Weather Ed- Cloud Caps and Lenticulars
The presence of a cloud over the top of a mountain such as Rainier makes for a great photo op for anyone who happens to see it from below; for climbers high on the mountain however, these clouds and associated winds can represent quite a challenge In this short overview we want to look at the fundamentals of how these clouds form and what the implications are for climbers.
|Aug 2016. Flow is from left to right|
There are two basic mechanisms that we will discuss. Keep in mind that these processes are come with respect to any large isolated mountain or mountain range. First, we will look at the generic mountain cloud cap which is produced by the forced ascent of air up the windward slope of a mountain. Then we will discuss the various types of lenticular clouds (lens shaped) which form in the crests of mountain waves.
We are going to start our discussion with a thought experiment. Let's say we have 20 mph west winds at 10,000 ft approaching Rainier (approx. upper Puyallup Cleaver-Tahoma Glacier). Note that wind direction is the direction from which the air is moving; in this example from west-to-east. What the possible flow patterns once the air reaches the mountain? It would seem pretty obvious that the air can go up, down or around the upper mountain. Under most circumstances (standard atmospheric properties) this air would have a difficult time moving down because it would be moving in to an area of higher pressure. Computer modeling studies of this nature show that part of the flow will go around a conical mountain like Rainier and some will go up and over (keep in mind that these flows may accelerate as well). The breakdown of how much goes over versus around depends on a number of factors chiefly, atmospheric stability which in turn is a function of the vertical change in air temperature with height. Without going into any more detail, it will suffice to say that the portion of the flow that is 'forced' up and over cools as it ascends. It cools because it expands into an area of reduced pressure hence the kinetic energy of the air molecules decreases (there are fewer collisions between the air molecules which is what we measure as temperature). The rate of cooling various a bit but typically ranges from 3 to 5 degrees F. per thousand feet.
|Upslope flow from left to right.|
If the air which ascends the windward slope contains only a small amount of water vapor (invisible to the human eye but is in the air nevertheless), then the cooling will probably not be sufficient to produce a cloud (the relative humidity [RH] remains below 100%). However, if the ascending air is moist to start with, as soon as the RH equals 100% cloud droplets or ice crystals form and we have the makings of a cloud. It is important to keep in mind that air is continuously moving through the cloud and that the cloud droplets (or ice crystals) are continuously condensing (deposition= vapor to a solid directly) and then evaporating (sublimating= solid to a vapor directly). Even though the cloud essentially remains fixed with respect to the mountain, the stuff that makes up the cloud is in a constant state of flux. This upslope flow produces a cloud directly over the summit and not above it or downstream.
So what happens as the air moves past the summit? Generally, it will start to descend the leeward slope; as it moves downward it is compressed and hence it warms (kinetic energy increases). As a parcel warms with no change in its water vapor content the RH drops below 100% and the droplets (crystals) begin to evaporate (sublimate)- this is the trailing edge of the cloud. The shape of a mountain cloud cap as seen from below is often symmetrical with respect to the leading and trailing edges but they don't have to be (photo above shows an asymmetrical cloud). Sometimes the edges are smooth and other times ragged. Flow around the sides of the upper mountain may produce some additional clouds as well. Cloud caps form and dissipate over the course of a few minutes and their duration (summer) is typically on the order of half-hour to hours. During the winter these may last for days during the passage of a storm.
|Massive outbreak of lee waves.|
Lenticular clouds on the other hand are produced when a wave in the atmosphere is generated by the flow of air over the summit of a mountain, ridgeline, or mountain range. This occurs in response to a fairly narrow range of atmospheric parameters hence they only develop from time-to-time. In short, energy is produced by air moving over the summit which is then transmitted upward through the atmosphere. Depending on the change with height of air temperature, windspeed and wind direction, this energy may be transmitted well into the stratosphere (miles above the source region), the result is a single wave above the mountain. At other times, a 'wave guide' may form in the atmosphere in which case a large part of the upward energy is reflected back down towards the mountain. If these two waves are in phase they will superimpose creating what we call a standing wave. Air is constantly moving through the wave yet the wave itself moves very little with respect to the mountain. One might be tempted to think these are the same as oceanic swells- in fact they are not. In the ocean case, wave energy moves horizontally while the water is only displaced vertically, hence the swell from a tsunami can travel 400-600 mph in deep water because only the energy is being transmitted long distances not the water. In the atmosphere the air is primarily moving horizontally while the energy moves primarily vertically.
