Tuesday, January 13, 2015

It's Raining, But the Radar is Clear. Why?

Saturday afternoon before taking a pre-game run, I took a look at the radar--no echoes over Seattle.

But hitting the trail, it was obviously raining lightly and my sweatshirt got quite wet.

Here is a photo at the time I left, around 2:15 PM, you see the drops on the tree?


Now here is the radar.  No rain over north Seattle where I live.


How could this happen?

And during the big Seahawks game it was clearly drizzling or raining lightly, but the radar showed nothing.

So is something wrong with the weather radar?   The answer is no...  The origin of this discrepancy lies in the type of precipitation and the limitations of weather radar.

On Saturday the precipitation was light rain and drizzle, and these small droplets were descending out of shallow clouds.  The vertical sounding at Quillayute suggested the low-level moist layer was only about 5000 ft thick (the layer where temperature and dew point were the same up to about 850 hPa):

This shallow layer produces small droplets and small droplets are hard to see in normal weather radar.  Also the intensity was light, again providing fewer droplets and thus less of a target.  The droplets got bigger as they descended from the cloud towards the ground.

But even more important is the height of the radar beam.  The closest radar to Seattle is the one at Camano Island, about 70 km (45 miles) away.   As a radar beam leaves the antenna, it rises above the earth surface.  Thus, the farther away from the radar, the higher the beam becomes (see figure).
The lowest radar beam from the Camano Island radar scans at .5 degrees above the horizon (dark blue color aloft).   The beam both rises and expands as it travels away from the radar.  As shown by the figure, by the time the bean gets 45 miles away (over Seattle) the lower part of the beam is at least 2000 feet above the surface, with the center of the beam between 3000 and 4000 ft above the surface.
Thus, the radar beam would miss the lower (and larger) raindrops, the ones that would have better chance of reflecting the radar beam back to the receiver.

Thus, it is not surprising that the radar missed the low-level drizzle that damped the surface.

So why does normal rain show well in weather radar?  Because the clouds and precipitation extend far higher and the drops are larger.

9 comments:

Unknown said...

Cliff,

I am a distressed skier wondering where our all our POW has gone. I've been tracking the Pacific Decadal Oscillation index for some time and noticed it's at +2.51 for December 2014. Quite high compared to the last 15 years or so. I also remember the 98-99 season where the index switched from a strong positive phase to strong negative phase, bringing RECORD DUMPS to the Washington Cascades. Of course, that also corresponded with a La Nina, the Queen Bee of POW gods. The ensuing years have been quite good for skiing staying mostly in the cool phase of the PDO, aside from a few El Nino's thrown it to cramp our POW-slaying style. Now, with wave after wave of tropical moisture streaming north, I am wondering if the PDO may be the cause ... ?

I am curious on your thoughts on the roles the PDO and ENSO play in mountain snow pack here in the Pac NW. Has topic already been covered? If so, can you point me to the location? If not, future blog topic?

Thank you for all your weather insight. Much appreciated!

Scott

Pete said...

Yes, what he said! Distressed pow lovers want to know... is there any hope?

Bruce Nourish said...

"by the time the bean gets 45 miles away"

*beam

Adam H. said...

So basically radar is useless in the Pacific Northwest?

Jeffrey Jacka said...

Distressed is putting it lightly.

Paul Nickelson said...

Hmmmmm..... wondering what aircraft weather radars might have shown on that day.

Elbegewa said...

Re radar: what about the inverse: A number of times it's been perfectly sunny but the radar on weatherunderground shows it to be raining.

And re snw! yEAH! aNY HOPE THIS YEAR?

Unknown said...

Hey Cliff!
We are wondering what other factors may be influencing our boring and no snow in the mountains weather.
Is the Pacific still warmer than usual in the Gulf of Alaska or off the coast as it was earlier this fall?
What role (if any) is a Rex Block or Omega Block playing in the unusual Jet Stream path?
And, most importantly, is there hope for more snow for our poor ski school kids at Snoqualmie later this month?
Thanks bunches! Jay

Neil Chasan, PT, MMT said...

Why is Obama making it warm in the PNW? LOL