High-Resolution Simulations of the Thermally-Driven Valley-Exit Jet of the Isarco Valley (Italy) and Comparisons with Wind-LIDAR Observations.

High-resolution numerical simulations with the Weather Research and Forecasting (WRF) model are performed to analyze, along with a rich observational dataset, the development of the thermally-driven valley-exit jet of the Isarco Valley and its spreading into the Bolzano basin, in the Italian Alps.

The Bolzano basin is located in the Central Italian Alps, at the junction of three Alpine valleys: the Adige Valley (S and NW), the Sarentina Valley (N) and the Isarco Valley (E). The wind field of this area is particularly complex due to the interaction of local circulations that daily develop in each of the tributary valleys and merge into the basin. In addition, especially during wintertime, the presence of strong ground-based temperature inversions affects the dynamics of the wind field. Under fair weather conditions, the drainage flow of the Isarco Valley accelerates at the outlet of the valley and spreads into the basin behaving like a valley-exit jet, with peak intensities between 6 and 13 m s^-1. In order to enlighten the key aspects of this phenomenon, a field campaign was carried out from January to March 2017. A Doppler Wind-LIDAR (WindCube100S, Leosphere) was installed in front of the outlet of the Isarco Valley and scanned, with different spatial resolutions, the wind field to capture the structure and the dynamics of the jet in both the Isarco Valley and the Bolzano basin. During this campaign, a SODAR and a MTP5-HE temperature profiler placed South of Bolzano operated together with several ground weather stations.

This contribution focuses on the study of two episodes of thermally-driven valley-exit jets. The first episode (Jan. 28-29) occurred with a strong ground-based temperature inversion inside the basin, while the second one (Feb. 13-14) occurred without any temperature inversion. Wind-LIDAR data allow the analysis of the internal structure of the jets and of the influence of the atmospheric stratification on the flow structure.

In order to complete the information given by the observational dataset and to get a three-dimensional picture of the development of the valley-exit jet, high-resolution WRF simulations are performed. Simulations are run with a horizontal resolution of 300 m and 20 vertical levels in the first 1000 m AGL. A good agreement between model results and observations is found. Simulations highlight substantial differences in the structure of the valley-exit jet between the two episodes, being strongly influenced by the thermal stratification. Moreover, it is found that the cold pool developing in the Bolzano basin influences the penetration of the wind into the basin in the low levels. In fact, when the cold pool is well developed, the jet floats over the stably stratified air in the basin, with very weak wind speed in the basin in the low layers.