|Gossamer cloud morphed into much larger cloud cap within an hour.|
Standing waves are generated individually or in a group; they may form above the summit, envelope the summit or downstream of the summit (standing lee waves). When they occur as a group the horizontal wavelength is often on the order of 3-10 miles. It is one thing for a wave to form it is another to be able to see it. Lenticular clouds form in the crests of standing waves if there is an abundant supply of water vapor. This is essentially the same process as occurs in upslope flow over the summit; in this case the wave 'forces' the air to ascend and then descend. Clouds that form in the crests of these waves (lee wave clouds) often have a very smooth appearance, sometimes they are 'stacked' on top of each other leading to some dramatic images if you are in the right place at the right time. Sometimes wave clouds show some additional smaller scale features within the primary wave- this of course suggest additional processes are in play. Wave clouds can form over the course of a few minutes and like cloud caps can last for hours or even a day or two. The vast majority of the time these clouds form over the higher summits of the Cascades but one or twice over the course of the summer conditions are ripe for lenticulars to form for hundreds of miles up and down the Cascade Range (clearly visible in satellite imagery). Many lenticulars form during the passage of an upper level weather front which in turn produces the aforementioned stability and wind profiles.
So what does this all mean to climbers' high on Rainier or any other peak that is prone to generate cloud caps and lenticulars? Two things should come to mind: wind and lower visibility. There is not magic windspeed at which cloud caps or lenticulars form: in general minimum speeds are on the order of 25-30 mph. Let me stress that that figure is a minimum, which you should interpret to mean that in many cases the winds are significantly stronger, sometimes far too strong to climb into. Secondly, these clouds often produce whiteout conditions due to the production of new snow (occasionally water droplets), or the re-suspension of snow from off the upper mountain. In the winter and spring it is also not uncommon for rimming (freezing of supercooled water droplets) to occur which will escalate the danger factor considerably. These clouds are very common in winter however there is so much intervening cloud cover that its uncommon to see them.
|Stacked lee wave-July 21, 2017|
Are cloud caps and lenticulars predictable? Yes and no. Don't expect to see predictions for these in your favorite mountain weather forecast or the REC forecast we use here on the hill. Why? The state of the art is slowly improving however the use of a high-resolution weather model does not guarantee a quality forecast of this nature because the outcome is very sensitive to the initial conditions. Recall that there is a fine balance between the water vapor content or air and the degree to which it is forced upwards. If the model(s) is off a little bit in moisture content- it will produce a cloud when it should not and neglect one when it should.
With that being said there have been many modeling attempts in conjunction with field studies that have looked at the generation of lee wave clouds (and downslope windstorms in particular). The results of these studies have shown there is some skill in forecasting (or hind-casting), however researches have almost always added smaller-scale data which is not readily available in the day-to-day forecast models. In our specific case, one would be tempted to look at the vertical cross-section (X-Z coordinates) which runs through Rainier from the GFS-WRF model and make a forecast. That would certainly be a good starting point, however a considerable amount of additional data and expertise would be needed in order to produce a relatively accurate wave cloud forecast each day. In other words, a forecast is possible but it would take a lot of additional resources ($$) which no one wants to pay for.
|July 22, 2017|
Cloud caps and lenticulars make for some memorable scenes when viewed from below but at times produce some challenging or even dangerous climbing conditions. In general these clouds are associated with moderate to strong winds and lower visibility or even whiteout conditions. As noted above, a wide spectrum of cloud types are possible; from gossamer like features wafting just above the summit to monster clouds which envelope the entire upper portion of the mountain to a series of downstream lee wave clouds. At times one type will transform into another over the course of to 30-60 minutes. If you're going to climb into one of these clouds give it some serious thought beforehand. If one of these forms around you give some serious thought to descending before conditions deteriorate. Only you know your own abilities, so choose wisely.