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Subseasonal to Seasonal Prediction with NCAR’s CESM2-WACCM

The goal of this joint project between NCAR, NOAA ESRL Physical Sciences Division and
NOAA/NCEP Climate Prediction Center (CPC) is to test and demonstrate the utility of NCAR’s
Community Earth System Model, version 2 with the Whole Atmosphere Community Climate
Model as its atmospheric component (CESM2-WACCM) as a subseasonal-to-seasonal (S2S)
prediction system that could improve NCEP’s operational multi-model S2S prediction capability.
CESM2-WACCM will be released by NCAR in Fall 2017. This whole atmosphere-ocean
coupled climate model has several improved features that address sources of predictive skill on
S2S time scale, including proper representation of stratospheric processes (e.g. Quasi-Biennial
Oscillation (QBO) and Stratospheric Sudden Warmings (SSWs)), Madden Julian Oscillation
(MJO), El Niño/Southern Oscillation (ENSO) and land surface processes. Furthermore, these
features address improved multiscale interactions between QBO, MJO and SSWs that can
provide rather untapped source of subseasonal predictability. Finally, using the Data
Assimilation Research Testbed (DART) to initialize WACCM allows for the utilization of high
altitude observations in S2S predictions.

In this proposed NOAA Climate Test Bed project, NCAR and NOAA/ESRL will work closely
with NCEP/CPC to:
1. Demonstrate the utility of a prototype of the CESM2-WACCM S2S system by running a
set of reforecasts for the 1999-2018 time period in compliance with the SubX and CPC
protocols (transition from Readiness Level (RL) 5 to 6).
2. Run CESM2-WACCM in real-time, operational mode (transition from RL 6 to 7).
3. Evaluate inclusion of the real-time, operational environment prototype forecasts from
CESM2-WACCM into CPC forecasts (transition from RL 7 to 8).

Improvements of the CESM2-WACCM system will be determined by comparing the new system
with reforecasts and real time forecasts from operational systems using CPC’s metrics for
evaluation as well as additional process-oriented diagnostics. These process evaluations will
focus on multi-scale/mode interactions between QBO, MJO, extreme stratospheric vortex events
and tropospheric modes of variability. In addition to comparing CESM2-WACCM to current
models available in the SubX database, we will utilize comprehensive reforecast data sets from
previous versions of CESM that will be submitted to the SubX project and differ in their
representation of stratospheric processes and tropospheric physics.

The proposed project directly addresses MAPP competition 3: NOAA Climate Test Bed –
Advancing NOAA’s Operational Subseasonal to Seasonal Prediction Capability by improving
the SubX multi-model ensemble prediction systems. Specifically, the utility of a new model that
can tap into potential sources of S2S predictive skill is tested. This project addresses NOAA’s
long-term goal: “Weather-Ready Nation Society is prepared for and responds to weather-related
events” by carrying out research on and providing tools for the prediction of extreme events on
the subseasonal time scale.

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