Lidar Observations and Numerical Simulations of an Atmospheric Hydraulic Jump and Mountain Waves

An atmospheric hydraulic jump was observed over the Alaiz mountain range and Elorz valley near Pamplona, Spain from radial velocity retrievals performed with two scanning lidars during October 5 and 6, 2018. The jump occurred on the lee side of the mountain close to its base and the jump location was observed more than 2 km further downstream of the mountain base inside the valley. Here, we simulate the two days using the multi-scale modeling capabilities of the Weather Research and Forecasting model. We find that the model is able to reproduce the hydraulic jump in high detail matching qualitatively well the timing and main features observed by both the scanning lidars and meteorological instruments on masts deployed throughout the area. The simulation results show that the jump starts at the beginning of the evening, right after the atmospheric conditions over the top of the Alaiz mountain become stable and the flow at the mountain top experiences a transition from subcritical to supercritical. The simulations also show that the jump lasts about 10 h until it moves close to the mountain top; then lee-wave activity dominates and lasts until late in the morning. The flow at the mountain top is only supercritical during the periods where the jump and the lee waves take place. The jump and lee-wave regimes can be distinguished from the simulation results by analyzing the ratio of the depth-average Brunt-Vaisala frequency to the depth-average mean wind speed both upstream and downstream of the mountain top.