The MatterHEX Experiment—Investigating Atmospheric Flow Patterns in Highly Complex Terrain Related to Banner Cloud Formation

Abstract Orographic clouds typically form on the windward side of a topographic barrier when approaching air is cooled as it is lifted, and moisture condenses into cloud droplets. Under special conditions, however, clouds can appear on the leeward side of a mountain. Attached to leeside mountain slopes below the summit like a banner to a mast, these cloud formations are known as “banner clouds.” They are often seen on steep singular mountains such as the Matterhorn (Cervino) of the European Alps. Their formation indicates that vertical displacement in the lee dominates over the lifting occurring upwind. Past investigations of the flow conditions associated with banner clouds have mostly relied on numerical simulations, as observations are difficult in the often extremely complex or inaccessible mountainous terrain. This article summarizes the main findings of the Matterhorn Experiment, MatterHEX, that was conducted at the Matterhorn near Zermatt, Switzerland, in fall 2023. The experiment was designed to overcome observational challenges due to topography and to collect the minimum data necessary to assess the flow conditions conducive to banner cloud formation. Upwind flow and stability conditions were observed with radiosondes; cloud conditions were monitored with webcams; and leeward flow patterns were remotely sensed with Doppler lidar from the best possible location still accessible by foot. Time-averaged lidar scans reveal strong leeside ascent associated with banner cloud formation, while individual scans resolve the turbulent features associated with leeside flow separation. Our analysis shows that flow conditions around the Matterhorn are frequently conducive to banner cloud occurrence, but that a lack of moisture can prevent their formation. Significance Statement Understanding weather in mountainous terrain is of high relevance owing to its sometimes-extreme character. Yet, observations are often very challenging due to the complexity of the terrain. The field campaign described in this paper addresses this challenge with the combination of careful campaign design and state-of-the-art instrumentation. Our focus lies on the leeside flow structures of an isolated steep mountain, the Matterhorn of the European Alps. Analysis of the data indicates leeside flow reversal and upwelling, consistent with theory. Our observations allow us to be the first to test existing theories on the formation of leeside banner clouds. The data are well suited for the evaluation of corresponding large-eddy simulations that resolve similar spatiotemporal scales in highly complex terrain.