Tuesday, July 18, 2017

Wetter in July

Ever since the 1981-2010 standard climate normals became available in 2011, July has been "officially" the wettest month of the year in Fairbanks.  As noted by Rick Thoman back in 2011, this is a new phenomenon, as August was traditionally the wettest month of the year; in fact, all other 30-year normal periods back to 1931-1960 had August as wetter than July, and in the early decades it wasn't particularly close.

Here's a comparison of the monthly mean precipitation from 1931-1960 with the modern climate normal period; interestingly the July-August total mean rainfall is nearly unchanged, as August rainfall has decreased about as much as July rainfall has increased.

The change of monthly rankings is not just a feature of monthly means and therefore susceptible to one or two huge outliers; the monthly medians show a similar result (see below).  Note that the medians are more stable in winter; for example, the 1931-1960 means were thrown off by the outlandish January precipitation of 1937.

If we add the most recent 15 year period, 2002-2016, the recent change really jumps out; the month of July has been extraordinarily wet in recent years compared to earlier normals.

The chart below shows a more continuous view of the evolving differences between July and August rainfall.  In the earliest decades, August really was a lot wetter than July; from 1930-1947, August was wetter in 15 of 18 years.  Contrast this with the past 18 years: from 1999-2016, July was wetter in 14 of 18 years.  This is quite a profound change in the climate.

So how do we explain the recent change to higher precipitation in July?  In a nutshell, heavy rain events have become much more common in July, and while the heaviest rain events are still rare, they are occurring frequently enough to cause a dramatic increase in climatological July rainfall.

Here's a look at the running 15-year frequency of daily rainfall events of 1" or more in July and August.  There have only been 27 of these days in July and August since 1930, so this is close to the top of the daily rainfall distribution.  Remarkably, August has not seen a single such day since 1990, but July has produced 8 such days since 2003 (3 of them were in 2014).

Given that normal July rainfall used to be only about 2" in Fairbanks, an increase like this in the frequency of 1" days is bound to have a significant effect on the long-term averages.  We can quantify this by dividing up the total rainfall into categories of daily rainfall amount - see below for the July breakdown.  Prior to the past 15 years, July rainfall was obtained nearly equally from events of 0.1-0.25", 0.25-0.5", and 0.5-1", but rain events of 1" or more were so infrequent that they contributed rather little to the overall total.  In contrast, the past 15 years have seen nearly equal contributions from the top 3 categories, and the change is most pronounced for the heaviest rain events.  Despite the rarity of 1" rain events, even in the new regime, these events are now contributing a substantial fraction of the total July rainfall.

Here's the parallel analysis for August.  In the past 15 years, the 0.1-0.25" events have contributed less and the 0.5-1" events have contributed more, but oddly there have been no days with daily rainfall above 1" in recent years. 

In conclusion, July has become easily the wettest month in recent years in Fairbanks, and this is largely because of a dramatic increase in the frequency of heavy rain events (above 0.5" and especially above 1").  Interestingly, August has seen a paucity of 1-inch rain events in recent years, and this shift appears to date back all the way to the 1970s.

From a physical standpoint, the increase in July heavy rain events is very consistent with the increased capacity of a warmer atmosphere for holding water vapor, and the same trend has been found in most areas of the U.S. and in many parts of the globe: see the 2014 U.S. National Climate Assessment and the 2012 IPCC SREX report (Table 3-2) at the following links:

The absence of a similar trend in August is more difficult to explain and therefore in one sense it's a more interesting result; I'd like to look at whether similar changes have occurred at other sites and dig a little deeper into the physical mechanisms responsible for the observed trends.

Sunday, July 16, 2017

Raws Warm Bias Continued

A couple of weeks ago I presented a few results from my latest project - an attempt to adjust RAWS temperature data to remove the warm bias that occurs during strong sunshine.  The goal here is to make the RAWS temperature data more useful for climate monitoring; we want to know the spatial and temporal distribution of temperature variations across Alaska, but the RAWS measurements are heavily affected by this warm bias that varies depending on sunshine and - to a lesser extent - wind speed.

In the previous post I showed the results of a bias correction based on the hourly quantity of solar radiation, for 3 different RAWS sites that are located close to reliable FAA instruments (ASOS/AWOS).  Now let's look at the effect of wind speed, which is also measured by the RAWS platform.  The charts below show the residual differences between the RAWS and FAA temperatures after the solar adjustment has been applied, with hourly mean wind speed on the horizontal axis.  The red markers indicate the median difference for each wind speed value; note that wind speed is reported to the nearest whole number in mph.

