Thursday, February 14, 2019

West Coast Storm

Earlier this week a strong storm moved through the northern Bering Sea and the Bering Strait, bringing strong winds to western Alaska and breaking up sea ice that was already weakened from unseasonably warm weather in the past few weeks.  According to NSIDC analysis, Bering Sea ice extent peaked at 567,000 km2 just a few days after my last update and has dropped by one-third in the 18 days since.  Here's a comparison of ice maps from the peak (Jan 26th) and today.



The most recent storm seemed like a strong one, with sustained wind speeds of over 50mph on St. Lawrence Island; and the balloon sounding from Nome on Tuesday morning reported a wind speed of 49mph at ground level.  However, the minimum central pressure of the storm was only about 980mb when it reached Alaska, which isn't particularly low by Bering Sea standards.


The chart below shows the typical monthly extremes in MSLP for a latitude/longitude box centered on the west coast of Alaska (depicted on the map underneath).  The months of October through February all typically bring at least one storm with minimum MSLP of 975mb or lower somewhere in this region; so the latest instance was a fairly run-of-the-mill storm.  What is not normal, however, is the lack of sea ice; only last year had significantly less ice on this date in the modern satellite record (but mid-February 1985 had about the same amount).



Thursday, February 7, 2019

Dawson Ice Bridge Problems

Readers may recall that in the past couple of winters I've mentioned the odd reluctance of the Yukon River to freeze up properly at Dawson in the Yukon Territory.  This has created a problem for residents of West Dawson who in years past relied on an ice bridge for seasonal access to the main town on the east side of the river.

https://ak-wx.blogspot.com/2018/02/yukon-river-at-dawson.html

https://ak-wx.blogspot.com/2017/02/yukon-river-at-dawson.html

https://ak-wx.blogspot.com/2016/12/ice-cold-and-solstice-sun.html

Perhaps not surprisingly, the same problem has occurred this winter, and efforts to stimulate ice growth across the stubbornly open channel were called to a halt last week.  Here are a couple of news articles on this winter's lack of success:

https://www.cbc.ca/news/canada/north/gov-halts-dawson-city-ice-bridge-1.5001387

https://www.cbc.ca/news/canada/north/dawson-ice-bridge-hopes-1.4974031

Here's a view from the webcam in Dawson a few days ago, looking out across the river; as in past winters, a narrow strip of open water is visible.



The lack of an ice bridge has been deemed sufficiently important that Canada's National Research Council looked into the issue last year, and a report was issued in October; it makes for very interesting reading.  The report discusses a variety of hypotheses about what may have caused the change, but there are no definitive conclusions; there may be multiple factors at play, some of which we've speculated about before on this blog (including good comments from readers).

As we've noted before, the change in ice conditions can't simply be pinned on warmer winter weather, because temperature data from Dawson doesn't show substantially warmer conditions in the winters when freeze-up failed.  The accumulated total of freezing degree days has been slightly lower this winter than the two-decade normal, but 2016-17 and 2017-18 were both near normal through this point in the season (see below).  Of course it is possible that ground and/or river water temperatures have risen, as noted in the NRC report.



Saturday, February 2, 2019

Radiation Normals

In the past week I've been digging deeper into the CERES radiation data for Alaska (see the previous post) and making some interesting discoveries along the way.  It really is quite fascinating - at least to me - to explore a new data set that deals with aspects of the climate that most of us don't often think about.

There's too much to discuss in one post, so here I'll focus on a few monthly averages over the period of record for just a couple of locations.  The chart below, for the Fairbanks area, shows the monthly average downward shortwave radiation at the surface, or simply the incoming "solar radiation" at the surface.  The CERES data set provides one set of numbers for a grid point near Fairbanks, and we can compare these to data from the high-quality Climate Reference Network (CRN) site just to the northeast of the city.


Interestingly the CERES numbers are consistently a bit higher, and especially so in March and April.  Some of the difference could arise from shading of the CRN site by nearby hills, but more investigation would be needed to find out how much solar energy is lost this way.  The larger difference in March and April looks more like a systematic bias that could be related to reflection and scattering of solar radiation from the snow-covered ground; perhaps there is locally more reflection and then downward scattering of the sun's rays at the CRN site than the CERES model estimates for the broad area contained in the (1x1 degree) grid box.

Here's the same chart for Utqiaġvik, formerly Barrow.  As expected the summer peak is narrower, but it's also higher than in Fairbanks; despite more abundant cloud cover on the Arctic coast, longer daylight hours allow Utqiaġvik to see more direct solar energy than Fairbanks in May and June (and July according to CERES).  The same positive bias is observed in the CERES data relative to the CRN site just a few miles outside Utqiaġvik.


