Thursday, March 3, 2011

A Rain Shadow...Literally

This will be a quick little post.  We've been getting a lot of rain recently in the Seattle area, and the mountains have been seeing a lot of snow.  So, the radar has been pretty active with lots of precipitation moving through.  The other day I was checking to see if there were any showers moving through the area and this was the radar view south of Seattle.
Fig 1 -  KATX 0.5 degree base reflectivity at 0250Z, March 1, 2011.
Seattle is just left of the top-center of this image.  However this radar reflectivity field struck me as rather unusual. There were a lot of showers moving through the area and that's obvious from the little cells of precipitation in the top part of the image.  What struck me as odd was the sudden abrupt shift to that wide area of stratiform-looking precipitation across center of the image.  An animation of the radar images showed movement within the area of stratiform precip so I know it wasn't an artifact of the radar.  It seems that there really just was a cloud deck at around 8000 feet that was producing strong enough returns to show up as that wide band.  I originally suspected that perhaps this was some sort of bright-band enhancement due to the radar hitting the melting layer at that level.  But a quick check of the Quileute sounding from 00Z on that day...
Fig 2 -- Sounding from Quileute, Washington at 00Z, March 1, 2011.
...shows that the freezing level was very close to the ground--not anywhere near 8000 feet (though that does begin to approach the dendritic growth zone...).  This sounding is also taken way out at the coast, which usually is warmer than what we see in the interior due to the slightly warmer ocean being right there (at least, in the winter).  So this really does look like simply a precipitating cloud deck aloft.  I'm just surprised that the edge of this is so straight--if the base of this precipitating cloud was at a constant altitude, we'd expect to see a curved edge to the precipitation area following that elevation of the radar beam.  So this structure is very intriguing.

What's more interesting (and the actual topic I wanted to talk about here) is that odd gap in the stratiform precipitation.  See that pie-shaped wedge of ZERO radar returns extending to the southeast in the middle of that stratiform area?  What would cause that?  Here's a big geographical hint:
Fig 3 -- Annotated version of figure 1.
Right at the tip of that pie-shaped void is the location of Mount Rainier!  So what we're seeing is literally the shadow of Mount Rainier in the radar returns.  The KATX radar itself is located off the top of this image.  Remember that I said (based on some standardized calculations for radar beams) that at the leading edge of that stratiform preciptation band, the radar beams were at about 8000 feet of elevation.  Mount Rainier is 14,411 feet tall--so those radar beams are definitely running into the side of the mountain.  You can even see a slight enhancement of the reflectivity right over Mount Rainier which is probably the radar returns from the beam bouncing off the mountain itself.  Of course, no radar beams can go through the mountain, hence we can't see what's behind it.   As radar beams travel outward, not only do they increase in elevation, but they also spread out.  This causes the "shadow" behind Mount Rainier to spread out too.

So when meteorologists talk about "rain shadowing", they don't actually mean this.  But I enjoyed how you literally could see the "shadow" of the mountain on the radar...

No comments:

Post a Comment