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

The Haslital is a valley located in the central part of the Alps, in the Bernese Oberland region. The complex topography surrounding the valley is the cause of many mountainous meteorological phenomena. In this 2 km wide alpine valley, temperature inversion and complex thermal wind cycles are common. Furthermore, strong synoptic wind like Foehn can be channelled by the valley and interact with the existing phenomena. In this context, a remote sensing campaign conducted by MeteoSwiss took place at the Meiringen airfield, located in the valley. A microwave receiver (MWR) and a Doppler Wind Lidar (DWL) have been installed from October 2021 to August 2022. In addition, a permanent SwissMetNet (SMN) automatic measurement station is present 4 km east of the measurement campaign site allows for analysis of multiple ground-based variables. The six months of data (February-August 2022) collected by the remote sensing instruments are used to estimate the performance of the NWP between ground level and the height of the surrounding moutains (2500 m.a.g.l.). In a more general framework, they are also used to better understand the complex phenomena induced by an alpine topography. In general, the temperature of the lowest level of the model and the SMN station shows small differences, which is most likely due to the fact that the SMN data are assimilated by the model. However, it shows a clear diurnal cycle with an overestimation during the night and a clear underestimation (1-2 °C) during the day. The nighttime overestimation is maximal in the case of fair weather days in winter favoring ground-based temperature inversions, whereas in the case of Foehn, the NWP always overestimates the ground temperature. The comparison between MWR and SMN temperatures indicates very small differences without diurnal cycles near the ground. In general, the difference between the MWR and the NWP profiles shows a small (± 0.5 °C) over/underestimation of the model for low/high altitudes, respectively. In special cases, however, such as fair weather days in winter or Foehn events, much larger differences are found. Ground inversions are well captured by the MWR but not by the model. This failure is partly explained by the difference in elevation (130 m) between the actual topography and the digital terrain model used by the model, which happens to be greater than the height of the inversion. The diurnal valley winds, on the other hand, are measured by the DWL from February to July. Their vertical extent is only a few hundred meters in winter but extends to the northern ridges (2000 m) in summer. These diurnal valley winds are very well represented by the NWP. The main differences observed with the measurements are a to early transition toward up-valley wind with overestimated wind speeds. At high wind speeds (> 20 km/h), the model simulates the main synoptic wind direction deeper in the valley. Finally, the PBL determined by the profiling instruments and the model are compared.