Friday, July 3, 2015

How Good is the Reanalysis?

As regular readers here know, the NCEP/NCAR reanalysis is a very useful tool for examining historical climate conditions across the globe.  The reanalysis contains a complete estimate of the state of the atmosphere every 6 hours all the way back to 1948; here's a link to the original publication that introduced the groundbreaking project.  On this blog we've often used the reanalysis data to explore Alaska climate, for example here and here.

A question that naturally arises is, how well does the reanalysis data correspond to reality?  In the paper linked above, the authors note that variables like upper-air temperature and wind are strongly constrained by the observations, and so are generally very reliable, but other variables are more influenced by the model's internal physics, because there are few or no direct observations of these variables.  Surface temperature is an example that is somewhat constrained by the observations, but precipitation is completely determined by the model (i.e. no precipitation observations are assimilated by the model).

In view of this suggestion that the temperature and precipitation analyses might not be that great, I thought it would be interesting to look at Fairbanks data to assess the fidelity of the reanalysis.  I took the monthly mean temperature and precipitation from the nearest reanalysis grid point to Fairbanks and compared it to the observations from Fairbanks airport; the chart below show the correlations by month, with rank correlation used for precipitation.

We see that the reanalysis temperature is very good (correlation above +0.9) from October through April, but the performance drops off very dramatically in the summer.  Remarkably, the reanalysis temperature in July is almost uncorrelated with what actually happens; but interestingly August is much better.

As expected, the precipitation analysis is less good, with correlation coefficients of only about 0.6-0.7 in winter, and very low correlations in June and July.

The chart below shows the results for 3-month periods, which we might expect to fare a little better because the random daily fluctuations tend to even out.  Unfortunately this isn't true, as the correlation values are similar to those for monthly data.

The reanalysis performance is so poor in summer that I had to look at the data more closely; see below for year-by-year comparisons of the temperature and precipitation in May-June-July, which is the worst-performing season in the chart above.  The temperature chart is fascinating, because it shows that while the reanalysis generally captures the sign of many of the changes from year to year, it has a serious problem with decadal trends.  Remarkably, the reanalysis temperatures became substantially colder from the 1970s to the early 1990s, while the observations showed the reverse, and since 1995 the reanalysis has become much warmer while the observed trend has been small.  I can't say for sure what might cause this, but it probably arises from changing systematic bias in the model either when new observations are introduced (e.g. satellite data from 1969 on) or when the ocean temperature patterns shift (e.g. the Atlantic or Arctic temperatures).  After all, the reanalysis is only a model, with lots of assumptions and physical parameterizations.

The May-July precipitation comparison shows more what we would expect for a system that has very little skill; the model is simply unable to reproduce the precipitation amounts in the Fairbanks area in summer.  This isn't surprising at all, because summer precipitation in interior Alaska mainly occurs from localized convective activity or from rather small upper-level features, which are either not resolved on the coarse grid or not well-represented in the model physics.  An interesting point is that the reanalysis has shown generally wetter summer conditions since 1995, and especially since 2004, but there is less change in the observations.

It would be interesting to look at other locations in Alaska to see what the regional performance is like; and I also want to look at the more modern Climate Forecast System Reanalysis (CFSR) from 1979-present, to see if it does a better job in the warm season.  In the meantime, however, it seems we should be very cautious about using or accepting results based on the reanalysis warm-season temperature and precipitation over the Alaskan interior.

Tuesday, June 30, 2015

Pattern Correlations with Temperature

This is a quick post to show a series of figures that I created recently while examining the connection between Bering Sea pressure anomalies and temperatures in Fairbanks.  Each of the maps below shows the correlation between monthly-mean sea-level pressure (left) or 500 mb height (right) with the monthly-mean temperature in Fairbanks.  It's interesting to see the seasonal progression of pressure patterns that tend to favor either unusual warmth or unusual coolness in Fairbanks.

Here are a few features that jump out at me; perhaps readers would like to suggest others.

- Upper-level heights over northwest Canada are strongly connected to Fairbanks temperatures in most months, but in summer the area of highest positive correlation shrinks and doesn't extend much beyond the borders of Alaska.

- In June the semi-permanent connection between Bering Sea low pressure and Fairbanks warmth (or high pressure and coolness) all but vanishes.  However, it starts to return already in July.

- There is an interesting difference between December and January, with December temperatures being most strongly connected to Bering Sea conditions, but with January temperatures being more heavily influenced by flow features to the east.

Friday, June 26, 2015

All-Time Record High Temperature Anniversary

100 year anniversaries don't come around very often. This is one of those rare exceptions. You see, 100 years ago, June 27, 1915, the weather observer at Fort Yukon wrote down a high temperature of 100°F in their monthly log (see Figure 1). That's right, a triple-digit temperature – in Alaska. In the 100 years since then, the record has been approached a few times but never equaled.

Figure 1. June 1915 scanned Cooperative form for Fort Yukon, Alaska.