At all 3 sites, increasing wind speeds cause the RAWS temperature to decrease relative to the ASOS temperature, which is what we expect; when a breeze is blowing, the thermometer is naturally aspirated and the airflow through the thermometer housing helps reduce the artificial warming from solar heating.  This means that the warm bias becomes less of a problem as the wind picks up, and therefore it also means that my solar adjustment is too great when the breeze is blowing: if I apply my solar adjustment without regard to wind speed, then my adjusted temperatures will be too low (as shown in the scatter plots).

The obvious next step is to model the wind speed effect in a similar manner to the solar effect, and I've done that using another analytical function to describe the relationship.  After optimizing the fit of the function for each site separately, the results look like this:

Happily we find that the average wind speed dependence has been largely removed, although of course this is not a perfect process; the temperatures still seem to be biased a bit high at low wind speeds at Eagle and Lake Minchumina.

So after all this we have a set of hourly adjusted temperatures for these RAWS sites, and we can now run a test to see whether the revised data show monthly or annual climate variations that are similar to those measured at the FAA sites.  Here we are interested not so much in the long-term average bias, which can always be removed by subtracting the long-term normals, but in the sign and magnitude of month-to-month and year-to-year changes.

Ideally we would find that such changes are very similar for each pair of sites; for example, when the adjusted Fairbanks RAWS data say that a month was 3°F warmer than normal, then we want to see that the ASOS data show the same anomaly.  If this is true, then the monthly temperature differences would remain constant over time - indeed the differences would be zero if the bias is fully removed - and then we could claim that the adjusted RAWS data provide a true estimate of the long-term temperature variations.

I'll start by showing results from Lake Minchumina, where the adjustment procedure seems to have paid off handsomely.  The first chart below shows the May, June, and July monthly means of daily high temperature before and after the RAWS adjustment; the FAA/AWOS temperature is also plotted in blue.  Note that these results are drawn only from the sample I used for the adjustment process - i.e. only "peak sunshine hours", so the high temperatures might be different from 24-hour values in some cases.  Clearly the adjusted RAWS numbers show very similar month-to-month and year-to-year changes to the AWOS data.  The unadjusted RAWS data also capture the major ups and downs, but notice that there's a trend in the differences: the unadjusted RAWS line is closer to the others in more recent years.

The chart below highlights the trend issue by showing the monthly mean differences of daily high temperature between the two sites, with the unadjusted differences indicated with solid lines and the adjusted differences shown with dashed lines.

The key thing to note here is that the adjusted differences don't change significantly over time - there is little trend and the monthly variance is much smaller than for the unadjusted data.  This means that, as we saw above, the adjusted RAWS temperatures essentially move in lockstep with the AWOS temperatures.  This is in contrast to the unadjusted RAWS data, which show a remarkable trend: the RAWS warm bias has diminished considerably in the past few years.  We might be tempted to speculate about instrumentation changes as a cause for this, but the fact that the bias correction eliminates the trend suggests that solar radiation has been reduced significantly in recent years.  Obviously I'll have to confirm whether that is the case; it would be an interesting result by itself.

In conclusion, the removal of the RAWS warm bias at Lake Minchumina appears to work very well as a means to improve the quality of the data for climate monitoring.  Unfortunately, the monthly mean temperature results are not as encouraging for Fairbanks and Eagle - see below.  The long-term average bias has been removed, but the monthly temperature differences are not significantly less variable than for the unadjusted RAWS data.

The disappointing results at Fairbanks could be related to the fact that the RAWS and airport ASOS sites are over 12km apart, in contrast to Lake Minchumina where the two sites are only a few hundred meters apart.  I looked into using Fairbanks' Fort Wainwright ASOS instead of the airport, but the Fort Wainwright historical data are not as complete.

In Eagle the problem could simply be that the solar warm bias is smaller, as shown in the first post, so there's less opportunity to improve the RAWS data.

Finally, as a measure of the degree of improvement, here are (1) the correlations of the monthly mean temperature anomalies before and after adjustment, and (2) standard deviation of the monthly mean temperature differences before and after adjustment.  The higher the correlation and the smaller the standard deviation, the better.

