Cloud Generation Downstream of Mountains

When we think about how mountains affect the production of clouds, we often assume that the air rises, cools, and produces clouds on the windward side of a mountain range (i.e., the side facing the flow) and sinks, warms, and dissipates clouds on the leeward side (i.e., the side facing away from the flow.  

Source: Whiteman (2000)

This is often a good assumption at low levels if the flow is moist and sufficiently strong to surmount the mountain range.  However, there are times when cloud generation occurs on the leeward side of mountain range.  

This happened overnight downstream of the Deep Creek, Schell Creek, and Snake Ranges of Nevada.  As shown in the photo below, mid- and high clouds formed or became enhanced downstream (east) of these ranges. 

In this instance, it is likely that the flow at low levels was sinking and warming on the eastern/leeward slopes of these ranges as depicted in the schematic above.  However, at mid and upper levels, it is likely that the air was rising, leading to leeward cloud generation.  

A schematic of this process is shown below.  The flow moves over the mountain barrier at low levels, resulting in cloud generation at low levels over the windward slope and dissipation over the leeward slope (this isn't happening in the satellite image above because the low-level airmass is too dry).  At upper levels, however, there is what meteorologists refer to as a vertically propagating gravity wave.  Such waves are sometimes generated by flow over mountains and they lead to an upstream tilt in the wave troughs and crests and rising motion over the lee slopes at mid and upper levels.  

Source: Durran and Klemp 1983/The Comet Program

This can lead to cloud generation downstream of the mountains.  In some situations, such clouds can extend well downstream of the mountain barrier.  

Instead of clear skies, you might get overcast skies.  Meteorologically you might say no biggie.  However, such clouds strongly affect power generation by solar farms and thus are a consideration for power management.  

Yesterday's Mountain Waves

Yesterday's MODOS imagery showed widespread evidence of clouds produced by trapped mountain waves over the Great Basin. 

Such clouds are produced in situations when the wind speed increases rapidly with height and the atmospheric stability decreases above a crest-level stable layer.  Such conditions were very apparent in the afternoon sounding form Elko.  Note how winds increase from 10 knots at about 725 mb to 130 knots at 300 mb, the existence of a sharp inversion near 725 mb, and then a decrease in statisc stability from 700 to 500 mb. 

This leads to a sequence of lee waves downstream of the mountains.  Clouds can form in the wave crests and dissipate in the wave troughs if the humidity is right, as was the case yesterday.

Source: Durran and Klemp (1983); COMET

Mountain waves were also evident over the Wasatch Range yesterday.  Note the wave-like undulations in the clouds in the photo below. 

 
Really, the mountains are generating waves pretty much all time time, but you need clouds to see them.  Their structure, however, varies.  Sometimes trapped lee waves are produced.  At other times, the waves may be confined to directly over the mountain.  Much depends on the characteristics of the flow and the atmospheric stability.

Salt Lake in the Lee of the Wasatch Range

If you were in the Salt Lake Valley and looked toward the Wasatch this morning around 8:30 you may have noticed that there was a cloud sitting several kilometers west of the Wasatch Range (note darker band of clouds below).  The cloud sat several km west of the Wasatch Range and didn't extend into the mountains.  Over the mountains, it was snowing, but not very hard.  

What you are seeing is a consequence of easterly flow.  There is a band of precipitation that currently extends from from SW to NE across the region, but this is embedded in easterly flow.  If you look carefully at the image below, the radar reflectivities at 8:18 AM were actually higher east of the Park City Ridgline (look along the border between Salt Lake and Summit Counties) than on the Cottonwood side of the Wasatch.  

So, in the photo above, the snow in the mountains is spillover.  Then, a harder, darker cloud base exists where a lee wave is producing rising motion.  This is something akin to that illustrated in the picture below, except there is more extensive cloud cover due to the precipitation band being generated on the north side of the close low that exists at upper levels to our south.  

Evidence for the easterly flow can be found in the morning sounding from the airport.  The low-level flow in the Salt Lake Valley is northerly, but there's northeast to east to southeast flow from 700 to 300 mb.  

Mountain Wave Clouds

I awoke this morning to a beautiful mountain wave cloud that was draped over the Traverse Range at the southern end of the Salt Lake Valley.

Mountain waves form during flow across topographic obstacles.  The Traverse Range has a maximum elevation a bit over 6500 feet, and a deep gap where it is bisected by the Jordan River, but is plenty high enough to generate such waves despite being small compared to the surrounding Wasatch and Oquirrh Mountains.  One often sees mountain wave clouds over the Traverse Range (and typically Lone Peak as well) in advance of approaching storms, so whenever I see one, I take a peak at the latest computer models.

Indeed we have a trough approaching northern Utah this morning and expected to move across the area tonight, bringing precipitation to all elevations.

