• What is Echo Tops?
• What is Vertically Integrated Liquid?

What is Echo Tops?

The Echo Tops product shows how high the precipitation echoes, or reflectivities, extend up into the sky. It is similar to cloud tops but not quite the same since in most cases, the top of the cloud will be somewhat higher than the top of the precipitation echoes. The heights in the radar graphic key are in thousands of feet above mean sea level, not above the local ground level. Heights are measured for precipitation echoes with reflectivities of 18.5 dBZ or higher. The lowest detectable tops are those at 5,000 feet. The highest detectable tops are heights of 70,000 feet. This product is extremely valuable for aviation users and it also carries information about the nature of individual thunderstorms and their potential for producing severe weather. For example, echo tops information can be useful in identifying a region of strong updrafts. The higher the echo tops, generally, the stronger the updrafts within the thunderstorm. In addition, severe weather events often coincide with the collapse of the top of the echo. The onset of severe weather, however, cannot be judged by the changes in the tops of the echoes alone. Severe weather can only be detected by use of all the NEXRAD products together in conjunction with other weather information. Also, the beginning of thunderstorm development can sometimes be seen as echo tops appearing at middle atmospheric levels before appearing as low-level reflectivities. The echo top heights will be misrepresented if the true top of the reflectivity echo is above the height of the highest elevation angle scan. The highest elevation angle scan is 19.5 degrees when the radar is in precipitation mode. At this angle, the radar beam is detecting reflectivities no higher than 56,000 feet at a distance of 30 miles from the radar. If the true echo top at a location 30 miles from the radar site is higher than this, it will not be detected, rather an echo top of 56,000 feet will be displayed at this point. When the radar is in clear air mode, the highest elevation angle scan is 4.5 degrees above the local horizon. (See Base Reflectivity for the range height table for this elevation angle.) The NEXRAD Doppler radars never scan directly overhead, so they never detect the true top of the reflectivity echoes directly over the radar site.

What is Vertically Integrated Liquid?

The Vertically Integrated Liquid product shows the amount of liquid water contained in a vertical column over each point on the display. This data is measured in kilograms per square meter, a measure of volume. The computer programs that create the Vertically Integrated Liquid (VL) product use as their input the reflectivity data gathered from all the elevation angles used during the volume scan (as is displayed in the four tilts of the base reflectivity product and the composite reflectivity product). The computer programs assume that all reflectivities are from liquid water and then use equations to convert the reflectivities to liquid water content. Hail has unusually high reflectivities (much higher than the largest raindrops) which can cause this product to overestimate the amount of liquid water actually contained in the clouds. For this reason, very high VL values in thunderstorms are a good indication that hail may be occurring. VL data is useful in distinguishing thunderstorms from rain showers. The thunderstorms will tend to have relatively higher VL values, although the threshold VL value for the difference between a rain shower and a thunderstorm will differ from one location to another, due to differences in atmospheric conditions and climate. Also as thunderstorm development progresses, relative VL values can help differentiate between strong thunderstorms that may be severe and thunderstorms that are not likely to be severe. However, VL data must be used in conjunction with the other NEXRAD products and weather data other than radar data to determine the likelihood that a particular thunderstorm may produce severe weather. Furthermore, the relationship of VL to the occurrence of severe weather is different and needs to be defined separately for different atmospheric conditions and climate regimes. The correlation that can be made using VL data is that the precipitation areas with the highest VL data levels are the strongest echoes within the radar's coverage area. For a line of thunderstorms (a squall line) examination of the VL values can help determine which storms in the squall line are the strongest and have the highest potential for producing severe weather.