SiteCorrelation: before (after)Standard deviation (°F): before (after)
Lake Minchumina0.93 (0.99)1.27 (0.43)
Fairbanks0.91 (0.92)1.56 (1.37)
Eagle0.98 (0.98)0.79 (0.70)

Monday, July 10, 2017

Modest Fire Season So Far

So far the Alaska fire season has been fairly subdued although certainly not inactive.  According to today's report from the Interagency Coordination Center, the statewide fire acreage has now exceeded 300,000 acres, which is just over half the 1996-2016 median for the date.  The acreage is increasing by about 25,000 acres a day at the moment, but this is mild in comparison to what happened two years ago, when about 2 million acres burned in the first 10 days of July.  See here for a blog post from the time.

The only two large fires in the state are burning within the Arctic National Wildlife Refuge, with one of them having crossed over from Canada (see more details here).  Widespread rain has fallen in the central and southern interior recently, but the northeast of the state has been hot, dry, and windy; for example, the Fort Yukon SNOTEL site has reported high temperatures in the 70s and 80s for 3 weeks now, with only 0.3" of rain in that time.  Similarly, Old Crow in the northern Yukon Territory has seen highs above 80°F on 4 of the last 5 days including today.

Friday, July 7, 2017

Satellite Perspective on Arctic Lightning

In yesterday's post on lightning over the Beaufort Sea I somehow neglected to look at the satellite imagery to confirm the presence of deep convective storms in the region where lightning was reported.  I realized this oversight when Rick Thoman sent me an image last night - thanks Rick!  After digging around on the UAF Geographic Information Network of Alaska (GINA) website, I found several images that clearly show the evolution of the thunderstorms.

Here's a particularly nice photo showing a curved line of storms in approximately the center of the image; the shadow of the deep cumulonimbus clouds is evident to the north of the line (the sun's position is in the southeast at the time of this shot).  For reference, Point Barrow is visible in the center left of the image, and Banks Island is towards the upper right.

Here's a zoomed-in photo.

In the image sequence below I've paired up a selection of satellite views with the lightning reports from approximately the same time, running from about 2am to 1pm AKST on Monday; the lightning strikes clearly correspond to the brightest (deepest and therefore most reflective) storm clouds.

Thursday, July 6, 2017

Arctic Lightning

The other day a reader pointed out that the Alaska Lightning Detection Network (ALDN) showed a notable cluster of lightning strikes on July 3rd over the Arctic Ocean north of the Mackenzie Delta.  Here's the 24-hour map of reported strikes:

There was also lightning in the same general area the day before and the day after, if the sensors are to be believed.

The 500mb charts show that an upper-level low pressure system drifted up from the southeast and lingered near the Mackenzie Delta for several days, so this may have provided the instability necessary for thunderstorm activity; the favorable meteorological setting increases the likelihood that thunderstorms really occurred and the reports were not just an artifact in the detection system.  Here are the 500mb maps at 24-hour intervals from Saturday morning (July 1) through yesterday morning (July 5).

To get a sense of how unusual it is to have this much lightning so far north, I examined the ALDN data since 2012.  The ALDN historical data goes much farther back, but in 2012 the ALDN switched to a new detection system with greater sensitivity and greater range (see this 2012 news article); so the data since 2012 are not comparable to the data from earlier years.

Looking at lightning strikes north of 70°N, it does appear that this week's event is unprecedented in the 2012-present history of the ALDN data.  On July 3 alone, 1521 strikes were reported north of 70°N in the ALDN domain, and the previous record was 1011 on June 24, 2012.  Here's the map from that day in 2012; note that the blue markers show strikes recorded by the old detection network.

It's very interesting to see that the same portion of the Arctic Ocean was affected in 2012, but a long-term study would be needed to see if this region is climatologically favored for electrical storms.

Here's a chart of the monthly strike numbers north of 70°N for May through August of 2012-present.  With only 5 days of data so far, this month already has more strikes than June 2012, and no other month comes close to this level of activity.

Is it a coincidence that 2012 saw high lightning activity and also produced the most recent record low sea ice extent in September?  Perhaps not; the ice extent maps below show that 2012 and this year both had widespread open water north of the Mackenzie Delta on July 1.  One would expect open water to provide a more favorable storm environment than an ice-covered ocean, via increased evaporation and greater low-level moisture.

Some of the intervening years were not too different (see below), but nevertheless the similarity between 2012 and 2017 is rather striking.