With the high sensitivity of the WSR-88D, it is possible to obtain reflectivity estimates of elevated clouds, which generally reflect at levels between minus 12 to plus 5 dBZe.

The depth of cloud layers maybe inferred from two processes. A four-panel display of the Reflectivity product for successively higher elevation scans can provide information on the depth as well as the structure of a layer. The top of a cloud layer and its depth can also be inferred from a Reflectivity Cross Section product, depending on the distance from the radar and the viewing angle. However, resolution is better with the Reflectivity product than the Cross Section product since the cross section integrates returns from the surface to 70,000 ft.

The following is a technique that will allow the determination of the top, base, and depth of a cloud layer. Using the Reflectivity product for the elevation that intersects the cloud layer, place the cursor at the point where the base appears to be and get a readout on the screen of azimuth, range, elevation, and height. Then, place the cursor at the apparent top of the cloud layer and get the same information. The depth can be computed from the difference in heights. If the cloud base or top is not uniform, this technique will have to be repeated several times to get average heights and thicknesses. This will also help ensure that the true radar cloud top and base are being observed. An increasing moisture source and gravity waves in the velocity field can also be detected. Gravity waves should coincide with the alternating increases and decreases in reflectivity. This may also be apparent in the Echo Tops product if the minimum reflectivity threshold of 18 dBZe is met.

In the Clear Air mode, cloud detection maybe best using VCP-31 due to a better signal-to-noise ratio. If the minimum reflectivity threshold of 18 dBZ is met, the Echo Tops product can provide an indication of the top of a cloud layer. Layer Composite Reflectivity products may show returns at given layers, indicative of clouds. These products should be supported with the use of the Reflectivity product to determine the existence and extent of the cloud layers. Stratus clouds and fog will usually not be detected by the radar due to small cloud particle size. The Velocity Azimuth Display Wind Profile product may be useful in determining moisture advection for development of cloud layers, depending on distance from the radar. For further discussion of preconvective, convective, multicell and supercell development as well as severe storm identification, refer to part D of the FMH-11.

THE STRUCTURE OF LARGE-SCALE PRECIPITATING WEATHER SYSTEMS

The character of precipitation is largely controlled by vertical air motions. Radar observations of the aerial extent, and lifetime of precipitation systems are evidence of the physical processes at work in the atmosphere. Depending on the dominant mechanism responsible for the vertical air motion, precipitation is usually classified into one of these two types:

1. Stratiform Widespread, continuous precipitation produced by large-scale ascent due to frontal or topographical lifting or large-scale horizontal air convergence caused by other means.

2. Convective Localized, rapidly changing, showery precipitation produced by cumulus-scale convection in unstable air.

The distinction between stratiform and convective precipitation is not always clear in practice. Widespread precipitation, for example, is often accompanied by fine-scale structures, or embedded convective elements. In fact, precipitation systems generally are composed of a wide spectrum of scales and intensities. Nevertheless, it is usually possible to classify precipitation patterns by their dominant scale.

Stratiform precipitation is most often produced in nimbostratus clouds or dissipating cumulus clouds. Upward air motions are weak and the vertical structure of the reflectivity pattern is closely related to the precipitation patterns by their dominant scale. The presence of stratiform precipitation facilitates wind measurements with the velocity azimuth display (VAD) to much higher altitudes than possible in clear air (part B of the FMH-11). The wind profiles observed during precipitation may be useful in determining the nature of fronts. A layer of warm air advection (veering of wind with height) is evident from the "S" pattern near the surface (at close slant ranges). Cold air advection (backing with height) is present at greater altitudes (far slant ranges). Generally, the reflectivity pattern depicts stratiform precipitation without prominent small-scale features.

On some occasions, mesoscale precipitation bands form within the stratiform precipitation area. The band is just ahead of and parallel to the wind shift line marking the location of the cold front. Other bands have different orientations with respect to cold fronts. Bands also occur in the vicinity of warm fronts and in precipitation areas away from frontal locations, particularly in the warm sector of a large-scale system.

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