Tuesday, June 8, 2010

Point-Specific Soundings

In a post last night, I wrote about upper-air weather "balloon" soundings and their utility in, among other things, determining rainfall potential in a given area.  At one point in the post I mentioned that the nearest weather balloon launch locations to the Austin-San Antonio corridor are Del Rio (DRT) and Corpus Christi (CRP).  I've had several e-mails from folks today asking why a major city like San Antonio and/or Austin is not a balloon launch site.
The above map shows the upper-air "weather balloon" network across the nation.  (I obtained this map from the RAP/UCAR website, which is an excellent source of raw weather data, by the way).

The primary goal of the upper-air network is to provide the "initial" weather data that feeds the computer forecast models.  The computer forecast models then produce their versions of weather forecasts for not only the nation, but much of North America as well.  Rather than focusing on the size of a city in determining whether a launch site is located there, the main focus is on the grid spacing between stations that feed the data to the various computer models.

With that said, I've often thought that there was an unusually large "spacing" between stations in Central and East Texas (i.e., Del Rio to Midland to Ft. Worth to Shreveport to Corpus Christi to Del Rio).  That leaves a pretty big "donut hole" (which happens to correspond to a heavily populated area) in my opinion.  But no one asked me, so that doesn't really matter...

Getting over my balloon frustration and moving on, I will say that several positive developments have taken place in the past several years (thanks mainly to computer technology) that allows us to access a "point specific" sounding "analysis" or forecast for any location we wish, regardless of whether an actual balloon was released in that location or not.  These point-specific soundings are produced by many of the same computer programs that produce the larger-scale forecast models.

So, where do you go to get that information?  There are several sources, actually, but one of my favorite "free" sources is direct from NOAA website located here.  Once you reach the main page, you are given several options for the type of model that you want to use to generate the point sounding.  When just taking a quick glance at things I typically use the default settings (which are already selected for you in the first 3 sections), then select the station location that I wish to view.  For example, if you wish to view a point sounding for San Antonio, you'd enter "KSAT" in the box in the 4th section.  For a list of some of the more popular surface observation stations (from which you may wish to select a point sounding), you can go here.  You can also enter a latitude/longitude combination if you wish.

Once the above are selected/entered and you click on the "Simple JAVA Plots" button (just my preference, you can use any of the 3 choices), wala, a point specific sounding is produced like this one (for Austin, TX today):

Now, how about all of that nifty "derived" information like Precipitable Water values and other things I mentioned in last night's post?  Well, "PW" is certainly calculated for you (note on the above image "PW=49") but the trick is this model calculates the field in kilograms per meter squared.  If you're like me and don't "do metrics", you need to then go somewhere and translate that value into "English" (i.e., inches).

There are many sites that can accomplish this task for you, my favorite (for the moment) is WolframAlpha.  Once there, you can enter the field into the box and hit the "=" (compute) button at the end of the box.  For example, in the above case, I entered "49 kg/m2", then hit the orange "equal" sign, which resulted in this output.  Scroll about halfway down the resulting page (under the "corresponding quantities" section) and you'll finally be able to tell that 49 kg/m2 = 1.9 inches of rain.

If you're lazy (like me) and don't mind paying for your point-sounding data, there are several programs out there that will calculate the fields for you in any "language" that you desire.  I won't list any here because I know Google is your best friend anyway...

Whew.  Glad that's over with.  I hope this helps some of you that inquired...

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