Mountain waves over Scandinavian Mountains

By Juuso Paajanen (FMI)

Favourable conditions for the formation of mountain waves:

• The wind is close to perpendicular towards the mountain range
• Strong winds
• Stable airmass over the mountain range
• Warm sector in front of warm or occluded front
• Jet right exit region

The synoptic view

On 5 June unstable weather dominated northern Europe. A large low pressure area occurred over Scandinavia, with multiple fronts (Figure 2, M = low pressure, K = high pressure). An occluded front (drawn as a warm front in the NSWC charts, Figure 1b and 1b) approached southern Norway with strong winds. On the surface in southern Norway, plus in the lower troposphere over the area, the wind was from east to southeast. As favourable conditions for mountain waves include strong winds perpendicular towards the mountains, and a warm sector in front of a warm or occluded front, mountain waves with moderate to severe turbulence occurred.

Observations and warnings

It is quite hard to get any kind of observations about mountain waves. Fortunately, some pilots reported turbulence on that day. Two airline pilots (Boeing and Embraer) reported moderate turbulence near Bergen between 7,000-9,000ft and near Molde at 30,000ft. Mountain waves can reach up to 15km or 50,000ft, so we can assume the latter report also happened due to mountain waves.

The received reports:

ARS B738 MOD TURB OBS AT 0535z 25NM SE OF ENBR FL090/7000FT=
ARS E290 MOD TURB OBS AT 1114Z 50 NM SE OF ENML FL300=

Meteorologists took active measures to warn pilots about this event. Both NSWC charts at 06z and 12z had a turbulence area over the mountains of southern Norway. On the 12z chart severe turbulence was forecast. Multiple Sigmets were also issued from early morning until late afternoon. Below is a Sigmet with severe turbulence that was issued around noon (Figure 3).

The new generation of satellite images

Colourful RGB images are useful for separating different cloud layers. Figure 4a and 7 are Cloud Overview RGB images from the time of the event. In these images the different cloud layers are somewhat visible. In the simulated Cloud Phase RGB image (Figure 4b) the cloud layers are much more visible. Still, for wave structure, there are better products to use, such as the Day Microphysics (Figure 5b & 5e).

As we know, the 24h Microphysics RGB displays upper clouds too heavily. This feature makes it hard to see what is going on in the lower troposphere. This problem appears to also be the case in the simulated image (Figures 5a and 5d). The FCI simulated Day Microphysics image shows low-level clouds far better than the 24h Microphysics. When comparing the SEVIRIs (Figures 5c and 5f) the wave structure is more visible in the simulated version of SEVIRI (Figure 5f).

When comparing the FCI simulated Day Microphysics RGB (Figure 5e) and the FCI simulated Cloud Type RGB (simulated from VIIRS, Figure 6a), the wave structure is easier to see in the Day Microphysics, but the latter one displays the absence of upper clouds more clearly over the mountains. Both of these images have added value, but in different ways. The Cloud Phase RGB FCI (Figure 6b) is also useful because because of the absence of upper clouds and visible structure of waves in lower clouds.

Summary

FCI

• In summer time the RGB products are useful, at least for these latitudes (southern Norway).
• The better resolution gives us added value that can be used to make more precise forecasts for turbulence. This will increase aviation safety overall and allow pilots to operate in a broader area if (severe) turbulence can be narrowed to a smaller area.

METImage

• Better resolution will make EPS-SG RGB images a viable option for number of situations, for example mountain waves.
• Despite better resolution, the new generation 24h Microphysics images still display upper clouds too heavily. For mountain waves, Day Microphysics and other products are more preferable in these situations.
• Overall, it seems that polar satellites are the best when it comes to mountain waves.