Wednesday, April 16, 2014

Will Global Temperature Records Fall?

Much has been made of the so-called global warming "pause" or "hiatus", a term that has been used to describe the slowdown in the rate of increase of surface temperatures over the past 10–15 years.  As we have discussed in previous posts (e.g., Global Warming Hasn't Stopped), this period has been marked by continued warming of the ocean, melting of ice, and warming of the land surface, so there is every reason to expect a return to more rapidly rising surface temperatures in the future.

In addition, there is some potential that we will see a new global surface temperature record in 2014 or 2015.  It is looking increasingly likely that El Nino will develop this summer or fall, with some indicators suggesting that it could be a moderate or strong event.  

El Nino is characterized by relatively warm sea surface temperatures in the tropical Pacific Ocean which, along with other accompanying changes in atmospheric and oceanic circulations, typically leads to an increase in global temperatures.  Conversely, La Nina is associated with relatively cool sea surface temperatures in the tropical Pacific Ocean and a decrease in global temperatures.  This can be seen in the graphic below, which categorizes years based on El Nino (red), La Nina (blue), or neutral/other conditions (a.k.a., "No Nino").  
Source: NCDC
That categorization doesn't distinguish, however, between weak and strong El Nino events.  1998 was characterized by a very strong El Nino and it remains one of the warmest years on record.  Note what an outlier it was compared to the period in which it is embedded.  There has subsequently been only one El Nino Year (2010) and it was a weak one.  However, the most recent 10-year period is clearly the warmest decade in the instrumented record so it is quite likely that a moderate to strong El Nino will likely yield record global surface temperatures.  

In the above analysis, NCDC classifies a year as El Nino or La Nina based on the first three months of the calendar year.  This reflects that fact that El Nino and La Nina typically reach their peak strength during the Northern Hemisphere winter.  However, this also makes the categorization a bit more difficult since the peak El Nino/La Nina conditions often straddle the start/end of the calendar year.  For example, some classify 2010 as a La Nina year since there was a rapid transition from El Nino to La Nina conditions during the Northern Hemisphere spring and summer.  

On the other hand, there is often a lag of a few months between peak El Nino/La Nina conditions and the accompanying increase/decrease in global temperature anomalies.  This is why we might need to wait for 2015 for a record.  Alternatively, we could bag this calendar year crap and instead look at a 12-month period encompassing the peak El Nino/La Nina period.  Here's what the global temperature trend looks like for July-June average temperatures.

Source: NCDC
In this instance, the hottest year is 2010 (i.e., July 2009-June 2010), which includes the moderate 2009–2010 El Nino, followed by 2007, which was characterized by a transition from weak El Nino to neutral or very weak La Nina conditions, and then 1998, which featured the strongest El Nino on record.

All of this suggests that if we can get a moderate to strong El Nino to develop, we will probably see record setting global temperatures for some 12-month period if not the calendar year.  Perhaps even a weak El Nino will do the job.  We'll have to see how things come together in the coming months and see if there are any surprises, like a huge volcanic eruption to cool things off. 

Tuesday, April 15, 2014

The Central American Cold Surge

We frequently talk about how the tropics and subtropics visit Utah in the form of cool-season atmospheric rivers, but there are situations in which high- or mid-latitude airmasses plunge southward into the tropics.  This is especially common to the east of the Rocky Mountains and Sierra Madre, which frequently act to channel relatively cool airmasses to Central America or, in extreme events, northern South America.

A great example of these Central American Cold Surges is affecting Mexico and Central America today.  Note in the loop below (especially the lower panel), the surge of relatively cool high pressure from the central United States into Central America.   To save bandwidth, I haven't plotted a longer loop to fully show the history of this event, but the airmass currently plunging into Central America originates in northwest Canada.  Brrr...

As these cold surges move southward, they are modified and warmed, but they are still relatively cold when they get to Mexico and Central America.  Check out the stiff north winds observed along the east coast of Mexico this morning.  The 59ºF with a wind of 23 mph at Tampico can't feel too comfortable for sunbathing.

Source: MesoWest
That 59 also represents the lowest temperature observed in Tampico since mid March, which was probably when they had their last major cold surge [note that wind gusts (green dashed lines) are also the strongest since that mid-March event].

Source: MesoWest
The cold surge passed Palenque, MX this morning and is about to reach Guatemala.

Source: MesoWest
Amongst the more remarkable mountain weather phenomenon that are produced by these Central American Cold Surges are strong gap winds that push through Chivella Pass in southern Mexico and extend over the Gulf of Tehuantepec.
Source: The COMET Program
This results in some very unusual phenomena.  First, one often sees a cold front pushing into the tropical eastern Pacific, which can sometimes be accompanied by a narrow rope cloud.

Source: Steenburgh et al. (1998)
Then there are very strong gap outflow winds over the Gulf of Tehuantepec.  These winds are very unusual because along the the center of the outflow jet they are inertially balanced, which causes them to curve rightward (relative to the flow) at a rate precisely predicted by the rotation rate of the Earth.  This is perhaps the cleanest example of atmospheric inertial flow anywhere in the world.   

Source: Steenburgh et al. (1998)
Finally, interactions between the strong winds and the ocean lead to an upwelling of nutrient-rich water to near the surface, which is an important aspect of the marine ecology of the Gulf of Tehuantepec.  

