Using HI-SCALE to Evaluate Parameterizations of Boundary Layer Cloud Fraction: the MYNN-EDMF Scheme

The Holistic Interactions of Shallow Clouds, Aerosols and Land Ecosystems (HI-SCALE) field campaign was conducted in the vicinity of the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site in north-central Oklahoma over the spring and summer of 2016. During the field campaign, the existing suite of routine atmospheric, soil, flux, and radiation measurements at SGP was supplemented by additional surface-based and aircraft platforms. In this study, we use satellite, lidar, and other HI-SCALE platforms to evaluate the performance of subgrid cloud prediction by a boundary layer parameterization in mesoscale simulations of the Weather Research and Forecasting (WRF) model, the Mellor-Yamada-Nakanishi-Niino (MYNN) eddy diffusion mass flux (MYNN-EDMF) scheme. The skill of the model configuration in predicting cloud fraction and surface irradiance as a function of scale, and relative to other parameterizations of subgrid cloud fraction, will be discussed

The MYNN-EDMF scheme uses a subgrid multi-plume model to represent upward vertical mixing in the convective boundary layer along with traditional gradient diffusion for non-convective mixing. This design allows the scheme to predict the evolution of and the transition between dry convection and shallow convection in a more unified and physically realistic manner than other formulations. MYNN-EDMF is now being used operationally in the WRF-based Rapid Refresh (RAP) / High Resolution Rapid Refresh (HRRR) system.

Boundary layer clouds, whether deep or shallow, strongly modulate the Earth's radiative budget through their suppression of surface downwelling shortwave radiation. They also can play a critical role in the vertical transport of heat, momentum, and tracers. Since their formation often occurs on scales too small to be adequately resolved by global- or even regional-scale atmospheric models, there is an ongoing need for parameterizations of subgrid cloud fraction that are appropriate to the whole range from dry (cloud-free) convection to shallow and deep convective cloud states.