The visual graphical representation of weather that you see on computer and TV weather displays starts out as a series of numbers generated from the radar echos obtained when the radar dish sweeps the sky. The most common visual presentation from these numbers is something known as “reflectivity data”. There are several different types of reflectivity data based on what part of the radar scan information is used.
As the radar dish sweeps around the sky, it makes a stepwise tilt up, sampling the atmosphere in layers, resetting eventually and starting the tilt cycle all over again. This happens because the dish focuses the beam, and it can only cover a narrow layer in each sweep. The reflectivity radar display you see on TV and on EWR represents a “stacked” composite of one or more of those tilt layers (indeed, one such full layer presentation is known as “composite reflectivity”).
Other reflectivity presentations may use only the bottom few layers. This is done for two main reasons: one, to speed up the time it takes to obtain the necessary scan data, and secondly, to focus on and illuminate storm structure nearest the ground – the area where tornadoes form. These are the various “base reflectivity” scans. EWR uploads two reflectivity scan sets in the Scan B scans on our main page. The principal display that you first see, is a full composite reflectivity scan, composed of all layers currently being swept by the radar dish. This scan is the slowest to update, because it has to collect all layer levels before data can be sent out.
If you click on any of the Scan B scans, you’ll see the first level Base Reflectivity scan (BR1) (you can distinguish it from the composite scan (CR) on our display by its black background). This scan shows only the first tilt level, and is important because it allows you to see the storm cell structure down where tornadic activity might be. The composite scan obscures this bottom detail by adding information from layers further above. Composite reflectivity scans give you a better sense of the size and overall content of the storm; base reflectivity scans give a better sense of the energy in the bottom layers close to where you live.
In impending severe weather, get into the habit of switching back and forth between the scans. If tornado activity is possible, watch the base reflectivity scan (BR1) particularly – it will present more timely data, and show you the characteristic hook structure a rotating supercell (where the tornadoes usually are) takes on when it’s gearing up to drop tornadoes. The composite scan obscures this detail with rain reflections.
Presenting these images is a bit of a visual art form. The US National Weather Service uses a colour scheme designed not only to show certain features, but to also be visible to those who have colour blindness issues. This compromises, to a degree, the extent to which the display can be used to highlight certain storm features. Our software, as is the case with others, allows us to design colour displays (“palettes”) for specific purposes.
EWR has designed some high quality palettes for winter displays that are capable of giving a good indication of the nature of the precipitation, once some experience is gained in their use, and we’ve correlated the displays with actual weather observations to further fine-tune their accuracy. Winter is tricky. Snow can be difficult to interpret from radar displays because its less reflective, and its buoyancy in the air floats it around, confounding the radar’s ability to discern the cloud structure. However, we’re comfortable with what our winter and mixed precipitation displays can tell us.
The next project will concentrate on summer storm weather. EWR will show a variety of different display palettes over the coming months as I explore how best to squeeze the most useful information out of the data. This data is a constant, and key display elements will not change – red into light purple in the display will still mean severe, potentially dangerous storm conditions (the light purple is an indication of a significant hail core). EWR alerts trigger at 60dBZ (strong echo return to the radar), well in the red zone. Red in the displays triggers at 50dBZ, and in any radar display, indicates that conditions have moved beyond merely strong to possibly severe, so caution is advised.
Most of the palettes changes we will be playing with will be below the 50dBZ level. Active rain begins around the 20 dBZ level, but this is variable and dependent to a degree on local topography and conditions. I’m looking to fine-tune this region in order to provide a clearer picture of whether there is active precipitation or just heavy cloud. This is especially important when its desirable to separate out dangerous storm characteristics from just plain soggy weather.