Nocturnal Low-level Jet Evolution in a Broad Valley Observed by Dual Doppler Lidar

Abstract

The temporal evolution of a nocturnal low-level jet (LLJ) in the 40km$40\,\text{km}$ broad Rhine Valley near Karlsruhe is studied, in the framework of a case study, with two heterodyne detection Doppler lidars using the new scan concept of “virtual towers”. For validation of this measuring technique, we performed comparative case studies with a tethered balloon and the highly instrumented 200m$200\,\text{m}$ KIT tower. The findings show capabilities of the virtual tower technique for wind measurements. Virtual towers can be placed at all locations within the range of Lidar measurements. Associated with nocturnal stable stratification, the LLJ, a wind speed maximum of about 9ms-1$9\,\text{m}\,\text{s}^{-1}$, develops at 100m$100\,\text{m}$ to 150m$150\,\text{m}$ agl, but the wind does not show the typical clockwise wind direction change that is reported in many other studies. This is attributed to the channeling effect occurring in broad valleys like the Rhine Valley when the boundary layer is stably stratified. Such channeling means a significant deviation of the wind direction from the Ekman spiral so that low-altitude winds turn into valley-parallel direction.

Keywords

• doppler lidar
• dual doppler lidar
• virtual towers
• wind measurement
• vertical profile