Mountain waves in satellite imagery
20 February 2002 00:00 UTC-18 December 2019 00:00 UTC
Africa, Europe, Asia, S America
MSG, Metop, MFG, Suomi-NPP, NOAA-20, Terra/Aqua, GOES
SEVIRI, MVIRI, AVHRR, MODIS, VIIRS, ABI
Water Vapour, Airmass RGB, Natural Colour, Infrared, Convection RGB, HRV, Day Microphysics, Dust RGB, Brightness Temperature Difference, Channel 7.3µm, Visible
Mountain Waves are oscillations to the lee side (downwind) of high ground, resulting from the disturbance in the horizontal air flow caused by the high ground. They can often be seen in satellite imagery, as this case shows.
Table of contents
18 Dec, Algeria
By Ivan Smiljanic
Interesting orographic cloud waves formed over the Atlas Mountains, and wider, in mid-December.
The following gravity waves, that were also related to severe turbulence, formed in a cloud-free area over Algeria. They formed along the circular motion of the air masses in the system, and were detected using the water vapour signal in the SEVIRI 7.3µm channel, blue, in places less organised, wavy formations in Figure 1.
The most intense wave formation can be seen over the highest orography areas, along the air mass advection path (e.g. south of the Atlas mountains).
The animation in Figure 2 is an extended view of the gravity waves in the water vapour imagery.
Figure 3 reveals the synoptic forcing on the wider scale, responsible for intense wind advection behind, and around upper level cold front that passed over Tunisia on 18 December. The Meteosat-11 Airmass RGB is overlaid with wind streamlines at 500hPa, roughly the height at which the 7.3µm channel is most sensitive.
15-17 Dec, Austria
By Yasmin Markl and Andreas Wirth (ZAMG) and Ivan Smiljanic
Strong winds from south-westerly directions created a Foehn situation over Austria on several days from 15 December 2019, with increased intensity on 17 December. The involved cloud types were very complex and changed during the day.
The Meteosat-11 Airmass RGB product, overlaid with the wind streamlines at 500hPa (Figure 4), indicates that the air mass advection over the Alps, mostly formed in the southwesterly direction.
The Meteosat-11 HRV and IR10.8 RGB of 17 December 500hPa geopotential shows the typical foehn 'nose' south of the Alps (see Figure 5). The complex cloud structure is also clearly visible in the Meteosat-11 HRV and IR10.8 RGB of 17 December.
In the morning of the 17 December, the HRV image (Figure 6, left) shows the typical stripe pattern of lee waves north of the Alps, but also in the south. The lower lee clouds located north of the Alps are superimposed by a cirrus veil. This cirrus veil prevents height assessment of the lee waves below as it is opaque for IR radiation (Figure 6, right). Given their black and white structure in the HRV image, however, it is highly likely that they consist of water droplets as these evaporate when being advected to lower levels (from wave crest to wave trough) while ice crystals usually prevail in this process.
The rather smooth cloud tops over the central Alps in the HRV image originate from higher, optically thick cirrus clouds. They consist of small ice particles as can be seen in the Severe Convection RGB with its bright yellow colour. This cloud shield propagates northward during the day and covers successively the lower lee waves.
The HRV loop (Figure 8) also shows a north-south lee wave pattern in the cirrus shield over the eastern part of Austria.
Lee waves show a different orientation when wind direction changes with height. This can be seen for the lee waves located north of the Alps and for those located in the eastern part of Austria (Figure 9 and 10).
Unusual high temperatures and strong wind gusts go hand-in-hand with this weather condition. Wind gusts up to 150km/h and temperatures above 20°C were recorded.
This video shows the some typical foehn clouds near Salzburg in the morning of 17 December 2019 with a distinct cloud gap towards the south.
20-21 Nov, Argentina
By Gabriela Ishikame (SMN Argentina), José Prieto and Fausto Polvorinos
Andes mountain waves are frequent in November, with one or more convection lines associated with them.
Usually around November, warm air masses reach Argentina where the subtropical and sub-polar jets reinforce each other, at the same time as the Bolivian High develops. The upper-level anticyclone pushes south the position of the subtropical jet and affects the central region of Argentina.
This example of November 2019, using GOES-16 Convection RGB (Figure 11) and channel 7.3µm (Figure 12) imagery, plus NWP analysis (Figure 13), shows the jets generated convection near the Andes along two or three lines. The waves propagated downstream, but parts of them were hidden by subsidence after the air dried up.
The scheme of the lee wave formation (Figure 14) shows that the turbulence created a gap on the lee side of Andes. Cirrus on the lee side was thicker and colder than on the wind side. On the wind side of Andes, gusts of more than 220km/h (120kt) at the 200hPa level originated on the cold front, with a centre in the southern Atlantic. On the lee side, the cloud was parallel to the mountain range, with perturbations moving with the flow.
