Wind-LIDAR Observations and High-Resolution Modeling of a Thermally-Driven Valley-Exit Jet at the Outlet of the Isarco Valley (Italy)

The Bolzano basin is a wide basin in the Central Italian Alps located 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.

The valley-exit jet of the Isarco Valley is one of the most remarkable circulations of the Bolzano basin. In fact, it has recently been observed that, 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 low-level jet, with peak intensities between 6 and 13 m s^-1.

In order to enlighten the key aspects of this phenomenon, a short 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 managed by the local protection agency (APPA Bolzano).

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 have been used to characterize the two jets. In particular, the vertical profiles of wind speed, obtained with the DBS scans (one profile every 5 s), allowed to analyse the internal structure of the jets and the influence of the atmospheric stratification on the flow structure. In completion, high-resolution numerical simulations performed with the WRF model provided a more complete three-dimensional picture on the development of the valley-exit jet and its spreading into the Bolzano basin.