Sunday, April 30, 2017

Spring Warming Trends (continued)

In my last post I suggested that annual dates of river break-up in Alaska may be able to provide quantitative information about long-term temperature changes at this time of year; this is possible because of the high degree of correlation between break-up dates and spring temperatures.  The annual date of "green-up" in Fairbanks can function in the same way as a climate marker.

After pondering the topic for a couple more days, it now seems clear that the method I used in the first post (read it here) was far from robust.  There's surely no significant difference between a correlation of (say) -0.86 and -0.87, so it's probably not reasonable to pick out the precisely optimal correlation and then claim to have identified a "true" temperature trend.  Accordingly, I think it's probably just coincidence that the results for Fairbanks and Nenana lined up so closely.

Not being one to give up, however, I moved on to a different approach by considering what range of temperature trends could be consistent with the observed correlation between temperature and break-up (or green-up) dates.  This is something that can be addressed with statistical simulation; so I produced 1000 simulated histories of southeast interior April-May temperatures by generating values based on the observed correlation with break-up date.  Each of the simulated histories has approximately the same correlation with break-up date as the actual reported temperature history, but the trends differ widely owing to the random component of the simulations.  Note that I'm assuming there are no other long-term changes that have systematically affected the break-up dates one way or the other.

Here are a couple of examples: the top chart shows the break-up dates and a simulated history that happens to have a very low (near zero) temperature trend, and the second chart shows an example with a very high temperature trend.

For both of these examples, the correlation of the simulated temperatures with break-up dates is very close to the correlation observed in reality, so these are outcomes that "could have" happened based on the physical connection between break-up and temperature.  However, both of these are unlikely outliers; the chart below shows the full distribution of trends from the 1000 histories, plotted as a cumulative distribution function.

It's nice to see that the 50th percentile of simulated trends lies almost exactly on the reported trend, so we can say that the changes in break-up at Nenana are entirely consistent with the temperature trend reported in the southeast interior climate division.  Bear in mind that the simulation process has no knowledge of the actual temperature trend; we have backed it out from the break-up dates.

Here's the same chart derived from green-up dates and April-May temperatures in Fairbanks.

In this case the simulated trends do not line up perfectly with the reported trend, as about 60% of the simulations have more warming than reported by the thermometers.  This is consistent with the simplistic result in my first post, but now we can quantify the probability that the trend is higher than reported.  Based on these results, and if my assumptions are correct, it is about 60% likely that Fairbanks has warmed more than the official climate record indicates (based on a 1974-2016 linear trend line).

Finally, here's an interesting result based on break-up dates of the Koyukuk River in Bettles.

Here we find that nearly all of the simulated histories have less warming than the reported April-May temperatures from the Bettles observing site; it appears to be 90% likely that Bettles has over-reported the warming trend.  In fact, more than 50% of the simulated trends are negative, and when we look at the break-up dates (see chart below) it becomes immediately obvious why this is the case: the linear trend-line for break-up dates shows a very slight increase over this period (although several years are missing near the beginning).  The lack of change in break-up date appears to be inconsistent with the reported warming at Bettles; more investigation is required.

Wednesday, April 26, 2017

Spring Warming Trends

Break-up season is about to get under way across interior Alaska, as seen in the following map from NOAA's Alaska-Pacific River Forecast Center; there is considerable open water on the upper reaches of the Tanana River, and the lower Chena River appears to be mostly open too.  This is supported by a webcam image from Fairbanks showing water pooling on or flowing over the ice.  The ice at Nenana is also deteriorating.

It is well known that break-up at Nenana has tended to occur earlier in recent decades, and of course this is directly related to the long-term warming trend.  Warmer spring weather has also caused trees to leaf out somewhat earlier in Fairbanks; we know this because the annual dates of "green-up" have been recorded since 1974 by visual observation of the east-facing slopes of Chena Ridge.  The chart below shows the green-up data along with the April-May mean temperature from Fairbanks airport.  Note that the high degree of variance prevents the trends from being statistically significant despite last year having the earliest green-up and warmest April-May period on record.

The chart below shows a corresponding analysis for break-up at Nenana, except that here I've plotted the April-May mean temperature for the Southeast Interior climate division (which shows a slightly higher correlation with break-up than temperatures in Fairbanks).  In this case the trends are marginally significant over this period of 43 years.