Several types of cloud can be best identified in the GOES -16 solar imagery (Figure 15): convective cloud near the Andes, roll cloud, billows on the wind side, and trapped waves on the lee side west of Neuquen, where the mountain range is lower. Billows are the result of Kelvin-Helmholtz instability and of shear between two separate air layers.
7 Apr, Locarno, Po Valley
By HansPeter Roesli
Orographic or wave clouds occurred in the lee of the Alps on 7 April 2017, under northerly winds.
Operational numerical models still have problems forecasting orographic or wave clouds in the lee of mountain chains, as was the case on 7 April.
In the middle-upper troposphere moderately humid air was advected towards the Alps during most of the day, as shown on the animation of the Meteosat-10 Water Vapour (WV6.2 channel) imagery, 7 April 00:00–23:45 UTC. The advection only weakened towards midnight.
The Meteosat-10 High Resolution Visible (HRV) imagery, 7 April 06:00–16:00 UTC shows that an important wave cloud covered the central and eastern parts of the Po Valley during daylight, marring the temperature and sunshine forecasts for the area.
The photograph taken in an easterly direction from Locarno, by MeteoSwiss, (Figure 16) shows the northern edge of the cloud that can be identified as a massive multi-storey cirrostratus lenticularis.
The imaging radiometer VIIRS on the sun-synchronous satellite Suomi-NPP viewed the area in two adjacent orbits, at 10:55 UTC (Figure 17, left) from an eastern position, and at 12:37 UTC (Figure 17, right), from a central position.
Both images are Natural Color RGBs at 375m spatial resolution, where the snowed-in alpine crests and some ice clouds stand out in a cyan colour.
What was surprising was that the wave cloud was white at 12:37 UTC, at least in its western part, hinting to consist of water droplets at its top.
The Milan radiosonde at 12:00 UTC showed a close-to-saturated layer between 420hPa (7.4km) and 200hPa (11.8km) with a temperature of -65 °C, suggesting that the wave cloud had a vertical extension of up to 4.4km and that its top had to be frozen. The cloud-top temperature was verified by the IR11.45 (I5) VIIRS band that also indicated -65 °C (208K), where the cloud was the most dense.
A second guess of the height was obtained from the shadow cast by the northern cloud edge: a reasonable match of ~10km was found.
So why do we see a white cloud top, although it must definitely be frozen? An explanation may given by looking at the temperature difference between the VIIRS bands IR11.45 (I5) and IR3.4 (I4).
On the IR11.45–IR3.74 difference image (Figure 18, left) the light blue streaks indicate that this part had an extremely high IR3.7 reflectivity, the difference even going beyond -80K.
This is confirmed by Meteosat-10 SEVIRI measurements in the Convection RGB with tropical tuning (Figure 19), which give a very high IR3.9r (solar component) reflectivity of around 15% and a NIR1.6 reflectivity of around 60% for the high-level wave cloud south of Switzerland (see cursor position in the image).
It could be that the ice crystals were so small that they also reflected well in the IR1.61 (I3) band (Figure 18, right), thus, whitening the RGB. In any case, further east where the difference is smaller at 12:37 UTC some 'correct' cyan colouring is evident.
High reflectivity at 1.6µm is unusual for ice particles. For reference, inside a typical frontal cloud, the differences between 0.8µm and 1.6µm for the ice cloud can reach 80 percent points because of 1.6µm ice particle reflectivity around 20%. In this case, however, the differences are much smaller, as shown on Figure 20, right panel.
If the crystals are less than 0.5µm in diameter, there cannot be much absorption at 1.6µm. Due to strong winds and low humidity, coalescence is restricted, and that explains why the crystal sizes are very small.
10 Feb, Europe, Mediterranean Sea, Po Valley
By Andreas Wirth (ZAMG)
Using a combination of Meteosat satellite data experts were able to to pinpoint lee waves over southern Europe and the Mediterranean Sea in February 2016.
Lee waves are stationary atmospheric waves, appearing as a group of parallel, white and narrow cloud bands in visible imagery (Figure 21).
In the infrared (IR) imagery (Figure 22), most of them appear as dark grey cloud bands due to their relatively warm cloud tops. The horizontal spacing of the waves is related to the wind speed over the mountain. The higher the wind speed, the longer the wave lengths.
Lee waves are best detected in High Resolution Visible (HRV) imagery during day because of the high resolution of HRV images and because of the high albedo of water clouds.
In the case of trapped lee waves, lee clouds propagate far downstream the mountain crests or any surface elevation where they are formed. These trapped lee waves develop in a stable layer at mountain peak level on top of which a less stable region has formed. Trapped lee waves present a considerable threat to civil aviation because they are connected to severe turbulence.
Unfortunately, lee waves are only visible in HRV imagery when water vapour condenses to clouds at the wave crests — when the humidity of the air is near saturation. In most cases, lee waves are invisible for the human observer and occurring turbulence is referred as CAT (clear air turbulence).
