A Line of Snow in a Cold Spring Shower

May 2nd, 2013 by Walt Petersen
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Our forecasters at Iowa State, and I believe via the National Weather Service, are calling this a “once in a career storm.” The heavy snow in central Iowareally, almost on top of us right now, is the reason. We ran NPOL all night long with the D3R radar in cold rain that started around 2 a.m. I awoke to some ice pellets this morning around 5 a.m. in my hotel room and then wind-driven rain. The wind has been blowing a steady 25 to 30 mph.

We had some funny things happen with the antenna last night. Wind stopped it a few times in big gusts; it is a big 28-foot “sail” after all, and then we had some miscommunication between the antenna controller and the software that controls scanning. The radar scientist on duty, Dr. Timothy Lang, came up with a good work-around. His solution was to run constant “PPI volumes,” which basically do a full 360 degree sweep at several elevation angles. This enabled us to keep collecting good data and to do it at our sub-3 minute cycle time. We set that time as the outside limit on how frequently we wanted to fully sample the rainfall field around us.

We got the antenna situation repaired this morning, and I was able to go back into alternate rain scan vs. range-height scanning mode with the D3R, so that we sample both the rain field and the structure of the precipitation with height at very high resolution along our ray of disdrometers and rain gauges that stretch in a line toward Iowa City.

May 2, 2013. A radar image similar to what you'd see in a weather report. Precipitation appears in colors ranging from light rain (blue to green) to heavier precipitation (yellow to red). Credit: NPOL radar / NASA

May 2, 2013. A radar image similar to what you’d see in a weather report. Precipitation appears in colors ranging from light rain (blue to green) to heavier precipitation (yellow to red). Credit: NPOL radar / NASA

One thing that is really impressive is the change in precipitation type, and the rapid drop in the height of the freezing level we are seeing as one moves west of the radar. This is illustrated nicely in the dual-polarimetric data that the radar collects. The first image (above) shows an example of the radar reflectivity field that we saw around 15:15 UTC (or 10:15 local time). This variable is typically what your TV Meteorologist shows you on the evening newscast. Notice the band of higher reflectivity just to the west of the radar that indicates heavier rain.

Now the second image (below) shows a variable called the “correlation coefficient” or, RHOHV. This variable takes the signal at both the horizontal and vertical polarizations and computes their correlation at each range sample along all the rays. It is a sensitive measure of the degree to which you have a mixture of liquid and frozen precipitation particles in a given sample volume.

This radar image combines two measurements to show what the mixture of liquid and ice is in the precipitation. Bright pink is liquid rain. Yellow to red shows that snow and ice are mixed with the rain. The line of this rain-snow mix stretches from the southwest (bottom left) to the northeast (top right) of the NPOL radar. Credit: NPOL radar / NASA

May 2, 2013. This radar image combines two measurements to show what the mixture of liquid and ice is in the precipitation. Bright pink is liquid rain. Yellow to red shows that snow and ice are mixed with the rain. The line of this rain-snow mix stretches from the southwest (bottom left) to the northeast (top right) of the NPOL radar. Credit: NPOL radar / NASA

Raindrops are usually very highly correlated and values will typically exceed 0.98 or so. However, when you get a mixture of snow and rain, the correlation drops rapidly. You can see this rapid drop in correlation to the west of the radar along the narrow line yellowish to orange colors (embedded in the solid pinkish colors of high RHOHV) that extend in a line from southwest of the radar up to the northeast of the radar. That is where the infamous rain-snow line is located in Central Iowa. We await the arrival of that line over NPOLthough it is only progressing eastward very slowly.

Dave Wolff is a radar scientist at NASA Wallops. He monitoring the radar data inside the science trailer on site at Traer, Iowa. Credit: Walt Petersen / NASA

Dave Wolff is a radar scientist at NASA Wallops. He is monitoring the radar data inside the science trailer on site at Traer, Iowa. Credit: Walt Petersen / NASA

Dave Wolff (above), my companion Radar Scientist this week, and also from NASA Wallops, has done a great job of getting some of our NPOL imagery online in real time, so that is a big help to the field operations. It seems that most of the equipment and networking is working- a testament to the hard work several of these folks are putting in.

I must say though, right now I am most anxiously awaiting a hot cup of coffee which one of our Radar Engineers has graciously offered to grab for me on his lunch run.

From May 1 to June 15, NASA and Iowa Flood Center scientists from the University of Iowa will measure rainfall in eastern Iowa with ground instruments and satellites as part of a field campaign called Iowa Flood Studies (IFloodS). They will evaluate the accuracy of flood forecasting models and precipitation measurements from space with data they collect. Walt Petersen, a scientist based at NASA’s Wallops Flight Facility, is the Ground Validation Scientist for the Global Precipitation Measurement (GPM) mission.

6 Responses to “A Line of Snow in a Cold Spring Shower”

  1. Dorian Janney says:

    This is really interesting stuff- fun to “be with you” as you explain the ins and outs of data collection. Thanks for making time to write this up!

  2. Jim in IA says:

    I am enjoying this series of posts. I learned something about the dual polarization radar and correlation coef.

    Thanks for the posts… Jim

    • Richard Ma says:

      Nice post series.

      The most interested part I learnt from these post is the determination of mixture of raindrop-showflakes with correlation coefficient among the particles. But how it works? multiple-particales comparisons?

      • Walt Petersen says:

        Richard,
        The correlation coefficient that the radar measures is essentially a correlation between the horizontally polarized and vertically polarized returns. For a collection of uniformly-shaped rain drops, you can imagine that the correlation between the two returns will almost always be very close to one (in fact, it is typically 0.98 or better). However, if you now through a mixture of particle types, sizes and shapes into the pulse volume, the correlation between the two returns decreases because the scattering at the two polarizations gets more complex when interacting with different particle shapes and phases (e.g., liquid water….frozen/snow etc.). So, what we see is a marked decrease in the correlation coefficient where the mixture occurs. The decrease is especially pronounced when there are both liquid and frozen particles (e.g., melting snow, or snow mixed with rain, wet hail etc.).

        Hope that helps,
        Walt

  3. Michael says:

    How are the different types of precipitate detected ? Do they reflect different wave-length differently ? Is this the result of a fourier-analysis ?
    And what are the artefacts at North-West by West on the map ?
    sincerely Michael

  4. Walt Petersen says:

    Hi Folks- thanks for the nice comments and the questions. Let’s see If I can address Michael’s question.
    1. Detecting precip: Each pulse of microwave radiation we send out is scattered by the volume of rain, snow or ice we are looking at. The “volume” of each pulse is determined by the beam width of the radar (about 1 Degree) adn the lenght of the pulse we transmit (about 120 meters). At a range of roughly 60 km, the pulse volume is out 1000 m wide by 120 m deep. So, there are huge numbers of precpitation particles in that single volume. In order to make sure we have a really good representation of the scattering from all those precipitation particles in the volume, we then collect about 60 different pulses from that one volume, and then repeat that for every pulse volume along an individual ray out to about 150 km from the radar. The radiation scattered back to the radar by the precipitation in that volume does indeed “reflect” differently depending on wavelength- and we use that information to say something more specific about both the size and type of precipitation. The processing of the data does sometimes invovle Fourier Transforms, but we also use a technique based on Autocorrelation (Pulse Pair Processing). You can google those for more info (too much to include in here).

    The “artifacts” to the north and west that I *think* you are referring to are missing regions where the low-level beams are blocked by a clump of trees located northwest of the radar. We just can’t see through them…………………..

    Hope that addresses your questions.
    Walt P.

Notes from the Field