How Carl-Gustaf Rossby Keeps Ruining Your Weekend

Carl-Gustaf Rossby was a Swedish-born meteorologist and to my knowledge is the only meteorologist to ever appear on the cover of Time Magazine.

Source: Time Magazine

Rossby was a towering giant in my field.  He studied at the famed "Bergen School" of meteorology in Bergen, Norway, which was the leading institute for meteorological research in the early 20th century and developed the Norwegian Cyclone Model that formed the foundation for meteorological analysis for decades.  He moved to the United States in 1925 and eventually served in many roles including Associate Professor at MIT, Research Associate at Woods Hole Oceanographic Institution, Assistant Director of the National Weather Service, and Chair of the Department of Meteorology at the University of Chicago.

Where Rossby enters into our story is in his pioneering work to describe and understand the jet stream and upper-level waves, the latter known today as Rossby Waves.  Most of us are familiar with waves.  For example, we've all observed waves at the ocean or on other bodies of water.  Such waves are gravity waves, in that gravity plays a dominant role in their evolution, including their speed.

Rossby waves are large-scale waves in the atmosphere.  The ridges and troughs found at upper levels in the atmosphere, for example, are Rossby waves.   Meteorologists commonly use maps of 500-mb geopotential height — basically a topographic map of the 500-mb surface — to identify these ridges and troughs.  Below is this morning's 500-mb analysis showing the Rossby-wave pattern over western North America and the North Pacific.  Notice the sequence of ridges and troughs that begin in Asia and extend to North America (a look at the opposite hemisphere would show that this wave-like pattern extends around the Earth).

In this pattern, the distance from trough to trough or ridge to ridge spans about 45º longitude, equating to a wavelength of about 3000–4000 km at this latitude.  A careful look reveals some waves with shorter wavelengths, but the large-scale waves dominate the current upper-level pattern.

In contrast to gravity waves on water, the primary driver of Rossby wave behavior is not gravity but the rotation of the Earth.  This rotation causes Rossby waves to move more slowly than the background flow, so that the air moves through the waves.  This effect is most pronounced for longer waves than shorter waves.  As a result, long waves in the midlatitudes tend to move more slowly to the east and in some cases they can even move westward (upstream).  Because of these differences in speed, short waves are often seen moving through the long wave pattern.  Meteorologists sometimes say that the short waves are "steered" by the long-wave pattern.

Now let's get back to how Rossby is ruining your weekends.  The wave pattern we are presently in has a period of about 7-10 days and the passing of ridges and troughs has produced the typical ups-and-downs in the local weather here in northern Utah.  Check out the weather last month.  A peak in temperatures from the 8th to 12th, a minimum, accompanied by precipitation, from about the 13th to 17th, a peak again around the 21st, and then a broad minimum with precipitation toward the end of the month.

The net result has been that I'll call "troughy" weather the weekend of the April 16 and 17, the weekend of April 23 and 24, and the weekend of April 30 and May 1.  The correspondence is not quite perfect, but it's close enough.

Of course, if instead of yard work, golfing, or biking you were looking for snow, Rossby has been your friend, not your enemy.

Congrats Jim Ehleringer!

Source: SustainableUtah

This morning I learned that Jim Ehleringer, Distinguished Professor of Biology at the University of Utah and, more importantly, regular reader of this blog, has been elected to the National Academy of Sciences.

There are only 2,291 active members in the academy, making election truly significant.  Jim received his Ph.D. from Stanford and has been at the University of Utah since 1977.  HIs full bio is available here, but I'll give a brief summary.  Nearly 500 articles.  Former chair of the biology department.  Director of the SIRFER stable isotope ratio facility.  Lead honcho for IsoCamp, a summer course in stable isotope biogeochemistry and ecology with hundreds of alumni from around the world.  Former director and visionary leader of the Global Change and Sustainability Center, which has become a catalyst for environmental research on campus.  Fellow of the American Geophysical Union.  Fellow of the American Association for the Advancement of Science.  Etc. Etc.

I'm not sure when I first met Jim, but at some point several years ago I found myself going to one of his graduate courses on stable isotope ecology.  At the time, I knew nothing about isotopes.  Today, I still know practically nothing, but I'm conversant just enough to appreciate the remarkable work that Jim and others do on campus using isotopes to understand everything from paleoclimate to transpiration, and for that I thank him.

Congratulations to Jim on this well deserved election and recognition.  

Windstorm Oddities

Forecasts of easterly downslope winds in northern Utah have improved considerably in recent years due in part to improved numerical modeling, improved monitoring, and better understanding.  Nevertheless, there's always more to be learned.

Amongst the oddities of our most recent event is the duration of the easterlies.  Below are 750 mb (near crest-level for the northern Wasatch) wind analyses for 1200 UTC (0600 AM MDT) yesterday and today showing a dramatic drop off in the strength of the easterly crest-level flow.