Mountain snow and valley rain will begin late today, with precipitation changing to snow at all elevations overnight.  For the mountains, this is looking like a pretty good storm tonight.  I'm thinking 6-12" in the upper Cottonwoods by 8 am tomorrow morning, with another 2-4" after that time tomorrow.  This is a bit more optimistic than the National Weather Service, which is unusual given my conservative forecast streak (they are calling for a 6-12" storm total).  Given that I'm still half asleep, perhaps you should keep your expectations low so you are pleasantly surprised tomorrow.  Low expectation are, afterall, the key to great powder days.

Normally I would call this a nice Goldilocks storm, but given the snowpack problems we've been dealing with, it's bound to cause further stress and problems.  Be careful out there.

This Conference Is a "Drag"

I've been to perhaps 50 major scientific conferences in my career and the one I'm attending in Innsbruck this week is one of the best for both professional and personal reasons.  The quality of the science has been very high, and the area provides beautiful views and great brews for late-night discussions.  In addition, I've had an opportunity to see some excellent presentations covering new results in areas that interest me but lie outside my core research area.

One of these areas is something known as gravity wave drag.  Most of us are familiar with gravity waves, which include waves on the ocean, as well as so-called hydraulics generated by river flow over rocks.  Similarly, gravity waves are generated in the atmosphere by flow over mountains.  Under the right conditions, these waves can propagate to jet stream level and even into the stratosphere and mesosphere further aloft.  In some instances these waves break (the water analog is waves crashing on the beach), resulting in gravity wave drag, a deceleration of the flow at upper levels.

Source: The COMET Program

It turns out that these breaking waves are a major problem for weather and climate modeling.  The models lack the resolution to fully account for the effects of major mountain barriers (e.g., Andes, Rockies, New Zealand Alps, Alps, etc.).  The resulting underprediction of mountain-induced gravity waves and associated wave breaking leads to an overprediction of the flow at jet-stream level, which ultimately affects forecasts of the strength and timing of weather systems.  

This has been know for a few decades, and techniques have been developed to artificially add gravity wave drag to both weather and climate models.  However, these techniques make numerous assumptions and sometimes work well over one mountain range, but not another.

During the southern hemisphere winter of 2014, a major field program called DeepWave examined gravity waves generated over the New Zealand Alps that produce gravity wave drag and significant changes to the circulation of the upper atmosphere.  Early results from this field program are now coming to fruition and are guiding the development of new techniques to deal with gravity wave drag, as discussed during several talks this week.

Yup, this conference is most definitely a drag, but hopefully that will yield better forecasts in the future.

Divergent Forecasts Continue

Like a child coming of age in Divergent, as a forecaster one choice can transform you and today that choice is do I join the GFS faction?

The GFS continues to be a dauntless like outlier amongst the various models with regards to the evolution of Hurricane Norbert.  It continues to hold on to a solution in which the remnants of Norbert make landfall near San Diego on Monday.

Source: Wundermap

This contrasts with the gold-standard ECMWF solution

Source: Wundermap

and even the, gasp, NAM.

Source: Wundermap

I'd show the Canadian, but it's pretty much a repeat of the ECMWF and NAM with Norbert tracking more slowly to the northwest and well off the SoCal coast on Monday.

The pros at the National Hurricane Center are clearly leaning toward the non-GFS solution and keep Norbert well south of San Diego through Monday.

Like Beatrice Prior in Divergent chosing Dauntless, perhaps Mother Nature will "chose" the GFS faction.  More likely she will stick with the safe choice and go with the ECMWF–NAM faction. Why the GFS is such an outlier is an interesting problem worthy of investigation, whether it gets it right or wrong.

BTW, if you are interested in reading about rare hurricane impacts on southern California, see The San Diego Hurricane of 2 October 1958 by Michael Chenoweth and Christopher Lansea.

A Menacing Frontal Passage

Some nasty looking clouds are accompanying the cold front as it moves across the University of Utah.  Radar shows precip is starting to move into the Salt Lake Valley, but the coverage is far spottier than I would like to see.  Nevertheless, the precip is about to move in and I just heard thunder. Whoot Whoot!

Salt Lake will get some snow out of this eventually, but the GFS has shifted the heaviest accumulations to the south just a bit compared to this morning's run, suggesting the big winners might be to the south.

We'll see how things come together tonight.

A Maalox Moment

Today is the kind of day when a meteorologist could use a bottle of Maalox.  The forecast through tomorrow is very difficult, with divergent views being produced by our two main forecast models: The GFS and the NAM.

To illustrate this, I've put together a paneled chart that shows the guidance being produced by the two models side by side.  The GFS is on the left, the NAM on the right.  The panels are the 6-h accumulated precipitation (snow-water equivalent) ending at the time indicated in the legend at color filled intervals of .01, .05, .10, .25, and .5 inches.  The forecasts start with this afternoon (1800–0000 UTC/1100–1700 MST) and end tomorrow afternoon.  Click to enlarge further.

