Wednesday, June 11, 2014

Olympic Wake Clouds

On Monday, clouds melted away over most of western Washington except for the central Puget Sound, where highly persistent stratiform (layered) clouds were observed all day (see satellite picture). Head north of Everett, bright sun!   Same for the coast.  Tacoma was sunny.

On Tuesday, clouds were more persistent everywhere, but they were most persistent in central Puget Sound.  (see picture).

Why have we been so unlucky?   Fallout from the higher minimum wage?

No, the origin is the Olympic wake, a a zone of relatively dead air behind the Olympics when the flow is from the northwest.

The Olympic Mountains are analogous to a big rock in a stream:  the water flows quickly on the sides, with some dead water behind the rock.   Some examples to illustrate:

The Olympics are a BIG rock and have large impact on the approaching winds.  On Monday afternoon the winds approach the Olympics was from the northwest as shown by the radiosonde sounding at Quillayute at 5 PM:

In contrast,  the winds like downstream of the Olympics at Sea Tac Airports were quite weak at low levels (this plot shows the wind from near the surface, 1000 hPa, to around 10,000 ft (700 hPa) from 5 PM on Sunday (09/00) to 5 PM on Monday (10/00)).   Below 900 hPa (around 3000 ft) the winds were generally less than5 knots.

The surface winds predicted by the UW WRF model  at 8 AM Monday show the light winds downstream of the Olympics clearly.

Not only are the winds light in the lee of the Olympics, but there is a tendency for the air to converge (come together) downstream of the Olympics, producing weak upward motion.  Weak winds and modest upward motion encourages the development and  persistence of low clouds.


Ansel said...

Hi Cliff,

Two questions about summer winds:

Despite your graph showing that Seattle's best winds are in midafternoon, I have often noticed that during fine weather, while out sailing, it is common that there will be light morning winds and then, around 2:00 to 4:30, there will be a period of doldrums, where the sound is frustratingly calm.

And then, often enough, it is also common to get a stiff breeze from the North, especially in the north Sound, 1-2 hours before sunset, lasting until about dark.

Why would this be? I would expect the sea breeze to be strong during the afternoon heat, then die towards sunset (common in the East).

Michael Schmid said...

I'm still fascinated by the "convergence zone" versus "rain shadow" dichotomy. Does it have to do with how deep the windward air mass is (deep -> downslope warming -> compression -> rainshadow versus shallow -> winds bifurcate -> reconverge -> uplift -> clouds/rain)?

Ansel said...

Hi Cliff,

I seem to be the only blogger this week! Anyway- I heard your radio spot this morning, I must say I am a bit confused: I thought that the lee of the Olympics causes the "blue Hole"- not this effect. Is the difference that somehow there is more subsiding air (hence drying and clearing) on the lee side of the mountains when the wind is from the SW, but not when it comes from the NW? Seems like the effect should be the same.