The SEVIRI infrared channels may give an indication of the presence of lee waves but are less suited for detection because of their coarser resolution and the mostly warm cloud tops. Nevertheless, as the HRV channel is missing, IR channels remain the only way to detect lee clouds during the night.
In many cases, water vapour imagery can complement wave detection in regions where cloud condensation does not occur and, therefore, no signal in infrared or visible channels exist.
On the HRV (Figure 23) and Water Vapour (WV) (Figure 24) images below it can be seen that while the Po Valley and the Mediterranean Sea, east of Corsica, are cloud free in the visible image, on the water vapour image there are distinct wave pattern in this areas. The WV7.3µm channel is sensitive to the water vapour concentration mainly between 700 and 500hPa.
As the total column water content of the atmosphere is constant whether this column is located in the wave trough or wave crest region, the varying absorption in this WV channel can only be explained by the different amount of water vapour in the layer which contributes most to the 7.3µm channel.
In case of very strong waves, drier upper tropospheric air protrudes down to lower layers and hence reduces the amount of water vapour within the highest contributing layers — the moisture is pushed down towards layers below 700hPa where the 7.3µm channel is more or less 'blind'.
10 Nov & 14 Apr
By HansPeter Roesli and Ivan Smiljanic
High-level orographic cloud pattern occurred in the lee side of the Alps, gradually seen as a pronounced feature in the Convection RGB loop, along with the rising Sun.
During sunrise high level mountain waves became easier to distinguish from the rest of the clouds in the Meteosat Convection RGB imagery, animation from 10 November 06:00–07:50 UTC and Figure 25.
The reason for that is the green component of the same RGB composite that includes a Sun reflection contribution in the 3.9µm microphysical channel of SEVIRI instrument.
This also applies to the Day Microphysics RGB composite (Figure 26) which, even more so, has only the reflected component of the 3.9µm channel (thermal Earth’s contribution extracted).
In the morning hours reflection from the very small ice particles, typical for high mountain waves, becomes gradually stronger. Reflection from the rest of the ice clouds naturally becomes gradually higher as well, but small ice particles always reflect much more.
From the Airmass RGB image (Figure 27) over the whole of Europe one could deduce the presence of the high-level north-westerly flow over the eastern Alps. This flow is responsible for the formation of observed wave cloud on the lee side of the mountains. This cloud looks very bright white in this RGB composite.
When we compare the different RGB combinations (Figure 25–28) it is obvious that the best contrast for wave clouds is given with the Convection RGB.
The High Resolution Visible (HRV) image, 14 April 10:00 UTC (Figure 29), shows the 'backbone' of the wave clouds was quite straight and oblique, in relation to the range of the Alpine crests, a rather unusual feature. Also, the structure was very complex.
Viewed from below, under the central section of the wave clouds, altocumulus lenticularis could be identified, topped by thick cirrus.
2 July, France, Spain
By Ivan Smiljanic
Pronounced high-level orographic cloud pattern appeared in the Severe Storm RGB behind the ridge of Pyrenees in July 2015.
The animation of the Severe Convection RGB imagery, 2 July 04:00–14:00 UTC shows the persistent high-level orographic wave cloud which developed on the lee-side of the Pyrenees mountains.
It is obvious from the synoptic view of the area (Figure 31) — especially looking at the position of high level jet (depicted by isotachs at 300hPa height) — that the formation of these clouds was connected to the strong high level flow. As is usual in such cases, the position of the cloud is to the right side of the jet axis.
On top of this large and long wave cloud (wavelength of hundreds of kilometres), which stretched almost perpendicular to the mountains, a formation of smaller orographic waves can also be seen. To depict these smaller patterns, images with better resolution are needed, such as the High Resolution Visible (HRV) (Figure 32).
Normally, Severe Convection RGB imagery is used for detection of convective systems, especially the most intensive cores or updrafts.
These cores are easily detected in this case because the most active parts of thunderstorms, especially the most intense cores/updrafts, have smaller ice particles at higher levels. However, in this case, no convective activity was present, only high level orographic clouds in the area of interest.
The typical yellow colour of these orographic clouds (also a typical colour of intense thunderstorm updrafts) comes from the small ice particles which form quickly at high levels.
These smaller particles reflect more sunlight back to the satellite than normal high-level ice clouds, at IR3.9. Therefore, in the RGB imagery (Figure 34) they appear as yellow shades not red.
30 March, Corsica, France, Mediterranean Sea, Sardinia,Spain
By Vesa Nietosvaara and Izabela Zablocka (IMGW)
Lee cloudiness over France, Spain and western parts of the Mediterranean Sea was seen by Meteosat-9 and 10, at the end of March.
