Schubert GFS

From NCAR WRF Tutorial