Crossing Multiple Gray Zones in the Transition from Mesoscale to Microscale Simulation over Complex Terrain

Abstract

This review paper explores the field of mesoscale to microscale modeling over complex terrain as it traverses multiple so-called gray zones. In an attempt to bridge the gap between previous large-scale and small-scale modeling efforts, atmospheric simulations are being run at an unprecedented range of resolutions. The gray zone is the range of grid resolutions where particular features are neither subgrid nor fully resolved, but rather are partially resolved. The definition of a gray zone depends strongly on the feature being represented and its relationship to the model resolution. This paper explores three gray zones relevant to simulations over complex terrain: turbulence, convection, and topography. Taken together, these may be referred to as the gray continuum. The focus is on horizontal grid resolutions from ∼10 km to ∼10 m. In each case, the challenges are presented together with recent progress in the literature. A common theme is to address cross-scale interaction and scale-awareness in parameterization schemes. How numerical models are designed to cross these gray zones is critical to complex terrain applications in numerical weather prediction, wind resource forecasting, and regional climate modeling, among others.

Current Challenges in Understanding and Predicting Transport and Exchange in the Atmosphere over Mountainous Terrain

“…The correct representation of atmospheric stability, particularly under stable situations, is, however, crucial for simulating exchange processes between mountains and the atmosphere aloft correctly. Detailed discussions of several issues related to high-resolution modeling in complex terrain can be found in Zhong and Chow [145], Doyle et al [146], and Chow et al [147] and are thus only briefly summarized here. The topic of data assimilation in mountainous terrain and related challenges are discussed in Hacker et al [148].…”

“…As a result, model performance in the gray zone has become a focus of ongoing research, highlighting, for example, that convection characteristics can become grid dependent and differ from reality [166] or that gravity wave drag parameterizations may require topography-dependent tuning [75]. Model gray zones for complex-terrain simulations are discussed in detail in Chow et al [147].…”

“…Other alternatives are finite-volume and finite-element methods, whose potential application to complex-terrain simulations is briefly discussed in Arnold et al [172]. Further discussion of vertical coordinates and topography can be found in Chow et al [147].…”

Random Force Perturbations: A New Extension of the Cell Perturbation Method for Turbulence Generation in Multiscale Atmospheric Boundary Layer Simulations

“…However, for the case of LES over highly resolved domains that are larger than these mesoscale heterogeneities, homogeneous boundaries are no longer an accurate representation of the heterogeneous forcing. For example, LES over very large domains (Schalkwijk et al, ) may resolve heterogeneous, mesoscale weather patterns, while complex terrain (Sauer et al, ) can produce complex heterogeneities with a wide range of scales (Chow et al, ).…”

High-Resolution Observations of Transport and Exchange Processes in Mountainous Terrain

“…The purpose of this contribution to a series of further papers overviewing the knowledge and challenges on mountain meteorology [1,11,14,15,18,42,43] is to address the necessary observational techniques which have to be made available in order to achieve the relevant information of transport and exchange processes in complex terrain. The focus will be on radiation, kinetic energy, heat and moisture.…”