Tuesday, March 24, 2009

Doppler Radar


This is lots of talk about Doppler radar...but the interesting thing is that most of you NEVER see the Doppler part. That will end right now! Normally, the only radar images you see show you where it is precipitating. More technically, the normal radar images show you reflectivity...how much of the microwave radar beam is scattered back to the radar. Heavier precipitation generally has a higher reflectivity...so that is why radar is useful.
But you don't need a Doppler radar to see precipitation. A Doppler radar has an additional capability...it can measure the velocity of the precipitation particles towards or away from the radar. And since precipitation is pushed around by the winds, that give you information about the winds. TV stations tell you about their super Doppler radars, their pinpoint Dopplers, their Storm-Tracker Dopplers, and for a short time a decade ago...their Wappler Doppler radar. At one point, there was a pinpoint, severe-weather center, storm tracker Doppler radar! But they never ever show the Doppler imagery on TV. I should correct myself..when KING 5 got their own radar, Jeff Renner showed the Doppler velocities for a few days. It was taken off VERY quickly, never to return. OK, want to see what you are missing?
Tonight there is a very nice convergence zone over central Puget Sound (see image). A nice example...and most of you probably recognize the band of cloud extending across the Sound (see image). Next, take a look at the Doppler velocity image (image). Both are for the lowest radar angle (.5 degree above the horizontal). Enough to drive you to drink?
Cool colors (e.g, blue and green) show velocites towards the radar and warm colds (yellow, red) indicate velocities moving away. The radar is located in the center of image at Camano Island. So air is moving towards the radar in the Strait of Juan de Fuca (since the colors are green and blue) and away from the radar in the very north Sound (see the small area of yellow). Those are northerlies. And you see the gray area over the Sound? There is no velocity towards or away from the radar....this is the center of the convergence zone. Anyway, it is lots of fun and not a little challenging sometime to figure out the wind directions from the Doppler image...since it is only giving you one component of the wind (towards or away from the radar). I bet you now understand why this is not shown on TV...my colleagues at the TV stations would spend all their time explaining the images!

16 comments:

Jim said...

What? No comments yet on that astounding revelation of the evolutionary nature of TV weather forecasting?

Sort of like the explanation of the impact on the economy of financial derivatives by local TV personalities on the business news?

Mark said...

I love reading your blog. Coming from the East coast a year ago, the weather here is quite a bit different.

HarrisonCZ7 said...

Quite the CZ tonight...wowzers

andycottle said...

Hey Jim....a bit off the weather topic, but have been meaning to say that I like your frog pic. Cool!

Weather wise....yeah, sure is quite the CZ. Almost up to half inch of rain for the day far since it started raining this afternoon. Currently have 0.45" with 41 degrees.

basil_x_fawlty said...

On the subject of radar "modes" -- can you talk a bit about "clear air" mode? I'm not sure if they do it in the Seattle area, but where I'm originally from (Midwest), the NWS would put the radar on the website into "clear air" mode whenever there was no precip in the area -- confused the heck out of me for a while before I accidentally discovered what was going on.

Eric Houston said...

Can anyone comment on the value of the TV station radars? I've always wondered if there's anything to be gained from these installations. Are they not detailed enough to enable valid conclusions to be drawn? What makes the NEXRAD radar at Camino island so much more effective?

As the NWS radar appears to be having issues again tonight it makes you wonder if there's anything to be gained by relying on these even in a backup role.

HarrisonCZ7 said...

Can't believe the persistence of this CZ. Just checked the webcams at both Stevens and Snoqualmie and it's just dumping...how much rain you have now Andy? I fathom rainfall amounts in some areas far exceed 1" inch.

Gary in Olympia said...

Harrison,

CoCoRaHS is a volunteer rain/hail/snow reporting network run by folks at Colorado SU. There are now more than 200 of us reporting our precip totals in WA daily.

You can see color coded maps at the national, state, county, & (in some states with lots of volunteers) metro areas.

