Sunrise or Sunset - Seeing Even More Gravity Wave Clouds

#2521 "Singleton Sunrise Morning Cloud"

Perhaps my years of paddling and walking on shorelines, not to mention "sitting on the rocks of the bay", have paid off in many more ways than just the enjoyment of the moment. You can learn a lot by watching waves. Gravity waves are the ultimate tool because they are absolutely perpendicular to the atmospheric frame of reference wind. I painted gravity waves in the above skyscape and this Blog will explain those as well as the patterns in August sunrise at Killbear that initiated this series of Blogs on Creative Scene Investigation. Please read on... 

Early satellite imagery of the weather from the seventies was the other key inspiration for me. I saw that atmosphere as an ocean of air very much like the lakes that I paddled on.  That statement is not a big stretch because it is true. The different air masses are also individual pools in that larger ocean. We just happen to live at the bottom of those air oceans. 

I applied the concepts of the different types of water gravity waves to the atmospheric fluid of air so that I could better appreciate the satellite imagery. There may be seven different types of ocean waves but I found it most helpful to keep it simple and just consider wind waves and swells. 

Wind-driven waves are created by the friction between wind and the surface water. As wind blows across the fetch of the water, the continual disturbance creates a wave crest. Gravity takes over from the wave crest and digs the trough. Wave height and wavelength both increase with the wind speed. 

Swells may originate as wind waves but continue propagating far from the original energy source of the strong winds. Shorter wavelength swells carry less energy and dissipate faster. Swells that are observable some distance from the source tend to be longer wavelengths. 

In my graphic for #2535 "Sunrise on Killbear Light", I conveniently labelled the main, large bands of cloud as simply gravity waves. I did not call them "swells" or mention the fine structure of the clouds within those atmospheric swells.  Let me explain... 

Swell Gravity Waves at a distance
from the strong wind source region

Atmospheric wind waves are generated where the atmospheric relative winds are strongest - between the companion swirls. This is also the local maximum in the jet stream. Atmospheric wind waves can be seen in the cirrus. As in the ocean, atmospheric swells propagate outward from this source. The shortest wavelength swells decay fastest. The longer wavelength swells travel great distances before "crashing" on the shore of the air mass at the deformation zone. If you can imagine the atmosphere in these terms, much of the science of oceanography can be transferred to the atmosphere. It all started when I first really began  to appreciate satellite imagery. 

In Sunrise or Sunset - Seeing Gravity Wave Clouds, I was actually describing atmospheric swells. What I did not describe were the details within those swells and how they got there. The CSI fun really starts with superimposing smaller scale wind waves on to the atmospheric swells. The signatures that result can be applied to really understand the weather. I constructed the following graphics to better explain these patterns thinking that each image might be worth a thousand words. 

Adding Wind Gravity Waves to Swells

Atmospheric gravity waves are most easily observed when the layer of moisture in the stable layer is rather thin. The crest of the wave associated with ascent and cooling air is more likely to be cloudy - condensed water droplets or ice crystals. The lowest point of a wave or trough associated with descending and warming air will be the thinnest. The best case is when the lifted condensation level in the atmosphere is near the midpoint between the crest and trough so that bands of cloud are separated by blue sky. Waves are occurring all of the time in the atmosphere but those motions are typically shrouded in cloud. 

There are many interesting situations that superimpose wind gravity waves on top of swells. The warm conveyor belt is probably the most interesting and informative about the weather. The system relative winds associated with the warm conveyor belt are best described by the deformation zone conceptual model but I will save most of those details for the next Blog and focus instead on the gravity of the situation. 

The wavelength of the wind waves is determined by the wind speed – stronger wind gives a longer wavelength.  The spacing of different portions of the wind wave are best described by differences in the wind speed which translate to differences in the wavelength. The simpler approach is to imagine that the stronger winds along the confluent asymptotes are simply tipping the gravity waves as depicted in the above graphic. That simplification is not correct. 

As a home study assignment, drop a pebble in a pond or your bathtub to really learn about dispersive swell waves. All of the energy in that first wave is supplied by the stone. The waves disperse in all directions from that source and significant ripples survive long distances before dissipating. … with no additional energy supplied. These are really swell swells! The direction of motion of dispersive swells like in your tub, might be diagnosed by the decay in amplitude, the arc shape of the swell or in the atmosphere, the direction of cloud drift.

We should not be surprised to see smaller and shorter wind waves superimposed on swells. They happen all the time in the atmospheric ocean.

Remember that nature's solutions to the laws of physics is always correct. Nature also constructs more accurate and beautiful graphics than I can. The next Blog will further explain gravity waves (both wind driven and swells) within the context of the deformation zone conceptual model. Stay tuned, looking at clouds is about to get even more exciting. I am sittin' on the very edge of that rock of the bay.. Otis Redding would be pleased. 

Keep you paddle in the water and warmest regards... 

Phil the Forecaster Chadwick