[wxqc] quick question

[Mark Wyman]

Just a guess that 1 second updates are probably great for plume
calculations. My UP24 pressure meter shows fluctuations in the 1Hz and
longer time frame, but not much faster than that unless a door closes or a
vehicle goes by. These events are very low frequency sound waves anyway and
over time neutralize. The <1Hz (>1 sec) pulses tend to be wind related and
don't neutralize, and trends can be developed from them for short and long
term changes.

http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=UP24

This indicates that anything shorter than 1 second unless in severe winds is
probably meaningless for what you are doing as it is sound waves.

I wonder if you could spread the range of the ultrasonics and place at a
45-degree angle to ground to try and get an average wind speed vs. elevation
rather than wind speed at a given height. Perhaps you can hit more than 3
feet at once. Probably just have to adjust the reading based on the changes
you make in distance.


-Mark

-----Original Message-----
From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Gerry Creager
N5JXS
Sent: Thursday, April 20, 2006 8:52 AM
To: Discussion of weather data quality issues
Subject: Re: [wxqc] quick question

I'm using a pair of the Vaisala WXT-510's, which incorporate an 
ultrasonic sensor.  I prefer the lack of inertia in my wind measurements:-)

The models I work with (WRF, MM5) are perfectly happy with hourly or 
even 3-hr updates for data assimilation.  You're correct: The weather 
models don't require the really frequent updates.  I can't (yet) speak 
to the plume models done as CFD on a finite element grid.  Worse, the 
folks I am helping here can't answer that question pool yet, either.

Thanks for the comments.
gerry

Mark Wyman wrote:
> 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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