2020-02-17 07:15 - 08:42 UTC (1h 27m)
Ricoh GXR-A16, interval 5 seconds, speed 150x
20200217_0715-0842utc_Praha-Libus_1920x1080.mp4 (88 MB)
Helmholtz cloud waves, Praha - Libuš. Waves in low clouds, which were formed by strong wind shear at levels around 2 km. Contrary to more frequent orographic waves, which form downwind of mountain ridges or islands and typically are quasi-steady or move very slowly, these wind shear-induced waves propagated very quickly in an eastward direction, even faster than the cloud base underneath. The wind shear can be nicely seen e.g. in this wind profiler plot from Temelín (south Bohemia, lower panel), and cloud height can be retrieved from the ceilometer plot from the same location (upper panel). The cloud height at the location from which the timelapse was taken (Praha-Libuš) can be seen in this ceilometer plot (no wind profiler there). In both plots, the vertical lines outline the period of the timelapse sequence. The traditional sounding plot from Praha-Libuš from 06 UTC and 12 UTC shows that at levels near 2 km there was no significant inversion. Source of these plots: CHMI and EUMETNET e-Profile.
The appearance of these waves from above can be seen in satellite imagery at higher pixel resolution. In this Meteosat-10 (MSG-3) HRV band image (at resolution of about 1x2 km for this region) from the beginning of the timelapse sequence, the individual waves can be resolved. The red dot indicates the location of Praha-Libuš (from where the timelapse was taken), the green dot marks location of Temelín, and the sector outlines the horizontal range and direction of the timelapse. In this Meteosat-10 HRV loop (taken at 5 minute rapid scan mode) the sector also marks the timelapse interval. Despite the fact that the waves can be resolved in the HRV images, their motion can't be resolved in the loop due to their fast eastward propagation. Somewhat better resolution is available in this Metop-B AVHRR 1 km image, captured at the end of the timelapse series. However, the clearest image of these waves was recorded about 30 minutes later by the Sentinel-3B satellite with its OLCI instrument at 300 m resolution, shown here in the RGB composite image of bands 8, 6 and 4. From this image it is possible to retrieve the wavelength of these waves, which ranges from about 2 km near Praha (red dot), up to about 4 km in the other more distant regions. From the timelapse series it is possible to estimate the period of these waves, reaching about 85 seconds.
The timelapse series, unfortunately terminated by the memory card capacity, was captured from my office window at the CHMI Observatory Praha-Libuš, Czech Republic, with a southwestward view.
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