Here's a scatter plot of green-up and break-up dates versus April-May temperature, showing the rather high degree of correlation for both physical phenomena.  Spring temperatures aren't the only influence on timing of green-up and break-up, but they do explain most of the year-to-year variation.

In view of the high correlations, it's interesting to consider what the green-up and break-up dates can tell us about long-term temperature trends.  For example, if there was no long-term trend in green-up and break-up dates, then it would be difficult to believe that temperatures have risen, even if the thermometers have reported a warming trend; we might infer that urbanization trends or other factors were creating "artificial" warming in the temperature data.  However, the fact that green-up and break-up dates have advanced supports the warming trend observed by the thermometers.

We can take this idea a step farther by examining what temperature trend would be needed to optimize the correlation between temperature and green-up or break-up dates.  I performed this analysis by systematically applying small adjustments to the temperature trend and re-calculating the correlations each time; the results are shown below.  Note that the correlation coefficients are around -0.85 or so, with small variations depending on how the temperature trend is adjusted.

The result of the analysis is rather fascinating: for both Fairbanks green-up dates and Nenana break-up dates, the correlation with temperature is optimized when the reported warming trend is increased by 0.2°F/decade.  This reflects a change from +0.4 to +0.6 °F/decade in Fairbanks April-May temperatures and from +0.6 to +0.8 °F/decade in the Southeast Interior climate division.  This appears to suggest that the actual rate of warming, which is reflected in the dates of green-up and break-up, has been higher than reported by the thermometers at this time of year.

I'll look at break-up dates from some other rivers around Alaska in a subsequent post.  For now, I'd welcome comment from readers on the idea I'm proposing here: that physical date markers with long-term histories can be used to infer true temperature trends and thereby test whether historical weather data is adequately measuring the changes that have actually occurred.

Saturday, April 22, 2017

Spring Progress

After a cold March (14°F below normal), Fairbanks has seen warmer than normal weather overall this month so far (4°F above normal).  Recent days have been relatively cool, however; the high temperature was only 35°F on Wednesday, compared to a normal of 47°F.

Total thawing degree days (accumulation of mean daily temperatures above freezing, in Fahrenheit) are up to 48 as of yesterday, which is the lowest for the date since 2011 and far behind last year's near-record pace.  However, we're actually only a couple of days behind the normal pace for thawing.  The snowpack is diminishing steadily and river ice is starting to look a bit rotten.

The chart below shows daily statewide maximum and minimum temperatures for the last several months according to data from NOAA's ACIS tool.  I've excluded Snotel stations, and for maximum temperatures I also removed RAWS sites because of their known warm bias in sunny weather.  It's striking to see the dramatically higher variance of statewide daily minima compared to maxima; this is largely because the warmest parts of the state in the cold season have highly maritime climates and therefore low temperature variance in comparison to the much colder continental areas.

Sunday, April 16, 2017

Snowy Weeks in Fairbanks

Rick T. here with a post about snowy weeks in Fairbanks.

Interior Alaska has a long snow season, easily from late September to early May, and occasionally even longer. The average number of days between the first measurable snow in the fall and last measurable snow in the spring is a bit shy of seven months (202 days). But of course, the snowfall is not evenly distributed during that time. On the seasonal timescale, early winter is, on average, much the snowiest time of year, while March and April are the climatological dry season. On shorter time scales, depending on what's happening with the flow aloft, we go through days or weeks that are drier and then snowier. However, over the years it sure seemed to me that a significant fraction of the seasonal snowfall comes in a relatively short window. Is that so, or is it just my back complaining from too much shoveling? To address that question, below is a plot of the maximum 7-day snowfall as a percentage of the winter total, i.e how much of the winter's snowfall fell in the snowiest week (any 7-day period) for the Weather Bureau/NWS era of observations (since 1929-30).

As I expected, a fair chunk of the total snowfall does indeed fall in short stretches. The 88 winter average is that almost 22 percent of the seasonal total falls in some seven day period. Although not statistically meaningful, it is interesting that the two highest percentages have occurred since 2010 and that there have been no winters since the early 1990s with unusually low (compared to the long term average) weekly snow percentages.

