Friday, July 25, 2014

Precipitable Water

As most readers of this blog know, precipitable water (PW) is a measure of the amount of moisture that is contained in a column of air extending from the surface to about 300 millibars. It also represents a measure of the volume of water that can contribute to a precipitation event. The PW amount is calculated from upper air balloon soundings released from each of fourteen sites around the state. It stands to reason that more moisture in the atmosphere will lead to more precipitation. In fact, anomalously high PW values are frequently noted in Area Forecast Discussions as indicators of heavy rain potential. So, let's put this to the test.

Since this June-July has been the wettest on record for Fairbanks (by far) I wanted to see if a correlation existed between the average daily PW value and the precipitation totals during this two month period. Using a June-July time period also has the advantage of eliminating snow events and it is also the season of peak thunderstorm activity.

When looking at it on an annual scale (June-July constituting a single year) there is no correlation between average PW over the 61-day period and the total rainfall over the same period (see Figure 1). For Fairbanks, this June-July is already the wettest on record but the average daily PW has been unremarkable.


Figure 1. Average June-July PW (orange) and total June-July precipitation (green) in Fairbanks between 1971 and 2014.

When we group days by the amount of precipitation that fell and calculate an average of those days, a strong correlation exists (see Figure 2). Days with no precipitation or only a Trace have substantially lower PW than those days with over 0.50" of precipitation. The three days this month with over 1.00" of rain in Fairbanks had a PW average of 1.16".

The apparent discrepancy between the two charts is primarily a result of the number of days with precipitation not taken into account in the first graph. If most of the rain falls in 3 or 4 days, the PW from those days is averaged with the PW from the other 57 or 58 days and thus blends in with the other values.


Figure 2. Average June-July PW value grouped by observed precipitation amounts in Fairbanks between 1971 and 2014.


Note: Here is a site ( http://www.crh.noaa.gov/unr/?n=pw ) that has PW climatology for all upper air stations in the U.S.

Thursday, July 24, 2014

Still Wet in Bettles

On Monday I noted that Bettles had seen measurable rainfall on 13 of the past 14 days, and including today's heavy rain showers this is now up to 17 of the past 18 days.  Rain has fallen on the past 14 days in a row, which is close to record territory; the warm-season record for consecutive days of measurable precipitation is 16 days (ending September 8, 2012).  The most remarkable aspect of the pattern may be the frequency with which the rain has been relatively heavy; 17 days since June 1st have seen rainfall of at least 0.25 inches, compared to a record of 11 days in the period June 1 - July 23, 1984.

The chart below shows the daily precipitation and high and low temperatures since May 1 in Bettles; it hasn't been much of a summer.  The lack of warm nights is quite notable: only 5 days have had daily minima above 50 °F, which is just about a record low pace through this date.

It goes without saying that all of this is an extraordinary turn-around from last summer's heat and dryness.


Wednesday, July 23, 2014

Brooks Range Snow Forecast

A significant snow event is about to get under way in the Brooks Range; the following statements were issued by the NWS in Fairbanks this morning:

AKZ206-240000-
NORTHEASTERN BROOKS RANGE-
INCLUDING...ANAKTUVUK PASS...ATIGUN PASS...GALBRAITH LAKE... SAGWON...FRANKLIN BLUFFS
428 AM AKDT WED JUL 23 2014

...SIGNIFICANT SNOWFALL THURSDAY THROUGH FRIDAY...

SNOW WILL DEVELOP THURSDAY ABOVE 3000 FT AND LOWER TO 1000 FT THURSDAY NIGHT. SNOW WILL CONTINUE THROUGH FRIDAY. STORM TOTAL SNOW AMOUNTS OF 3 TO 6 INCHES ARE POSSIBLE WITH THE HIGHEST AMOUNTS ABOVE 3000 FT.



AKZ218-240000-
SOUTHEASTERN BROOKS RANGE-
INCLUDING...ARCTIC VILLAGE...WISEMAN...COLDFOOT... CHANDALAR D.O.T. CAMP...INIAKUK LAKE
432 AM AKDT WED JUL 23 2014

...SIGNIFICANT SNOWFALL THURSDAY EVENING THROUGH THROUGH FRIDAY FROM THE PIPELINE WEST...

