Abstract
This paper examines current understanding of the influence of orographic flow dynamics on the turbulent transport of momentum and scalar quantities above complex terrain. It highlights three key low-level orographic flow phenomena governed by gravity-wave dynamics: Foehn flow, atmospheric rotors and gravity-wave modulation of the stable boundary layer. Recent observations and numerical simulations are used to illustrate how these flows can cause significant departures from the turbulent fluxes, which occur over flat terrain. Orographically forced fluxes of heat, moisture and chemical constituents are currently unaccounted for in numerical models. Moreover, whilst turbulent orographic drag parameterisation schemes are available (in some models), these do not represent the large gravity-wave scales associated with foehn dynamics; nor do they account for the spatio-temporal heterogeneity and non-local turbulence advection observed in wave-rotor dynamics or the gravity waves, which modulate turbulence in the boundary layer. The implications for numerical models, which do not resolve these flows, and for the parametrisation schemes, which should account for the unresolved fluxes, are discussed. An overarching need is identified for improved understanding of the heterogeneity in sub-grid-scale processes, such as turbulent fluxes, associated with orographic flows, and to develop new physically-based approaches for parameterizing these processes.
Highlights
The influence of hills and mountains on both the local and large-scale weather and climate is well known and orographic flows have been the subject of extensive research over the past few decades
There is a wide range of dynamical processes, many of which are related to orographically generated internal gravity waves, which can significantly modulate the exchange of momentum and scalar quantities between the atmospheric boundary layer and the land or sea surface, and between the atmospheric boundary-layer and the free troposphere
These aspects are not well represented in models, if at all. This is problematic for regional numerical weather prediction (NWP) in complex terrain, for air-quality modelling in mountainous regions, and for global Earth System Models, which require the representation of complex processes such as atmospheric chemistry and aerosols among others
Summary
The influence of hills and mountains (orography) on both the local and large-scale weather and climate is well known and orographic flows have been the subject of extensive research over the past few decades. There is a wide range of dynamical processes, many of which are related to orographically generated internal gravity waves, which can significantly modulate the exchange of momentum and scalar quantities between the atmospheric boundary layer and the land or sea surface, and between the atmospheric boundary-layer and the free troposphere These aspects are not well represented in models, if at all. Gravity waves are known to cause mixing, either explicitly through wave breaking, or indirectly by increasing vertical wind shear, which in turn leads to Kelvin-Helmholtz instability [6] This wave-induced mixing of temperature and other atmospheric constituents is unresolved in weather and climate models, but unlike wave drag it is generally not parametrised either.
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