flow perpendicular to the mountain range
After the cold front passage on March 14, 2023 (see
blog post
a ridge approached from the west. The upper air flow turned to the northwest and increased! significantly in strength. The following day, the polar jet crossed! the Alpine region eastwards.This situation, with the polar jet directed almost vertically towards the Alpine arc! is usually accompanied by a strong current in the same direction at the height of the Alpine ridge! The strong overflow of air led to the formation of a northerly foehn ! on the leeward side (i.e. on the southern side! of the Alps), which reached as far as the lowlands of Ticino. At the same time, the associated drying! brought cloudless conditions with excellent visibility.
Waves caused by the north wind on Lake Lugano south of the connection! between Melide and Bissone on 15.03.2023 at 10:40 recorded by the webcam! of Monte San Salvatore.
Waves caused by the north wind on Lake Lugano south of the connection between Melide and Bissone on March 15, 2023 at 10:40 a.m. recorded by the Monte
pressure gradient and north foehn
Since the wind is mainly determined by the pressure gradient, one is interested in the evolution of this gradient between very specific points in order to predict the arrival of the foehn and its strength. As can be seen in the figure below, the pressure gradient changed from south to north in the second half of the day on March 14, 2023.
Animation of the pressure difference between Kloten and Lugano in hPa. The measurement is shown in blue, the prediction and uncertainty of the model (IFS HRES and COSMO) in orange.
Animation of the pressure difference between Kloten and taiwan phone number library Lugano in hPa. The measurement is shown in blue, the forecast and the uncertainty of the model (IFS HRES and COSMO) in orange. (Source: MeteoSwiss)
waves downstream of the Alps
When crossing the Alpine ridge, the northwesterly current is inevitably diverted upwards, and depending on the stability of the air mass, wave crests and troughs form downstream. On March 15, the air was so dry that these were not visible. If the humidity is sufficient, the waves in the area of the wave crests often appear as so-called lens clouds (altocumulus lenticularis), also known colloquially as foehn clouds.
Cross-section showing the mechanism of lee waves being
Cross-section showing the mechanism of triggering lee online workshop for unwcc researchers waves when flowing over a mountain ridge. (Graphic: Aude Untersee)
The wave, a welcome “lift” for gliding
These waves are of particular interest to aviation, as they adb directory spread at very high altitudes and are not particularly popular with commercial aircraft due to the associated turbulence. This is different for gliders, which use the rising areas of the waves, the so-called wave crests, without an engine. The wave is a real atmospheric elevator with climbing speeds of up to 10 m/s (about three times the speed of a conventional elevator). The resulting gain in altitude is in turn used to cover distances.