Thursday, April 28, 2022

Radiation Normals

Recently I've been taking a look at the (infrared) radiation climate for Fairbanks, based on data from the state-of-the-art ERA5 reanalysis.  I suppose that many people may find it a rather abstract topic, but to me there's nothing more relevant in the world of weather and climate than radiation, and particularly the long-term trends thereof.  After all, solar radiation ("shortwave" radiation) is the fundamental driver of weather, and the imbalance between incoming and outcoming radiation controls weather and climate variations on many time scales.

The chart below shows the monthly normals for the familiar shortwave radiation budget near Fairbanks, according to ERA5.  Solar input (red columns) is close to zero in November through January, it rises quickly in spring under relatively clear skies, and then its decay in autumn is more gradual because increased cloud cover in July and August brings a premature decline (e.g. July has more daylight than May, but much more cloud cover).

The blue columns show the normal upward (reflected) shortwave radiation at ground level, and this peaks in April because most of the month is usually snow covered under relatively strong sunshine.  However, the albedo (i.e. the fraction of incoming solar radiation that's reflected) does drop off markedly in April as snow disappears from trees and eventually from the ground, and it remains near 0.1 until October, when snow cover typically returns.

This much is pretty straightforward to understand.  However, the longwave radiation budget is less intuitive, and in particular it can be a surprise to see that the rate of energy transfer for both incoming and outgoing longwave radiation is much greater than for shortwave - see below.  Probably not many of us would guess that much more radiation is warming us from the sky above than we receive directly in the form of sunshine, even in summer.  Of course this is largely because we're bathed in longwave radiation at all hours of the day and night (emitted by clouds and the atmosphere), with little change from hour to hour, but intense sunshine is confined to only a portion of the day.

But although the incoming longwave is large, the outgoing is even larger at all times of the year, because the ground temperature is higher than the average emitting temperature of the clouds and air above.  The longwave radiation flux is proportional to the fourth power of temperature, so both upward and downward components track very closely with the seasonal temperature cycle.  The only obvious departure from a simple seasonal cycle that I can see is that the downward flux doesn't increase from January to March as quickly as the upward flux, and that's because the surface warms up more quickly than the air aloft (and also because the air stays very dry well into spring - water vapor is very efficient at absorbing and emitting these wavelengths).

So we have a net loss of longwave energy at all seasons, and a net gain of shortwave in all but winter.  What does the overall net look like?

Here we see that there's a net gain of radiative energy from March (barely) through September, and a small loss from October through February; and overall it's a significant gain over the course of a year, which is perhaps a bit surprising at a latitude of nearly 65°N.

Now someone may ask why the temperature in Fairbanks drops so dramatically in the early autumn when there's still a net gain of radiation - even in September, according to ERA5.  The answer is that this radiation budget pertains to the ground surface, not the air above.  The atmosphere doesn't absorb or emit shortwave radiation, so the longwave balance is the only thing at play - and consider that the atmosphere only gains longwave energy from below, while it emits it both downward to the ground and upward to space.  This implies a significant net radiative loss for the atmosphere year-round, and when heat transfer from the surface (by mixing/convection) drops off in the autumn, there's nothing to stop the air from cooling rapidly.

In another post I'll take a look at long-term trends in the different radiation components.

Thursday, April 21, 2022

Breakup Briefing

Here's a link to Tuesday's UAF/ACCAP/NWS river breakup briefing:

The breakup portion starts on Slide 14.  To probably no one's surprise, the key message is, "We expect a dynamic breakup with above average potential for ice jams and flooding."

I thought this was a helpful graphic to explain the difference between dynamic (mechanical) breakup and thermal breakup:

The higher the ice resistance at the point where it is overcome by driving forces, the more "dynamic" the breakup is said to be; at the dynamic extreme, it's a violent process involving what we might call an irresistible force and an immovable object.  At the other end of the spectrum, ice resistance crumbles before the driving force rises a lot, and the whole process is relatively uneventful.  Most breakups occur somewhere in the middle, but this year's abundant snowpack and this month's chilly conditions have tilted the odds significantly towards the dynamic end.

Here's an NWS graphic showing estimated flood potential for settlements across Alaska.  Circle is the only location with a "high" risk rating, with an estimated breakup window of May 9-15.  Note that the risk pertains to both ice jam flooding and snowmelt flooding, with the latter involving the greatest discharge.