Even though the 100°F temperature is recorded on June 27th, it most likely occurred the day before – June 26th. This observer made their readings at 2 p.m. every day and so when they inspected the min/max thermometer at 2 p.m. on the 27th, the 100°F temperature indicator was probably left there from the previous day. The typical time for a high temperature in June is around 5 p.m. Standard Time. Having a 1-day offset of temperatures is a quite common occurrence for Cooperative stations all over the U.S. That being said, it will always be shown as a June 27th observation.

Data Quality

Is this temperature reasonable? The basic answer is yes. Rick did an analysis for a conference in 2009 that described the Fort Yukon reading as plausible. He noted that it was near the solstice, skies were clear, and other very warm temperatures were observed. Figure 2 shows the highest temperature observed in the last few days of June in 1915.
Figure 2. Warmest high temperature during the last six day of June in 1915.

In Fairbanks, the high temperature reached a scorching 95°F on June 26th (likely the same day that Fort Yukon reached 100°F). Remarkably, this temperature occurred at or shortly after noon. According the the Fairbanks Sunday Times, a thunderstorm with hail formed (hail noted on Cooperative observer form) and cooled off the city (see Figure 3) at noon.

Figure 3. Fairbanks Sunday Times story from June 27, 1915.

If a thunderstorm had not cooled off Fairbanks so early in the day, it is entirely possible that they would have warmed by another 5°F. Also, since the 100°F temperature at Fort Yukon is only 5°F to 8°F warmer than the nearby stations, it is possible that a lack of clouds or a just-right wind direction allowed the temperature to jump a few degrees warmer than would ordinarily be expected. It is also possible the the reading is not valid. However, there is far too little evidence to toss the reading out. Therefore, it is the accepted state record.

So, congratulations to Fort Yukon on the 100th anniversary of the warmest official temperature in Alaska history!

Thursday, June 25, 2015

Cooling from Smoke

Parts of interior Alaska have been blanketed by smoke from wildfires in the past few days, and the smoke has become increasingly dense in Fairbanks.  The visibility reported at the airport was only 3/4 mile this morning, but the situation was even worse in Tanana, which was reporting less than 1/4 mile visibility owing to nearby fires.

It's been nearly 6 years since a comparable period of sub-1 mile visibility in smoke was reported in Fairbanks; early August of 2009 saw several days with these conditions.  Prior to that, August 2005 also brought heavy smoke, and of course 2004 was the summer of smoke; but this is the first time (since 1950) that such a heavy smoke episode has occurred so early in the year.

The chart below shows the climatological frequency of smoke being reported in the hourly observations; the peak is in the first half of July, with nearly 10% of all days reporting smoke.  However, the frequency peaks later in the summer for smoke that is more dense, as August smoke days are more likely to have low visibility.  In other words, when it's smoky in August, it's usually bad (median visibility of 1.5 miles in late August), but smoke is typically light in early summer (median visibility of 4.0 miles in late June).  This year's situation is very unusual.

With all the smoke obscuring the sky in Fairbanks, daytime temperatures have been significantly suppressed relative to what they would have been otherwise.  For example, the computer MOS forecasts for the high temperature in the past 3 days were 86, 85, and 85 °F, but the high temperature actually only reached 82, 77, and 70 °F respectively (the MOS scheme does not account for smoke).  The National Weather Service did better, with forecasts of 82, 81, and 79, but still missed yesterday's high temperature by nine degrees.

Last year Brian discussed the cooling effect of smoke with an example from the Kenai peninsula.  I thought it would be worthwhile to look at the history in Fairbanks to see if a clear relationship exists between smoke and temperature.  First, I looked at the history of MOS temperature forecasts since 2000; the following chart shows the difference between the forecast and the observed high temperature, for summer days with smoke reported between 10am and 10pm, and with visibility on the horizontal axis (note the logarithmic scale).  The visibility is calculated as the mean between 10am and 10pm.

The chart shows the expected relationship, with most days seeing cooler temperatures than predicted when smoke is dense.  The linear regression fit is not particularly good, but the regression line does have a near zero y-value for visibility of 10 miles (the highest reported by the ASOS platform), which is what we would expect if the forecasts are unbiased for smoke-free conditions.

Another way of looking at the problem is to calculate the difference between surface and 850 mb temperatures and examine the relationship with visibility on smoke days.  See below for this analysis.  The idea is that smoke would not significantly affect 850 mb temperatures but would cool surface temperatures, so we expect to see a relatively cooler surface when smoke is dense.  Note that this analysis goes back to 1950 and uses pre-ASOS visibility observations, which were often greater than 10 miles.

The cooling effect of smoke is clear, and the relationship is a little better than with the MOS analysis.  An alternative regression fit, using a second-order polynomial, is shown below; this seems a little more satisfactory in terms of capturing the drop-off in temperatures at sub-1 mile visibility.  Based on this analysis, it seems clear that smoke-caused temperature deficits can reach or exceed 10 °F in Fairbanks when the visibility drops below 1 mile; and this matches very nicely with what happened yesterday.

