Friday, June 10, 2016

Thunderstorm Season Approaching

Fairbanks has yet to observe a thunderstorm this year, according to the ASOS lightning sensor at the airport, but this will soon change if history is any guide.  Two years ago we looked at the climatology of thunder in Fairbanks and saw that the frequency ramps up very quickly in early June (see the chart below, reproduced from that post).  The median date for first thunder is June 6, with "near-normal" ranging from about May 30 to June 15.  Last year the thunderstorm season began on June 7, and it started on June 9 in 2014, but back in 2013 the first thunderstorm didn't occur until June 21, which was remarkable given the hot early summer weather that year.

[Update June 11: and just like clockwork, thunder was reported today at 4:05pm AKDT.  Remarkable.]



The chart below shows the dates of all 408 days with "thunderstorm" reported in the hourly observations at Fairbanks since 1950.  The ASOS platform was introduced in December 1997, so that could explain the lack of outlier dates in recent years, although the ASOS lightning detector is reported to have a higher rate of reporting than human observers (and a very low false alarm rate).


In my post of 2 years ago I looked at seasonal changes in humidity as a possible explanation for the very rapid increase in thunderstorm frequency in early June, but the conclusion was that humidity alone probably isn't enough to explain the phenomenon.

To take this investigation one step further, I recently calculated a measure of vertical instability by taking the difference in equivalent potential temperaturee) between the surface and 500 mb.  The idea here is that a large vertical gradient (decrease) in θe indicates greater thermal instability and more potential energy for deep convection (i.e. warm, humid air lying below cold, dry air).  There are many other measures of convective instability that could be used, but the θe difference provides a first-order estimate of potential instability.  Note that I used the 3pm AKST Fairbanks sounding data to get the 500 mb conditions, and I restricted myself to thunderstorms observed between 9am and 9pm AKST.

The chart below shows how the frequency of thunderstorms is related to the daily (9am-9pm) maximum in θe difference.  As we would expect, thunder is more frequently observed when the atmosphere is more unstable, and thunder is very rare when there's no instability (negative θe difference).


Now we can address the question of whether there is a notable early June increase in instability that might explain the arrival of thunderstorms; the chart below shows the frequency of moderate (≥6°C θe difference) instability by date.  The answer appears to be a resounding "no", as substantial instability is surprisingly infrequent in early June and doesn't peak until a month later.  It seems that rising instability is not an adequate explanation for the climatological surge of thunderstorm activity in Fairbanks; and so the search for a suitable explanation will have to continue.


One redeeming aspect of the chart is that the gradually decreasing instability in July and August does mirror quite nicely the drop-off in thunderstorm frequency in late summer.  So it seems we can explain the end of the season in terms of the elimination of instability; it's the beginning of the season that is a bit of a mystery.

16 comments:

  1. How does the minimum potential change over time? Any negative lapse rate could cause a thunderstorm if there is sufficient agitation.

    ReplyDelete
    Replies
    1. I'm not quite sure what you're asking with respect to the change over time, but I completely agree that storms can develop with minimal instability if there is suitable forcing. In fact the atmosphere destabilizes when it is lifted, so an approaching front or upper-level disturbance can quickly transform the stability profile and create more favorable conditions than expected.

      Delete
    2. Sorry I wasn't more clear. Things often sound better inside the head than on paper.

      Your brief study only looked at potentials over 6. Presumably because we moderate instability for the thunderstorm to develop. My point was that we don't need to filter out the results to above 6. If a thunderstorm developed than it could be a combination of temperature height differences and forced uplift. What is the minimum potential over the daily period? This could give us a view into how much lift is from cold fronts, etc.
      Then plot this minimum value over the year like you did for the potential > 6.

      Delete
  2. I linked this in the previous discussion...is there something lacking in their explanation for example "intense solar heating and little vertical wind shear" over elevated terrain? There are other ingredients in the soup of course.

    Solar heating intensity may no correlate perfectly with photoperiod.

    http://docs.lib.noaa.gov/noaa_documents/NWS/TM_NWS_AR/TM_NWS_AR_14.PDF

    Gary

    ReplyDelete
    Replies
    1. I forgot to add the delayed albedo of mature vegetation that's complete by mid-Summer might explain the asymmetric shape to Figure 1. More mature vegetation that can absorb insolation = more surface heating.

      Gary

      Delete
    2. And, mature vegetation also...