In 1998, I wrote a paper on these gap outflow winds with David Schultz and Brian Colle.  It was an entirely curiosity driven research project for which we had little-to-no financial support.  This paper has become the most popular of my research career, with citations in everything from oceanographic to renewable energy journals.  It is a prime example of why I often tell my students to never let the best laid plans get in the way of good serendipitous research.  

Sunday, April 13, 2014

Overnight Blow

Some strong winds accompanied a band of showers and thunderstorms that moved through the Salt Lake Valley just after midnight MDT/0600 UTC last night.

Source: NCAR/RAL
The award for highest lowland wind gust goes to the Meteorological Solutions Inc. observing site in Olympus Cove where a peak gust of 63 mph at 0105 MDT/0705 UTC. Note the dramatic drop in temperature and increase in wind speed.

Source: MesoWest
Other peak gusts include 60 mph on the Eyring Science Center @ BYU, 60 in Vernon, 57 at the University of Utah, and 52 at the Salt Lake City International Airport.  

Two inches of snow at Alta combined with the cooler weather should yield some "interesting" dust-on-crust conditions in the Wasatch for today.  

Friday, April 11, 2014

Observational Oddity Continues

The curious "noontime notch" continues this week in temperature time series from the Alta-Collins observing site (see An Observational Oddity for earlier discussion).

Given the clear skies that have predominated during this period, I think we can eliminate cloud and precipitation influences from the hypothesis list.  I also believe there are no problems with the sensor, so I think we're looking at an example of local microclimatic effects due either to terrain or vegetation shading.

On a meteorological scale of 1-10, this rates about a zero, but it's still a learning opportunity.

Thursday, April 10, 2014

El Nino to Return?

Source: NOAA/Climate Prediction Center
Lots of hedging still in the forecast as it is hard to predict El Nino at this lead time (historically, El Nino predictions are more uncertain when made in the spring).  Of course, for the Wasatch, it doesn't provide us much predictive skill, as we discussed in the fall (see Outlook for the 2013–14 Ski Season).  On the other hand, if El Nino does end up going big, then the southwest might finally get some badly needed precipitation.  We should know more by summer.

Tuesday, April 8, 2014

A Cool Look at Snowfall in Little Cottonwood Canyon

It's a beautiful day in Salt Lake, but snow continues to dominate our thoughts.

In November of 2011, we had the opportunity to observe winter storms over northern Utah with a Center for Severe Weather Research (CSWR) "Doppler on Wheels" Radar.  For a meteorologist, it doesn't get any better than playing around with this beastie.  

The CSWR DOW6 in a northern Utah winter storm.  Photo: Trevor Alcott.
Today we learned that a paper based on our efforts was accepted by Weather and Forecasting, a leading journal in the atmospheric sciences.  Leah Campbell was the lead author and did a remarkable job describing the fine-scale precipitation structures that we observed in Little Cottonwood Canyon prior to, during, and immediately following the passage of a cold front. 

For the event we positioned the DOW near Daybreak in the southwest Salt Lake Valley where it had an unobstructed view directly up Little Cottonwood Canyon.  We collected high resolution "horizontal" radar scans at various tilts to the horizon (known as PPIs), as well as vertical slices (known as RHIs) up Little Cottonwood Canyon and over the Cottonwood and Alpine Ridges to the north and south.  Several University of Utah students planned and executed the observing effort.  

a) Position of the DOW and orientation of the RHIs over the Salt Lake Valley and central Wasatch Mountains.  b) View of the central Wasatch Mountains from the DOW site.  Source: Campbell and Steenburgh (2014)
Prior to the frontal passage, we observed a pronounced "barrier-scale" mountain wave that resulted in stronger radar returns in the upper Cottonwoods and along the Wasatch Crest as one often anticipates during winter storms.  Most interesting in this case was that the strongest radar returns were displaced to the east of Lone and Twin Peaks.

Source: Campbell and Steenburgh (2014)
A storm period like this produces greater accumulations at Snowbird and Alta than over the big lines farther down canyon like Lisa Falls, Coalpit Gulch, and the Y-couloir.  This is true not only on the canyon floor, but also at upper elevations.

The DOW is also a polarimetric radar (meaning it transmits and collects radar signals in both the horizontal and vertical planes), which allows us to infer the types of snow crystals in the storm.  Below you can see where and when ice crystals (small ice particles), aggregates (clusters of snowflakes), and low-density graupel are found.  

Following frontal passage, we observed something completely different.  Behind the front, the northwesterly flow was very shallow and was unable to surmount the high Cottonwood and Alpine ridges around Little Cottonwood Canyon.  As a result, the strongest radar returns were over the lower canyon, especially the northwest facing sidewalls of both Big and Little Cottonwood Canyons.  

In a situation like this, accumulations are greater not at Alta and Snowbird, but in the big lines further down canyon like Coalpit Gulch and the Y-Couloir.  In Big Cottonwood, lines like Bonkers in Broads Fork would also do better, whereas on the south side of Twin Peaks, Lisa Falls would get less.  

Don't make your backcountry ski plans for the next cold-frontal passage based solely on this event, however.  A rich spectrum of precipitation structures accompany cold frontal passages, so you can't count on a pattern like this to be featured after every cold frontal passage.  

Instead, what this research illustrates is the tremendous potential to use small, well-positioned radars to better observe precipitation in the central Wasatch.  Yeah, I know you want to see this done simply for your powder-hunting pleasure, but there are some potentially important applications for public safety, especially the ability to monitor snowfall rates and ice crystal types in the starting zones of major avalanche paths that cross the Little Cottonwood Canyon highway.