Although this dropoff can also be seen in mountain-top and pass observations from the Wasatch Range, the corresponding dropoff in wind speed at the base of the Wasatch Mountains is weaker.  For example, at the top of the Strawberry Bowl at Snowbasin Saturday night, winds were generally around 50 mph with gusts to 70.  Meanwhile, in Farmington (US-89 at Park Lane), winds were just a bit stronger.  During the day yesterday, winds at both sites weakened commensurately.  However, last night and this morning we clearly entered a new regime.  While the flow at the top of Strawberry Bowl weakened, winds of about 30 mph with gusts to 40-50 mph persisted at Farmington.

This potentially suggests two differing regimes for flow dynamics across the Wasatch.  In the first, strong flow near the crest plunges into the lee, but experiences only a small acceleration (Saturday night and Sunday morning).  In the second, the plunging flow strongly accelerates into the lee (last night and this morning).  At least in this event, the latter does not lead to damaging winds, but that's not necessarily the case in all events.

Of course there is always the possibility that the localized nature of the Strawberry Bowl observing site makes it somewhat unrepresentative of the flow moving across the Wasatch Range immediately east of Farmington.  It's unfortunate that we don't have a wind observing site in that part of the Wasatch Range and perhaps installing one should be a high priority for the state given the severe impact these events have on the northern Wasatch Front.

It would also be helpful to have soundings taken during future events from along highway 66 south of Morgan.  My colleagues at the University of Utah have done this for a couple of events, but it would be great if we could find a way to do it regularly during events.  Combined with high-resolution modeling, we could tease out some of these oddities and potentially better predict some of their more fine-scale aspects.

Windstorm Update

As of 8 am, wind gusts ≥ 60 mph reported to Mesowest:

• Bountiful Bench (BBN): 61
• Fruit Heights (AS519): 63
• Centerville (C8948): 63
• Kaysville (AT097): 63
• Centerville (UP028): 65
• Hill Field (KHIF): 67
• Layton (AS186): 69
• South Ogden (C5505): 70
• Brigham City (KBMC): 74
• Centerville (UP138): 80
• Centerville (CEN): 81
• Farmington (AP611): 87
• Farmington (UTPKL): 91

Those observations are based on measurements from a variety of sensors with a variety of capabilities and limitations.  The 91 mph at Farmington is from a site operated by UDOT at US-89 and Park Lane and is likely reliable.  Overall, these observations show high winds covering nearly the entire Wasatch Front from Bountiful north to Brigham City.  

Observations from UTPKL show winds began to gust over 60 mph beginning around 1900 (7 PM) yesterday, reaching their peak from 0400–0600 (4–6 AM) this morning.  

Rocky Mountain Power reports more than 28,000 people currently without power.  

In the Salt Lake Valley, peak gusts include 58 mph at the Triad Center, 53 mph in the upper Avenues, and 53 mph at the University of Utah.  Those peak gusts occurred late yesterday or earlier in the overnight hours.  Along the east bench where winds were strongest, winds weakened in the late night hours.  

Although not as strong as overnight, downslope winds will persist this morning from Bountiful North.  The event will wind down this afternoon.  Latest forecasts at http://www.wrh.noaa.gov/slc/. 

Tonight's Downslope Windstorm: Don't Be Complacent

I'm pretty blown away (pun intended) by the model forecasts for tonight and tomorrow (Sunday) morning.

The 0600 UTC NAM is generating strong easterly flow across the northern Wasatch Front with 700-mb (near crest-level) winds reaching 50 knots in places.  There is also a strong pressure gradient from Evanston to Ogden.  Coming overnight and in the early morning, these are ingredients for a strong downslope wind event.

The National Weather Service has issued a high-wind watch for the Cache Valley and northern Wasatch Front for sustained winds between 30 and 40 mph with gusts in excess of 60 and localized gusts in excess of 75 mph in some areas, especially Davis and Weber Counties.

Take the time today to secure loose outdoor items if you are in areas affected by these downslope winds and monitor forecasts at http://www.wrh.noaa.gov/slc/.

This Is the Biggest Storm Cycle of the Season

Here's one for you.  When did we have the biggest storm cycle of the ski season?  December?  January? February?

Nope.

The answer is right now, and it isn't even close. 

The chart below shows the running 5-day accumulation of water equivalent at the Snowbird SNOTEL station since October.  In terms of water equivalent, the 5-day accumulated total through today is 4.3 inches.  The next highest is December 25th with 2.8 inches.

All I can say is what an event, especially Tuesday and Tuesday night when we picked up 16 inches and about 2.25 inches of water at Alta-Collins.  Although snow was in the forecast, that was impressive.

And we got a bit more last night and will get a bit more today.  Note the inexact terms "a bit more."  I'm feeling beaten and battered by these closed lows and their whimsical precipitation patterns.