Blocking Error in 10-day to 1-year Global Model Forecasts and Dependency on Resolution and Model Numerics

Blocking error in 10-day to 1-year global model forecasts and dependency on resolution and model numerics Stan Benjamin, Shan Sun, Rainer Bleck, Xue Wei, John M. Brown, Michael Fiorino, Randall Dole, Kathy Pegion NOAA Earth System Research Laboratory Boulder, CO Deficiency in blocking in global climate models is a well-known, significant problem (e.g., Scaife et al. 2010, D'Andrea et al. 1998), also present in global weather models. The best-documented factors related to this systematic behavior include horizontal resolution and internal model capacity for blocking. Initial results from Project Athena confirmed this resolution dependency. CFSv2 and GFS occupy NOAA's operational suite slots for subseasonal and medium-range prediction, respectively. Like other global models, these models show some modest deficit in blocking frequency versus that which is observed. Many international laboratories and centers are developing a next-generation set of global models as partly represented in the Dynamic Core Model Intercomparison Project (DCMIP - http://earthsystemcog.org/projects/dcmip-2012/ ). A NOAA participant in DCMIP is the Flow-following finite-volume Icosahedral Model (Bleck et al., 2010, Mon. Wea. Rev.). FIM uses an isentropic-sigma hybrid vertical coordinate designed for improved conservation of potential vorticity and accuracy in 3-dimensional transport. A related coupled atmospheric-ocean model linking FIM with an icosahedral version of the HYCOM ocean model and a variation of GFS/CFSv2 physics is also now being tested for subseasonal-seasonal application. FIM has been tested substantially for 1-14-day weather forecasts showing equal or improved skill over GFS when run at similar resolution. The development of FIM and other next-generation global models (listed among DCMIP participants), enabled by improvements in computational speed, opens the door for new studies to investigate GFD issues related to blocking. Initial results run earlier in 2013 showed little difference in blocking frequency from 15-30-day forecasts from FIM using 60/30/15km resolution but improved block duration at 15km. At the conference, we will present new results for blocking frequency for integrations from 10-360 days in duration from comparison of FIM model (AMIP and CMIP) runs at different horizontal resolutions (120/60/30/15km). Results will also be shown for dependence on vertical coordinate, comparing use of a more-PV-conserving hybrid ϴ-𝛔 coordinate vs. a sigma-pressure hybrid coordinate as used in GFS and IFS, and on strength of momentum diffusion. This research will be the beginning of an expected sustained effort to improve global modeling on blocking and related extreme weather (drought, flooding, sustained heat/cold events) and understanding of processes related to onset and cessation of blocking (e.g., MJO, stratospheric events) under Demonstration #1 of a US Earth System Prediction Capability (ESPC) program.