Stable Boundary Layers in an Arctic Fjord-Valley System: Evaluation of Temperature Profiles Observed From Fiber-Optic Distributed Sensing and Comparison to Numerical Weather Prediction Systems at Different Resolutions

Abstract

Stable boundary layers (SBLs) commonly form during the Arctic polar night, but their correct representation poses a major challenge for numerical weather prediction (NWP) systems. To enable detailed 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 (EC) measurements during contrasting synoptic forcings in a fjord-valley system in Svalbard. The FODS-derived temperature variances and static stability profiles are used to investigate the spatial and temporal evolution of different inversion types. 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 and 0.5 km. The higher-resolution model captures cold pool and low level jet formation during weak synoptic forcing, resulting in a well-represented vertical temperature profile, 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, the higher-resolution model is more sensitive to misrepresented fjord-scale wind directions and performs less well. The results indicate the importance of the ratio between nominal horizontal model resolution and valley width to represent SBL features. Our results underline the substantial benefit of spatially resolving FODS measurements for model verification studies as well as the importance of model and topography resolution for accurate representation of SBLs in complex terrain.