Thursday, October 23, 2014

Disappearing Sun; Barrow Update

Today marks the day when the sun's angle above the horizon at solar noon has declined half way from the equinox to the winter solstice; or equivalently we have traveled three-quarters of the way from the summer to winter solstice in terms of the sun's elevation at noon.

What does this mean?  It means we hereby enter the dark third of the year in the northern hemisphere; and of course this fact is more inescapable the farther north you go.  We can illustrate the lack of solar energy across Alaska in winter by calculating the theoretical solar insolation under clear skies.  From this theoretical standpoint, the total solar energy received in Fairbanks over the next 4 months is less than is received in 6 days in the height of summer.  In Bettles the dark third of the year receives less radiation than in 4 days in summer.  However, in Anchorage the winter sun provides the equivalent of about 10 days in summer.

Here's the view at close to solar noon yesterday from the Alaska Climate Research Center webcam on UAF West Ridge.  The weakness of the sun is illustrated by the fact that some snow remains on the ground despite most days getting above freezing in the past two weeks; the official snow depth in Fairbanks has been at 1 inch for 10 days now.

On another note, Barrow has had a chilly and very windy time of it in the past several days, with a very strong pressure gradient importing cold Arctic air from the northeast.  Here's the surface analysis from Monday afternoon when winds were sustained at about 40 mph for a time.

The high temperature on Tuesday was only 17 °F in Barrow, which is the coldest day so early in the season since 2002.  As we've mentioned many times before, Octobers since 2002 have been extremely warm in Barrow compared to previous decades, and so this kind of chill would have been completely normal in the last century.  For example, the 1930-2000 normal for coldest high temperature to have occurred by October 21 in Barrow was 10 °F; and a high temperature of 17 °F would normally have been observed by October 12.  In 1996, when sea ice was firmly established from Barrow eastward by late September, the high temperature was -7 °F on October 11!

So, it's been a little cooler in Barrow in the past few days - but even this is only approaching normal from earlier decades.  October 2014 is still running well above both the 1981-2010 and 1971-2000 normals.

Wednesday, October 22, 2014

Anchorage Forecast Performance

This is a follow-up to Richard's excellent post on Fairbanks' forecast performance. What I want to focus on is the comparison of the official forecast to both climatology and persistence. Unlike Richard, I have not been proactively saving forecast products. Instead, I utilize the Iowa State text product finder. One of the forecast products is called the State Forecast and represents point forecasts for first-order stations. The product is issued twice daily and includes a minimum and maximum for the next 7 days (morning issuance) and 6.5 days (afternoon issuance). Figure 1 shows a sample State Forecast product.

Figure 1. State Forecast product issued by the Anchorage NWS Office on October 22, 2010.

In Figure 1, you will notice a series of minimum, maximum, and precipitation probability forecasts. Unfortunately, only the Anchorage and Juneau offices issue State Forecasts; hence, our analysis will focus on Anchorage (I know, not very Deep Cold). A choice must be made as to whether to use the morning or afternoon issuance. In this case, we used the afternoon forecast product, which effectively gives us a 6 day forecast. This option provides the shortest time window for assessing the Day 1 forecast (best case scenario) and makes the day-to-day comparison more meaningful. It also allows for a baseline to conduct a persistence forecast comparison. On the negative side, it eliminates Day 7 as a forecast period.

Forecast temperatures vs. Actual Temperatures

When looking at days 1 through 6, we see decreasing skill in the temperature forecast. This is not surprising, as we would expect this result for every forecast issued anywhere in the world. The question is how much value do we get from the forecast compared to another method. In Figures 2 and 3, we see the 2009-2013 difference between the forecasted temperature and the actual temperature for Day 1 through Day 6. Figure 2 is a summary by year and Figure 3 is a summary by month.

On both charts, we see decreasing skill the farther out in time we get; however, the actual forecast always exceeds the no skill forecast (climatology). The most dramatic forecast skills are in the winter months when the skill for Day 1 through Day 3 is especially high. A noticeable drop off is observed by Day 4. We see in Figure 2 that on average, the Day 1 forecast provides 3°F of improvement versus climatology and the Day 6 forecast provides 1.3°F of improvement.

