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What do these plots tell you? For close spacing there isn't a lot of difference in precipitable water from one point to another. Not too surprisingly, as the spacing increases, so too does the spread.
I wondered as I looked at these plots: is the difference a function of
spacing? To visualize that, I plotted the percentage difference between
adjacent points vs. the distance. Those scatterplots are below and they
will show you that as the distance between the two points, the percentage
difference can also increase.
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I sent this url to some satellite folks who work on the floors below me
at the Space Science and Engineering Center, and here is part of the
e-mail that I received:
Scott,
I think the biggest differences are due to comparisons between retrievals over
land vs ocean. Unfortunately we don't (yet) store a land/sea flag in the
MDfile.
We have seen how our retrievals more favorably compared to raobs at smaller
match distances. In fact, at large distances (say 1 degree), the guess is
better than the retrieval (because it's so smooth). But, by say 0.5 km the
retrievals are better.
So I recomputed the scatterplot, restricting the comparisons to over land.
I also used only GOES-8 or GOES-10 data -- I won't be comparing data
from two different birds, as that could introduce variability as well.
Here is a map of the locations included
in the comparisons. A table with all the scatterplots follows. Recall
that GOES 8 is over the equator at 75 W and GOES 10 is over the equator
at 135 W. I also include here in the table, for simplicity, the scatterplot
that contains all the data, i.e., those at the top of this webpage.
A couple of things I notice about these scatterplots. The most obvious is that there is apparently more scatter in the GOES 10 instrument. This could be a function of many things and I mention it only as something that deserves further study. Note that the precipitable waters in both GOES 8 and GOES 10 fields of view are roughly equivalent, even though there are synoptic patterns where the deep south is significantly moister than the Rocky Mountains. These data were collected when an intense storm was off the east coast of the US and the eastern half of the country was under strong northwesterly flow, so the Gulf of Mexico and its moisture was shut off. (The date is 1 Nov 1999 -- the date the Egypt Air plane went down off Nantucket). (If you notice something interesting, please e-mail me! I also redid the plots showing the difference as a function of distance. They are below.
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Scott, When one looks at the instrument noise, GOES-10 is cleaner than GOES-8. If GOES-10 has more scatter, I think it's due to the variability of the terrain. Sfc pressure is very important for TPW determinations. To test this, do your scatter plots sorting on the sfc pressure differences.
(Note above: TPW is total precipitable water, which we are trying to measure). Now this made me think to redo the difference scatterplot with surface pressure as the y-axis; furthermore, I first did this in the region where the GOES-8 and GOES-10 satellites overlap -- the high plains of the USA. It is the intersection of the two areas shown in this figure -- between 35 and 41 North and between 100 and 107 West.
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Note above how as the pressure increases the spread difference between adjacent points increases. Can we conclude from this that the GPS/GOES comparison will be better over flat land? I think so. One might argue that the difference between adjacent points as the surface pressure varies is an error -- one could also argue, however, that the difference is real. As the surface pressure drops, the amount of atmosphere holding water vapor is reduced, so the precipitable water will of course decrease. At any rate, this suggests that if there is a big difference between a GPS and a GOES data point, it could be related to pressure differences in the data -- and this effect is much more likely over regions of variable topography (mountains).
Finally, I made a using all data (GOES 8 and GOES 10) and looked at the percentage difference in precipitable water at adjacent points as a function of pressure difference between the adjacent points. There is an obvious relationship between the two.
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