From each major cell is drawn a horizontal line, one end of which touches the cell to identify it. Written on this line is the rain-echo top altitude in hundreds of feet, and any additional information such as if hail is suspected.
For example, the cell near the top center of this figure is moving toward the northeast at 7 knots, has a echo top at 45, feet Fig H 7.
Radar Summary Chart for Arkansas, for a time roughly 1. Broad regions of low radar reflectivity indicate widespread drizzle from stratiform clouds. These often form in advance of warm fronts and north of low pressure centers. This differs from thunderstorms, which often form along cold fronts.
An example of a region of light rain from probably stratiform clouds is shown in Fig H 8 , near Texarkana in the southwestern portion of Arkansas. Note the smoothly curved lower-left edge of this reflectivity area. This is not the actual cloud edge, but just the limit of range that is visible from the Little Rock radar. So the widespread area of drizzle probably spreads further southwest, even though we can't see it from this one radar. Widespread regions of stratiform clouds and drizzle or snow are more common in winter over N.
Fig H 8. Large region of drizzle and light rain, probably falling from a deck of nimbostratus clouds. These are not thunderstorms. When the radar reflectivity data is viewed as a vertical slice through the storm [namely, as a Range-Height Indicator RHI display] the result is as shown in Fig H 9. In this figure, range along the bottom axis is in units of nautical miles, and height along the vertical axis is in units of thousands of feet.
Thus, the vertical scale is greatly stretched compared to the horizontal scale. Namely, the vertical distance between each grid line is about 1. In this figure, orange and red colours indicate the heaviest precipitation. The tan region just above the ground is below the lowest radar scan; namely, there is no data there. So there might be heavy precipitation there under the storm even though it was not scanned.
Fig H 9. RHI scan through a thunderstorm, showing reflectivity. Remember that the radar see through the storm to see what is happening inside. If the same storm was viewed with the naked eye, the result might look as sketched in Fig H Namely, this particular RHI display is set up to show only the moderate to heavy rain, and does not show the small cloud droplets and ice crystals that make up the rest of the cloud that we would have seen by eye.
Fig H Sketch of thunderstorm outline as might have been viewed by eye. Hurricanes consist of spiral bands of thunderstorms that converge into an intense ring of thunderstorms called the eye wall, just outside of the clear eye of the storm.
A negative dBZ means that the radar is detecting very small hydrometeors. As mentioned above, this is great way for forecasters to detect very dry light snow or drizzle which have lower reflectivities. It may also be useful to detect outflow boundaries and drylines. One of the disadvantages of clear air mode is that any dBZ value under 10 gets filtered by the datalink weather. However, the XM weather image shown below only includes the returns that are greater than 10 dBZ.
The areas shown in the taupe color have been effectively filtered out due to their lower dBZ values. Scott Dennstaedt. Weather Systems Engineer. All Release notes EZNews. Recent Posts See All. A closer look at forecasts for SLD. Post not marked as liked. November EZNews. Use the closest airport with a TAF, is that a good idea?
The question that needs answering is what values to associate with what colors. Because data are digital, the computer can use any combination of colors and image values. Typically "hot" colors are used for higher values while "cool" colors are used for lower values. The color bars shown at the left side of Figure 3 and at the bottom of Figure 5 are the standard National Weather Service color scale. All NWS radar products use the same color coding.
For example, red represents 50 dBz reflectivity or higher, which means thunderstorms. Blue starts at 15 dBz which is considered the lower limit associated with liquid precipitation falling to the Earth's surface. The digital nature of these data means that each sample volume has a numerical value that can be color-coded by a computer as the radar operator wishes.
The implication of this option is that unless a color bar is included with a radar image, it is very difficult to determine the reflectivity values of what is being shown.
The bottom line is: All radar displays are not created equal. This is only guaranteed for NWS radar displays. NWS radar displays do not. Basic radar data do not indicate precipitation type, only reflectivity. The modification of the basic data to display precipitation type is another algorithm.
Details of these algorithms are not common knowledge, but they likely combine basic radar data with surface observations and computer forecast model precipitation type forecasts to show the likely precipitation type. It should be remembered that this type of display is an educated guess at precipitation type and not an observation. During the winter some radar displays show precipitation type and some do not.
When viewing a radar display, be aware of this. Although a thorough discussion of radar interpretation is beyond the scope of this web page, there are a couple of things that can be described. Intensity : The higher the reflectivity detected by the radar, the stronger is the intensity of the precipitation.
The following table lists the dBz values and their corresponding intrepretation:. Showers versus Rain : Pattern recognition is a big part of radar interpretation. For example, showers and thunderstorms tend to be relatively small, quasi-circular areas, 10 to 30 miles across, of relatively high dBz value.
The cluster just to the left of center in Figure 3 and the cluster in Figure 6 are examples of thunderstorms. Rain is more widespread and fairly uniform in intensity. There are some occasions when showers and thunderstorms are embedded within a wider area of rain.
Figure 5 has a broad area of moderate rain over Kentucky while a line of thunderstorms extends along the south edge of the precipitation area. Rotation and other Algorithms : The National Weather Service radar system employs dozens of algorithms to process the three types of data noted earlier into useful information.
These algorithms cover a wide variety of features from storm tracking, to storm rotation, to one- and three-hour precipitation estimates. A lot of these algorithms are used in determining whether severe weather is present and serve as the basis for severe thunderstorm and tornado warnings issued by the National Weather Service.
The details of these algorithms is beyond the scope of this web page. This web page has described some basic properties of radar displays.
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