Examining turbulent scalar exchange during a strong wind event

Various research has been conducted into the turbulent momentum fluxes in orographic flow, and is in some models parameterised to simulate orographic form drag, yet the role of turbulent exchange of heat and moisture presents a somewhat opaque picture, as these processes are generally not resolved or parameterised in NWP models.  We present results from rare, low-level leeside turbulence observations in a strong mountain wind event.  These observations were obtained in 2018 during the Iceland-Greenland Seas Project (IGP) field campaign, a coupled atmosphere-ocean project, which included two research flights over the steep and complex orography of two Icelandic peninsulas with the aim of investigating leeside turbulent exchange processes.  High resolution regional model MetOffice Unified Model (MetUM) forecasts are run and compared to in-situ observations from 12^th and 19^th March 2018 over the Westfjords and Snaesfellsness peninsula in north-western Iceland.  With sub-kilometre horizontal grid-cell lengths flow features such as a downslope windstorm coupled with a hydraulic jump and a wave-breaking region directly aloft are well resolved and provide suitable cases for testing different MetUM science configurations.  The evaluation of control forecasts has shown a consistent 2 K bias in the lower atmospheric boundary layer sourced from the global driving model.  Sensitivity tests are run with the aim to ultimately parameterise scalar transport of heat and moisture in the leeside orographic flow.  This presentation will outline current progress and, to a degree, will attempt to answer the question of the significance and importance of turbulent scalar exchange within a strong wind event.

Keywords: MetUM, tuurtbulent exchange, orographic, NWP, Iceland