The areas of lee clouds over the Pyrenees, Massif Central (France), and mountainous areas in Corsica and Sardinia can be clearly seen on the HRV imagery (Figure 35).
Typically lee cloudiness is characterised by narrow, middle-level cloud bands perpendicular to the wind direction (Lee Waves Clouds). They can also consist of cirrus clouds that extend from a mountain chain over the leeward side, and continue for up to 1,000 km (High Lee Cloudiness).
The wind direction doesn't necessarily need to be perpendicular to the mountain chain, but it must have a normal component with respect to the mountain.
In this case lee cloudiness is associated with the Pyrenees mountains on the Iberian Peninsula, Massif Central, and mountain areas in Corsica and Sardinia. On both the Meteosat-10 infrared images, Figure 36 and 37, the areas with high wind speeds can be clearly seen. These are the areas where lee cloudiness is well developed.
The airmass image (Figure 38) shows a very strong jet stream (yellow lines) extended over Great Britain and north-eastern France with wind speed higher than 90m/s. The right side of the jet stream is a typical location for lee cloudiness.
Moreover, on the the infrared image (Figure 39) there was a big area of positive Showalter Index values (yellow lines) over Western Europe. Values higher than 3 indicate areas of stable stratification of the atmosphere, which is a necessary condition for lee cloudiness to appear.
In this case lee cloudiness can also easily be observed in visible and infrared channels (Figure 40).
Two areas of lee cloudiness developed, as shown in Figure 40 and 41.
The first was an area spreading from southern France to Corsica and Sardinia. These lee wave clouds consisted mostly of water droplets and are white on the visible image, grey on the infrared image (Figure 40), blue in Severe Convection RGB and grey/brown on 24h Microphysics RGB (Figure 41).
A separate area of cloudiness over eastern part of Spain, white on both the visible and IR images, is more easily distinguished on the 24 Hour Microphysics RGB and the Severe Convection RGB (Figure 41). On the 24 hr Microphysics RGB (right) the dark blue areas are thin cirrus clouds and the dark red areas are thick cirrus clouds. On the Severe Convection RGB (left) the area is bright yellow. That means that over this region the high lee clouds contained small ice particles.
Lee cloudiness is visible evidence of existing turbulence in the atmosphere, often quite severe and dangerous for aircraft. Figure 42 shows the areas of SIGMET warnings which were issued for severe turbulence or mountain wave. For instance, the small area indicated near Barcelona is the area where the severe mountain wave was forecast between flight level 060 (about 1,800m) and 290 (about 8,800m).
22 Dec, Czech Republic, Baltic Sea, Poland
By Petr Novak, Milan Salek, Martin Setvak and Pavla Sknvankova (CHMI)
The Meteosat-8 image below shows two types of waves above the Czech Republic, interfering with each other.
Yellow arrows point to the first wave of a series of lee waves, forming downwind of the Krusne Hory/Erzgebirge mountains on the Czech-German border.
This type of low-level waves is quite common in the area, forming under suitable stratification and northwesterly winds.
However, the second set of waves, labeled by the red arrows, probably represents gravity waves propagating across the country from north to south, interfering with the lee waves. Higher resolution MODIS images show further details of the clouds formed by these waves, namely their turbulent cloud-top structure — see 500m resolution True Color image or 250m resolution band 1 image.
The evolution and propagation of the waves can be seen nicely in the animation obtained from the Rapid Scan Service of Meteosat-8 imagery. The gravity waves seem to be associated with the passage of a cold front across the area (see surface map, source: DWD).
The position and shape of the gravity waves roughly corresponds to the location and shape of the cold front itself, also shown in the animations of the Airmass RGB product (see overview animation, or more detailed zoom on Central Europe). Furthermore, a loop of radar reflectivity images shows that the location of the main precipitation band corresponds to the shape and location of the cold front.
However, detailed analysis of the radar and MSG imagery shows somewhat more complex patterns. The shape of the main precipitation band (see 12:10 UTC radar image) shows that the eastern part of this band has different curvature from that of the gravity waves shown in the corresponding MSG image below.
This can be even better seen in the composite image loop showing MSG rapid scan imagery, overlaid with partially transparent radar imagery, both scanned at corresponding five minute intervals. Here it is obvious that the precipitation band starts to develop (at about 10 UTC) well ahead of the gravity waves, roughly matching them in the western part around 12 UTC, and staying behind them later on. In the eastern part the precipitation band and the gravity waves are not at all co-located throughout the entire episode.
So, if the precipitation band is related to the cold front, it is probable that the gravity waves were generated at higher levels by some other mechanism, and not directly by the propagation of the cold front itself. The Prague Libus 12:00 UTC sounding shows the presence of a very strong inversion layer between the 2.5 to 3km levels, at or inside which the gravity waves most likely have propagated southward.
