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2012 Winter Banter Thread #3

yoda

By yoda,
in Mid Atlantic

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tcutter

tcutter

Posted
Posted

The 18z model runs? My understanding is that these point and click forecasts are direct outputs of model data with little or no human intervention.

Supposedly not anymore.......................

I debated whether or not to try and go through all this again, but I'll try. First, in terms of total volume of observations, you're first statement is dead wrong. The radiosonde network only forms a very tiny portion of the global observing system (though it is hugely important).

I have no idea what you mean when you say that the radiosonde data is "out of date" for 6z and 18z. At NCEP, we (currently) use a 3D analysis scheme with a 6 hour time window. If you use 12z as an example, we only assimilate observations that are taken between 09z and 15z, for that given analysis. There are ways to use observations within a longer time window, and actually take their time information (and propagation) into account (4DVAR), but we'll save that subject for thread.

In terms of aircraft data, your comment is sort of true if you focus only on a small portion of the globe (i.e. CONUS). I took the liberty to get a rough estimate for the number of aircraft observations that we assimilated, by cycle, from 00z-18z today, and here are the approximate numbers:

67610, 52744, 53490, and 72177.....not as much disparity as people think. This is partly because these are global numbers. However, we actually get a lot of observations from package carriers that operate aircraft (think Fedex and UPS). Here is an example of a distribution from 06z on the 24th:

w.acarsw.300.150.gif

w.cftwnd.300.150.gif

This is only for the cruise altitude (150-300 mb) observations that were taken;, nor have I shown the obs for type 231 (that's why the numbers don't match up with what I provided above).

Keep in mind that for any given 06h window, we basically have global observation coverage (tons and tons of multi-channel IR, MW and visible satellite data, GPS radio occultations from space, satellite derived atmospheric motion vectors [satellite winds], surface observations, ships/buoys, aircraft, wind profiler, radar, in addition to the radiosonde network). Because of hyperspectral satellites, the number of observations we have access to for any given cycle is on the order of hundreds of millions (though there is a lot of redundant information)....and the number assimilated is on the order of several million (I'm too lazy and tired to get the exact counts right now).

Back on topic....now, I feel that the biggest differences between 06z/18z and 00z/12z can be seen for individual, high impact events....where better in-situ sampling can make a difference [example: some critical shortwave that is important for some downstream development or something entering the North American Raob network].

I personally don't think that in a time mean sense the skill between any of the cycles is statistically significant (mind you, this certainly wasn't always the case). However, every time I try to put together evidence to make my case, I can always find metrics, levels, or periods that seem to suggest one thing is better than the other.

We use AC scores too much, so bear with me. Here is a long time series showing the GFS AC at 500 hPa for all four cycles (the top panel is a three month moving average, and bottom panel shows the difference relative to 00z):

acz_wave120_NH500mb_day5.png

Notice how before 2008 that the red and blue curves are almost exclusively below the 0 line, but within the past few years the red/blue do poke above the zero difference line (and the spread has shrunk somewhat). I can assure you that the differences (at least from the middle of 2007 and beyond) are not statistically significant. Also note that for the last year that 12z seems to be doing a little better than the other cycles. However, to argue against myself.....and as the above plot would suggest, you can find individual periods that would seem to suggest some cycles are better than others (here is one recent example....a small sample size where the differences are not statistically significant):

acz5.gif

I don't have time to do an extremely thorough evaluation using other metrics/levels, etc., but the bottom line (in my opinion) is this notion of 06/18z versus 00/12z is more myth than fact (at least in the recent NWP era, and in a time-mean sense).

Another thing to keep in mind, these metrics are looking at forecasts of the same length (120h forecasts in this case). In the short range, an updated forecast is always going to be more skillful than an older forecast valid at the same time because of the assimilation of observations. I'll use a 2 day forecast as an example:

The 42h forecast initialized at 6z is going to be better than the 48h forecast initialized at 0z (regardless of what you think of 6/18z in general). In fact, if no observations were assimilated at all at 6z, the two forecasts would be identical (since the assimilation scheme is an update to a short term guess, or 06hr NWP forecast).

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tcutter

tcutter

Posted
Posted

I believe he was just referring to the fact that the NWS zone forecasts are computer outputs. Nothing about the 6z/18z vs 0z/12z debate.

i really should have posted the original quote that dtk replied to........sorry about that

Well, there is less available data for the 6z and 18z (particularly the 6z) models to ingest. Also, the radiosonde data is a bit out of date for the 6z and 18z runs.

Also, with the 6z run, there are few aircraft in the sky in the middle of the night (outside of cross-country red eyes), so there is not much aircraft data for the models to ingest.

I would be interested to see a statistical analysis of the 0z/12z runs versus the 6z/18z runs, though. I feel like the difference probably isn't all that great and, instead, we generally just say that the 6z or 18z run is trash just because we don't like it! :lol:

EDIT: Disregard this and look at dtk's post below.

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BristowWx

BristowWx

Posted
Posted

Reading the HPC discussion it's the first time this season I have seen these words...THIS WAVE HAS THE POTENTIAL TO PRODUCE

SIGNIFICANT SNOW ON ITS NORTHERN EDGE OVER THE MID ATLANTIC STATES

DAY 5...has a nice ring to it...I don't even want to look at the 12z runs

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nw baltimore wx

nw baltimore wx

Posted
Posted

Filter ftw

Up to this point this winter I have been amused by the random, ridiculous posts, and even the reactions to them. We'll have to see how bad it gets in the real threads before I use the ignore button. I've only used it once for an un-named b, b, boy but like a train wreck, I found myself reading them anyway just for laughs.

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