CCSM Atmospheric Model Working Group Report J. J. Hack, D. A Randall AMWG Co-Chairs CCSM 2001 Workshop, Breckenridge CCSM 2001 Workshop, Breckenridge

What has been happening since the last time we met? n Indicated our intentions for a final model configuration –two-time level semi-Lagrangian dynamics with reduced linear grid –26 levels –prognostic cloud water –generalized cloud overlap (plus requisite changes to LW SW) –updated long wave water vapor absorption formulation –new convection parameterization –netCDF native output –improved topographic filter n Were planning to conduct initial coupled simulations –new ocean, sea ice, land, and coupler

Cumulus Convection Parameterization n Variant of Arakawa-Schubert –developed by Marat Khairoudinov (CSU) –multiple cloud bases/cloud tops –simplified closure, computationally efficient n equilibrium cumulus kinetic energy closure –can handle arbitrary tracer transport –framework can be directly coupled to stratiform cloud and boundary layer parameterizations

Strengths/Weaknesses of Proposed Model n Improvements in a number of simulation features –precipitable water, implied ocean heat transport, polar thermal bias, variability, eastern ocean solar energy budget n Degradations in several important areas –Wintertime stationary wave structure –Excessive precipitation maxima –No improvement in Arctic Surface wind field –Cold tropical tropopause –Non-conservative advection of tracers

Tests of the proposed configuration n Uncoupled climatological forcing n 17-year AMIP II simulations n No coupled simulations to date –immediate priority was to enable a coupled evaluation

Ongoing Diagnostic Analyses n NCAR –standard WGNE diagnostic suite n Stevens and Hack –variability and other mean properties n Hurrell and Shea n PCMDI (Wehner, Taylor, Sperber, Boyles, Achutarao) –performance portraits, mean state, variability –comparison to other models and observations n Variability Analyses (through intraseasonal) –MJO (Demott, Waliser, Ricciardulli) –diurnal characteristics (Dai)

In late May we reconsidered options n Tested a modification to the current ZM scheme –incorporates a precipitation evaporation mechanism n Proposed alternative configuration to AMWG –model was same except for following n 3-time level spectral dynamical core with reduced grid n modified Zhang-McFarlane convection n Put this decision to a “vote” from the AMWG –extensive diagnostic studies were completed and available n Comments favored alternative configuration

Strengths/Weaknesses of New Model n Clear improvements to several simulation features –precipitable water –implied ocean heat transport –polar thermal bias –eastern ocean solar energy budget –eastern Pacific surface stress n Degradations in several important areas –No improvement in Arctic Surface wind field –Cold tropical tropopause –Non-conservative advection of tracers (no different than CCM3)

Summary Development of new atmospheric component is complete and undergoing continued testing! n Improvements to physical and dynamical framework n Improvements to the simulation n Improvements to computational performance n Demonstrated the AMWG development process works