Some large-scale precipitating systems begin as combinations of a number of convective elements. The resulting system, although still feeding on unstable air, takes on a character much different from typical cumulus-scale convection. Regions of strong convection and heavy showers can become randomly distributed within a larger area of developing stratiform precipitation, or the strong convection can be limited to the large systems leading edge with the rest of the system primarily composed of stratiform rain. When organized in a linear fashion, the convective cells are typically distributed along a band about 20 km (11 nmi) wide and hundreds of kilometers long. The bands are usually related to low-level convergence and wind shear, but the Earths topography also affects the structure of the rain areas. The characteristics of precipitation bands have been categorized, but it is not completely understood why precipitation has the strong tendency to become organized into the characteristic scales and patterns that are observed.

Precipitation areas can be grouped into categories of size and lifetime. Observations show that synoptic areas that are larger than 29,000 nmi2 have lifetimes of one day or longer; large mesoscale areas that range from 2,900 to 29,000 nmi2 last several hours; small mesoscale areas that cover 54 to 216 nmi2 last about an hour; and elements that are about 5.4 nmi2 in size usually last no longer than half an hour. Although the smallest elements have the highest rain rates, the major contribution to the total rainfall over large areas comes from the small and large mesoscale features.

Refer to part B of the FMH-11 for further discussion of mesoscale convective systems.

In stratiform precipitation where lower portions of the echo are at above freezing temperatures, a thin layer of relatively high reflectivity (bright band) is often observed just below the level of 0C. As snowflakes descend into this layer, they begin to melt and stick together. The radar reflectivity of the large wet snowflakes is higher, principally because of their large size and because the dielectric constant of water exceeds that of ice by a factor of five. Descending further below the bright band, the snowflakes become more compact, break up, and become raindrops. The raindrops fall faster than snowflakes so their concentration in space is diminished. This decrease in size and number density of hydrometers accounts for the lower reflectivity just below the bright band.

The last section of this chapter will deal with the Next Generation Weather Radar (NEXRAD) program.

WEATHER SURVEILLANCE RADAR 1988-DOPPLER (WSR-88D)

LEARNING OBJECTIVES: Recognize criteria for the three groups of alert thresholds. Understand data access procedures for the WSR-88D. Be familiar with the various user functions, as well as archiving procedures. The following discussion will deal with weather radar alert areas and thresholds, the editing and sending of data, and the various user functions of the WSR-88D.

Radar product generators (RPGs) can automatically issue alerts upon detection of user-specified meteorological phenomena. Automated alerts relieve the operator ffom constantly monitoring the Principle User Processor (PUP) for significant meteorological phenomena and allow automatic generation of paired products when a particular phenomena occurs. The forecaster determines various thresholds for meteorological phenomena. The PUP operator then selects which phenomena and associated threshold values to use, based upon local watch/warning criteria.

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