Thursday, August 17, 2017

Summer Wanes

Summer is waning quickly now in Alaska's interior, and some cooler temperatures are finally showing up.  There have been no freezes in the Fairbanks area yet, but the first freeze occurred this morning at Chicken (29°F).  This is the 3rd latest first freeze in the 21 years of data from Chicken; the record latest was on August 21.

Similarly, the Chalkyitsik RAWS saw its first freeze this morning (32°F); this is also the 3rd latest on record (19 years of data; the record latest is August 22).

Last year at about this time I commented on the persistent warmth at the Goldstream Valley Bottom (Ester 5NE) coop site near Fairbanks, and it's been a similar story this summer.  From July 1 through August 15, the lowest temperature was 38°F, compared to 40°F in the same period last year.  In every other summer in this site's 20-year history, the temperature dropped to at least 34°F in this period, and indeed the average date for the first freeze is August 2.  The unusual warmth has been very persistent in the last few weeks.



Looking back at summer conditions across the entire state, the highest reliable temperature measurement was 94°F at the CRN site southeast of Tok, although a more remarkable heat wave occurred just 12 days ago at Skagway, when the temperature rose to 93°F - an all-time record for the site.  These were the highest temperatures in Alaska since June 2013, when Talkeetna smashed its all-time heat record with an astonishing 96°F.

Wednesday, August 16, 2017

Minchumina Follow-Up

Yesterday I posted what I thought was a bit of a mystery regarding solar radiation and temperature data from the Lake Minchumina RAWS, but within just a few minutes reader Gary pointed to a possible solution: increasing shade from vegetation that may have grown up right next to the RAWS instruments.  Here's a 2004 photo from the Western Regional Climate Center website (click to enlarge):


As Gary noted, the photo faces approximately east, so the tree growing up on the right side appears to be roughly southwest of what look like the thermometer and pyranometer in the middle of the arm.  Obviously if this and other vegetation hasn't been controlled in the 10+ years since the photo was taken, then it may have provided increasing amounts of shade over the instruments in recent years; and this would explain the reduction in both solar radiation and warm bias.

Interestingly the hourly solar radiation data support the idea that shading has developed from objects to the south and southwest.  The chart below shows the mean hourly solar radiation (units of langleys) during May on a kind of polar plot; the distance away from the center indicates the radiation amount in each hour, and the angle from the vertical corresponds to the average position (azimuth) of the sun in that hour.  So over the course of the day the solar radiation starts small in the east, increases as the sun moves towards the south, and decreases as the sun goes west.  The blue line shows the averages for 2009-2013 and the red line is for 2015-2017.


The plot makes clear that the reduction in sunshine is fairly small in the morning until about 11am in May, but then it appears that the shading effect is pronounced by around 1-3pm, when the sun is just west of south.  This is nicely consistent with the apparent location of vegetation in the photo.

The charts below show similar results for June, July, and August.  Interestingly the month of June is the only month in which there appears to be no shading from the southeast, i.e. around 9am-noon, and this makes sense if we consider that the sun rises highest in the sky near the solstice; so whatever vegetation has grown up to the southeast, it's apparently not yet high enough to cause shading in June.




In conclusion, I think the problem is just about solved - it looks like the Minchumina radiation data have been seriously affected by shading in recent years, and this has also altered the temperature bias relative to the nearby airport thermometer.  Final confirmation will await a site visit: anyone want to take a field trip?

Tuesday, August 15, 2017

Increasing Clouds at Minchumina?

Some weeks ago when I was exploring the artificial warming reported by RAWS thermometers on sunny days, I noted a remarkable reduction in the RAWS warm bias at Lake Minchumina over the past several years.  The chart I showed earlier is reproduced below, with the solid lines indicating the difference in monthly means of daily high temperature (RAWS minus airport AWOS).  In 2009-2012 the RAWS high temperatures averaged about 4-5°F warmer in May through July, but in 2015 and 2016 the difference was only around 2°F.


In the previous post I suggested that the systematic trend towards a smaller warm bias could be related to increasing cloudiness; if solar radiation has been lower in recent summers, then there would be less artificial warming of the RAWS thermometer.

To explore this hypothesis, I looked at solar radiation data from the Lake Minchumina RAWS for May through August - see the chart below.  To minimize issues related to missing data, I first calculated the mean solar radiation for each hour of the day in a given year and month, and then I took the 7am-7pm mean while requiring fewer than 10% missing data points to arrive at a valid monthly number.