A caveat regarding 2009 and 2013 in Figure 2. Those years had very large temperature anomalies and so the forecast skill for those years suffered accordingly.

Figure 2. Difference (absolute value) between forecasted temperature and measured temperature for Day 1 through Day 6 in Anchorage, Alaska, between 2009 and 2013. All months are aggregated for each year.

Figure 3. Difference (absolute value) between forecasted temperature and measured temperature for Day 1 through Day 6 in Anchorage, Alaska, between 2009 and 2013. All years are aggregated for each month.

Forecast temperatures vs. Climatology

If you had no access to television, radio, or the Internet, one option for generating a 6-day forecast is to predict that each day will be exactly normal; i.e., just use the numbers from the NCDC normals table. As it turns out, climatology appears to be a factor in the NWS forecast. As a forecaster, you would feel comfortable predicting that Day 1 is 20°F above or below normal based on the numerical models is the situation was warranted; however, the comfort level with a forecast of 20°F above or below normal for Day 6 is much reduced. Therefore, the forecast is tempered somewhat by trending it toward climatology. Figures 5 and 6 show the difference between the forecasted daily temperatures and the NCDC published temperature.

Looking at Figure 4, the Day 1 forecast is slightly more than 5°F different than the published normal temperature. However, by Day 6, the forecasted temperature is 3.7°F difference than the published normal temperature. In Figure 5, we see the breakdown by month. In every month and in each year, the forecast trends toward the climatological daily normal.

Figure 4. Difference (absolute value) between forecasted temperature and the NCDC published normal temperature for Day 1 through Day 6 in Anchorage, Alaska, between 2009 and 2013. All months are aggregated for each year.

Figure 5. Difference (absolute value) between forecasted temperature and the NCDC published normal temperature for Day 1 through Day 6 in Anchorage, Alaska, between 2009 and 2013. All years are aggregated for each month

Which Forecast is the Best?

So how do the point forecasts for Days 1 through 6 compare to a no skill forecast? For this analysis, we add a second type of no skill forecast called persistence. This is where you forecast that the temperature tomorrow will be the same as the temperature today. This can be extended all the way out through Day 6. When we do this, the results are shown in Figure 6.

We see that over the course of out 5-year period, the NWS forecast for the Anchorage International Airport is off by slightly more that 2°F (see Figure 3 for an NWS forecast breakdown by month). This is 1.1°F less (better) than a strict persistence forecast and 3.3°F less (better) than a climatology forecast (see orange line in Figure 3 for a climatology breakdown by month).

By Day 3, the climatology forecast catches up with the persistence forecast. Looking out to Day 6, the NWS forecast sill exceeds climatology by 1.3°F. The maximum differential between the NWS forecast and the no skill forecast is at Day 3.

Due to high variability of temperatures from year-to-year, it is impossible to assess the relative forecast improvement over this short time period. However, there is a large skill improvement when using the NWS forecasts for temperatures as compared to the alternatives.

Figure 6. Difference (absolute value) between actual temperature and three forecast methods for Day 1 through Day 6 in Anchorage, Alaska, between 2009 and 2013.

Monday, October 20, 2014

Fairbanks Forecast Performance

For some time I've been meaning to take a look at the long-term performance of the National Weather Service temperature forecasts for Fairbanks, and particularly with one question in mind: do the forecasts show enough variance at the end of the short-term forecast period, i.e. 5-7 days in the future?

The question is motivated by the idea that sometimes the computer models indicate a pronounced temperature anomaly from about a week in advance, but the early NWS forecasts for the same time show only a small departure from normal.  A recent example was seen in the early October cold spell, when the ECMWF and GFS deterministic forecasts of September 29 both showed a notable cold anomaly in place by October 5, but the NWS forecast for the high temperature on October 5 was 38 °F, only 3.6 °F below normal.  In this case, as time went on and the forecast became more certain, the forecast dropped and the observed high temperature was 31 °F.  However, there are many cases when the computer forecasts are badly wrong from 7 days out, and so it is entirely justifiable for the official forecast to show only a small anomaly at longer lead times.  Indeed, it would be most undesirable for the raw model forecast to be reflected in the official outlook, because the numbers would often swing wildly from day to day.  The question is, does the NWS have the right balance?

