You are currently browsing the monthly archive for March 2012.

Hopefully the weather will behave this week – this puts the GH-GTA severe weather alert system off air until they return to service.

NOUS61 KBUF 261037

636 AM EST MON MAR 26 2012

03/26/2012 1036 UTC





I’ve increased the size and coverage area of the Scan B displays on the Ephemerata Weather Radar main page. Coverage is still dependent on the range of the four radar sites I use for the southern Ontario sweeps, as does the particular VCP mode used (VCP modes are a set of sweep algorithms the radar dish follows, along with specific processing, in order to achieve certain information and display characteristics. The interpretation notes on the main page talk about VCPs).

Previously, the B scans displayed the Composite Reflectivity sweep as the native display, and if you clicked on the display, the display switched to the Base Reflectivity sweep.

Update: The step-through click sequence for the available uploaded products has been replaced with a tablular selection array for each product EWR uploads (dependent on weather). This makes it easier to see what products are currently being uploaded, and to select the panels of interest. At a glance, you can see what uploads are active, and which are not. Additionally, several products may be available on an ad-hoc basis from the new dual-polarization capability of the radar system. These new products (Correlation Coefficient, Differential Reflectivity and Specific Diferential Phase) are extremely technical in nature, and are intended to be analysed in context with other scan products. I am including them in the new product arrays for those who are interested, but they will only be available when I am interrogating specific storms.

Rather than clicking through, the click-returns now take you back to the arrays, or to the relevant main EWR page. This new arrangement also works better on tablets too.

Composite Reflectivity (CR) – As its name suggests this is a composite of all reflectivity layers from the bottom scan to the top scan, and shows the total content of the complete sweep cycle. It is derived at the last scan, which can take several minutes to reach, therefore Composite Reflectivity is always a few minutes behind Base Reflectivity. CR is the general view that shows you how much water is distributed in the cloud deck.

Base Reflectivity (BR1)
– As its name suggests, Base Reflectivity shows the radar reflectivity in a single layer. Usually, the choice is for the 5 degree tilt, the first layer (BR1), as it looks at the cloud structure nearest to the ground. This is the choice for visualizing storm structure in the search for rotation and tornadoes. Composite reflectivity will hide these structures under the higher layers. If weather reports indicates the possibility of supercell formation with tornadic activity, this is the reflectivity scan to choose. BR1 comes up on the first click of the main page B scan image. This scan is the most current from the radar unit as the radar computers send out this data at the beginning of the scan set.
When the HCA product (discussed below) is turned on, the BR1 screen will show the circular “melting layer” rings (see the dual-polarization echo type classification discussion further down the page for a discussion of melting layer).

Storm Relative Velocity (SRV1) – Click on the BR1 image and you are taken to the Storm Relative Velocity screen SRV1 scan (click again and you are taken to the Base Velocity BV1 screen). Weather radars not only capture the reflected radar beam to tell you how much water is in the air, they can tell you how fast the water is moving in the air, through the use of Doppler time measurement (google it!). There are two velocity parameters, Base Velocity, and Storm Relative Velocity. Base velocity measures water movement velocity relative to the radar site itself, and will tell you something about the speed and direction of wind as felt on the ground.

Storm Relative Velocity measures water movement within the storm, relative to the base velocity, not the ground. What this allows, is the visualization of rotation within a supercell, and will detect the motion and speed of tornadoes forming at the base of a supercell (supercells, by definition, contain rotating updraft columns, or “mesocyclones”). A potential tornado shows up as a close coupling of two colours from opposite ends of the scale, indicating winds moving in different directions very close together, as what happens in a tornado. The stronger and more tightly defined the couplet, the higher the probability that a tornado is forming at the layer level the radar sees. This doesn’t necessarily mean that a tornado has reached ground, only that very tight rotation is occurring at the cloud level visualized, from which a tornado may descend. Base Reflectivity BR1 and Storm Relative Velocity SRV1 are used together to interrogate a storm for tornadic possibility.

Dual polarization Radar Echo Type Classification (HCA mode) – Click on the Storm Relative Velocity image (or click on continue if no image is present) and you’ll move to a new screen, the Dual polarization Radar Echo Type Classification screen for the selected radar site. Recent technological advances to the weather radar hardware now incorporate the ability of the radar sweep scan to sweep a vertically polarized beam as it sweeps horizontally around the sky. This “dual-polarization” echo signal, when coupled with sophisticated computer algorithms, allows, amongs other things, an estimate of the type of precipitation or radar target, that the radar site is seeing. Ordinary single polarization radar can’t distinguish rain from snow, for example. EWR is showing the “HCA” mode, which attempts to distinguish the nature of the echo target, and report it as a colour coded display according to a set of possible targets. Dual-pol, as its known for short, can do many things, including provide more sophisticated imaging of tornadic cells. This is leading-edge weather radar technology and is in its infancy still. Interpretation of the more sophisticated uses of dual-pol is beyond the scope of EWR’s presentations at the present time. Of the radar sites we monitor, all are dual-polarization capable except for KDTX, Detroit, which therefore does not have the HCA display in the sequence. As with the other sequenced displays, for bandwidth considerations, HCA is invoked when the weather conditions are appropriate to include it. It won’t necessarily be available at all times.
Mousing over the legend scale below the image will bring up a chart of colour codes and indicators depicting how the radar system is interpreting the radar data.
When turned on, the “melting layer” rings will also be evident. The ring circles indicate the “melting layer” – the vertical zone in the atmosphere where the precipitation changes state from frozen to liquid. The area between the thin yellow lines represents the vertical thickness of the radar beam as it crosses the melting layer – the inner yellow ring is where the top of the beam enters the melting layer, and outer yellow ring where the bottom of the beam exits the melting layer. The thicker white lines represent where the centre of the beam crosses through the melting layer. The widths of these areas between lines gives you an idea of the thickness of the melting layer. The HCA product determines what the state of the precipitation is within the beam. The farther the distance from the radar site the higher the beam is, and why the inner ring state of the HCA product may show one state, and the outer ring state show another (programmed that way). Therefore, beyond the outer white ring, the precipitation may not be, at the ground, what the HCA product indicates higher in the atmosphere.

One Hour and Storm Total Precipitation are derived displays that estimate the amount and rate of rainfall that a storm cell is producing. One Hour rate (OHR) is an estimate of how much rain will fall in one hour, and Storm Total Precipitation (STR or DSP) is the estimated amount of total precipitation during the specific storm event. The STR display is reset by the radar operators to reflect discrete storm events. A one hour gap of no precipitation is the standard for resetting Storm Total Precipitation, but in practice the operators set different intervals depending on the duration of storm events.

For reference, the click sequence of product images (as available) is as follows:

Default: Composite Reflectivity (always available, main page)
Base Reflectivity, lowest tilt (always available, first sequence click)
HCA Dual-Polarization display (based on weather, not available for Detroit, at the moment)
Base Velocity, lowest tilt (based on weather)
One Hour Precipitation rate (based on weather)
Storm Total Precipitation (based on weather)

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.


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RSS Last Alert Issued:

  • GH-GTA Scan Zone Severe Weather Alert #ONStorm October 2, 2016
    SEVERE WEATHER ALERT — 01:35 PM EDT Oct 02 2016 This is an automated alert of potentially severe weather for the Golden Horseshoe/ Greater Toronto/Niagara Peninsula/South-Central Ontario Monitored Area, from Ephemerata Weather Radar. See attached scan image. The alert triggered at 01:35 PM EDT on Oct 02 2016, from radar data analyzed from NWS radar site KBUF […]

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