Cold Front in Water Vapor Imagery

[Patrick Marsh]

I just posted a blog post about the cold front advancing through the central plains. It's fairly long, so I don't want to rehash it out here, so here's the link: http://www.patricktmarsh.com/2010/08/day-235-central-united-states-cold-front/

Of particular interest about this event is how well it shows up in water vapor imagery. The blog post explains why as well as why the front is "shaped" the way it is. I hope people find it interesting and educational.

 

[Jeff Duda]

So that's what that was that I saw on the WV loop today. Cool stuff, Patrick.

I did a small write up last winter about a cold front that could be seen on infrared satellite: http://www.meteor.iastate.edu/~jdduda/cold_front_IR.html

 

[Patrick Marsh]

In the spirit of full disclosure, I want to post this comment I received after discussion with colleagues. It has been added as a comment to the blog post. (For those who don't know him, Kim Elmore is a research scientist with NSSL.)

Beautiful depction! As I consider myself a perpetual student, I went you your blog to read what you had to say. Your discussion is good, but I must take issue with your explanation of why we see these boundaries in the water vapor imagery.

Water vapor imagery comes from channel 3 in the GOES I-M scanning radiometer and that channel is centered somewhere between 6 and 7 microns in wavelength. These wavelengths are where water vapor is most absorptive and so where it can obscure radiation from earth's surface. So, white areas are where less radiation reaches the radiometer and dark areas are where more radiation reaches the radiometer. Thus, white areas are where there is more water vapor mass in the vertical column and dark areas are where there is less.

Water vapor imagery results from imhomogeneities within the vertical distribution of water vapor in the mid-troposphere. These inhomogeneities come about through horizontal advection, vertical motions, and horizontal deformation. In your explanation, you say that what we see is due to a density gradient, but I don't see how that can be. Since the areas associated with the front are darker, more radiation is making it through the vertical column, which means there is less vertically integrated water vapor mass. I think what we see depicts something about mesoscale circulations set up by the boundaries you note. I'll guess that we're seeing subsidence and possibly frontolytic deformation (post passage) associated with these propagating boundaries.

As to why the secondary boundary dissipates, your hypothesis is as good as any I've heard.

Kim Elmore

And my response back:

Hi Kim,

Thanks for taking the time to read the longer discussion on my blog
and providing feedback.

I don't disagree with anything you said. In fact, when explaining
this to the synoptic class yesterday, I did attribute the darker band
to subsidence within the frontal circulation. However, since I was
(am?) unsure of explaining the details of these mesoscale
circulations, I (wrongly) decided to just be "generic" on the blog
post and mention the parent density gradient. It was lazy writing,
admittedly, which is why it's a good thing I'm attending the Eloquent
Science workshop on Friday! =)

I went ahead and updated the blog with your information and added a
note directing readers to the post's comments, where I have added your
comment in its entirety.

Thanks for the discussion! I hope to have more of it on here!


(PS...After 235 consecutive days of weather posts, my writing (with
respect to the blog) has gotten sloppier. I appreciate any and all
feedback as it can only improve my communications skills!)


Patrick

 

[Jonathan Whitehead]

Patrick, I read your blog just now. I think it is one of your top 3 blogs you've written all year. I think this really ties in well with the "Cold Air Damming" COMET module that I went through last week. Unfortunately I didnt do so well on that quiz but this your blog really helped in reinforcing some things I read in that module. As for the images in your blog, unfortunately I could only make out the front on the first image (without the annotation). The others it was really really difficult if not impossible to pick out.

This blog post topic will definately be a hot topic (or cold topic in this case) in the dead of winter when we get the really monsterous polar blasts.

Good job.