A surprise to me was the lack of correlation between the total seasonal snowfall and snowiest week. In the graphic, each year's dot is sized according to the total seasonal snowfall: bigger dots indicate higher seasonal totals. Just eyeballing it you can see the size of the dots varies randomly above and below the average line (correlation only +0.03, effectively zero). I expected that low snowfall winters might tend to have higher percentages fall in a few days, but this analysis says otherwise.

———Update April 17, 2017

Inspired by Richard's question on extremes of timing of the snowiest week, here's a histogram of the date of the the snowiest week by half-months. Note that there is no well defined peak: the late November spike is likely no more than random variability.
And for completeness, here's a histogram of the amount of snow that fell in the snowiest week each winter. Not surprisingly, there is a concentration of values around a foot but with a long tail of higher amounts.


Friday, April 14, 2017

Arctic Update

It's been more than two months since my last update on basin-wide Arctic warmth, but not much has changed; even the remarkable cold spell in Arctic Canada in early March didn't put a dent in the overall high-latitude temperature anomaly.  According to the mean temperature from my set of 19 long-term surface observing sites, both February and March were more than 4°C warmer than the 1981-2010 normal.  The coolest of the last 6 months relative to normal was December, at "only" 3.4°C above normal.

This winter's November-March average temperature was the highest since at least 1971 for the 19 stations, as shown in the chart below.  Remarkably, Vize Island (on the northern side of the Kara Sea) was more than 10°C above normal for the 5-month average.  To get a sense of how enormous this anomaly is, consider that 10°C is greater than the difference between the record coldest and record warmest November-March periods in Fairbanks.

If we look at the temperature in terms of standard deviations away from normal, the warmth really hasn't taken a break since the remarkable events of January 2016.  Monthly mean temperatures now seem to be remaining near levels that were very unusual only a few years ago.

Not surprisingly, March average Arctic sea ice extent was the lowest in the satellite era according to the NSIDC, and estimated sea ice volume is also well below previous records.

Interestingly, however, the Danish Meteorological Institute estimates that the Greenland ice sheet has gained considerably more mass than usual so far this cold season.  This was probably caused by the unusual North Atlantic circulation pattern, as low pressure and storminess have been more dominant than usual near southern Greenland.

Monday, April 10, 2017

Chena Basin Snowpack

The April 1 snowpack update from the National Resources Conservation Service (NRCS) shows the cumulative effect of a very snowy winter in the east-central interior, as the snowpack is considerably greater than normal in the vicinity of Fairbanks - see below.  Fairbanks airport has received 83.1" of snow in total, the most since 1992.  However, most of the measuring sites to the south of the Tanana River did not do nearly as well, and it appears that nowhere else in the state saw such a snowy winter in relation to normal.

The chart below shows the 1981-2010 normal snow water equivalent for the SNOTEL sites near Fairbanks, revealing that peak water content is typically reached in mid to late April, depending on elevation.  The higher sites receive more snow in total owing both to orographic forcing and to the longer period with sufficiently low temperatures; so Munson Ridge, at 3100' elevation, tends to build snowpack all the way until the end of April.  In 1982, Munson Ridge was still reporting snow on June 13!

Friday, April 7, 2017

North Slope Winter Warmth

Yesterday NOAA released the U.S. climate division monitoring data for March, and the results for Alaska show a bit of a puzzle.  I was expecting to see significantly below-normal March temperatures for most of the state and a significant warm anomaly for the North Slope district.  However, it turns out that the climate division data show the North Slope division with a -3.0°F anomaly relative to the 1981-2010 normal, and -1.3°F relative to the 1925-2000 normal (as shown in the map below - click to enlarge; note that the normal is 1925-2000, not 1901-2000 as stated in the legend).

In contrast, the graphic below (courtesy of Rick Thoman) shows that Barrow, Deadhorse, and Umiat all saw temperatures in the upper two-thirds of the 1981-2010 distribution; the month of March was significantly warmer than normal at these sites and almost certainly at many other locations north of the Brooks Range.

