A Mesoscale Model-Based Climatography of Nocturnal Boundary-Layer Characteristics over the Complex Terrain of North-Western Utah.

Stefano Serafin,Stephan F J De Wekker,Jason C Knievel

doi:10.1007/s10546-015-0044-6

Abstract

Nocturnal boundary-layer phenomena in regions of complex topography are extremely diverse and respond to a multiplicity of forcing factors, acting primarily at the mesoscale and microscale. The interaction between different physical processes, e.g., drainage promoted by near-surface cooling and ambient flow over topography in a statically stable environment, may give rise to special flow patterns, uncommon over flat terrain. Here we present a climatography of boundary-layer flows, based on a 2-year archive of simulations from a high-resolution operational mesoscale weather modelling system, 4DWX. The geographical context is Dugway Proving Ground, in north-western Utah, USA, target area of the field campaigns of the MATERHORN (Mountain Terrain Atmospheric Modeling and Observations Program) project. The comparison between model fields and available observations in 2012–2014 shows that the 4DWX model system provides a realistic representation of wind speed and direction in the area, at least in an average sense. Regions displaying strong spatial gradients in the field variables, thought to be responsible for enhanced nocturnal mixing, are typically located in transition areas from mountain sidewalls to adjacent plains. A key dynamical process in this respect is the separation of dynamically accelerated downslope flows from the surface.

Highlights

Granite Peak, located in the Dugway Proving Ground in north-western Utah, is an isolated mountain rising about 800 m above the surrounding terrain (Fig. 1)
Two years of simulations from a limited-area weather prediction model are analyzed in order to provide insight into yet poorly understood aspects of nocturnal planetary boundary layer (PBL) circulations in an area with complex topography and land cover, viz. Dugway Proving Ground in north-western Utah
Our study focuses instead on nocturnal PBL phenomena and aims to quantify the impact of mesoscale topography on airflow patterns

Summary

Introduction

Granite Peak, located in the Dugway Proving Ground in north-western Utah, is an isolated mountain rising about 800 m above the surrounding terrain (Fig. 1). Sonic anemometer and scanning lidar measurements provided observational evidence of these convergence lines during MATERHORN (e.g. Lehner et al 2015; Fernando et al 2015) Such events generate vigorous mixing even during the night, in contrast to the typical behaviour of the stable boundary layer over flat terrain (Fernando et al 2015). These MATERHORN observations have been interpreted almost exclusively as evidence of “collisions” of drainage flows, caused by differential thermal forcing along slopes and adjacent valleys. We test this hypothesis by analyzing operational high-resolution numerical weather prediction products, which are continuously available for Dugway Proving Ground from the 4DWX model system.

Observational Data

The 4DWX Model Forecast System

Comparison Between Observations and Simulations

Wind Climate Simulated by the 4DWX Model System

Findings

Discussion and Conclusions