Thick cloud and light rain were of course the direct cause of the temperature suppression yesterday. The 500 mb height analysis from 4pm AKDT (below) reveals a trough along the 140th meridian and a weak ridge over southwest Alaska. In the northwesterly flow between the two features, some subtle vorticity (cyclonic flow) features and warm advection appear to have caused the rain and cloud cover.
Here are the 500mb and 850 mb vorticity maps from 10am yesterday; yellow and orange shading shows enhanced vorticity, which when transported across a region typically brings cloud and precipitation.
The diurnal temperature range yesterday was only 3°F (high of 55, low of 52), which is only the second time in Fairbanks' history that the temperature has remained so steady for an entire day in July. The previous occasion was July 11, 1990, when 1.24" of rain fell.
It is rare for the temperature to remain so steady in the height of summer, because the long daylight hours and large solar radiation nearly always allow the temperature to jump up by at least a few more degrees. So it's even more remarkable that yesterday's small diurnal range occurred in conjunction with only 0.08" of rain. This has actually never happened before from May through August; the previous smallest amount of rain associated with such a small diurnal range in these months was 0.23" on August 21, 1978. So in this sense yesterday was a record-breaking day.
I think the small diurnal range might have to do with warm air invection. The following morning, there was a small wind and it was relatively warm. Almost like a Chinook.
ReplyDeleteAfter reviewing yesterday's pilot reports indicating multiple cloud layers and tops to around 25K, depending on direction, I suggest part of the reason the temps varied little was the relatively thick layers blanketing the Interior. Such depth may be atypical for this time of year (?).
ReplyDeleteSource of reports: http://aawu.arh.noaa.gov/index.php?tab=4&hour=24 There may be other ways to determine this assumption.
Gary
Perhaps a look a solar radiation in W/m*m for the preceding day(s) and average summary for this time of year would be informative. It might support or discount my hypothesis for reduced diurnal insolation due to yesterday's cloud cover...wide coverage and atypical thickness?
DeleteGary
It would certainly be interesting to compare the day's insolation to average and previous record values at the CRN site (Fairbanks 11NE). I'll take a look.
DeleteThe results are in... the CRN site reported the third lowest daily solar insolation in July (data since 2003). The average of the 24 hourly observations (midnight to midnight) was 28 W m-2; there were two days in July 2014 with lower insolation (17 and 25). The average for all days in July is 193 W m-2.
DeleteIt was just a guess based upon what appeared to be a darker than usual day Sunday 7/19/15 at the surface. The PIREPS during that period indicated vertical cloud layers of some magnitude.
DeleteI wonder if the Skew-T plot (or whatever ?) would indicate the sources of the insolation filters? I don't have a good grasp of that data yet.
Gary
Gary, the Skew-T will often indicate layers of higher humidity associated with cloud. However, in a situation with precipitation and multiple cloud layers, there is often little to distinguish between cloudy and cloud-free levels, i.e. humidity is high through a deep column.
DeleteA site for monitoring the most recent Skew-T plots is
http://weather.rap.ucar.edu/upper/
I'm sure you know that the closer together the temperature and dewpoint traces, the higher the relative humidity. Note that it is common for cloud layers to appear sub-saturated although they are not.
Here's something that might interest you, Gary. I don't know if I'm going to be able to go myself.
ReplyDeleteUp at UAF in the Elvey Globe Room, 3:00 pm - 4:00 pm, 24 Jul 2015, there is a Master's defense:
"Using self-organizing maps to detail synoptic connections between climate indices and Alaska
weather". In particular, from the summary: "Self-organizing maps (SOMs) are a way of categorizing
multidimensional data, like daily sea level pressure (SLP) into commonly occurring patterns. In this
paper, we use SOMs to describe the synoptic patterns during winter in the North Pacific and then
explore the commonly occurring patterns during different phases of the PDO, ENSO, and the "Pacific
blob." Results suggest that the patterns common during a given phase of the PDO include subtle
differences that would result in Alaska weather that is very different from what would by expected
based on the canonical PDO. These subtle differences would not be evident in the overall average used to produce the canonical PDO description. Our study also finds evidence that supports recent studies suggesting that the pattern responsible for the 2014 "Pacific warm blob" is linked to tropical SST forcing."
If anything, it will be interesting to read when it gets published. (Or if the research can be found elsewhere like in a poster or something)
Thanks Eric I may attend...should but am trying to get my plane on floats so that might interfere. The results would make great reading when approved and published. SOM: https://en.wikipedia.org/wiki/Self-organizing_map I'd need to learn more about the process to appreciate the defense.
DeleteGary