Comparison between ground-based remote sensing observations and NWP model profiles in complex topography: the Meiringen campaign

Abstract. Thermally driven valley winds and near-surface air temperature inversions are common over complex topography and have a significant impact on the mesoscale weather situation. They both affect the dynamics of air masses and pollutant concentration. Valley winds affect it by favoring exchange between the boundary layer and the free troposphere, and temperature inversion by concentrating pollutants in cold stable surface layers. The complex interactions that lead to the observed weather patterns are challenging for Numerical Weather Prediction (NWP) models. To study the performance of the COSMO-1 model anaylsis (KENDA-1), a measurement campaign took place from October 2021 to August 2022 in the 1.5 km wide Swiss Alpine valley called Haslital. A Microwave Radiometer and a DopplerWind Lidar were installed at Meiringen, in addition to a multitude of automatic ground measurement stations observing meteorologic surface variables. Near the measurement’s sites, a low altitude pass, the Brünig Pass, influence the wind dynamic similarly to a tributary. The collected data shows frequent nighttime temperature inversions for all months under study, which persist during daytime in colder months. An extended thermal wind system was also observed during the campaign, except in December and January allowing an extented analysis of along and cross valley winds. The comparison between the observations and the KENDA-1 data provides good model performances for monthly temperature and wind climatologies but frequent and important differences for particular cases, especially in case of foehn events. Modeled nighttime ground temperature overestimations are common due to missed temperature inversions resulting in bias up to 9 °C. Concerning the valley wind system, modeled flows are similar to the observations in their extent and strength, but suffer from a to early morning transition time towards up valley winds. The findings of the present study allow to better understand the temperature distributions, the thermally driven wind system in a medium size valley, the interactions with tributary valley flows, as well as the performances and limitations of a model in such complex topography.