Here's an image from the SNPP satellite yesterday afternoon; Fairbanks is under the expansive, dense patch of milky white just to the right of center.  Individual fire plumes are visible in numerous locations.

Monday, June 22, 2015

North Slope Heat

Extreme warmth has enveloped the North Slope in the past couple of days, rivaling past record events.  Here are a few highlights:

- 82F at Deadhorse Airport, and 80F at the new Deadhorse CRN site.  According to the NWS, this is a new all-time record in the 20-year history at Deadhorse, although my data shows 82F on August 5, 1999.  Also, observations from Prudhoe Bay list 83F on two occasions, including June 21, 1991.  The Prudhoe Bay site was a few miles closer to the coast and thus generally cooler, so I think it's safe to say this is not an all-time record for the general area.

- Daily minimum temperature of 46F at Barrow on June 20.  This ties June 20, 1991, for highest minimum temperature so early in the year.  The all-time highest minimum temperature is only 53F.

- 73F at Wainwright airport.  This is a record high temperature for June (but historical data are missing from 1969-1998).

- 85F at Umiat yesterday.  The record for June is 90F, set on the same day in 2013.

Here's this morning's uncharacteristically sunny scene from the UAF sea ice cam in Barrow.  The temperature is back in the 30s now that the wind has gone around to the west.

Friday, June 19, 2015

Stagnant Flow Aloft

The blocking high pressure system that we noted on Monday has weakened somewhat and migrated a little to the west and north, but continues to have a strong influence over Alaska's weather.  It is residing near the Gulf of Anadyr, as seen in this morning's 500 mb analysis (see below).

The location of the persistent blocking high is unusual, as the Bering Sea is a climatologically favored zone for low pressure; blocking highs are much more common over eastern Alaska and northwestern Canada at this time of year.

With stagnant flow aloft and clear skies, the seasonally intense solar radiation is generating very warm temperatures throughout the interior and north of Alaska.  Many locations across the western interior have reached at least 86 °F today, including McGrath, Tanana, Kaltag, and Huslia, and even the North Slope is hot, with at least 63 °F in Barrow and 70s in other locations.  The 63 °F at Barrow is warmer than any temperature observed throughout last summer; the highest temperature for 2014 was only 58 °F.

The chart below shows how persistent the unusual warmth has been in Fairbanks since late winter.  Unfortunately - from the point of view of fire weather - it looks like relief will not arrive soon.  The map below shows the forecast 500mb height anomaly from 3 different computer models for 7-10 days from now, and all three show a very similar pattern of continued above-normal heights (and thus above-normal temperature) over central Alaska.

Update June 20: here's a map showing the climatological frequency of cut-off high pressure centers at 500 mb in June.  Northeastern Alaska and northwestern Canada lead the Northern Hemisphere in the frequency of these events, with an annualized rate of over 9 per year, which translates into nearly one per June on average.  This is also the highest frequency of any location in any month of the year.  However, the frequency is a lot lower over the Bering Sea.

Monday, June 15, 2015

Southwest Warmth

Another dramatic change of the weather pattern has occurred, as a strong and expansive upper-level high over southwest Alaska is now bringing unusual heat to the interior and especially to southwestern and south-central locations.  The map below shows the 500 mb height analysis from 3am AKST this morning, courtesy of Environment Canada.  Twelve hours earlier the 500 mb height was even slightly higher at Bethel and McGrath: 5870m, which is close to record territory.  It's actually the earliest in the summer than such a high 500 mb height has ever been observed at either location.

The pressure gradient at the surface kept a warm breeze blowing last night in many southwestern locations and led to some remarkably high overnight minimum temperatures.  For example, the lowest hourly temperature report was 68F at Dillingham, and Sparrevohn at 1600' elevation only dropped to 64F.  Note that the surface wind direction was northerly and thus blowing off the warm land mass rather than the cold ocean.  The temperature has risen to (at least) 88F this afternoon at Dillingham, which is close to the all-time record high of 92F (which occurred during the June 1953 event that set the all-time 500 mb height records in Bethel and McGrath - no surprise there).

The unusual warmth was also evident at Kodiak yesterday, where the high of 73F and low of 56F produced a daily mean departure from normal of +3.8 standard deviations.  This would be pretty extreme if the temperature distribution were Gaussian, but actually it's nowhere close to Gaussian, as we've noted before.  At this time of year, large positive temperature anomalies are much more frequent than large negative temperature anomalies, if we use the NCEI (formerly NCDC) 1981-2010 mean as the "normal".  For example, since 1981 the largest negative temperature anomaly was -1.9 standard deviations, but +2.0 standard deviations has been exceeded 60 times.  From a physical standpoint, this reflects the fact that there's no way for Kodiak to get much colder than normal in June, but it's not difficult to get unusual warmth advected from the mainland.  The current situation is a great example of this.

The chart below shows the histogram of June temperature anomalies within the 1981-2010 reference period.  Sometimes a Gaussian distribution assumption is a very poor one even for temperature.