As discussed in the previous post, longwave (infrared) radiation is just as important for the climate as direct solar radiation.  I'll illustrate this in more detail in the next post, but for now the chart below shows the overall surface radiation budget (longwave and shortwave combined) for the same two locations.

The most striking difference between Fairbanks and Utqiaġvik is seen in spring, as Fairbanks quickly sees a building surplus of energy in April and May, while Utqiaġvik really struggles to gain any ground in terms of radiative transfer until high summer - at least according to the CERES data.  The main reason is quite obvious: the universal snow and ice cover around Utqiaġvik create a high albedo in April and May, so the spring sunshine is largely reflected in the north, whereas the generally wooded environment and earlier snowmelt near Fairbanks allow for much more efficient absorption at this time of year.

The time series of CERES and CRN data also reveal some very interesting results over the ~15 year history, but I'll document these in another post.

Saturday, January 26, 2019

Satellite Radiation Data

In many previous posts, Rick and I have commented on the seasonal changes in solar radiation that are such a key part of understanding Alaska's climate, and I've also discussed the importance of "longwave" radiation, for example:

https://ak-wx.blogspot.com/2016/09/cloud-cover-and-temperature.html

As a reminder, "longwave" radiation is the infrared radiation that is constantly emitted and absorbed naturally by all objects, including blog readers and their immediate surroundings at this very moment.  This is in contrast to "shortwave" radiation, which is only emitted in significant quantities by very hot objects like the sun.  The earth's climate system is fundamentally driven by the ever-changing spatial distribution of shortwave radiation from the sun, but longwave radiation transfers between the surface and atmosphere (and out to space) are also a critical part of the energy balance.

I recently took some time to acquire a very nice satellite-derived radiation data set from NASA's CERES project ("Clouds and the Earth's Radiant Energy System").  Based on data from polar-orbiting satellite instruments, CERES provides global coverage with radiation data ranging from hourly to monthly time scales.  With this, we can take a look at how the radiation energy budget works in Alaska and surrounding areas.

First, here's the most tangible element of the radiation transfer - the incoming shortwave at the surface - in mid-winter and mid-summer.



There isn't much to say about winter, but the July map highlights just how cloudy it is across a huge area of the North Pacific.  It's also interesting to note the relative maximum in available solar energy in July over the high Arctic owing to 24-hour daylight.  Of course this is part of the reason for concern over ice-albedo feedback in the Arctic Ocean; as ice extent decreases in summer, the albedo (reflectivity) of the ocean surface decreases dramatically and more solar energy is absorbed.  Here's a map of July average albedo in the CERES data.


As a result of high albedo in the recent climate, the Arctic Ocean surface absorbs less solar radiation than surrounding land areas, so the net shortwave gain is relatively low:


Now let's look at longwave radiation, which is less intuitive but equally important for understanding the climate.  First, the downward longwave; this is the infrared radiation emitted by clouds, water vapor, and other radiatively active gases in the atmosphere like carbon dioxide, methane, and ozone.  The amount of longwave radiation emitted by the atmosphere is closely tied to the average temperature and absolute moisture content (e.g. precipitable water), so a warm and humid air column emits much more than a cold, dry one.  Over Alaska, the coldest and driest atmospheric column is over the Brooks Range in January, but more extreme cold is found over eastern Siberia as well as the high Canadian Arctic.



The upward longwave emission from the earth's surface is just a direct reflection of the average temperature of the surface - see below.  The relative warmth of the far North Atlantic (the Norwegian and Barents Seas) in winter is pretty striking.



Adding up the downward and upward longwave at the surface gives the net longwave radiation.  The surface loses longwave energy on balance, but much more so in places (a) where it's warmer and so there is more longwave going around, and (b) where clear skies and dry air tend to prevail (more upward than downward emission).



Finally, putting all the pieces together to get the total net radiation balance yields the maps below.  As expected, the high latitudes lose energy via radiation on average in January, with the largest losses found in relatively warmer regions, i.e. over the oceans (even if ice-covered).  But note the relative maximum near the pole, where upward emission is low (thick ice cover and very low surface temperatures) but cloud and moisture aloft (probably derived from the North Atlantic influx) provides a bit more downward radiation than we might expect.



An interesting aspect of the July map above is that much of mainland Alaska gains less energy from radiation overall than the Gulf of Alaska and parts of the Bering Sea; I would not have guessed this, because I would have focused on the sunshine part of the equation.  In reality the ocean surface is colder than the land in summer, so longwave emission is reduced over the water; and the summer ocean has a very low albedo that allows it to capture most of the available shortwave.  Evidently these two factors more than compensate for the higher cloud cover over the ocean - so even though it's cloudier, the ocean near Alaska captures more energy from the summer sun than does the land.