The most plausible mechanism to generate the gravity waves seems to be the rapid descent of the dynamic tropopause (determined as 1.5 PVU) above the Baltic Sea and Poland between 00 and 12 UTC (see Aladin model analysis). This rapid decrease of the dynamic tropopause height in combination with a jet stream may generate strong vertical motion, creating waves in the stable air of the temperature inversion and above it. Here, at 12 UTC, the gradient of the dynamic tropopause height in the area of the Czech-Polish border closely matches the shape and propagation direction of the gravity waves, spreading southward, which tends to confirm this hypothesis.
11 April, Italy, Mediterranean Sea, Greece
By Jochen Kerkmann
The images below show a high-level plume-like cloud extending downwind from Mount Etna for about 200km to the east.
The plume cloud is very cold and has a relatively high reflectance in channel IR3.9r, it appears with a strong yellow colour in the RGB composite.
Principally, there are two possible explanations for this plume-like cloud: a) a cloud from a volcanic eruption or b) a mountain wave cloud. The first possibility is unlikely because there are no reports of an eruption of Mount Etna on this day.
Also, there are no signs of volcanic ash or sulfur dioxide (see Met-9 RGB composites) and the cloud temperature of -55°C speaks more for a wave cloud — a cloud from an eruption of Mount Etna would normally be lower, at 4–5km height, unless the eruption was very strong (which would have been reported on the news). Finally, the synoptic situation on 11 April, with strong westerly winds over Sicily, was favourable for the formation of lee wave clouds over Mount Etna.
27 Jan, Czech Republic, eastern Germany
By Martin Setvak (CHMI)
On 27 January 2008, a strong northwesterly flow formed well-developed, low-level waves to the lee of the Ore mountains (also known as Krušné hory or Erzgebirge mountains, on the border between northwest part of the Czech Republic and eastern part of Germany.
As the loops of MSG images (below) show, the waves persisted for most of the day, hardly changing their position and extent. In the parts of waves with descending flow the clouds dissipate, forming cloud-free bands. In contrast, the clouds form and persist in those parts of the waves where the air is ascending.
More details can be seen in the images from polar orbiting satellites, like MODIS images (see Aqua MODIS band 1 image at 11:50 UTC (250m resolution) and the corresponding RGB composite (500m resolution, bands 01-04-03)), and AVHRR images (see NOAA-17 image below and NOAA-15 image at 15:17 UTC (night microphysics RGB composite)).
The animations given under 'see also' (e.g. Airmass and Snow RGB products) show that similar mountain waves formed on 27–28 January over most of central Europe — also to the lee of the small Carpathian mountains. (spreading over Slovakia and Hungary) and in the lee of the Alps and Dinaric Mountains. In addition, stau cloudiness north of the Alps and orographic cirrus related to high-level waves south of the Alps can be observed (the latter best in the Airmass RGB , 00:00–23:45 UTC).
Animation Airmass RGB Composite (00:00–23:45 UTC)
Animation RGB Composite VIS0.8, NIR1.6, IR3.9r (06:30–15:30 UTC)
Animation RGB Composite HRV, HRV, IR10.8 (28 January, 07:00–12:15 UTC)
Aqua MODIS band 1 image at 11:50 UTC (250m resolution)
Aqua MODIS true-colour RGB composite at 11:50 UTC (500m resolution)
NOAA-15 AVHRR night microphys. RGB composite at 15:17 UTC (1.1km resolution)
7 Sep, Italy
By Jochen Kerkmann
High-level mountain wave clouds over the Po Valley as seen in Meteosat imagery in September 2007. The two images below show a high-level mountain wave cloud seen from both satellite and ground. As seen from both views, the wave cloud had a very sharp border on its northern (inflow) side, while the southern border was more gradual and thin.
Optically, on its northern side the cloud was thick and dark, formed of small ice particles, as indicated by the high reflectance in the IR3.9 channel of about 10%. This high reflectance gives the cloud an orange colour in the RGB composite when compared to the high-level ice clouds further to the east over Croatia (large ice particles, dark red colour).
1 June, Argentina, Paraguay, Bolivia and Brazil
By Jochen Kerkmann, Jose Prieto and HansPeter Roesli
The Meteosat-8 images below show a case of stationary, high-level wave clouds over Argentina, Paraguay, Bolivia and Brazil produced by the Andes mountains during a situation of strong westerly winds in June 2007.
By Jochen Kerkmann, Jose Prieto and HansPeter Roesli (EUMETSAT)
It should be noted that the Andes mountains are located very close to the limit of the Meteosat-9 field of view.
Typically, during such a situation strong foehn winds can occur on the eastern slope of the Andes, also called 'zonda' winds (in Spanish, viento zonda).
As can be seen in the bottom image, the wave clouds extend extremely far downstream over the South Atlantic. A reason for this could be the existance of very small ice particles which can stay up in the air a very long time without sublimating or fall out. More information about wave clouds over the Andes can be found in the case study from 29 June 2005 (High-level wave clouds over the Andes) .
