[meteorite-list] How to compute NEXRAD radar hit times

From: Jake Schaefer <jakeschaeferml_at_meteoritecentral.com>
Date: Sun, 23 Jan 2011 15:31:03 -0800
Message-ID: <AANLkTimPAru=actdkFPVJu6HPzug6J9TStK8hUp2cBuB_at_mail.gmail.com>

Rob, I have verified that the 0.5 and 1.5 and even sometimes 2.5 scans
do get swept twice. Within the actual data file there is a time code
(accurate to the millisecond) for every sweep and azimuth angle as it
spins around. You also have to be careful when the radars are in storm
mode, some things change slightly. Another oddity is the elevation
angles are not constant through the sweep, they very slightly (I'm
guessing do to ground interference since every station handles the 0.5
scan a little bit differently.)

Using the NOAA provided tool can be difficult at times. It's not the
best way to look at the radar data in my opinion. I've had troubles
with the latest version of the software reading in recent files, they
switched to full high resolution for all scans and got rid of the
lower resolution high elevation angle scans and are doing them all in
high resolution now. Get some weird errors when loading data so I have
gone back to an older version of the NOAA tool when i do use it.
Mostly I use my own tool I have developed. I am working on a way to
implement the time code into it so you can play it like a movie and
watch the sweeps go around for each of the stations. I have a blog on
the tool I have developed and hope to release an initial version in
the coming months for people to use. The original goal was to make the
software to make it easier to look through the archives and another
goal is to someday make an automated search routine to look for
meteors in the data in realtime.

http://3dradar.wordpress.com/about/

-Jake






On Sun, Jan 23, 2011 at 2:15 PM, Rob Matson <mojave_meteorites at cox.net> wrote:
> Hi Jim,
>
>> I find the use of Doppler Radar very interesting. I downloaded all
>> the NOAA free stuff I could find to view Doppler from my laptop
>> using their archived data. However, I can not produce the times
>> that are specific to seeing Ash Creek. I was going to use Ash
>> Creek as my test to determine if I was doing things right. I am
>> not! And, I contacted the Phoenix NWS and they can not help with
>> the times. The issue is the time. Only certain specific times are
>> archived, so how or where did the data come from for the specific
>> time?
>
> You probably DO have the right data -- it's just that you have to
> do a bit of extra work to compute the approximate times corresponding
> to a particular radar sweep and target location. Strangely, there is
> NO information on the web about how to do this -- likely because
> meteorologists don't care very much about knowing the scan times
> of a particular cloud front.
>
> Here is how I do it. Mind you, I have no confirmation that my
> technique is correct, but it is mathematically sensible and is
> based on some logical assumtions:
>
> 1. ?The NEXRAD radar sweep rate (in azimuth) is constant.
> 2. ?The sweep direction is clockwise as viewed from above.
> 3. ?Each elevation scan takes the same amount of time to
> ? ?complete.
> 4. ?Each full scan starts at the lowest cut angle and ends
> ? ?at the highest cut angle, with transitions in cut angle
> ? ?occurring when the radar is transmitting due north.
> 5. ?That the time-tag of each file corresponds to the start
> ? ?time of the lowest cut angle, and that at that time the
> ? ?radar is pointing due north.
>
> (I would love to have some confirmation that all of these
> assumptions are correct -- Marc Fries can likely confirm or
> correct me.)
>
> One remaining uncertainty I have is what physically happens
> with the radar when it generates two sweeps at the same cut
> angle (e.g. 0.5-degrees and 1.5-degrees are nearly always
> swept twice each.) In these cases, does the radar complete
> two full revolutions for each cut angle, or are the double
> sweeps really just a single sweep, but processed twice (in
> software) at different sensitivities? I have a feeling it's
> the latter, since I usually see very little movement in
> scene features between scans at the same cut angle.
>
> In either case, what you need to do is count up how many
> sweeps are done for each full scan -- it's usually at
> least 5, and can be quite a bit more when in storm mode.
> Divide the time between consecutive full scans (typically
> around 10 minutes) by the number of sweeps per scan.
>
> Example:
>
> Image #1 timetag: ?02:12:06
> Image #2 timetag: ?02:21:56
> 5 sweeps per full scan
>
> (2:21:56 - 2:12:06) = 09:50
> 09:50 / 5 = 1 minute 58 seconds (1.967 minutes)
>
> This tells you that the radar sweep rate is:
>
> 360 degrees / 1.967 minutes = 183.1 degrees/minute, or
> 3.051 degrees/second.
>
> Now, to determine the time of a specific radar "hit", all
> you need is its azimuth (i.e. bearing from the radar),
> which is displayed at the bottom of the Toolkit screen
> when your cursor is over the hit.
>
> Let's say you see your target in the second sweep out of
> five (e.g. the 1.5-degree cut angle) and that it's at
> azimuth 227 degrees. Using the above example, the time of
> that hit is computed as follows:
>
> 02:12:06 + (1 sweep * 1.967 minutes) + (227/360 * 1.967 min)
> = 02:12:06 + 1.967 minutes + 1.240 minutes
> = 02:15:12
>
> Try this with Ash Creek, and I think you should come very
> close to the time that Marc Fries did.
>
> Good luck!
> Rob
>
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Received on Sun 23 Jan 2011 06:31:03 PM PST