These panels illustrate a common dilemma facing meteorologists in the Intermountain West.  The GFS (left-hand-side) is a lower resolution model.  It doesn't handle topographic effects very well.  For example, note the lack of structure in the precipitation forecast compared to the NAM on the right.  However, the GFS runs later, sometimes ingests data that the NAM hasn't, and frequently does a better job handling the movement of large scale features like fronts.  For this forecast, that could be very very important.

Notice how the GFS develops an elongated west-east oriented frontal precipitation band that sags very slowly through northern Utah and Nevada.  If such a forecast verifies, there would be major accumulations not just in the mountains, but also the lowlands of Salt Lake and Utah Counties, as well as portions of western Utah and central Nevada.

In contrast, the forecast produced by the NAM is very different.  Beyond the greater detail owing to its higher resolution, the frontal precipitation band is weaker, especially over western Utah and central Nevada, and moves more quickly into southern Utah.  Does one lean toward this given the better handling of topographic effects?

One thing is for sure.  Snow is coming.  Major accumulations are likely in the mountains, but let's hope the GFS verifies so that we get a pasting in the valleys too.

This Forecast Is a Turkey

Source: http://davidlansing.com

The prospects for a deep-powder day for the holiday weekend are not zero, but are fairly low.  The latest computer model forecasts keep the action largely to our north.  The total accumulated precipitation forecast by the GFS between 1200 UTC this morning and 2100 UTC Sunday afternoon shows heavy precipitation over the northwest, and dry conditions over the southwest.  The Wasatch sit just to the south of the goods.

Source: NOAA/NCEP

Two weak systems graze northern Utah during this period, one Wednesday night, the other late Saturday and Sunday.  We'll have to hope these storms are more productive than currently forecast if we're to get a good freshening up during the holiday weekend.  Yes, it's a turkey of a forecast, but there is something to be thankful for.  The snowpack SWE at Snowbird is 140% of average.  At upper-elevations in the central Wasatch, we will have an above average snowpack for this time of year.  

Source: NOAA/NWS

Change You Can Believe In

No, not Obama.  This is a science blog.  The change is a meteorological one, with the deep upper-level trough swinging into the western US later this week.  For Utah, it will bring the coldest air of the season thus far.    

Mountain and valley snow?  You betcha.  How much?  On that, the polls are still open.

Nevada Mountain Waves

The flow over mountains constantly induces waves in the atmosphere.  Sometimes clouds indicate their presence, other times, the air is clear and they are undetectable to the naked eye.

Recall from the previous post that a close low formed over Utah yesterday and was forecast to move southward to the Gulf of California by Wednesday.  The circulation center is presently parked over the Lower Colorado River Valley, with easterly–northeasterly upper-level flow over southern Nevada where several dark bands cut across the flow in the water vapor imagery.  

These dark bands result from mountain waves induced by flow over the ranges of southern Nevada.  The water vapor channel is very sensitive to the amount and temperature of water vapor in the upper troposphere.  The dark bands are generated in areas where the mountains cause the air to sink, warm, and dry.  The band near the head of the arrow appears to be related to flow over Mt. Charleston and other high topographical features east of the Amargosa and Pahrump Valleys along the CA-NV border.  

These mountain waves are only briefly detectable in conventional infrared satellite images because the air is too dry and clouds are only generated for a short time at the beginning of the loop in areas where the mountain waves are causing rising motion.  For most of this period, the atmosphere is "severe clear."

Mountain waves are sometimes associated with aircraft turbulence, which can be particularly problematic when there is wave breaking in the atmosphere, which is similar to wave breaking on the beach.  

Source: Whiteman (2000)

Thus, water vapor imagery can be useful for helping anticipate areas of clear-air turbulence.  

Mountain Induced Cirrus

We have a nice example this morning of upper-level cirrus clouds being generated near or downstream of the Sierra Nevada and Cascade Mountains of northern California.

In particular, note how the coverage of cirrus clouds increases near Lake Tahoe and how you can see a back edge to these clouds near or just downstream of the Sierra–Cascade crest.

Here's an even better image (thanks Dale Durran) of mountain induced cirrus downstream of the High Sierra from another event.

Most of us think about clouds forming on the windward side, but some mountain waves tilt upstream with height and, while the low-level flow might be sinking downstream of the mountains, at upper-levels it is rising.  For example, in the model simulation below, the flow moves from left to right and roughly parallels the black contours.  Note how contours slope downward in the lee of the barrier at low levels, consistent with sinking motion.  The mountain wave, however, slopes upstream with height, so that the strongest sinking motion sits over the barrier when you get to around 5000 m or higher.  Further, downstream of this subsidence, the air rises, much like one sees when water flows over a rock in a river.

Source: Jim Doyle, NRL

 It is in this area of downstream rising motion that mountain induced cirrus clouds form.  It is also in this area that gliders can reach extreme altitudes.