Saturday, April 16, 2022

River Ice Wecams

For those interested in breakup progress, here's a nice resource for daily-updated webcam views:

Yesterday's photos are linked below via Twitter.  There's no significant breakup yet; the ice is getting soft under the sun with warm afternoon temperatures, but nights have been cold.  Fairbanks has yet to see a daily mean temperature above freezing, and this is unusual: in the past 30 years, only 2013 and 2002 had zero thaw degree days by April 15.  Of course 2013 was the coldest April on record in Fairbanks, with a record late breakup at Nenana, and 2002 was also very chilly, with Nenana breakup on May 7.

Saturday, April 9, 2022

March Climate Data

With March climate data now available, here's a look back at the major climate anomalies.  For the month as a whole, the broad circulation pattern around Alaska was quite similar to that of February - compare the two maps below - and most of southern Alaska was again wet, cloudy, and mild.  However, there wasn't enough sustained ridging to the west to keep cold locked in over northern Alaska, and it was easily the warmest month of the winter for the state (and the only month that was significantly above normal statewide).

Here are the temperature and precipitation rank maps from NOAA and ERA5 respectively:

And to confirm the widespread warmth, Rick's ground-truth temperature anomaly map:

Wind and solar:

According to the ERA5 data, it was an unusually windy winter overall from the west coast to the northern interior, but it wasn't Bering Sea storms that produced this anomaly: MSLP was generally higher than normal from the Chukchi Sea to the Gulf of Alaska (see below).  Looking at ASOS wind data from Kaltag as an example, virtually all of the windiest days had winds from the northeast; this is the favored interior wind direction, of course, but it appears the strength of these winds was considerably enhanced by repeated episodes of high pressure to the north and northwest.

Winter temperatures overall were not as low as might be expected during a significant La NiƱa, and south-central Alaska was actually warmer than normal.

Extremely widespread and very unusual warmth in the northwestern North Pacific may help explain the lack of sustained cold in most of the state despite a negative PDO phase and a circulation pattern (Bering Sea ridge) that was relatively favorable for cold northerly flow.  Sea surface temperatures were far above normal to the south of the Aleutians, and that's a source region for Alaskan air during more southerly episodes such as late February and early March.

Extreme precipitation was the biggest story overall, especially for the interior.  As Rick Thoman pointed out, snowpack water content on April 1 in Tok was nearly 50% higher than the previous record from way back in 1967 - see below.

Click to enlarge the April 1 snowpack map below.  According to NRCS, four subbasins have 250% or more of normal snow water equivalent: Nenana River, Tanana Flats - Tanana River, Healy Lake - Tanana River, and Salcha River.  Meltout and breakup continue to be a very pressing concern in terms of flood risk, especially with the forecast continuing to favor below-normal temperatures this month.

Friday, April 1, 2022

Sea Ice Update

First a quick comment again on northern Alaska, where winter is holding on.  Temperatures have dropped into the -20s °F widely across the North Slope the last 3 nights, and not because it's been calm; strong winds have pushed wind chills down into the -40s and even -50s in some spots.

According to airport ASOS data, Point Lay on the west coast has seen nearly continuous blizzard conditions (blowing snow, presumably) since late Tuesday morning.  Winds have been sustained at 30-40mph or greater the whole time, with a temperature between -10°F and -20°F.  Not bad for April.

But of course we're nowhere near record cold for the time of year across the region.  Daily record lows at Umiat are still in the -40s, and just last year it was -38°F on April 3.  The Umiat thermometer notched -50°F on April 5, 1986, and that stands as the latest observed -50°F in the state of Alaska.

And now for an update on sea ice, a follow-up to this post from two months ago.  Bering Sea ice has remained relatively abundant in terms of areal extent, at least compared to the recent history.  Ice extent has mostly been well above last year and it has been far above the extreme lows of a few years ago.

The chart below shows the strong recovery from 2018, with this year's January-March extent reaching 6% above the 1991-2020 median.

The map below shows today's analysis from the NWS.  On the plus side, full ice coverage is found as far south as Saint Paul Island, but on the flip side Norton Sound ice is already in poor shape, with a lot of open water visible on satellite (per Rick Thoman's Twitter comments).

As for other seasonally ice-covered basins of the Northern Hemisphere, the major anomaly has been in the Sea of Okhotsk, where a January-March shortfall of nearly 200,000 km2 (over 20% of normal) has far exceeded the small Bering Sea surplus relative to normal.  The Greenland Sea has also been running a deficit of about the same size as the Bering Sea surplus.