Thursday, May 17, 2012

Mysterious Ocean Precipitation Under Clear Skies

For a number of days, starting on Friday, the new Langley Hill coastal radar has been showing lots of precipitation offshore, some if it heavy.  Yet the skies have been clear or nearly so.  Then on Wednesday the rain disappeared.  What is going on?   And let me warn you now...these mystery showers are going to come back!

First, let me show you what I am talking are two radar images for 8:38 PM on Friday and 6:49 PM on Tuesday.  Lots of apparent rain offshore, but very little over land...very strange!

And lets compare these radar images to the visible satellite imagery at nearly the same time:

All that rain is not coming from clouds!  So if this is not some sort of alien intervention, there must be another explanation!  And there is.

Last weekend and early this week the land surface temperatures have been relatively warm, and most importantly the lower atmospheric temperatures have been warm.  But the surface temperatures of the eastern Pacific have remained quite cool--roughly 50F.   With warm temperatures aloft and cool sea surface temperatures, a low-level inversion (temperature warming with height) formed over the ocean.    We don't have a balloon-launched radiosonde over the ocean, but I can show you the simulated conditions from the UW WRF modeling system there.  The red line is temperature and you can see a shallow, cool marine layer surmounted by an inversion.
So why is this important?  It turns out that such low-level inversions can bend the radar beam downward towards the surface...we call this superrefraction (see figure).   Radar beams are generally bent a bit (refracted) by the normal atmosphere, but inversions greatly enhance this refraction, enough so that the bean is bent back to the earth.  In fact, under inversion conditions (like this weekend), the refraction is so complete the radar beam is trapped near the surface (this is called ducting).

It appears that the radar beam from the Langley Hill radar was not viewing precipitation in the air at all, but rather intersecting the sea surface and reflecting back!   The ducting allowed the radar beam to skim the surface for hundreds of kilometers. To see this, look at this radar image I got from my colleagues in the National Weather Service...the radar beam was hitting and returning from coastal terrain way down the Oregon coast.

This problem was only apparent for the lower radar scans (.2 and .5 degrees above horizontal).  For the higher scans..1.5, 2.5 and higher...there was nothing out there--consistent with the superrefraction mechanism.  Today I was on a conference call with NWS radar experts in Norman, Oklahoma.  They told us they had never seen such a profound, sustained superrefraction/ducting for the record books.

The thing is, this is not the end of this issue.   When we get warm again, the mysterious ocean rain will be back.  The TV weathercaster folks will have to keep this in mind when they show the coastal radar imagery.

Annual Solar Eclipse Forecast

The event will be centered south of us, but the event will occur around 6 PM on Sunday.   Unfortunately, it looks like most of our region will be clouded over at that time.  Sorry.  Will update on Sunday AM. 


Kyle said...

For the eclipse, the map from NASA/Google is awesome. Allows clicking on your exact location to get timing and intensity of the eclipse.

Fixed Carbon said...

This is fascinating. Thanks, Cliff.

Anonymous said...

Amateur radio enthusiasts are quite familiar with this phenomenon, because the same ducting effect can be used to make VHF radio contacts far beyond the usual line-of-sight range. Hawaii-to-California contacts on the 2 meter (144 MHz) band via ducting are reasonably common certain times of year. It's fascinating how the atmospheric conditions affect radio waves.

Brad Smull said...

Presumably one must admit that the much-heralded ~0deg elevation angle capability employed for the LGX/Langley Hill (as previously described at is exacerbating this problem beyond what is routinely experienced at other coastal sites...

JewelyaZ said...

Happy Mt. St. Helens Day! Of course, it's not REALLY weather, but this is a fascinating report, all the same: Mt. St. Helens Recovering from Blast 32 Years On

I had noticed those radar traces and wondered about them, Cliff. I guessed they were a weird sort of ground clutter since there were no real clouds, and I guess I was more or less right. :-)

Anonymous said...

Although the closest place to see the Annular Solar Eclipse is a band along the OR/CA border, a wider area can see a partial eclipse where the sun will appear as a crescent. With rain/clouds in the forecast for Western Washington and Oregon, can anyone tell me what is the closest place with a good chance (60%+) of visible sun, even if through a little haziness? Parts of Eastern Washington are looking cloudy for Sunday afternoon too, so could I go further East?

Unknown said...

Regional radar on often shows heavy precip in the middle of the Strait of Georgia on days with little or no precip elsewhere. Same phenomenon perhaps?

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badmomgoodmom said...

NRL and NPS in Monterey, CA see this type of ducting behavior--extending over hundreds of km--frequently.

Unknown said...

There's actually someone who forecasts tropospheric ducting.