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Global to Local Impacts of Flow over Orography

Final Report Summary - GLIMFLO (Global to Local Impacts of Flow over Orography)

Developing accurate forecasts for weather and related meteorological natural hazards has become a priority due to the ensuing savings in human lives and property damage. Mountain waves affect the atmosphere over a wide range of scales, causing severe local weather phenomena such as downslope windstorms, lee-wave rotors and Clear-Air Turbulence (CAT), which are also important aviation safety hazards. However, the triggering mechanisms controlling these phenomena are still largely unknown, and methods used to model them operationally have a weak physical basis. The research developed in this project aims to bridge the conceptual gap existing between our understanding of such phenomena at low and high amplitude, via a combination of theory and numerical simulations. It addresses the triggering mechanisms and controlling parameters of downslope windstorms, lee-wave rotors and CAT. At larger scales, the drag force associated with mountain waves decelerates the atmospheric circulation, and must be parametrized in global weather and climate models, leading to temperature errors in excess of 10K in the polar stratosphere if omitted. Substantial imbalances in the modelled angular momentum budget of the Earth suggest that the impact of vertical wind shear on mountain wave drag, currently neglected in drag parametrizations in all global forecast models, should be included. The theory necessary to do this has been developed previously by the PI. Through partnerships with the UK Met Office and ECMWF, drag parametrizations that take wind shear into account will be verified using high-resolution numerical simulations, implemented, and their impact on forecast skill tested.

The project had 4 main objectives: 1. To Investigate the triggering mechanisms for downslope windstorms; 2. To develop diagnostics for the onset of lee wave rotors; 3. To understand mountain wave breaking in directional wind shear and its implications for CAT generation; 4. To improve the formulation of mountain wave drag parametrizations, by including wind shear effects. Towards objective 1, numerical simulations and linear theory have been used to tackle high-amplitude mountain waves generated by 2D orography, and understand the linear-nonlinear transition of this flow, with the aim of achieving a better description of high-drag states and the associated mechanisms for downslope windstorms. This impacts not only on forecast methods for these storms, but also on drag parametrizations for weather and climate prediction models, where high-drag states play a prominent role. Concerning objective 2, a correlation between the onset of rotors downstream of elongated mountain ridges and the drag force produced by the associated trapped lee waves had been identified for idealized two-layer atmospheres. A flow stagnation condition that explicitly diagnoses the onset of rotors was derived using a linear model with a bulk representation of the boundary layer, with implications for rotor prediction based on the basic parameters of the incoming flow. Objective 3 was pursued largely via a PhD project funded by the present Career Integration Grant. Numerical simulations of a flow where the wind turns with height at different rates over 3D mountains of various heights were carried out, to understand under what conditions wave breaking (which leads to the generation of turbulence) occurs in the simplest flow configurations with directional shear, and determine the dependence of these conditions on the basic input flow parameters. Regime diagrams describing these results were produced, and the importance of directional shear in a specific turbulence event over the Rocky Mountains was also investigated, leading to 3 publications in international journals. Towards objective 4, contacts have been established with the UK Met Office to obtain recent versions of the subroutines that implement the mountain wave drag parametrization of the Met Office’s Unified Model, so that shear effects can be implemented. The program of research began by using ERA-Interim Reanalysis data to calculate measures of the importance of shear at representative heights, followed by high-resolution simulations to calibrate the optimal level at which shear corrections should be evaluated, and offline tests of the drag parametrization to assess its performance. All of these results are novel, and contribute to the advancement of Mountain Meteorology.

My appointment to the University of Reading as a Lecturer (in one of the top meteorology departments in Europe) provided optimal conditions to increase the impact of my research, interact with highly skilled colleagues, become updated with the newest trends and priorities in the field, and maintain, as well as enhance, my collaborations abroad. In the context of this project, I established a new collaboration with a colleague from Meteo-France/CNRS, Toulouse, specialized in laboratory experiments relevant to trapped lee waves and rotors, which is complementary to my ongoing collaborations with numerical modellers. I also pursued my ongoing collaborations with colleagues from Portugal, which are highly relevant to this project (and from which many of the deliverables resulted). I have started building a small group on Mountain Meteorology, comprising myself and two PhD students (one of whom was funded by this project). I have been teaching since 2013 an undergraduate (BSc) course on Fluid Dynamics and a postgraduate (MSc) course on Boundary Layer Meteorology at the Department of Meteorology of the University of Reading, and in 2015 and 2017 I also taught lectures/courses in two Summer Schools. I became an Associate Editor of the open-access journal Frontiers – Atmospheric Science, where I published an Inaugural Review Article on “The physics of orographic gravity wave drag”. I also published an invited paper in the Enok Palm memorial volume of the European Journal of Mechanics B. Both studies aimed to draw the attention of the scientific community to the importance of Mountain Meteorology. In November 2016 I edited, along with other Mountain Meteorology experts, a special issue of Frontiers in Earth Science on the Research Topic “The Atmosphere over Mountainous Regions”, which is also contributing to increase the visibility of this area of research. In total, 13 publications in international journals, 6 invited talks or seminars and 16 presentations in conferences resulted from this project. Having successfully completed my probationary period as a Lecturer, I am now a permanent, fully integrated, member of staff at the Department of Meteorology of the University of Reading.

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