17 May, Spain
By Mária Putsay and Ildikó Szenyán (Hungarian Meteorological Service)
On 17 May very bright white clouds could be seen in the airmass RGB images over Spain. Over the Pyrenees a stationary cloud formed into a sharp line and extended far to the south. This cloud stayed at the same location during the whole day in spite of the strong wind.
Similar stationary clouds were formed over the Cevennes in France and over the Mediterranean Sea, see the arrows on the Airmass RGB (Figure 53).
All these features can be seen clearly in the airmass RGB animation (Figure 54).
The long lifetime, the stationary and the sharp cloud edge is even more obvious in the convective storms RGB (Figure 55 and recipe).
The clouds over the Pyrenees and Cevennes are bright yellow, which indicates the presence of very small ice particles (homogeneous freezing processes at very low temperatures typically lead to the formation of very small ice particles).
In the dust RGB image (Figure 56) the wave clouds over Spain and France are dark brown (indicating thick ice clouds) and black (indicating thin ice clouds). Thin and thick ice clouds cover Spain and the western part of the Mediterranean Sea, much more than over other parts of Europe. The dust RGB is very sensitive for thin clouds.
Both the WV6.2 image (Figure 57) and the IR10.8 image (Figure 58) confirm that these are high-level clouds. The clouds over the Pyrenees are very cold, about -60 °C.
The 00:00 UTC SatRep analysis refers to these clouds as lee clouds.
Lee cloudiness forms due to the lifting of the airflow over the mountains. This lifting process does not necessarily stop at the mountain top but can propagate to much higher levels. Depending on the atmospheric conditions two types of wave clouds can form on the lee side of a mountain chain.
- Lee wave clouds consisting of narrow middle (or low) level cloud bands perpendicular to the wind direction and parallel to the mountain chain.
- High lee cloudiness consisting of cirrus clouds extending from the mountain chain over the leeward side, and continuing (in some cases) for up to several 100km downstream (see conceptual model and graphics ). The clouds often have a sharp edge over the mountain chain. Sometimes in strong wind conditions they become detached from the mountain and appear as if they have developed far away from the obstacle.
In the present case study the second type of mountain wave clouds, ie the high level lee cloudiness, are clearly observed.
As the wind plays a key role in the formation of high level lee cloudiness, the ECMWF wind analysis is shown overlaid on the satellite image (Figure 59) (the 200hPa wind field was superposed as the cloud top pressure over the Pyrenees at 10:00 UTC was 200hPa according to the SAF Nowcasting cloud top pressure product).
At this height the ECMWF analysis shows strong north winds, almost perpendicular to the mountain chain. For comparison, the Atmospheric Motion Vectors (AMV) (produced by Nowcasting SAF software) are also shown (Figure 60).
By comparing the ECMWF wind analysis with the AMVs, one can see a lot of similarities. However, distinctive differences are found for the area of lee cloudiness where the AMVs calculated from the motion of the clouds are obviously too small (ie despite of the strong wind the cloud appears almost stationary, as it is formed at the same location, whilst the air parcels move away downstream).
9 March, Sicily, southern Italy
By Jochen Kerkmann, Kornel Kollath (Hungarian Meteorological Service), and HansPeter Roesli
Close-up of a frontal cloud system related to a cyclone over Tunisia in March 2007. Southern Italy and, in particular, Sicily, appears fully covered with thick clouds with cloud top temperatures of about -50°C. Interestingly, a high-level plume-like cloud extends downwind from Mount Etna for about 150km to the west north-west. As this plume cloud is very cold (-62°C) and has a relatively high reflectance in channel IR3.9r (about 6%), it appears with a strong yellow colour in the RGB composite.
There are no signs of volcanic ash or sulfur dioxide (see Meteosat-9 RGB composites below) and the cloud temperature of -62°C speaks more for a wave cloud (a cloud from an eruption of Mount Etna would normally be lower, at 4–5km height, unless the eruption was very strong. The synoptic situation on 9 March 2007, with strong easterly winds over Sicily, was favourable for the formation of lee wave clouds over Etna. Looking carefully at the animation (Figure 48) one can actually see some high-level lee wave clouds over the Algerian Atlas mountains.
Meteosat-9 Day Microphysics RGB Composite (9 March 2007, 12:00 UTC)
Meteosat-9 Airmass RGB Composite (9 March 2007, 12:00 UTC)
Meteosat-9 Dust RGB Composite (9 March 2007, 12:00 UTC)
Meteosat-9 Enhanced IR10.8 image (9 March 2007, 11:45 UTC)
17 Feb, Iran, Afghanistan and Pakistan
By Jochen Kerkmann
The Meteosat-8 images below show a case of stationary, high-level wave clouds over Iran, Afghanistan and Pakistan produced by the Zagros mountains during a situation of strong south-westerly winds. It is interesting to note that the high-level wave clouds do not form right above the Zagros mountains, but at a certain distance on the downwind side extending far downstream into Afghanistan and even into Pakistan.
