Saturday, February 23, 2013

Radar, Wind, and Snow

The front has gone by, the winds have lessened, and heavy snow is falling on the western slopes of the Cascades.    But first, let me answer a frequently asked question:  what is the persistent echo seen just offshore of Hoquiam in many images from the Langley Hill radar? (see below).   This pattern is seen persistently in the lower radar scans and looks like a half circle.    The answer: the lower portion of the radar beam is hitting the surface and reflecting back to the radar, which is located a few miles northwest of Hoquiam.   Such a phenomenon is known as ocean or sea clutter.   The image below is from the half-degree elevation angle scan of the Langley radar, which means the center of the beam is a half-degree above the horizontal.  The width of the beam is roughly 1 degree, so the lower portion of the beam skims the surface in the vicinity of the radar.

Some of you might ask, doesn't the radar also hit the ground and terrain?  Why don't we see a return there as well?  The answer is that you would except for the fact the radar has ground clutter suppression software that subtracts that out.   Land and mountains stay put and so the radar can be trained to ignore returns from them. Not so easy with an ocean surface with waves and swell.  We knew we would get some sea clutter with the new radar, but it is a small price to pay for the capability to see farther out into the Pacific.  In fact, due to the intercession of Senator Cantwell, our radar has the capability to scan lower than any other NWS radar in the country:  .15 degrees.  Yes, more ground clutter, but more offshore range.

Let's compare the radar imagery at .15, .5, and 1.5 degree elevation angles this morning (7:02 AM).  First .15 degrees....lots of sea clutter, but you can see the shallow convective showers way offshore.



.5 degrees--the range pulls in a bit.

1.5 degrees:  no sea clutter, but the horizontal range is much less.

With taller targets, the .15 degree elevation angle can see 300 km offshore or more.

This radar image also shows weak convective showers over the ocean in the cold, unstable air and substantial enhancement of the precipitation as the air is forced to rise by Olympics and coastal mountains.

Finally, the winds.   The vigorous cold front and the strong northwesterlies than followed produced some strong winds over the region--but nothing truly damaging or exciting.  Here are the max winds (mph)during the past 24 hr.  Gusts above 50 mph in and downwind of the Strait and some 40s over the Sound.  Notice how quickly the winds weakened over land.


 And yes, snow.  This kind of cool, unstable, northwest flow pattern is great for snow:  expect snow totals of 1-2 feet above 3500 feet.  The mountains needed some fresh snow.   Skiers will be happy.

Reminder:  if any of you want to attend the Northwest Weather Workshop on March 1-2 (next weekend) in Seattle, you can get more information and register here.

3 comments:

Gpacharlie said...

Why does it seem that 18000 feet or 500hPa is the altitude used to observe weather patterns?

What technology is used to observe at 18000 feet and how are the winds and pressure so well discriminated?

Great post Cliff. It really got me thinking - ouch-

What is the average elevation used for radar returns.

What is the effective azimuth in degrees at differing elevations forr this radar and if a full 360 what physical limitations are there such as mountains, hills, etc...

How much negative impact is caused by beam divergence in the vertical and horizontal; or where can one find a discussion of more detail regarding this radar-
How much manual input is required by meteorologists in the post processing of publically released radar returns. In other words does it require additional observed factors such as temperatures and ground observations to post process a more comprehensive and accurate display of precipitation?
Is there a website where one could read more detailed info on this radar systems capabilities and limitations; and is it dedicated to weather service use.

Can the site be visited and toured by the public?
Old NIKE Radar Nerd

smokejumper said...

We must be playing 20 questions....

Good storm. Will help our overall snowpack near the crest. Kinda missing or begging for some rain on the leeward side of the mts though. Recorded less than a tenth of an inch since Christmas. Not a good pattern for us.

Curious about the far extended. 99 percent unlikely, but if a high could amplify upstream, cold trough dropping south and something cutting underneath. There's been constant hints of that possibility.

Gpacharlie said...

Couldn't help myself. Radar has always fascinated me.