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

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.

• downslope flow
• complex orography
• small remotely piloted aircraft
• observational nudging

7. Acknowledgements

We thank Joachim Reuder at the University of Bergen for making the SUMO RPAS available during the MOSO-project. This study is a result of a successful and on-going collaboration with Bjarni G. P. Hjarðar at Sorpa waste management and with ISAVIA, the Icelandic Aviation Authority. The work was in part funded by the Icelandic Centre for Research (RANNÍS), through the Icelandic research fund and the Icelandic technology development fund (SARWeather project). We acknowledge three anonymous reviewers for their constructive comments.