      1. Collects, stores, and later releases moisture from rain, surface runoff, and a portion of the photosynthesis process. That moisture increases surface humidity which can then be lifted.

      2. Acts as both a warming mechanism to surface soils and insulation to slow the cooling of above surface air from below during summer.

      Exposed surface soils also eventually warm and reradiate heat by mid-later summer.

      Gary

      Delete
    3. Thanks Gary. Yes - there are various factors that either favor or hinder deep convection, and instability is only part of the puzzle. I'll ponder your comments as I dig into further analysis.

      Delete
    4. Don't fret over them...they're just observations of changes in surface dew point, vegetation, and terrain over time. Our thunderstorms are mainly air-mass types...localized unlike those common to the Lower 48 Plains States that depend on vertical shear from winds for significant strength and later lateral movement.

      I suspect localized increases in humidity (often delayed) via precip, vegetation, and solar lifting due to the effect of insolation on dark terrain start the seasonal process. The tops of the hills are often scant to devoid of vegetation yet initiate cloud formation. I assume the moisture is then fed from lower elevations (?).

      Any significant reduction in insolation via cloud cover moderates the thunderstorm formation process...like today's cloud cover.

      Gary

      Delete
    5. And yet, Gary, only a few minutes ago I heard a clash of thunder here at my home near the airport. And just now another. Summer is officially here.

      Delete
    6. I finally figured out the cause...as soon as I finished polishing and buffing the plastic windows in my airplane clouds formed followed by thunder and lightning.

      Actually the upper deck of clouds dissipated after noon and it got hot and sunny...to be soon followed by lifting and thunderstorms. The flow was NW from hills SE of town towards Delta Jct. It's still going on as they forecast a few days ago for this weekend.

      Gary

      Delete
    7. Today's and early 0000 06/12/16 Skew-T may be informative as to thunderstorm formation:

      http://knik.iarc.uaf.edu/rtwrf/SKEW-T-FAIRBANKS.htm

      Gary

      Delete
    8. Well that was quick! And yes - good idea to look at the Skew-T from the first day of storm formation each year.

      Delete
    9. If you look quickly the storms can be seen forming to our SE and passing towards Fairbanks:

      http://radar.weather.gov/ridge/radar.php?rid=APD&product=NCR&overlay=11101111&loop=yes

      Gary

      Delete
    10. Very interesting Gary. Looks like some spots are seeing a lot of rain.

      The 00UTC sounding is fascinating, as there is very little instability on paper. The plot thickens.

      http://weather.rap.ucar.edu/upper/pafa.gif

      Delete
  3. Below select Maps...Lightning...Fairbanks zone for today 06/11 and 06/12/16. About 200 strikes so far:

    http://fire.ak.blm.gov

    The Skew-T can be walked back a few days from current here. My analytical skills at this stuff are weak:

    http://twister.sbs.ohio-state.edu/upper/skew-t/PAFA

    Gary



    ReplyDelete
  4. Might as well capture today's surface obs at PAFA prior to and after the thunderstorms (TRW) passed. Times are local AST. I was there from 13:00 until 16:00. Note the drop in dew point, relative humidity, and pressure...and brief increase in air temperature associated with the sunny interval, initiation of TRW's, and the subsequent steady rain event now occurring in Fairbanks.

    Gary


    Prior:
    11 12:53 Calm 10.00 Mostly Cloudy FEW060 SCT090 BKN120 65 48 54% NA NA 29.72 1006.7
    11 13:53 Calm 10.00 Mostly Cloudy FEW055 BKN140 BKN220 67 47 49% NA NA 29.70 1006.1
    11 14:53 Calm 10.00 Mostly Cloudy SCT055 SCT140 BKN200 70 44 39% NA NA 29.67 1005.1

    Start of thunderstorms just before this obs to the east:
    11 15:53 Calm 10.00 Light Rain BKN060 BKN140 BKN200 69 45 71 60 42% NA NA 29.67 1005.0
    11 16:53 W 15 G 23 10.00 Thunderstorm Rain FEW043 BKN055CB BKN140 BKN200 60 50 70% NA NA 29.68 1005.3 0.33
    11 17:53 W 12 10.00 Mostly Cloudy FEW027 BKN060CB BKN095 BKN200 62 51 67% NA NA 29.68 1005.3 0.01
    11 18:53 NE 7 10.00 Rain BKN036 BKN060 OVC090 55 50 83% NA NA 29.70 1006.3 0.23 0.57

    ReplyDelete