The impact of assimilating data from a remotely piloted aircraft on simulations of weak-wind orographic flow

Hálfdán Ágústsson; Haraldur Ólafsson; Marius O. Jonasse; Ólafur Rögnvaldsson

doi:10.3402/tellusa.v66.25421

The impact of assimilating data from a remotely piloted aircraft on simulations of weak-wind orographic flow

Tellus

Volume 66 (2014): Issue 1

By: Hálfdán Ágústsson, Haraldur Ólafsson, Marius O. Jonasse and Ólafur Rögnvaldsson

Open Access

|Dec 2014

Abstract

Orographic winds near a 914 m high mountain in Southwest-Iceland are explored using unique observations made aloft with a small remotely piloted aircraft, as well as with traditional observations and high-resolution atmospheric simulations. There was an inversion well above mountain top level at about 2 km with weak winds below. Observed winds in the lee of the mountain were indicative of flow locally enhanced by wave activity aloft. Winds descended along the lee slope with a prevailing direction away from the mountain. They were relatively strong and gusty at the surface close to the mountain, with a maximum at low levels, and weakening and becoming more diffuse a short distance further downstream. The winds weakened further aloft, with a minimum on average near mountain top level. This situation is reproduced in a high-resolution atmospheric simulation forced with atmospheric analysis as well as with the observed lee-side profiles of wind and temperature below 1.4 km. Without the additional observations consisting of the lee-side profiles, the model fails to reproduce the winds aloft as well as at the surface in a region in the lee of the mountain, as was also the case for the operational numerical models at that time. A sensitivity simulation indicates that this poor performance is a result of the poorly captured strength and sharpness of the inversion aloft. The study illustrates, firstly, that even at very low wind speed, in a close to neutral low-level flow, gravity waves may still be a dominating feature of the flow. Secondly, the study presents an example of the usefulness of lee-side atmospheric profiles, retrieved by simple model aircraft, for improving numerical simulations and short-term weather forecasting in the vicinity of mountains. Thirdly, the study confirms the sensitivity of downslope flow to only moderate change in the sharpness of an upstream inversion.

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Articles in this issue

DOI: https://doi.org/10.3402/tellusa.v66.25421 | Journal eISSN: 3035-9554

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Language: English

Page range: 25421 - 25421

Submitted on: Jul 9, 2014

Accepted on: Nov 2, 2014

Published on: Dec 1, 2014

Published by: Stockholm University Press

In partnership with: Paradigm Publishing Services

Keywords:

downslope flow,

complex orography,

small remotely piloted aircraft,

observational nudging

© 2014 Hálfdán Ágústsson, Haraldur Ólafsson, Marius O. Jonasse, Ólafur Rögnvaldsson, published by Stockholm University Press
This work is licensed under the Creative Commons Attribution 4.0 License.

Volume 66 (2014): Issue 1