The results are rather striking, with a pronounced drop-off in solar radiation since 2014-2015 at the Lake Minchumina RAWS.  Broadly speaking the decrease in reported solar radiation corresponds to the diminution of the RAWS warm bias, so this seems to be physically consistent.  But is it possible that the solar radiation can have dropped off so significantly and in such a sustained manner in each month from May through August?  This would imply a really significant shift in the summer climate and I'm inclined to be skeptical: a more likely explanation, perhaps, might be that a systematic error has been developing in the solar radiation measurements.  For example, perhaps the sensor has somehow become progressively more obscured or inefficient.

We might have a lot more confidence in the Minchumina RAWS data if other nearby RAWS sites showed a similar reduction in solar radiation.  The map below shows the area we're dealing with.


Looking first at data from Fairbanks, there is little evidence of a sustained increase in summer cloudiness and in fact the overall trend in solar radiation is slightly upward since 2007.  But of course Fairbanks is a long way from Lake Minchumina.


The data from the Telida RAWS (southwest of Minchumina and at similar elevation) do suggest a downward trend in solar insolation in each of the summer months - see below.  The magnitude of the trend is less than at Minchumina, and in particular June and July of this year were comparable to earlier years, which is very different from the situation reported at Minchumina.  So the Telida data seem only mildly supportive of the Minchumina trend.


Looking farther to the southwest, the Farewell RAWS has also reported a modest decrease in summer sunshine over the past 10 years, but again the change in the past few years is nowhere near as dramatic as at Minchumina.



The McKinley River and Wein Lake RAWS also show some indications of diminished solar radiation, but the trends are not pronounced and consistent across all months as at Minchumina.  So again, this is not conclusive.




What about other independent sources of information on cloudiness and humidity?  The airport AWOS at Lake Minchumina has reported cloud coverage at sub-hourly intervals for some years, so this should be of some value - see below.  No significant trend is evident, but we must bear in mind that the AWOS ceilometer can't detect high-altitude cloudiness; and moreover the bi-annual oscillation in the cloud cover is very odd and more than a little suspicious, so I'm not sure the data are trustworthy.


Finally, a quick look at reanalysis data suggests that, contrary to expectations, relative humidity has been lower than the long-term normal over our area of interest in recent years.  Of course, the NCEP global reanalysis is incapable of reproducing the local flow patterns around the Alaska Range and so its broad analysis may not correspond to local trends, but nevertheless it does not support the idea of increased summer cloudiness.



In conclusion, the situation at Lake Minchumina unfortunately remains a puzzle.  The significant reported decrease in solar radiation is quite consistent with the dramatic reduction in the RAWS warm bias, suggesting that recent summers have been consistently and significantly more cloudy than earlier summers; but data from other sites and sources give a mixed message as to whether we should believe the Minchumina trend.  Judging from the multi-site consensus of the RAWS data, it does seem that summers have been relatively cloudy of late in the upper Kuskokwim valley, but that may be the only conclusion we can make.

Saturday, August 12, 2017

The Fairbanks Flood of 1967: The Rainfall

Hi, Rick T. here. You don't have to live in Fairbanks very long to hear about the great flood of August 1967. Even for a community long accustomed to significant flooding, this was extreme. Something like 95% of the city was flooded, causing millions of dollars in damage. Rasmussen Library at UAF has posted a number of videos from the flood on Alaska Film Archives YouTube channel. The NWS Fairbanks Forecast Office and the Alaska-Pacific River Forecaster Center have also put together a very nice online storybook with a short description of the meteorology and hydrology and lots of photos. There has also been work by the NWS, the City of Fairbanks and FNSB Borough to survey and put up high water mark signs around town, and many of these have been installed in the past couple weeks, such as this one downtown on the north bank of the Chena River.


