Extension and Evaluation of University of Washington Moist Turbulence Scheme to Gray‐Zone Scales

With the rapid improvement in computational resources, it has become possible to structure numerical weather model simulations to use sub‐kilometer grid spacing. Traditional turbulence‐mixing closures have become obsolete within the context of the gray zone of turbulence. Here, a new scale‐aware algorithm (SA‐UW) is developed based on the UW (University of Washington Moist Turbulence) scheme. By dividing both the local and nonlocal vertical turbulence flux into sub‐grid and resolved parts, the partition of sub‐grid turbulence is tuned by a set of well‐acknowledged scale‐dependent relationships. Idealized simulations confirm that the newly developed scheme can adequately reproduce sub‐grid turbulence transport at gray‐zone scales. The inclusion of a nonlocal transport term in the SA‐UW scheme is essential, as it vastly improves the distribution and intensity of coherent structures. The SA‐UW scheme recovers the conventional UW scheme when grid spacing is larger than the turbulence length scale, thereby evincing the sale‐aware capability of the new method. For real‐case simulations, the SA‐UW scheme can capture the diurnal cycle and provide a more accurate vertical structure of temperature and humidity. Plain Language Summary Turbulence mixing within the planetary boundary layer is a key process which needs to be parameterized in numerical weather models. Owing to the increasing computational power, operational numerical weather prediction centers around the world have begun to carry out simulations with horizontal resolutions of the order of several kilometers to sub‐kilometers. However, traditional turbulence‐mixing parameterization schemes are not designed for these so‐called “terra incognita” or “gray zone” resolutions. This study aimed to develop a turbulence‐mixing scheme suitable for simulations from the gray zone to mesoscale models. Compared with some traditional schemes, the new scheme improves the vertical structure of the convective boundary layer, including wind speed, temperature, moisture, and turbulence fluxes. The new scheme shows promising prospects for improving the model performance at gray‐zone scales. Key Points A scale‐aware planetary boundary layer scheme (SA‐UW) was developed to address simulation issues at gray‐zone scales The new scheme fully takes into account the scale‐awareness of both the local and nonlocal transport in the mixed layer Idealized and real case simulations show considerable improvement in the new scheme compared