Improving the turbulent mixing & non-local transport in the MYNN PBL scheme for improved consistency across spatial scales

Initial attempts to produce scale-insensitive simulations in the Limited Area Model (LAM) component of the Grey Zone Experiment revealed some struggles in capturing the cloud morphology in a cold-air outbreak case. The most troubling behavior of MYNN PBL scheme (as is exists in WRF-ARW) was the tendency to produce too much stratus at coarse (dx=16 and 8 km) resolutions while producing too much small-scale cumulus at high (dx=2 and 1 km) resolutions. An assessment of these results concluded that a lack of non-local transport was the primary cause of these deficiencies. Efforts were made to improve the simulations by adding a mass-flux scheme to the MYNN as well as a cloud-specific mixing length. Results will be presented to show that these modifications helped to improve several aspects of the cold-air outbreak case. These modifications have also been implemented into experimental versions of NOAA's Rapid Refresh & High-Resolution Rapid Refresh. General improvements to these forecast systems will also be highlighted.