# Gravity Waves

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## Chapter II: Lee Waves

### Table of Contents

- Chapter II: Lee Waves
- Introduction
- Lee Waves In Satellite Images
- Lee Waves And Convection
- Summary
- Exercise

### Introduction

Lee waves are a very common atmospheric phenomenon over land. The example before shows the large impact of lee waves and related turbulence on air traffic. The example also shows that lee waves are not always visible to the pilot or in satellite imagery. Atmospheric waves will be visible for the human observer only if the humidity of the air is near saturation and air temperature falls below the dew point due to lifting motion at the wave crest. Because of the varying amount of humidity in different atmospheric layers, lee waves will be visible only in those layers where the above conditions are fulfilled. Hence lee waves and turbulence are not restricted to the height of the characteristic cloud bands seen in satellite images but occur both above and below them.

Figure 1: SEVIRI image loop from Meteosat 10 (high resolution visible (HRV) channel) on 10 February, 2016. Lee clouds developed over many places in southern Europe.

### Lee Waves In Satellite Images

Lee waves are stationary atmospheric waves and are depicted as a bunch of parallel, white and narrow cloud bands in VIS images (figure 2). In IR imagery, most of them appear as dark grey cloud bands due to their relatively warm cloud tops.

Lee waves are internal gravity waves in a stable atmosphere; the clouds are generated by topographic elevations like mountains or coasts. Wind passing over such features receives a vertical pulse and kinetic energy is transformed into potential energy. The horizontal spacing of the waves is related to the wind speed over the elevation in question. The higher the wind speed, the longer the wave length. Stronger winds are also related to stronger turbulence. Atmospheric conditions during the winter season are more favorable to the development of lee clouds because of the stable stratification in the lower troposphere.

To diagnose mountain waves from satellite imagery, the highest available resolution should be chosen (e.g. HRV channel for Meteosat or AVHRR images from NOAA/Metop satellites). A coarser resolution of the image data will blur the cloud pattern and the waves will appear as a homogenous cloud shield. Lee clouds are best seen in the visible channels during the day. They offer a higher contrast between clouds and land than IR or WV imagery. Night time lee cloud detection with IR or WV channels is more challenging.

Figure 2: Comparison between MSG IR10.8 and HR-VIS image for 10 February, 2016 at 12:00 UTC (use the slider).

In many cases, water vapor imagery can complement wave detection in regions where cloud condensation does not occur and therefore no signal in IR or VIS channels exists. Figure 3 shows both, an HRV and a WV7.3 μm image. While the Po Valley and the Mediterranean Sea east of Corsica are cloud free in the visible image, the water vapor image shows a distinct wave pattern in these areas.

The WV7.3 μm channel is sensitive to water vapor concentration, especially between 700 and 500 hPa. As the total column water content of the atmosphere is constant whether this column is located in the wave's trough or wave's crest, the varying absorption in this WV channel can only be explained by the different amount of water vapor in the layer which contributes most to the 7.3 μm channel. In case of very strong waves, drier upper tropospheric air protrudes down to lower layers and hence reduces the amount of water vapor within the highest contributing layers (the moisture is pushed down towards layers below 700 hPa where the 7.3 μm channel is more or less blind).

Figure 3: Comparison between MSG WV7.3 and HR-VIS image for 10 February, 2016 at 13:00 UTC (use the slider).

Though the MSG Severe Storms RGB does not include the high resolution HRV channel, it is very suited for detecting lee clouds composed of ice crystals. Typically, Severe Storms RGB imagery is used for detection of convective systems, especially for the most active cores or updrafts. Small ice particles, characteristic of high level lee cloudiness, are shown in the same bright yellow colors (figure 4). These high level lee clouds consist of the same type of small ice crystals as active thunderstorms.

Figure 4: Meteosat-10 Severe Storms RGB, 2 July 2015 at 08:30 UTC. High lee clouds formed north of the Pyrenees.

### Lee Waves And Convection

Atmospheric lee waves may also act as a trigger for convection. The vertically lifted air at the wave crest is cooled adiabatically, but at the same time, when cloud droplets form, latent heat is released. In a potentially unstable stratification of the surrounding air, this little extra heat can be crucial for initiating convection. Cloud lines mark the wave crests of the atmospheric gravity waves all over the Iberian Peninsula. At some locations further downstream, the HRV satellite image shows convective cells instead of thin cloud lines (red circles in figure 5). These are the regions where convective initiation takes place.

Figure 5: MSG SEVIRI image (HRV) from 6 March, 2016 at 15:00 UTC. The red circle points out some of the many convective cells initiated by atmospheric gravity waves. For the image loop click here.

It is sometimes difficult from the satellite perspective to discern whether the reason for convection is orography or gravity waves or both. In general, orographic convection is attached to a fixed place (windward side of the mountain) while convective initiation in lee waves moves downstream. In the image loop above (figure 5) both cloud types can be observed on the northwestern part of the Iberian Peninsula: stationary lee clouds and convective cloud cells moving southeast.

Severe convection usually acts as inhibitor to the propagation of gravity waves. While shallow convection (cumulus humilis) will not influence the wave propagation much, deep convection stops this process completely.

On the other hand, shallow convection is able to trigger so-called thermal waves. The vertical pulse of rising air acts similarly to a mountain barrier and obliges the air stream either to pass over the cumulus cloud or around it (figure 6).

Figure 6: Schematic illustrating how shallow convection initiates thermal waves (WMO, 1978).

This effect is often visible when cold stable air arrives over flat warm land. Shallow cumulus convection starts to appear soon and takes on a wave like pattern. The HRV image below shows thermal waves (figure 7, red circle) above the Benelux in the cold Atlantic air behind a cold front. When convection gets stronger, the wave pattern disappears (figure 7, green circle).

Figure 7: MSG HR-VIS image from 2 March, 2016 at 12:00 UTC. The blue arrow shows the main wind direction.

### Summary

Various cloud patterns are summarized under the name of lee cloudiness: rotor clouds, lenticularis clouds, cirrus clouds, etc. What they all have in common is that they are generated in a stable atmosphere by a surface elevation which makes the air rise above condensation level. The image below gives an overview of the different cloud types, though note that they do not all need to occur simultaneously.

Figure 8: Schematic of the different lee cloud types (WMO, 1978)

### Exercise

The HRV satellite image below shows a frontal system over Western Europe and an older low pressure area centered over Greece.

### Choose the right position

Find three regions with lee cloudiness by clicking on the image above. MSG HRV image from 4 March, 2016 at 12:00 UTC.

For a larger image click here

- Solution: Click here for the solution.