Evaluation and Improvement of an Inflow-Nudging Technique for Idealized Simulations of Convective Boundary Layers

Abstract

Recent idealized modeling studies have highlighted the importance of explicitly simulating realistic convective boundary layer (CBL) structures to assess and represent their influence on mesoscale phenomena. The choice of lateral boundary conditions (LBCs) has a substantial impact on these turbulent structures, including the distribution of kinematic and thermodynamic properties within the CBL. While use of periodic LBCs is ideal, open LBCs are required for nonuniform domains (e.g., multiple air masses or land surface types). However, open LBCs result in an unrealistic, laminar CBL structure near the upstream boundary that undoubtedly impacts the evolution of any simulated phenomena. Therefore, there is a need for a modified open LBC option to mitigate this unrealistic structure, while still permitting users to simulate phenomena in nonuniform domains. The Pennsylvania State University–NCAR Cloud Model 1 (CM1), version 19.8, includes an optional inflow-nudging technique to nudge inflow to the base-state wind profile. For the present study, the authors modified this method to one that nudges toward a continually updated, horizontally averaged profile so that the technique may be used for phenomena under evolving conditions. Simulations using LBC choices, including nudging to either the base state or horizontal average, were evaluated relative to respective dual-periodic LBC control simulations with or without vertical wind shear. The horizontal average nudging technique outperformed the traditional open LBCs and nudging to the base state, as demonstrated using a histogram matching technique applied to grid points within the CBL. Ultimately, this work can be used to assist modelers in assessing which LBCs are appropriate for their intended use.