FROM THE PIPELINE WEST...SNOW WILL DEVELOP THURSDAY AFTERNOON ABOVE 5000 FT AND LOWER TO 2000 FT THURSDAY NIGHT. SNOW WILL CONTINUE THROUGH FRIDAY. STORM TOTAL SNOW AMOUNTS OF 2 TO 5 INCHES ARE POSSIBLE WITH THE HIGHEST AMOUNTS ABOVE 3000 FT. THIS COULD MAKE TRAVEL DIFFICULT OVER ATIGUN PASS THURSDAY NIGHT.


Snow has already been falling in the western Brooks Range, as evidenced by the Red Dog FAA webcam photo from earlier today (elevation 962' MSL):



I took a quick look at the latest high-resolution (6km) NAM model forecast for the area, and indeed the model indicates substantial snowfall in the higher elevations over the next two days.  Below is a map of the estimated total snow accumulation through Friday evening (note: the model predicts snow accumulation in liquid equivalent, and I used a 10-1 ratio to estimate total depth).  I added markers for some of the FAA webcam locations, including Anaktuvuk Pass (red), Chandalar Shelf DOT (blue), Coldfoot (green), and Arctic Village (turquoise).


For historical context, here's a map that Brian put together a few days ago, showing all Alaska observing locations that have reported measurable snow accumulations in July (per GHCN data).  The Brooks Range location with 2.5" is Chandalar Lake, and that event occurred on July 17, 2003.



Tuesday, July 22, 2014

Precipitation Percentiles

A few weeks ago Rick started a discussion (via e-mail) regarding precipitation percentiles. Since temperatures are pretty normally distributed, the terms 'above normal' and 'below normal' are universal when the time scale is a single day. However, precipitation is an entirely different matter. If a station receives precipitation on 20% of days throughout the year, then 80% of days will experience below normal precipitation. From a statistical perspective this is counter intuitive. Using a probabilistic measure provides a meaningful baseline from which to compare a single day's precipitation to determine whether or not it is a high or low probability event.

Figure 1 shows the daily precipitation values for the 75th, 80th, 85th, 90th, 95th, and 99th percentiles for Fairbanks between 1915 and 2013. Measurable precipitation values for 2014 are overlaid as blue diamonds. The daily values were interpolated from a spline of monthly values. Importantly, all values were included when developing the percentile lines – including days with no precipitation.

For example, on March 1st, the 99th precipitation percentile is 0.25". In other words, there is a 1% chance of observing a precipitation value greater than 0.25" on March 1st. Since there are 365 days in a year, we expect to exceed the 99th percentile 3.65 times annually. Figure 2 shows the number of exceedences of the 99th percentile for every year since 1915. So far in 2014 the 99th percentile has been exceeded 6 times. In fact, those 6 exceedences occurred in a span of 20 days. The only time that occurred in a shorter time period was back in March of 1918 when 6 exceedences occurred in a span of 11 days.


Figure 1. Precipitation percentiles for Fairbanks (1915-2013) with 2014 precipitation shown as diamonds.

Figure 2. Number of days that the 99th percentile was exceeded for Fairbanks (1915-2014). 2014 data is through July 20th. 

Monday, July 21, 2014

Cool Summer Days

Summer 2014 is turning out to be cooler than normal in interior Alaska, as highlighted by reader Gary who mentioned yesterday that for the first time in many years it's been necessary to run the heating stoves in July in Fairbanks.  The temperature anomaly is mostly seen in the daily high temperatures, which have been more than 3 °F below normal on average since June 1.  In contrast, the low temperatures have actually been above normal on average; the compressed diurnal range is directly related to the abundant cloud cover and precipitation.

Below are charts showing the daily high temperatures compared to normal at Fairbanks and Keystone Ridge, and also at Bettles where they have been having a very wet time of it.  Not only is it the wettest summer-to-date on record at Bettles, they have had measurable precipitation on 34 out of 50 days since June 1, which is also a record.  The last two weeks have been particularly bad, with measurable rainfall on 13 of 14 days and with daytime temperatures dropping to levels more typical of early September.




Here's another way of looking at the distribution of high temperatures since June 1: the charts below show the counts of days exceeding a range of threshold temperatures.  We can see that the frequency has been especially lacking for days on the warm side, with (for example) only 6 days reaching 77 °F at Fairbanks, compared to a normal of 15 days.  Up on Keystone Ridge, only 2 days have reached 75 °F, compared to a normal of 11 days.  The anomalies appear particularly large on Keystone Ridge, but part of this could be due to the station history beginning in August 1996; the "normal" period only includes the (presumably) warmer half of the 1981-2010 period, so current anomalies will look cooler relative to normal than for stations with longer histories.