In this post, I want to look at the rainfall that lead to this flooding from a climate perspective. The rainfall August 11-13, 1967 stands out as the highest of record for daily and multi-day totals. The only rainfall records this event does not hold are short duration records, which are all thunderstorm related. To set the stage, here is the background: the second half of July 1967 saw well normal rainfall: 3.07" fell between July 16 and 31. That's still the fifth highest "second half of July" total. However, that was followed by a dry start to August: only 0.02" of rain fell during the first week of the month. But then the skies opened. Below is a plot of the hourly and cumulative rainfall for the week of August 8-15, 1967 (data extracted from Fairbanks August 1967 Local Climatological Data). More than half an inch of rain fell on the 9th. This was followed by about 36 hours with very little rain. Starting late in the afternoon on the 11th, moderate rain fell without much of a break until the morning of the 13th, though light rain continued to dribble on into the 15th before the fire hose finally shut down.
The following totals were recorded at the Airport, all of which still stand as the highest of record:
  • 24 hour: 3.44" 11pm AKST August 11 to 11pm AKST August 12
  • 36 hour: 4.40" 4pm AKST August 11 to 4am AKST August 13 
  • 48 hour: 4.76" 3pm AKST August 11 to 3pm AKST August 13
  • Single calendar day: 3.42" August 12  
  • Two consecutive calendar days: 4.29" August 11-12
  • Three consecutive  calendar days: 4.98" August 11-13
So beyond "highest of record", what's the climatological context? Was this a one-in-a-million event, or is there some reasonable likelihood it will be broken?  To answer this I've compiled annual extremes of a few of these parameters and then fitted a generalized extreme value (GEV) distribution. If that's greek, no worries: GEV is a standard technique for analyzing extreme event frequency and generating estimates of return periods.

Maximum 24-hour precipitation (not necessarily calendar day) has been recorded in Fairbanks since the Weather Bureau office opened in the summer of 1929. For two and three day totals, I've included the cooperative data from the Ag Experiment Station starting with 1915, when daily precipitation began to be regularly recorded. In the graphic below I show an example of the annual times series, in this case the maximum 24 hour precipitation (upper left) and then the GEV analysis for the annual  maximum 24 hour precip (upper right) and annual maximum two and three day consecutive days (bottom row). The red line shows the fitted return period, while the open circles are the observations (which are plotted in rank order).  I should point out that there is no significant trend in any of the annual values.
So what are the return periods for the precipitation amounts that occurred in August 1967?

With  87 years of data:
  • 3.44" in 24 hours is expected to occur on average once in 203 years
With 102 years of data:
  • 4.29" in two consecutive days is expected to occur on average once in 269 years
  • 4.98"  in three consecutive days is expected to occur on average once in 352 years
I'm actually not a fan of return periods. It's not technically wrong, but many people would look at those numbers and say "gee, I'll never see that." In fact, the return periods are just an alternate way of expressing the probability of occurrence. So if I tell you that there is 13% chance that Fairbanks Airport will receive precipitation totaling 3.44" or more in 24 hours hours once in the next 30 years, that's equivalent to saying the return period is 203 years, but the take-home message is different. Now 13% in 30 years is not high (and that makes the dubious assumption that there is no change in extreme precipitation events in a warming world), but it is hardly unthinkably rare.

Postscript:
The GEV analysis I've presented here differs somewhat from that published in the 2012 NOAA Atlas 14 primarily in that I used a longer period of record (for extremes analysis, the longer the better), and, as near as I can tell, the maximum 24 hour precipitation (as distinct from calendar day) was not used in compiling that work.

The maximum 24 hour precipitation for August 1967 is listed as 3.42" in the August 1967 Local Climatlogical Data publication and has been carried through ever since. In fact the hourly precipitation data in the same publication shows that the correct amount is 3.44", 11pm on the 11th to 11pm on the 12th.



Thursday, August 3, 2017

Extent of Humid July

In the previous post a question arose as to whether the very high humidity observations from Fairbanks airport in July were accurate or if perhaps the sensor might be malfunctioning to some extent.  To shed a bit more light on this, I looked at other sites around the interior and also - thanks to a suggestion from Rick T. - looked at the surface dewpoint reported twice per day on the Fairbanks upper-air sounding.

The sounding data clearly support the record July humidity reported by the ASOS instrument - see the chart below.  Last year saw the most humid July on record at the Fairbanks upper-air site (adjacent to the airport), and this July was even more humid.

Looking at hourly data from Fort Wainwright and Eielson AFB, the same thing is observed; both of these sites also saw a record high monthly mean dewpoint.  This is a record for any calendar month, not just for July; and at all three sites the previous record was in July 2016.


I also pulled out the historical data for several other sites across the interior to see how far afield the record moist airmass extended.  The charts below show the July mean dewpoint for 12 sites divided broadly into eastern and western groupings, with the record high values indicated by markers.  Among the "eastern" sites, Nenana and Northway observed record humidity in July 2017, although the 1973-74 data from Delta Junction and the 1962 record from Fort Yukon look highly suspect, and if we take these out then the record also occurred in 2017 at these sites.


