Diagnosing topographic forcing in an atmospheric dataset: The case of the North American Cordillera. (2nd December 2019)

• Record Type: Journal Article
• Authors: Hartung, Kerstin; Shepherd, Theodore G.; Hoskins, Brian J.; Methven, John; Svensson, Gunilla
• Is Part Of: Quarterly journal of the Royal Meteorological Society. Volume 146:Number 726(2020)
• Page Start: 314
• Page End: 326
• Publication Date: 2019-12-02
• Subjects: large‐scale atmospheric circulation -- shallow‐water model -- stretching term -- topographic forcing -- trajectories; Meteorology -- Periodicals; 551.5
• DOI: 10.1002/qj.3677
• Languages: English
• ISSNs: 0035-9009

Abstract

Abstract: It is well known from modelling studies that surface topography influences the large‐scale atmospheric circulation and that several model biases are associated with incorrect representation of topography. The textbook explanation of topographic effects on large‐scale circulation appeals to the theoretical relationship between surface forcing and vortex stretching along trajectories in single‐layer models. The goal of this study is to design and use a simple diagnostic of the large‐scale forcing on the atmosphere when air is passing over topography, directly from atmospheric fields, based on this theoretical relationship. The study examines the interaction of the atmosphere with the North American Cordillera and samples the flow by means of trajectories during Northern Hemisphere winter. We detect a signal of topographic forcing in the atmospheric dataset, which, although much less distinct than in the theoretical relationship, nevertheless exhibits a number of expected properties. Namely, the signal increases with latitude, is usually stronger upslope than downslope, and is enhanced if the flow is more orthogonal to the mountain ridge, for example during periods of positive Pacific–North American index (PNA). Furthermore, a connection is found between an enhanced signal of topographic forcing downslope of the North American Cordillera and periods of more frequent downstream European blocking. Abstract : We diagnose the large‐scale topographic forcing on the atmosphere directly from reanalysis data based on the stretching term of the barotropic vorticity equation. Sampling the flow by means of trajectories over the North American Cordillera, we find such a signal of direct topographic forcing, which increases with latitude, is usually stronger upslope than downslope, and is enhanced if the flow is more orthogonal to the mountain ridge and during periods of more frequent downstream European blocking.