And for a final map, the annual average net radiation gain at the surface.  Apparently the region very near the pole actually gains a small amount of energy over the year - another surprising result.


With nearly two decades of CERES data now in hand, I'll plan to take a look at seasonal and year-to-year variations in Alaska in a subsequent post.

Sunday, January 20, 2019

Sea Ice Update

The past six weeks have seen some fairly persistent cold weather over the Bering Sea and western Alaska - much more so than over the rest of the state, where prior to the recent cold spell the winter had been relatively warm.

In response to the cold in the west, sea ice has expanded in the Bering Sea, and the ice area exceeded half a million km2 just the other day.  In the 1981-2010 period, this milestone was typically reached just after the new year.  So the ice growth is still a bit behind "normal", but it's a big recovery from last year.

NOAA's latest daily sea ice analysis shows the ice pack extending to the south of Nunivak Island but not yet reaching St Matthew Island farther to the west.


The NSIDC analysis is very similar.


The chart below shows the recovery off last year's record-low ice extent in the Bering Sea - we already have more than last winter's peak extent - but we're nowhere near the high ice coverage of just 7 years ago.  The Bering ice extent of 2011-2012 was the highest in the modern satellite record for most of the mid-winter and meltout period, so sea ice has been highly variable in the Bering Sea over the last decade (to say the least).



The NSIDC provides ice extent data for other regions of the Arctic basin, so it's interesting to compare the long-term trends in both March (seasonal max) and September (seasonal min).  See the chart below, showing the 40-year linear trends in percentage terms, i.e. the trend divided by the 40-year mean ice extent for each region.  Of course some of the regions melt out completely every September, and many freeze up completely every March, so those cases have undefined and zero trends respectively.



Obviously the changes in ice extent have been much more dramatic at seasonal minimum, but nevertheless there have been highly statistically significant decreases in March ice extent in some of the basins - notably in the North Atlantic sector (e.g. the Greenland Sea and the Barents Sea).  However, it's the Chukchi Sea that has the dubious distinction of the greatest percentage loss in September - see below.  (Note that while the Bering Sea loss is also very high in September, the actual numbers involved are very small; another viewpoint would be to look at the absolute change in ice extent over time.)


Saturday, January 12, 2019

Cold Frequency This Century

The cold snap is on its way out across Alaska now, as Pacific low pressure approaches from the southwest and a blanket of clouds aloft provides a warming influence.  McGrath saw the temperature rise from -50°F this morning to "only" -18°F this evening as the sky turned overcast.  However, severe cold is hanging on across the north, with -40s in the Yukon flats, and Umiat is currently sitting at -52°F on the North Slope.

Here's a chart showing some statistics for winter cold in Alaska since the turn of the century.  The black markers show each winter's statewide minimum temperature, and the columns indicate the number of daily reports in which the daily minimum temperature reached -40°F and -50°F.  This is based on data from NOAA's ACIS tool, which includes daily temperature data from about 400 sites (but closer to 300 at the beginning of the century).


The frequency of extreme cold has been on quite a roller coaster, with (for example) -40° or colder showing up somewhere in the state on 89 separate days in winter 2011-2012, but only 25 days in 2015-2016.  (But this ignores the confound of mismatched observation times, such as morning coop reports.)

See here for an earlier post about statewide minimum temperatures on a longer time scale:

http://ak-wx.blogspot.com/2016/03/statewide-minimum-temperature.html


Wednesday, January 9, 2019

Cold Update

Here's a quick update on the colder reports so far in this notable but not yet extreme cold spell that is reminding much of mainland Alaska what real winter feels like.

Fairbanks airport made it down to -39°F and had a couple of days with midnight-to-midnight high temperature below -30°F, but cloud and light snow brought temperatures back up to near zero today.  Elsewhere in the central and eastern interior it's mostly colder, with -40s hanging on in Tok and the most sheltered valleys of the Fortymile region; and fresh, very cold air is working its way down across western Alaska, with lots of -40 reports showing up late today.  Click below to enlarge.


As is often the case, Chicken has been the "winner" for cold, with a -56°F low temperature reported as of Monday morning.


The hourly data from the Fortymile River HADS site at the Taylor Highway showed similar numbers for Monday, with afternoon temperatures rising just a couple of degrees to -50°F.  (As an aside, note the elevation: 1558 feet; this is part of the reason why the narrow valleys of this region get so cold.  Chicken is at 1800 feet.)


Other sites that have dipped below -50°F are Tok and Fort Yukon.

As impressive as these numbers seem in comparison to the last year or so, we don't have to go back even two years to see colder conditions - the map below shows the minimum temperatures on January 18, 2017 (courtesy of http://xmacis.rcc-acis.org/).  But the next few days may rival this - will someone see a -60°F for the first time in several years?