The map site is:
http://www.cocorahs.org/Maps/

After you pick a precip type, you can select locations in the drop menus.

Jim said...

Sorry about the sarcastic/off topic comment last night.

I've used different doppler views that I found on the web for trying to make plans on outdoor activities, especially short-term ones. Until I read this explanation, I really never understood the underlying science. I'm not sure I could explain it to someone else, but henceforth I'll look at those images with a different appreciation.

Thanks for providing all these little mini weather workshops. Also, thanks to the many posters who provide additional links to other sites where even more information is available.

AnselAndy said...

Thanks for the Doppler explanation and example. Hope to see some more real Doppler images here in this blog!

Kevin Purcell said...

basil_x_fawlty said... "On the subject of radar "modes" -- can you talk a bit about "clear air" mode? I'm not sure if they do it in the Seattle area"

In clear air mode they slow the rotation rate of the scanner to increase the sensitivity of the radar to anything that's in the air. You'll see more "clutter" though that clutter maybe be real things (migrating birds, for example, weather radar has taught us a lot about bird migration).

The upshot is you get updates every 10 minutes rather than every 6 minutes for a full volume scan.

And they do use clear air mode in the Puget Sound at Camano. You can tell (on a blue sky precip free day) because the scale at the side of the plot will show from -28dBZ to +28dBZ rather than from 0dBZ (ND) up to 75dBZ and the updates come in slower. You'll also see a fair amount more light gray clutter.

There is a very nice explanation at the NWS (but note none if it really involves the doppler shift)

With two dopper radars scanning the same volume you can get absolute velocities and directions (i.e. a vector field) but aside from storm chasers I'm not sure if this is done regularly in the US).

http://weather.noaa.gov/radar/radinfo/radinfo.html

mjgrota said...

Cliff's comment on the way radar is used on TV to inform the public is very important. Too often we (the technical users of measurement systems) lose site of what the "customer" relates too when getting weather information. The misuse of terminology (doppler, double doppler, super giga widget, etc) to imply a particular tool used by a media outlet is better than another is a problem that confuses the way the public can use the final product.

Keeping the graphics clean and current is vital (some stations show old images because they sell the story better but in fact the event is over). Showing coverage, intensity, precipitation type and movement is all the public really needs. 3D projections of cloud tops and radar winds don't add clarity visually but should be included verbally in the description of the base image. If the current weather event is about wind rather than precip, create a graphic to show the winds in a way that the effected pubic can use the info. Even the simplest radard images may be too technical for most viewers. It would be good if the NWS, U of WA or the TV stations would create a RADAR 101 type tutorial for the general public.

Michael said...

Prof. Mass any news yet on snowpack for this year? A lot of late winter/early spring snow this year...

WeatherNerd said...

I'm not the professor :) but I recently ran across some info to answer your question. As of March 15th, so 10 days ago, the Central Cascades were between 70% and 90% of their normal snowpack. They have been hit particularly hard the past week so they should be closer to 100% I would suspect. The South Cascades are running around 110%. Finally, the North Cascades are running the furthest behind at around 70%. With the current forecast all those values will be creeping up through the end of next week.

HarrisonCZ7 said...

Thanks Gary...good stuff!

3PM update -- 44F sunshine and some cumulus in Spokane. Brisk northerly wind at 09G18 HR

Joe said...

With two dopper radars scanning the same volume you can get absolute velocities and directions (i.e. a vector field) but aside from storm chasers I'm not sure if this is done regularly in the US).

See, that was the first thing I thought of when reading Cliff's discussion. The (seemingly) obvious thing to do would be to combine the outputs from two (or more) of these DoubleCheeseDoppler radars to get an actually useful (to the lay public) rendering of absolute wind speed and direction. But I suppose that would require the TV stations to work together and somehow split the expense, and then none of them would have an exclusive claim to the technology, so there's no incentive to do it since it would only benefit the public good rather giving them any competitive advantage.

Or we might just discover how bad their radars actually are.