Comparison of Different Approaches NCAR Earth System Laboratory National Center for Atmospheric Research NCAR is Sponsored by NSF and this work is partially supported by the Willis Research Network and the Research Program to Secure Energy for America Greg Holland Note: This discussion applies only to regional climate simulations and interpretations

Summary of Different Approaches Model Dynamics and Physics Spectral Nudging Horizontal Boundary Conditions Surface Boundary (ocean and land) Length of Forecasts Potential Impacts of Different Approaches: Level I: results in scatter, but does not substantially change the overall outcome or interpretation; Level II: may substantially impact the overall interpretation but this cannot be fully quantified; Level III: probably will substantially impact the overall interpretation. Holland Comparison of Approaches RPSEA 0310

Model Dynamics Dynamical Core – Discretization of the equations – Grid solvers – Accuracy and long-term drift Level I impacts on changes Holland Comparison of Approaches RPSEA 0310

Cloud Physical Processes For coarse grids, dx>15-20 km, require cumulus parameterization Issues – Changing the parameterization scheme, or even tuning the inherent parameters can make a substantial difference to the results – This is sorted empirically by experience and comparison with known climate (we did this in NRCM Phase I) Level I impacts on changes. Holland Comparison of Approaches RPSEA 0310

Cloud Physical Processes ctd Issues: – For very fine grids, dx<4-5 km, cloud physical processes directly (called resolvable convection) – From 4-20 km there is no good approach. – GFDL and others have used explicit clouds and no parameterization in the km zone – NCAR does not generally do simulations for 4<dx<12 km, above 12 km we use cumulus parameterization, occasionally with some explicit clouds also. Level II impacts, requires some further investigation to determine actual impacts Holland Comparison of Approaches RPSEA 0310

Other Physical Processes Boundary-layer transfers, radiation budget, etc All are different, but all are also carefully compared to “reality” Probably Level I impacts. Holland Comparison of Approaches RPSEA 0310

Spectral Nudging Nudging the interior domain to force it to follow the global model, with a spectral cut-off – e.g. GFDL use global wave numbers 0,1 and 2 for spectral nudging Issues may result in a false sense of accuracy when a good fit is obtained for current climate using global analyses, but it then can introduce substantial errors when forced to follow the global climate model for predictions Level II and Level III impacts. Holland Comparison of Approaches RPSEA 0310

Example Impact of Spectral Nudging 2005 Aug-Sept-Oct (mm dd) Observation 15 Storms Spectral Nudging 9 Storms Control 12 Storms

Horizontal Boundary Conditions The global model forces the regional model across the horizontal boundaries, different approaches include: – Hand all information from the global domain – Use current weather and add climate increments of, e.g. temperature, humidity, mean wind conditions, etc – Bias correct the global model for known errors Issues: small domains, use of combination of current analysis and climate perturbation Level II and Level III impacts. Holland Comparison of Approaches RPSEA 0310

WRF 12 km WRF 36 km Image by Steve CCSM ~ 150 km Nested Regional Climate Model Specifics Holland Comparison of Approaches RPSEA 0310 Outer domain size chosen to maximize internal generation of relevant weather systems, such as easterly waves. And thus to minimize impacts of known biases in the global climate models.

Surface Boundary Condition Fixed Ocean (defined by bias-corrected global model) – Careful testing leads us to the conclusion that this is not a major problem in the Atlantic….Level I Impact However – Eastern Pacific climate model bias definitely leads to Level III impacts on the Atlantic…. Holland Comparison of Approaches RPSEA 0310

NCEP/NCAR Reanalysis CCSM Windshear Bias in CCSM (Vecchi and Soden 2007)

Yamada et al (2010) Their Conclusion: “Consistent with recent studies, frequency is reduced over the North Atlantic due to intensified vertical wind shear.” Holland Comparison of Approaches RPSEA 0310

Length of Forecasts Computing limitations mean that compromises have to be made on the length of climate predictions at high resolution: – We decided on 3x11-y time slices – GFDL use 3-months Aug-Sep-Oct for each year – Yamada et al (2010) did a single 5-month global simulation. Level I-III impacts: – For NRCM, Level I Holland Comparison of Approaches RPSEA 0310

Summary Holland Comparison of Approaches RPSEA 0310 The choice of model configuration and the way in which model biases are handled can have a substantial impact on the prediction. For existing studies the level of impacts are: Model Dynamics: Level I Model Physics: Level I-III Spectral Nudging: Level III Horizontal Boundary Conditions: Level I-III Surface Boundary (ocean and land): Level III Length of Forecasts: Level I