The effects of terrain

After popping into work for a couple hours yesterday afternoon, I took a quick look at the visible satellite and one thing clearly stood out. Why is there an anomalous clear patch in the middle of a state-wide deck of clouds?

Exhibit A: look over southwest Alaska near the coast (Bristol Bay area)

The answer lies in something that most people don’t think about in terms of it’s effect on weather: topography.

The Kilbuck Mountains are part of the reason for the clearing. While not terribly vertically imposing at 2,238 ft. above sea level it goes to show that you don’t need much terrain to have  sensible changes in the weather.

The Kilbucks lie in the western portion of the image. Anchorage is off the map to the east.

So why is the clearing happening? Here is a model derived cross section that is packed with information. The cross section looks at a vertical slice of the atmosphere, in the case from Bethel to Dillingham.

What is in this image? (for the non-meteorological types) First the vertical axis looks from the surface all the way up into the stratosphere, measured by pressure in millibars logarithmically. The green lines and color fill denote vertical motion. Warm colors denote upward motion, cold colors denote downward motion. Orange lines are equivalent potential temperature, think of it as a combination of temperature and moisture, used to diagnose stability or resistance to vertical displacement. Wind barbs are pretty self explanatory, plotted in polar coordinates.

The first thing that sticks out is the strong unidirectional northwest flow throughout the column. The packed isentropes (equivalent potential temperature) near the surface denotes strong moist static stability. Any air parcel in this region will resist vertical motion and want to return to it’s original elevation when displaced. When the northwest flow encounters the mountain barrier the air is mechanically forced up the slope, condensing out moisture and cooling along the way. Once the parcel reaches the top of the barrier it is going to want to return to it’s original vertical position. As the parcel descends down the lee slopes, compression and adiabatic (no outside heat introduced) warming dry and heat the parcel as it accelerates. These are the physical processes that lead to the clearing in the midst of a very large stratus deck. Another feature to note in the visible satellite are the trapped waves the intermittently show up in the clearing. These come about from the air parcels oscillating between the ground and inversion above as they try to reach equilibrium.

Upstream of the barrier. Afternoon sounding from Bethel (PABE)

Features to look for: unidirectional northwest flow through the column, moist low levels, and surface based temperature inversion reaching the top of the flow obstruction. (Maybe a bit of wet-bulbing in there)

Downstream afternoon sounding from King Salmon (PAKN) on the lee of the barrier.

Notice the warming and well mixed nature of the near surface air, keeping in mind that King Salmon is a good 60+ miles removed from the Kilbucks.

What does all this mean for an operational setting? The warming is going to increase temperatures as well as the lack of cloud cover promoting more shortwave radiation reaching the surface to further enhance warming.

The two highest temperatures for the whole state of Alaska yesterday were 74 at Manokotak and 73 degrees at Dillingham which is pretty damn impressive considering their coastal locations right along Bristol Bay.

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About mtlawson07

I'm a meteorologist living and working in Anchorage, Alaska
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5 Responses to The effects of terrain

  1. Mary Miller says:

    I was in Anchorage from August 10 to August 14 and had unbelievably great weather. Just a tourist but a very happy one. I could see Mount McKinley from Anchorage at 6 a.m. on the 11th and took a flight-seeing tour around the mountain that day. Had phenomenal views with nearly clear skies, except on the north slope, which was sopped in and completely covered from view. Now I’m working on a book of photographs from that day and would like to mention a scientific explanation for my great luck. You did a great job with this explanation for the clear patch around King Salmon, so I’m hoping you can explain the clearing from Anchorage to Mount McKinley on the August 11, in a month that normally sees cloudy skies and rain.

  2. mtlawson07 says:

    I’d have to look back at the meteorological setup again to be sure, but in order to have that kind of clearing from Anchorage to Denali we must have been under an upper level ridge. The post here was more of a small scale effect due to the localized wind flow. I’d have to guess we were just between weather systems (good timing on your part) to have such nice weather inbetween the two areas. I do also remember being able to see McKinley and Foraker from here in Anchorage just after that time. If I remember correctly we received 2-3 inches of rain in the first 10 days of August so the good weather was a climatological anomaly.

    • Mary Miller says:

      Thanks for replying. Below is the paragraph where I’m planning to quote you in the introduction to my book of photos taken during a flight-seeing tour of Denali. Please let me know that it’s okay to quote you directly. Otherwise, I can paraphrase your comments with no attribution.
      Introduction, paragraph 11:
      By late afternoon the sky was clearing over Anchorage. Michael Lawson mentioned in his blog on meteorology that this was a “climatological anomaly” that occurred while the region was under an “upper level ridge . . . between weather systems.” As he called it, good timing on our part – really just pure luck.

  3. mtlawson07 says:

    That is just fine Mary. I looked back a little more and in the first 10 days, we received around 2 inches of rain in Anchorage. In fact, there were only 4 days in the month where Anchorage didn’t receive measurable precipitation, and two of them were the 10th and 11th. So luck was definitely a factor in the good weather you received.

  4. Mary Miller says:

    Michael,
    Just a reminder that I included your name in the Acknowledgments in my book “An Hour Over Denali” and it’s now available for sale at Amazon.com. I’d be happy to send you a free copy as a gratuity for your approval to use your comments about the weather we experienced that day. Contact me at maryinfl@bellsouth.net.

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