P25     Recent development of the MYNN turbulence parameterization for RAPv5 / HRRRv4.

Kenyon, Jaymes, Joseph Olson, Wayne Angevine, CIRES and NOAA/ESRL, and John Brown, NOAA/ESRL

The Mellor–Yamada–Nakanishi–Niino (MYNN) turbulence parameterization in WRF–ARW has been under intensive development since 2014, driven by imperatives to advance forecast skill in NOAA's RAP and HRRR models.  In its present form, the MYNN is a non-local scale-adaptive moist-turbulent boundary-layer scheme that parameterizes turbulent fluxes using an eddy-diffusivity / mass-flux (EDMF) approach, while also providing a representation of subgrid-scale (SGS) cloudiness for coupling to the RRTMG radiation scheme.

Numerous improvements and new capabilities have been incorporated into the MYNN scheme as part of the RAPv5/HRRRv4 development effort, including: (1) improved representation of convective and nonconvective SGS clouds; (2) the representation of top-down turbulent mixing linked to radiative cloud-top cooling; (3) the capability to cycle TKE; (4) the representation of TKE dissipative heating; and (5) the option to mix second moments from the microphysics scheme.  These features are available to the WRF–ARW community as of the version 4.1 release.

This presentation will summarize MYNN development efforts and provide an update on  "best practices" for MYNN users in the broader community.  Drawing from test cases and retrospective simulations, we demonstrate the impact of this MYNN development in the context of improvements to RAP/HRRR forecast skill.  Finally, we will outline future directions for MYNN development as part of the RAP/HRRR physics suite.