Radiation Climate Trends

Following up on a couple of previous posts about the balance of infrared radiation (both shortwave and longwave) that shapes Alaska's climate (see here and here), it's interesting to look at trends since 2001, when the CERES data begins.  Let's start with annual shortwave radiation data from near Utqiaġvik (formerly Barrow) - see below.  The blue line in the chart below shows the measured average flux of incoming shortwave energy according to the ground-truth CRN instrument, and the gray line indicates the estimated value from the CERES satellite data.  As we noted before, the CERES numbers are biased a bit high.  The green line shows the estimated amount of solar radiation that is reflected at the surface.

The CERES and CRN data are not well correlated year by year, but both series show a statistically significant downward trend in average incoming solar radiation.  This is consistent with increasing cloud cover associated with a warming Arctic in which reduced ice cover and a warmer ice-free ocean surface tend to produce more evaporation and higher atmospheric moisture content.

The CERES estimate of reflected solar radiation is more variable from year to year than the downward flux, but it too suggests a downward trend.  If annual average albedo is decreasing (owing to a shorter ice/snow season), then we would expect reflected solar radiation to decrease even more rapidly than incoming radiation, and in fact the linear trends do support this.  According to this short period of record, the surface is absorbing slightly more solar energy even though cloudiness has increased markedly.

The situation in Fairbanks is of course much different.  There are no significant trends in the shortwave radiation data, although it's interesting to note that 2018 was the least sunny year in the CRN data set by some margin.  2013 stands out as being a sunny year, but that's mostly because of the extraordinary weather of May and June that year.

How about longwave radiation?  First I'll emphasize how closely tied the longwave energy transfer is to the surface temperature; in both Fairbanks and Utqiaġvik, the annual average emission of infrared radiation upward from the surface is correlated with the annual mean temperature at about R=0.9; this is just the Stefan-Boltzmann law in action.

With longwave radiation being closely tied to temperature, it's no surprise that (a) there is not much variation in absolute terms from year to year, but (b) values have been noticeably higher since 2016 at Utqiaġvik - see below.  This means that the surface is losing energy upward at a greater rate; it's a natural consequence of warming and it's a negative feedback, although the downward longwave has also increased nearly the same amount.  The CERES data hint at a small increase in the net energy loss from the surface at these wavelengths.

As with the shortwave data, the trends are smaller in Fairbanks; it appears there has been a small increase in net energy loss - similar to Utqiaġvik - but it's very marginal in terms of statistical significance (p~0.1).

What happens when we add up the net shortwave and longwave contributions?  See the chart below.  For Fairbanks, there is a hint of a downward trend in the overall annual radiation gain owing to the longwave changes (increased upward emission), but again it's not really significant.

Utqiaġvik's total radiation gain is noisy, but here the interesting result is the absence of a trend; it turns out that the small gain in shortwave radiation (lower albedo) is almost exactly offset by a small increase in net energy loss via longwave.  Does this suggest that the ice-albedo effect is less of a concern than we might have thought?  I don't think so, but it would be worth examining more closely for other parts of the Arctic; there is always more to learn.

One caveat associated with all of this is of course that the surface CERES data is computed with a radiative transfer model that depends on all sorts of inputs, both measured and estimated.  The model undoubtedly has some deficiencies, so the results - and especially the trend analyses - could be sensitive to details of the model; in other words, I frankly have no idea what the error distributions might be on the CERES data.

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.

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.

Radiation Budget

In Monday's post I alluded to the rapid net loss of infrared radiation in interior Alaska at this time of year, and reader Gary inquired about the processes that affect this mechanism of energy transfer; so I thought it would be helpful to go into a bit more detail on the answer.

The balance, or more accurately the imbalance, of infrared radiation across the Earth's surface is the fundamental driver of climate and weather variability, on timescales ranging from seconds to millenia.  Short-wavelength (shortwave) infrared radiation from the Sun heats the Earth's surface, and the surface emits long-wavelength (longwave) radiation back out to space.  Clouds and atmospheric gases such as water vapor and carbon dioxide also emit longwave radiation that is absorbed by the ground, and this is a major component of the surface radiation budget.  The relative magnitude of each of these radiative fluxes plays a large role in determining temperature changes at the surface, with the most obvious contrasts being those between day and night, and between summer and winter.

The rate of heating by shortwave radiation from the Sun is obviously affected by the time of year and of day, a location's latitude, the extent and character of cloudiness, and other factors like atmospheric aerosols (smoke, haze) and the degree of reflectivity (albedo) of the surface.  As for the longwave radiation emitted by the ground and by the atmosphere, the rate of emission is essentially determined by the temperature of the respective components, as per the Stefan-Boltzmann law.  So for example a warmer ground surface emits much more radiation upward than a cold surface; and a layer of low (warm) clouds aloft emits much more radiation downward than a clear sky.  (This is why cloudy nights are usually warmer than clear nights, and clouds bring warmth to interior Alaska in winter.)

It's an interesting exercise to develop a rough estimate of the radiation budget in the Fairbanks area.  We can do this using observed shortwave radiation data from the CRN site 11 miles northeast of Fairbanks; this site has measured shortwave radiation since mid-2002.  There are undoubtedly some differences between the CRN site and Fairbanks itself, but we'll ignore that for now.

The next assumption we make is that the radiation budget is in balance at both the summer peak of temperature and at the winter minimum of temperature; in other words, the following equation is satisfied on these dates:

absorbed shortwave + downward longwave = upward longwave

We know what the normal mid-summer and mid-winter temperatures are according to the 1981-2010 normals (63.7°F and -8.7°F at Fairbanks airport), so using Stefan-Boltzmann we can calculate the longwave radiation emitted upward from the ground at these temperatures.  Then given a balanced budget, this immediately gives us the downward longwave on these two dates.  The table below shows the results in terms of daily energy transfer; note that I've assumed a shortwave albedo of 0.132 on both dates.  (This is about right for mixed coniferous and deciduous forest in summer; it certainly should be higher in winter, but forested areas still have a fairly low albedo even with snow on the ground.  Of course, in the depths of winter the albedo doesn't make much difference as there is so little solar insolation.)

Summer temperature maximum
Absorbed shortwave	4382 Wh
Upward longwave	9726 Wh
Downward longwave	5344 Wh

Winter temperature minimum
Absorbed shortwave	17 Wh
Upward longwave	5362 Wh
Downward longwave	5344 Wh

With the arbitrary but realistic albedo choice that I made above, the estimated downward longwave radiation is (by design) exactly the same in summer and winter.  I don't know how close this is to the truth, but I suspect it is not a bad assumption; albedo constraints suggest that it can't be far from the truth.  If we proceed with the assumption that the downward longwave is constant throughout the year, we can then calculate the radiation imbalance for any day of the year.  The result is shown below.

Although I've made a few assumptions along the way - and this is admittedly a crude approach - the chart shows the basic features of the infrared radiation processes in interior Alaska.  The springtime gain in shortwave radiation outpaces the longwave losses, as the surface remains cold, and this leads to a radiation surplus and a warming trend.  In autumn, the direct solar radiation drops off more rapidly than the emitted longwave, because the surface remains warm, leading to a net loss of radiation.  We also see that autumn's peak rate of energy loss is greater than springtime's peak rate of gain, and so the temperature change is a bit more rapid in autumn than in spring.