Similar cases of large downsteam cirrus wave clouds (slightly detached from the mountain barrier) have been observed over the Andes, the Alps and the Norwegian mountains. One interesting aspect in the images below is the high values of the brightness temperature difference (BTD) between the IR3.9 and the IR10.8 channels with values reaching more than +70 K. Such high values indicate the presence of very small ice particles inside the cold, high-level wave clouds.
6 Feb, France
By Jochen Kerkmann
Wave clouds over the Massif Central, France on 6 February 2006. The Meteosat-8 image below shows the development of lee cloudiness in a situation of northerly flow over France. In the animation, two types of lee clouds can be observed to the lee of the Massif Central: a) low-level wave clouds with a relatively short wavelength of 10 to 20km, and b) a high-level wave cloud that extends further downstream as far as the Pyrenees.
The high-level wave cloud casts a distinct shadow on the lower-level clouds. This shadow gets longer during the afternoon. Also, below the high-level lee cloudiness low-level fog/stratus can be seen in the Garonne valley that dissolves during the afternoon.
29 June, Argentina
By Jochen Kerkmann and Jose Prieto
The Meteosat-8 images below show a case of stationary, high-level wave clouds over Argentina produced by the Andes mountains during a situation of strong westerly winds (it should be noted that the Andes mountains are located very close to the limit of the Meteosat-8 field of view). Typically, during such a situation strong foehn winds can occur on the eastern slope of the Andes, also called 'zonda' winds (in Spanish, viento zonda).
Although this phenomenon can occur along the entire length of the extratropical Andes, it is most frequently detected near the cities of Mendoza [32°S, 69°W , 704m above sea level] and San Juan (32°S, 68°W, 598m above sea level), which represent important urban regions of western Argentina (see map of Argentina, source: Marcelo E. Seluchi et al., 2003).
One interesting aspect in the images below is the high values of the brightness temperature difference (BTD) between the IR3.9 and the IR10.8 channels (see lower left image) with values reaching more than +70K. Also other channels, and differences between channels, reach extremely high values for these high-level ice louds, for example:
- -19K for both the BTD IR12.0–IR10.8 and the BTD IR10.8–IR8.7, indicating thin, very high ice clouds.
- +26K for the BTD IR9.7–IR10.8, indicating very high clouds.
- +33% reflectivity for the IR3.9 channel, indicating very small ice particles.
- +50% reflectivity for the NIR1.6 channel, indicating very small ice particles.
It is interesting to note that the high-level wave clouds do not form right above the Andes, but at a certain distance on the downwind side extending far downstream into Paragua,y and even into Brazil. Similar cases of large downstream cirrus wave clouds (slightly detached from the mountain barrier) have been observed over the Alps and the Norwegian mountains.
However, in Europe the wave clouds usually do not reach such a high altitude (probably 16km in this case). The extremely high values of reflectivity in the IR3.9 channel can only be explained by homogeneous freezing processes at very low temperatures, which lead to the formation of very small ice particles. A similar case has been observed over the Carpathian mountains (Romania) in November 2003. In the RGB composite VIS0.8, IR3.9r, IR10.8 these clouds appear in an intense green (thin cloud) to yellow (thick cloud) colour and could be confused with a super-cooled water cloud. However, it is clear from the IR10.8 temperature well below -40°C that these clouds are actually ice clouds.
Remapped Meteosat-8 images
Mercator projection centred at 30°S, 55°W
RGB composite VIS0.8, IR3.9r, IR10.8
RGB composite WV6.2-WV7.3, IR9.7-IR10.8, WV6.2 (Airmass)
RGB composite IR12.0-IR10.8, IR10.8-IR8.7, IR10.8 (Dust)
Eta–CPTEC Regional Model (Marcelo E. Seluchi et al., 2003)
Remapped IR10.8 image with 500 hPa geopotential field
Remapped IR10.8 image with surface pressure field
Remapped VIS0.8 image
Remapped Difference image IR3.9–IR10.8
Remapped RGB composite WV6.2–WV7.3, IR9.7–IR10.8, WV6.2 (Airmass)
Spain & the Balearic Islands
28 Dec, Spain, western Mediterranean
Detection of mountain waves over Spain and the Balearic Islands in the WV7.3 channel of MSG.
The detection of lee clouds associated with mountain waves is an important element of operational meteorology insofar as lee clouds can indicate the presence of areas of turbulence. Usually, lee clouds can be quite easily detected in visible and infrared satellite images, but sometimes, depending on the altitude of the lee clouds, they also appear in water vapour images. The best way to detect them is to animate a sequence of images where the lee clouds tend to be stationary, as if they were attached to the mountain barrier.