Update July 22: reader Eric asked about corresponding charts for last summer and last winter.  Here they are for the standard three-month seasons.  Summer 2013 was really excessive in terms of the number of days above 80 and 85 °F.



Saturday, July 19, 2014

Barrow Upper-Air Climatology

As a companion to yesterday's post, here are the 1981-2010 normals for the temperature aloft at Barrow, as observed by the twice-daily balloon soundings.  The color scales are the same as in the Fairbanks plots.



Looking first at the cold half of the year, we see that a deep low-level temperature inversion is characteristic of the atmospheric profile at Barrow, and in this respect the climatology is similar to Fairbanks.  The warmest temperature in the "normal" sounding is found above 3000 feet from late November all the way into May.  However, the inversion strength at the surface is nowhere near as strong as at Fairbanks, because the near-constant wind mixes the low-level air and usually prevents radiative cooling from getting out of hand.

In the afternoon climatology we see that the surface inversion ceases to be present in early April as solar heating picks up, and the afternoon surface inversion doesn't return until late October.  However, the inversion aloft persists through the entire summer and disappears only from late August through September and October.

It is very interesting to observe that there are two distinct maxima in low-level lapse rate, in which temperatures are relatively high at the surface compared to aloft.  The first, and lesser, maximum is found in May when surface temperatures are warming rapidly, and solar heating is relatively strong, but the air aloft is still cold.  The second, more significant, maximum is in late August through early October, when the open ocean nearby keeps surface temperatures relatively high while the air aloft is cooling.  Between these two periods, particularly in early July, lapse rates are notably lower (more prone to inversion) in both morning and afternoon; this period is close to the time of peak warmth aloft.

Lastly, I'll point out that the average morning sounding shows no surface-based inversion throughout September and the first few days of October, because the ocean warming is sufficient to offset nighttime radiative cooling during this period.  Contrast this situation with that in Fairbanks, where even in the height of summer the morning sounding still shows a healthy inversion.

Friday, July 18, 2014

Temperatures Aloft (Climatology)

In my last post I looked at recent temperatures observed by the balloon soundings from Barrow and Fairbanks, as compared to the 1981-2010 climatology (normal).  I thought it would be interesting also to look at the temperature climatology itself to see if there is anything to learn.  Starting with Fairbanks, the images below show, in the top panel, the normal temperature throughout the year in the lowest ~8000 feet of the atmosphere,
and in the lower panel, the lapse rate (multiplied by -1 for easier interpretation).  The first image shows the normals for 12 UTC (3am AKST), and the second image shows 00 UTC (3pm AKST).  Click on the images for a better view; discussion is below.





There is not too much surprising in the Fairbanks climatology, for those familiar with the march of the seasons in the Alaskan interior.  From late October through early March the "normal" sounding shows a low-level temperature inversion even in the afternoon, and the inversion remains strong throughout the day in December and January.  For three months of deep winter (mid-November to mid-February), it is usual for the temperature lapse rate to be inverted to above 4000 feet elevation, i.e. the warmest temperature in the sounding typically occurs above 4000 feet.

The most interesting feature of the warm-season climatology may be the "super-adiabatic" lapse rate near the surface that is seen at 3pm from late March all the way through the beginning of October.  A super-adiabatic layer has temperature decreasing with height at more than 9.8 °C/km, which means that the layer is absolutely unstable to dry convective overturning.  The phenomenon is ubiquitous over land surfaces around the world when there is adequate solar radiation, because strong solar heating of the ground causes the lowest layers of air to gain heat more quickly than any heat removal mechanism (radiation, conduction, convection) can transport it elsewhere.  The implications of such a layer are that thermally-driven turbulence will be active, a fact that aircraft pilots are well aware of.

The 3am lapse rate climatology shows that low-level inversions are typically found in the early morning throughout the year, but are shallow from March through most of October.   The morning inversion is weakest in early August, and also shows a slight secondary minimum in early October; both of these times correspond to increased cloud cover and precipitation frequency, but usually without snow on the ground.  As snow cover typically becomes established later in October, the morning inversion strength rapidly picks up owing to efficient radiative cooling.

Given the length of this post, I'll discuss the Barrow temperature climatology in a separate post soon.