Farther to the west, July 1998 was the most humid July (and calendar month) on record at Galena, Indian Mountain, and Bettles, and July 2004 was the most humid month at McGrath, but a new record was set last month at Minchumina and Tanana.


In conclusion, the data suggest that record humidity occurred last month at least throughout the Tanana River valley, as new calendar month record dewpoints were observed at every site I looked at from Northway down to Tanana (assuming the 1974 Delta Junction data is wrong). 

The July 500mb and MSLP patterns (see below) do not show an amplified pattern over Alaska, but the modest upper-level ridge over northern and western Alaska was persistent and prevented cool, dry air from reaching the interior from the north.  Daily minimum temperatures were almost entirely above normal in Fairbanks as shown in the chart below.

 


It seems to have been the stagnant weather pattern, more than anything, that allowed humidity to pool over the Tanana River valley, although the widespread above-normal sea surface temperatures (and therefore enhanced evaporation) surrounding Alaska presumably played a role.  Here's a map of recent SST anomalies, as shown by Rick in his recent climate briefing.


Saturday, July 29, 2017

Summer Humidity

As a follow-up to recent posts on summer temperatures, let's take a quick look at summer humidity trends in Fairbanks.  This topic is timely, as Rick drew attention yesterday to a very remarkable statistic: the number of hours this July with a dewpoint of 55°F or greater in Fairbanks is higher than last year, despite the month not being over yet - and last year the number was apparently far higher than any other year in recent decades.  Here's a chart from Iowa State University.


When I first saw this I really thought there had to be a mistake somewhere, but this is what the hourly observations from the airport have recorded.  There does seem to be a chance that the ASOS sensor is malfunctioning to some extent, as the Eielson ASOS data (see below) show similarly high humidity in a number of earlier years; but there's little doubt that this month has been much more humid than normal.


For a longer term look at each of the summer months, the chart below shows monthly mean surface dewpoint and column precipitable water.  The precipitable water is the total moisture in the atmosphere above a given location and is expressed as the depth of liquid that would result if all the moisture were condensed, i.e. the amount of moisture that is theoretically "precipitable".



The long-term upward linear trends in July dewpoint and precipitable water are highly statistically significant, although for precipitable water there has been little increase since about 1980.  June has also seen increases, but less pronounced, and the long-term changes in August have been quite small.

As noted in the previous post, higher humidity provides an obvious (although perhaps not sufficient) explanation for higher daily minimum temperatures during summer in Fairbanks, because water vapor is a powerful "greenhouse" gas.  See this previous post for more discussion on the topic.


Tuesday, July 25, 2017

Changes in Summer Warmth Odds

Rick's post on the climatological chances of reaching various warm temperature thresholds in the remainder of summer led me to wonder how these "exceedance probabilities" have changed over time.  The chart below takes a very simple look at this by showing the probability curves for years prior to 1976 (solid lines) and years since 1976 (dashed lines).


Perhaps surprisingly, the 90°F threshold is the only one with a distinct systematic difference between the two periods.  Clearly 90+ temperatures have become more common in recent decades, but none of the other curves show pronounced or consistent differences throughout the summer.

Part of the explanation for the lack of difference is that summer-time daily high temperatures have not warmed very much over the long term in Fairbanks.  In terms of high temperatures, June through August in the latter period (1976-2016) was less than 1°F warmer than the earlier period; whereas daily low temperatures were nearly 3°F warmer.  If we created a similar chart for probabilities of warm nights rather than warm days, I'd expect to see greater differences.  I'll see if I can add that chart tomorrow.

[Update July 26] Here's the chart for daily minimum temperatures, so now instead of looking at the chance of a threshold high temperature being reached after each date, we're looking at the odds of daily minimum temperatures being above a threshold after each date.


As expected, the differences are more pronounced for warm nights than for hot days (the term "night" being used loosely for the height of summer).  The probability of any given summer having a daily low temperature of 60°F or above nearly doubled from 39% to 76% between the two periods, and this trend has continued in more recent years; the only year in the last 10 years without a 60+ minimum temperature was 2011.  It's worth noting also the stark difference for August: a 60+ minimum temperature was not observed in August prior to 1974, but it has happened 15 times since then.

The stronger warming trend in daily minimum temperatures is probably caused by a combination of urbanization influences (more heat-storing asphalt etc) and higher humidity derived from warmer ocean surface temperatures surrounding Alaska.  These effects are less significant during the day because the lower atmosphere is generally well-mixed and the afternoon surface temperature is closely tied to temperatures aloft and the amount of sunshine at this time of year.