It's possible to answer this question using a history of NWS forecasts that I have collected for Fairbanks airport since November 2011.  First, here is the basic "skill" of the forecasts for lead times of 1-6 days, i.e. the forecasts for "tomorrow" through "6 days from now".  Averaged over all seasons, the average error of the high and low temperature forecasts is similar and rises from just over 4 °F to nearly 8 °F over the six days.  Not surprisingly, the errors are much larger in winter, but it is interesting to see that the winter low temperature forecasts improve more significantly at shorter lead times, whereas the winter high temperature forecast error remains over 7 °F even for "tomorrow".

Here's a similarly-formatted chart showing the bias of the forecasts, i.e. the mean difference between the forecast and the observed temperatures.  Negative values indicate that the forecasts were too cold on average.  We see that the winter high temperature forecasts have been several degrees too cold on average in the past 3 years, even at shorter lead times, but the bias is much smaller for the low temperatures.  It would be interesting to investigate this further in search of a possible explanation.

Let's now consider the scaling of the temperature forecasts.  I've examined this by calculating the mean absolute error (MAE) that would result if the NWS forecast anomaly (departure from normal) were multiplied by values ranging from 0 to 2.  On the low end of this range, the forecasts would deviate very little from climatology and the forecast would just show normal values each day; but on the high end, the forecasts would show greater deviations from normal than they currently do.  The chart below shows the results of this experiment for day 7 temperature forecasts from all seasons of the year.

The data from the last 3 years show that (on average through the year) the high temperature forecasts are perfectly scaled at day 7, i.e. there is no way to improve the MAE by arbitrarily reducing or increasing the forecast anomaly.  We conclude that the NWS shows just the right amount of variance on average in the day 7 high temperature forecasts; this is not to say that we can't improve on any given forecast using additional information, but we can't reduce the error by simply adjusting the departure from normal across the board.

The day 7 low temperature forecasts are not quite optimally scaled, according to these results, as the NWS shows marginally too much variance.  In other words, the forecasts would be marginally (but only very slightly) better if they showed smaller departures from normal.

There is one other aspect of the problem that interests me, and that is whether we can show that the forecast variance is too small when the computer models show a large anomaly (as opposed to any size anomaly) and/or when the computer models agree with each other.  I'll return to this idea in a subsequent post.

Thursday, October 16, 2014

Barrow Temperature Update

The midpoint of October has been reached, and the mean temperature so far this month in Barrow is 25.4 °F, which is right in the middle of the pack in terms of the "new normal" since 2002.  Of course it is well above normal relative to the climate in former decades.

The chart below shows the daily mean temperature anomalies in Barrow between August 1 and November 30, for 2014 (black line) compared to the past 12 years, which were all very warm in September and October.  As we noted here, the anomalous warmth in recent years has tended to peak at the end of October; we can see that daily temperatures were above the 1981-2010 normal almost all the time in the second half of October in the past 12 years.  This year we've been following the same script since mid-September, and with sea ice still a good distance offshore, there seems to be no reason to expect a sudden change (see the ice analysis below, courtesy of the NWS in Anchorage).

Tuesday, October 14, 2014

Snow Pack Onset

Fairbanks has reported at least 1" of snow on the ground every day since October 4th. Today makes 11 consecutive days with measurable snow depth. However, the total for today was down to 1" and the forecast for the next few days suggest that it might dip below 1". If that occurs, the snow pack onset date will the date when the snow depth exceeds 1" again and stays that way for the rest of the season. Figure 1 shows the snow pack onset date for Fairbanks.

Figure 1. Date where snow depth stayed at or above 1" for the remainder of the season.

In is not unheard of for snow to be measured for at least 10 days and then drop below 1" before being reestablished some number of days later. Here are the instance when it occurred:

In 1919, at least 1" of snow was on the ground from October 1st through October 11th (11 days), but then the snow went below 1" for all but one day until October 29th.