Here's a time series of the March mean temperature in the North Slope district according to the climate division data:

Oddly the March 2017 temperature is well below the March 2016 temperature, even though Barrow and Umiat were warmer this year than last year in March.  Looking back over the past 5 months (November-March), the same discrepancy is observed: the climate division data indicate that this winter was colder than 2015-2016, but in fact this winter was the warmest on record at Barrow and appears to have been warmer than last winter at Umiat too (although there is a fair amount of missing data).

The two charts below show a closer comparison between the two winters for Barrow, Deadhorse, and Umiat.  The January through March mean temperature was slightly cooler this year, but November and December were much warmer this winter than last winter.  Notice the +54°F daily temperature anomaly at Umiat on January 2 of this year: with a high temperature of 38°F and a low of 29°F, this is the largest departure from normal of any day in Umiat's history.  The 1981-2010 normal for Umiat on January 2 is -20°F, the lowest of any station in Alaska for that date.

What can we conclude from all of this?  First, it's plain to see that climate monitoring is a challenge in the data sparse regions of northern Alaska; different analysis methods can give very different answers even for something as "simple" as year-to-year temperature changes.  It's possible that NOAA's methodology for the climate division calculations allowed this winter's North Slope result to be influenced by colder temperatures south of the Brooks Range and outside the boundary of the district; this would be unfortunate, but these kinds of issues are not uncommon in the world of near-realtime climate monitoring.

An interesting question to consider is, was this the warmest winter on record for the North Slope as a whole?  According to the climate division data it wasn't even close, but my cursory analysis of the numbers from Barrow, Deadhorse and Umiat suggest it may well have been so.  A more thorough investigation, including a careful accounting for missing data, and additional observing sites, would be required to say anything with greater confidence.

Saturday, April 1, 2017

Sub-Seasonal Forecast Skill

I'll be traveling for the next few days, but some readers may be interested in a quick glance at some results I found recently with regard to sub-seasonal temperature forecasts for Fairbanks.  Sub-seasonal forecasts are made about 2 to 6 weeks in advance and therefore deal with prediction time scales in between more traditional medium-range weather forecasts (~5-10 days ahead) and seasonal forecasts (a month or more in advance).

Sub-seasonal forecasting has long been regarded as a very challenging problem, but increasing demand is driving research and investment in this area; the National Academy of Sciences published a report last year on strategies for research into improving sub-seasonal and seasonal forecasts.  I'll be giving a talk in Anchorage on May 2 at the Climate Prediction Applications Science Workshop; hence the new research reported here.

The result that's interesting to me is illustrated in the chart below, which shows a simple measure of performance (R-squared correlation) for forecasts of weekly mean temperature anomaly (departure from normal) in Fairbanks for week 2 (days 8-14 mean), week 3 (days 15-21 mean), and week 4 (days 22-28 mean).  I've taken the ensemble mean of forecasts from the U.S. CFSv2 model and the European (ECMWF) model, and then used a simple mean of the two models.  Generally the ECMWF model is superior to the CFSv2, but the average of the two models is better than either model by itself.

It's interesting and intriguing to see that the forecast performance is considerably better in spring and autumn than in winter; I would not have expected this, because large-scale flow anomalies are largest in winter, and I would expect predictability to be higher as well.  It seems possible that unusual boundary condition forcing (i.e. sea surface temperature and snow/ice cover anomalies - which are a key source of model skill) have greater influence in spring and autumn relative to the magnitude of random/chaotic variations in the flow.  It's also possible that the near-permanent surface-based temperature inversion in winter in Fairbanks makes long-range forecasts particularly difficult in that season.

The best month of the year for sub-seasonal temperature forecasts in Fairbanks is April, and especially at weeks 3 and 4 relative to other times of the year.  I'm quite impressed, actually, that forecasts issued in April capture nearly 50% of the variance at week 3 and almost 30% of the variance at week 4.  The skill is nowhere close to this for most of the rest of the year, although October is also fairly good at week 3.

So what are the current forecasts showing?  As of yesterday morning, when Thursday's extended-range ECMWF forecast came out, the multi-model ensemble is showing warmer than normal conditions persisting into the second week of April, and there is a suggestion of ongoing warmth over southern and western Alaska into weeks 3 and 4, but the signal is not particularly amplified.

A warm outlook is consistent with the CPC's forecast for April - see below - but given the very warm start to the month, it would be surprising to see anything else in the one-month forecast.