Friday, January 18, 2013

Dual-Polarization coming to Houston!

A big upgrade is happening to the NWS Houston radar. The radar is being upgraded with Dual-Polarization or DP technology. What does this mean? It means the meteorologists at the NWS will be able to determine what type of precipitation is falling to the ground. There is even the possibility of detecting the debris from a tornado.

So how will the radar do this? I will try and not get to technical. Basically a radar works by sending out horizontal pulses of microwaves (yes the same ones you cook with). The pulses hit rain, snow, or debris and are bounced back to the radar. It then produces an image we have all seen on tv, the greens, reds, and yellows representing rain and all that other jazz. Even the tv radars work like this. These types of radars are called "Conventional Radar" (See fig.1).

 A DP radar works by not only sending out a horizontal pulse, but also a vertical pulse of microwaves. This not only tells the meteorologists its raining, but it can also tell them the approximate size, shape, and rate its falling at. Meteorologists will be better able to determine where its snowing and raining.

If you look at fig.2. Its an example of DP data showing the tornado debris signature from the Branson tornado. You will notice that Branson is surrounded by reds and yellows forming a sort of hole around the city. Thats the debris the radar pulses are bouncing off of producing that signature.
fig.2 Branson Tornado

Now there are some limitations to all this great technology of course. That "tornado Debris Signature" or TDS, can really only be detected about 40 or 50 miles from the radar ( I will be speaking with a meteorologist from the NWS soon, so I may have to correct that). Quote from "Dual-Pol Radar Operations Guide", "Since most of the debris cloud will be shallow and occur at a relatively small scale, the tornado usually has to be at close range to produce this signature. EF-1 tornadoes have been detected at ranges as far as 40nm, and EF-3/4 tornadoes out to 60nm. it may be possible to see debris outside of 60nm, but it would be very unlikely."  Remember the tornado MUST hit something to produce a signature!

 I'm thinking of writing another post next week going a little bit deeper into each of the new products DP has to offer. Stay tuned for that!

 While it will be easier to detect snow, it can still be difficult to determine on the radar without knowledge of the surrounding environment. There is still some work to be done, but from what I have seen so far, this is going to be a great upgrade. Fig. 3 is another DP example from the NWS office in New York.

Fig. 3 Radar image of a mixed precipitation weather event in New York. Dual-Pol helps forecasters distinguish between rain, snow, hail, and other objects.
The NWS radar upgrade started on January 10. They are expected to finish on January 21. Here are a few images from the Huntsville office when they upgraded their radar.

All the long tubes you see here (including the long one in the box in the upper left of the photo and the one wrapped in pink bubble wrap) are called waveguides. These tubes will be added to the existing tubes to guide the vertically oriented signal to and from the radar dish.
Technicians turned the radar straight up (called birdbath-ing) so they could climb into the dish and replace some parts. They are seen here inside the dish installing the new feed horn for the radar. The feed horn transmits the signal into the dish, which then bounces it out to the sky surrounding the site.
This is where important changes to the radar signal is made so that it can send out beams in the horizontal and vertical. This is located on the back side of the radar dish.

Here are some helpful links. Click on the WDTB link to learn the technical ins and outs of DP radar. They also have a "Storm of the Month" series going on.

WDTB Dual-Polarization Radar Training:
The WSR-88D dual-polarization upgrade deployment began in September 2011. The Deployment Schedule is available, but it will likely change over time.

Tuesday, January 15, 2013

The Sky Tonight via Sky & Telescope

Jan 15, 2013

In this coldest time of the year, the dim Little Dipper (Ursa Minor) hangs straight down from Polaris after dinnertime.

NOTE: No astronomy photo was posted yesterday due to a family medical emergency. Things should return to normal next week.

Monday, January 14, 2013

The Sky Tonight via Sky & Telescope

Jan 14, 2013

Before moonlight returns in force to wash out low-surface-brightness telescopic sights, try tonight for the Bubble and Pac-Man Nebulae in Cassiopeia — using Ken Hewitt-White's article "Hot Gas in Cass" with finder chart and photos in the January Sky & Telescope, page 65.

Here is the moon at 823am.