[wxqc] quick question

[Mark Wyman]

I'm not coming from the weather side of things, but the engineering side.
The wind measurement rate is going to be dependant on the mechanical nature
of the sensors. It takes some time for them to accelerate and decelerate.
Large wind cups will take a lot longer than say a hot-wire anemometer to
respond to a change in wind speed simply due to their mass vs. wind
friction. It will also depend on your needs. 

Weather models may not require sub-30 second measurements but you may
require 1 second to determine the volume of air to pass through a certain
area in a given time.

Probably the fastest response time air measurements will be the ultrasonic
variety, which would also be the safest in a combustible environment. They
are also mighty expensive.

-Mark

-----Original Message-----
From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Gerry Creager
N5JXS
Sent: Wednesday, April 19, 2006 8:29 AM
To: Discussion of weather data quality issues
Subject: [wxqc] quick question

OK, so this is a little off topic, but I'm trying to get a lot of things 
done quickly and this group tends to have some good expertise...

I'm instrumenting a site for some liquified natural gas dispersion 
experiments, and we are gonna do it tomorrow.  I'm putting up a pair of 
Vaisala WXT-510s, one at 2 meters, one at 10 meters, in as close to an 
unobstructed area as I can get near the test site ("prop") as possible 
while staying out of the anticipated plume (if the plume ignites, and 
the WXT-510s are in it, they'll become molten pieces of thermoplastic 
slag...).

OK, the question: Does anyone have a good idea of the Nyquist frequency 
for wind measurements?  I've got a request for 1-sec measurements to 
capture all the dynamics.  I can do this but suspect the noise will 
overcome the data and I'll end up having to decimate the data to get a 
reasonable value.  I guess I can look at an FFT in the frequency domain 
and see if anything drops out on the floor, but that's introducing a lot 
of work.

I seem to recall from the depths of my brain, that wind data should be 
collected at a rate not greater than once per 30 sec (twice/min), but 
now I can't find documentation on that.

Adding insult to injury, the folks I'm working with on this don't know 
what sort of continuous data will really aid their analysis.  They do 
know they need to initialize the FEM3A codes they're running with 
surface met data (temp, pressure, humidity, anemometry) but there's more 
than a little confusion as to whether it takes continuous data while 
it's running or if it's an isolated measurement.

I'll be looking at data for weeks after this, so there's no real time 
constraint on answers, if someone's got something!

Thanks, Gerry
Texas Mesonet
AP009
-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.862.3983
Office: 1700 Research Parkway Ste 160, TAMU, College Station, TX 77843
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