From horst Wed Feb 17 13:41:56 1999
Subject: Seattle fetch
I am continuing with the evaluation of the sonic tilts and have begun
looking at station 2. The plot of wind elevation vs azimuth for
Seattle from April 23 - May 30 shows a very prominent deviation for
winds from approximately ENE (the sonic boom pointed to the North).
The data indicate a positive elevation angle (positive vertical
velocity) of 1-2 deg for winds from 60 deg E of N and a negative
elevation angle of 1-2 deg for winds from 75 deg E of N. This feature
is present throughout the period. Does anyone recall any obstruction
or terrain feature to account for this?
Tom
From: guestps@ibis.met.nps.navy.mil (Peter S. Guest)
Subject: Re: Seattle fetch
Date: Wed, 17 Feb 1999 14:09:37 -0800 (PST)
Tom, I was there then. Unfortunately, I didn't get any close-up
pictures of the Seattle site. Drats! (I took 360 deg pictures of all
the other sites at least once!) I think I actually made it out to the
Seattle site only once, but Jeff Otten went out at least a couple more
times during this period. There was considerable lead activity; in
fact we tried several times unsuccessfully to reach the site toward
the end of my stay. Leads prevented us from getting there. I recall
there was a lead fairly close (10- 20 meters?) to the North. It was
very active and threw up several large chunks of ice. I think I recall
there was a large mound in about the ENE direction about 15 m away from
the tower, perhaps even closer. I remember because I climbed it to look
for polar bears. I would suspect this is causing the effects you note,
but I can't say for sure without an accurate direction fix from
pictures.
After a closer look, I found a couple of pictures from the ship's
bridge which show the Seattle site in the distance. There does appear
to be a new ridge to the north and a "bump" just behind and a little to
the left of Seattle from the bridge perspective. I think this (bump)
is about ENE from Seattle. Again, the view is not clear so I can't say
for sure, but it does seem to be consistent with my recollections
stated above. One thing I am sure of is that there was a lot of lead
activity and ridging very close to Seattle and it does not surprise me
that there were some 1-3 degree deviations in vertical wind speed. I
also know there were no huts or other man-made obstructions that could
be the cause. Hope this helps....
-Peter
Date: Wed, 17 Feb 1999 17:14:04 -0500
From: Ed Andreas
Subject: Re: Seattle fetch
I've described Seattle several times as a "fortress." It had broken
ice all around it at various distances. I was on the ice camp from May 12
through the end of May and found one photo I took of the Seattle PAM site
May 16. The view is eastward, not northeastward, but there is an obvious
ridge encroaching into the picture from the north, fairly close--within 50
meters. I'd say this is pretty good evidence that the offset you're seeing
is topographically controlled.
Peter was on the station just before me, and I shared duty with Dave
Costa. You might query them, too, for photos.
Ed
From horst Wed Feb 17 15:55:45 1999
Subject: RE: seattle fetch
Ed and Peter:
Thanks for the input. John also recalled the ridge to the north of
Seattle. His recollection is that the sled was perhaps to the WNW
of the station. Does that sound right to you?
What is really remarkable to me is the extremely distinct change in the
elevation angle of the wind from +2 deg to -2 deg over only a 10-15 deg
change in wind direction. A more complete description is roughly
azimuth elevation
50 deg 0 deg
65 deg 1.5 deg
70 deg 0 deg
75 deg -2 deg
85 deg 0 deg
Does that seem possible, given the terrain (icescape? help!)? I
certainly have no other explanation. I guess that I have spent too
much time collecting data over sites that resemble billiard tables.
Tom
From: "Persson, Ola"
Subject: RE: seattle fetch
Date: Wed, 17 Feb 1999 16:42:01 -0700
John and I set Seattle up on April 20. My logbook from that day says that
"320-338 deg true affected by sled". I also have a picture of Seattle on
April 20 towards about 15-20 degrees (assuming the sonic arm is due north)
showing substantial ice rubble/ridges about 40-50 m to the NNE. As Ed
describes it, Seattle was a "fortress", with extensive ice ridge rubble
especially close to the sonic in the NW through ENE directions (clockwise).
Unfortunately, the picture probably does not show the 65-75 deg sector. I
did find a slide I got from Dominique showing Seattle from the west. I'm
not sure if the contrast will be good enough to see details on the opposite
side of the tripod, but once I find a slide projector (they seem to have
been packed for our move), I'll let you know.
Ola
Date: Thu, 18 Feb 1999 08:34:56 -0500
From: Ed Andreas
Subject: RE: seattle fetch
The changes in elevation angle do seem pretty wild, but I have never
done an analysis like yours. So who knows how to explain it? I have seem
similary dramatic changes in the drag coefficient with angle of attack over
sea ice.
A logbook entry (also should be in the PAM electronic long) on May
30 states that the Seattle sled was -36 degrees from head-on to the
sonic--i.e. northwest of the tripod.
Ed