A Different Approach for Scale-Awareness in Convective Parameterizations

Convection parameterizations are sub-model components of atmospheric models that aim to represent the statistical effects of a sub-grid scale ensemble of convective clouds. This is done in models in which the spatial resolution is not sufficient to resolve the associated convective circulations. When horizontal resolutions where convection is neither resolved nor unresolved are used, we speak of “convection allowing” resolutions. Using convective parameterizations on these so called “gray scales” has gotten much attention over the last decade. A frequently used approach to make a convection parameterization scale-aware was introduced by Arakawa (2011) by rederiving the equation for the vertical eddy fluxes in terms of the fractional area covered by the active cloud draft and the vertical eddy flux term given by conventional (non-scale aware) tendencies. This led to a simple scaling of the original tendencies from the convection parameterization. Some type of scaling approach is now used in many other convective parameterizations to implement scale-awareness. We will show that any scaling approach has significant physical shortcomings. We will introduce a different method to provide a possible solution. Our new scale-aware approach is implemented in the Grell-Freitas convection scheme, and tested in the Model for Prediction Across Scales (MPAS) in a convection-allowing setup (dx ~ 3km).