In 1940, at least 1" of snow was on the ground from October 20th through October 30th (11 days); the snow pack was established for good on November 8th.

In 1949, at least 1" of snow was on the ground from October 9th through October 24th (16 days); the snow pack was established for good on October 31st.

In 1951, at least 1" of snow was on the ground from October 6th through October 25th (20 days); the snow pack was established for good on October 27th.

In 1955, at least 1" of snow was on the ground from October 7th through October 22nd (16 days); the snow pack was established for good on October 24th.

In 1972, at least 1" of snow was on the ground from September 29th through October 16th (18 days); the snow pack was established for good on October 19th.

In 1981, at least 1" of snow was on the ground from October 4th through October 14th (11 days); the snow pack was established for good on October 27th.

Monday, October 13, 2014

Arctic Ice Minimum - Part 2

This is a follow-up to last month's post on the seasonal Arctic sea ice minimum, which occurred on about September 17 based on spatial extent.  It's worth looking also at estimates of sea ice volume, thickness, and age, to see how this melt season compared to recent years.

First, volume: the University of Washington's Polar Science Center uses a numerical model to estimate Arctic sea ice volume based on various observations and calculation of known physical processes.  There is obviously a good deal of uncertainty in the volume estimates, but the results are a lot better than nothing.  The chart below shows the monthly mean volume estimates for April and September since 1979 (blue and red lines) along with the difference, i.e. the estimated volume of melt from April to September.

We see that the estimated volume in September increased this year for the second consecutive year and reached a level similar to 2009.  However, the volume remains very much below the volume of earlier decades, and the volume in April 2014 was comparable to the lowest values of recent years.  The relatively small difference between April and September this year indicates that the amount of melting this summer was significantly smaller than in recent years; in fact the melt volume was less than the 1981-2010 average.  It will be interesting to see if this translates into any recovery in April ice volume next spring.

The PIOMAS estimated ice thickness is shown in the following chart from U-Washington.  It's interesting to note that the estimated thickness this summer was considerably higher than in the past four years, which are closely clustered together at the bottom of the chart.  If the model is correct, this suggests that ice thickness remained very low during the 2013 recovery in ice extent; and but this year has seen a recovery in ice thickness, although total extent remained similar to 2013.

Finally, the estimated sea ice age compared to last year, courtesy of NSIDC:

The extent of 3+ year-old ice appears to be similar to last year, but there was more second-year ice this year in September.  This indicates that a significant fraction of last year's first-year ice (formed in the winter of 2012-2013) survived through its second summer, and this appears to be perfectly consistent with the volume and thickness changes shown above.

Saturday, October 11, 2014

Freeze-Up Progress

After a much colder start to October (first 10 days) than in the past five years, freeze-up is getting under way on area rivers a little early this year.  Through October 10, Fairbanks airport has seen 37 freezing degree days, compared to a 1981-2010 median of 9.5 by this date.  The total of over 3 times the normal for the date sounds like a significant anomaly, but owing to the typically rapid drop-off in temperatures at this time of year, it is only 4 days ahead of normal.

Here are a few webcam images of freeze-up progress at various locations, starting with today and going back about a week:

Tanana River at Nenana today:

Koyuk River from Koyuk today:

Koyuk River on Thursday Oct 9:

First widespread ice on Teshekpuk Lake, close to the Arctic coast, on Tuesday Oct 7:

 Teshekpuk Lake the next day, Wednesday Oct 8:

First ice cover on the lake at Inigok, between Umiat and Tesh Lake, last Saturday Oct 4:

And finally, a grainy shot of Toolik Lake (near the haul road just north of the Brooks Range) freezing over last Friday Oct 3:

Here's a chart showing the lake and air temperatures during the freeze-up of Toolik Lake.  I'm not sure of the depth of the temperature sensor in the lake, but it shows nicely how the temperature stabilized at about 2 °C in tandem with the freeze-up.  No ice was evident on the lake until this temperature threshold was reached, but since freezing began there has been only a tiny amount of additional cooling.  This nicely illustrates the heat exchange processes that are so important in the freezing of fresh water lakes around the world.