Investigating Stable Boundary-Layer Temperature Profiles Observed from Fiber-Optic Distributed Sensing on a Tethered Balloon and comparing them against NWP Systems at Different Resolutions for an Arctic Fjord-Valley System in Svalbard

Stable boundary layers (SBL) commonly form during the Arctic polar night, but their correct representation has been posing major challenges for numerical weather prediction (NWP) systems. To enable innovative model verification, we performed measurements of the lower atmospheric boundary layer with airborne fiber-optic distributed sensing (FODS), a tethered sonde, and ground-based eddy-covariance measurements. Here we contrast findings across two representative synoptic forcings leading to structurally different inversion types in a fjord-valley system in Svalbard, namely inflow and outflow conditions during the arctic polar night in early 2024. The strong gradients of the inversions are accompanied by an increased temperature variance, which is related to enhanced buoyancy fluctuations. The observed vertical temperature and wind speed profiles are compared to two configurations of the HARMONIE-AROME system with different horizontal resolutions at 2.5 km and 0.5 km.

The higher-resolution model captures cold pool and low-level jet formation during weak synoptic forcing and valley outflow, resulting in a well-represented vertical temperature profile down to the snow surface, while the coarser model exhibits a warm bias in near-surface temperatures of up to 8 K due to underestimated inversion strength. During changing background flow to valley inflow conditions, the higher-resolution model is more sensitive to misrepresented fjord-scale wind directions and performs less well, while the coarser NWP system has a seemingly better agreement with the observations lending to the underrepresented interaction with the topography.

The results indicate the importance of the ratio between nominal horizontal model resolution and valley width to represent stable boundary layer features in a physically meaningful manner. Our results underline the substantial benefit of the innovative spatially resolving FODS measurements for model verification studies as well as the importance of model and topography resolution for accurate representation of stable boundary layers in complex terrain.