A special case of mountain waves are 'trapped' mountain waves, which can propagate much farther downstream (several hundred kilometres) than non-trapped waves. The conditions for trapped waves are a very high vertical stability above the mountain tops that extends over a large area downstream, and an increase of the wind speed with height in this stable layer.
On 28–29 December 2004, an strong cyclone over the Western Mediterranean generated intense winds and turbulence over the Iberian Peninsula and the Balearic Islands. In the Meteosat-8 infrared and water vapour images below, lee clouds associated with mountain waves can be seen over Spain and the Western Mediterranean, extending far downstream from the mountain ranges that generate them. Some thin, high-level lee clouds can also be seen to the lee of the Iberian mountain range over eastern Spain. Furthermore, the WV7.3 channel shows evidence of lee mountain waves generation and related turbulence even in clear areas (eg in the area between Spain and the Balearic Islands). Thus, in some cases, WV7.3 images can help to identify areas of clear air turbulence (CAT).
Other image sources
Leewave cloudiness over Spain (Met-6, VIS, rapid scans, 3 April 2003, 08:30–11:50 UTC)
Absolute topography 300 hPa with WV7.3 image (29 Dec 2004, 00:00 UTC. Source: INM)
Absolute topography 700 hPa (28 Dec 2004, 12:00 UTC. Source: Deutscher Wetterdienst)
Radiosounding of Madrid (29 Dec 2004, 00:00 UTC. Source: Univ. of Wyoming)
21 Oct, Lesotho, South Africa
On 21 October 2004 Meteosat-8 observed a large cloud band with embedded convective cells extending from Angola to South Africa.
Marked wave patterns, low-level short-wave lee clouds as well as extended high-level lee clouds, were observed over and extending south-eastwards from the Drakensberg mountains (Lesotho-South Africa border) to the coast. High-level lee clouds often occur along the anticyclonic side of a jet stream, and sometimes in the jet exit region.
The high-level lee clouds over Lesoth and South Africa are best visible in the animated RGB composite VIS0.8, IR3.9r, IR10.8, where these clouds appear with a yellowish to orange colour, indicating that these clouds are formed of small ice particles. In this RGB composite, cold, thick ice clouds with large ice particles appear as a red colour and thin ice clouds as a more olive colour.
20 Feb, France, Germany
The visible, infrared and water vapour images below show an example of high lee wave clouds (type 2) forming in the lee of the Massif Central (France) on 20 February.
Atmospheric circulations resulting from airflow over mountains exist on many scales. The size of a given circulation is generally related to the size of the forcing obstacle, but also the atmospheric conditions play an important role, for example, vertical stability and wind variation with height.
Depending on the atmospheric conditions two types of lee wave cloudiness can be observed:
- Lee wave clouds — characterised by narrow middle level cloud bands perpendicular to the wind direction and parallel to the mountain chain. The wave length ranges from about 3 to 40km.
- Lee wave clouds consisting of high cirrus clouds extending from the mountain chain over the leeward side, and continuing (in some cases) for up to 1000km (large wave length).
The latter type often occurs along the right (anticyclonic) side of a jet stream, and sometimes in the jet exit region.
Key parameters for the nowcasting/forecasting of lee wave clouds are:
- the wind direction with respect to the orientation of the mountain chain (the wind direction need not necessarily be perpendicular to the mountain chain but there must be a normal component);
- the stability of the atmosphere (a stable layer in the middle or upper troposphere);
- the humidity profile (sufficient humidity near the wave crests).
In the images below the lee wave clouds, which extend downstream down to the Alps, can be easily distinguished from the frontal cloud band that covers large part of France and Germany.
The lee clouds are whiter (colder and denser) and remain attached to the obstacle (Massif Central). They appear in the right exit region of a strong jet stream over Western Europe (see absolute topography at 300hPa) in an area of marked atmospheric stability above 700hPa (see sounding Lyon-Satolas).
A second, smaller area of lee wave clouds can be observed downstream (to the east) of the Western Alps. Within the lee cloudiness over France, some cloud stripes aligned parallel to the wind direction can bee seen.
They look like waves, but they are, in fact, perpendicular to the typical direction of lee waves. A possible explanation is that the single cloud stripes are generated separately by the highest peaks of the Massif Central, ie the cloudiness follows the orography, as indicated in the conceptual model below.
The animated infrared images below show a second interesting example of high-level lee wave clouds, which was observed on 23 April 2002. As in the first example, lee clouds form along the anticyclonic side of a jet stream, but this time to the south of the Massif Central being the wind at high levels from the north. Some lee wave clouds also form on the southern side of the Pyrenees.
An impressive development of lee wave clouds (type 2) can also be observed in the lee of the Norwegian mountains. They extend downstream to the Czech Republic covering a vast area of about 500,000km2. These lee clouds are quite common in Scandinavia during the whole year.