Radar Meteorology

Radio detection and ranging (radar), developed for aircraft detection during the Second World War, was swiftly applied to tracking precipitation areas from the radar echoes. Radio waves transmitted by an antenna in the cm wavelength range (typically 3 and 10 cm) are back-scattered by raindrops and ice particles, as well as by cloud droplets, particulates, insect swarms and flocks of birds. The return signal and its time delay provide information on the objects in the path of the beam and their direction, distance and altitude. The need to detect tropical rainstorms led to the first training programmes in radar interpretation in 1944. In 1946 to 1947, the Thunderstorm Project led by H. R. Byers used radar to track the growth and organization of thunderstorms in Florida and Ohio. Gradually, indicators of storm severity were devised based on the shape and arrangement of echoes, their vertical extent and the strength of the back scatter measured in decibels (dB). Much of this process is now automated. Specifically designed weather radars for the US Weather Bureau became available only in 1957. In the 1970s the Doppler radar, which uses the frequency shift produced by a moving target to determine the horizontal motion relative to the radar location, began to be used for research on hail and tornadoes. Dual Doppler systems are used to calculate the horizontal wind vector. The Next Generation Weather Radar (NEXRAD) deployed in the 1990s in the United States, and similar systems in Canada and European countries, are modern Doppler instruments. The vertical profile of winds in the atmosphere can be determined with vertically pointing Doppler radar operating in the VHF (30 MHz) to UHF (3 GHz) ranges. The wind velocity is calculated from variations in the clear air refractive index caused by turbulence.

A major application of radar is in estimating precipitation intensity. R. Wexler and J.S. Marshall and colleagues first established a relationship between radar reflectivity and rain rate in 1947. The reflectivity, Z, was found to depend on the droplet concentration (N) times the sixth power of the diameter (D6). The basis of this relationship has recently been questioned. Estimates are generally calibrated with reference to rain gauge measurements.

Reference

Rogers, R.R. and Smith, P.L. (1996) A short history of radar meteorology. In J.R. Fleming (ed.) Historical Essays on Meteorology

1919-1995. American Meteorological Society, Boston, MA, pp. 57-98.

maximum expected precipitation for storms of different duration and frequency in the USA. The maxima are along the Gulf Coast and in Florida.

b Areal extent of a rainstorm

The rainfall totals received in a given time interval depend on the size of the area that is considered. Rainfall averages for a twenty-four-hour storm covering 100,00 km2 may be only one-third to one-tenth of those for a storm over a 25 km2 area. The curvilinear relationship is similar to that for rainfall duration and intensity. Figure 4.13 illustrates the relationship between rain area and frequency of occurrence in Illinois, USA. Here a log-log plot gives a straight line fit. For 100-year, or heavier falls, the storm frequency in this region may be estimated from 0.0011 (area)0896 where the area is in km2.

c Frequency of rainstorms

It is useful to know the average time period within which a rainfall of specified amount or intensity may be expected to occur once. This is termed the recurrence interval or return period. Figure 4.14 gives this type of information for six contrasting stations. From this, it would appear that on average, each twenty years, a twenty-four-hour rainfall of at least 95 mm is likely to occur at Cleveland and 216 mm at Lagos. However, this average return period does not mean that such falls necessarily occur in the twentieth year of a selected period. Indeed, they might occur in the first or not at all! These estimates require long periods of observational data, but the approximately linear relationships shown by such graphs are of great practical significance for the design of flood-control systems.

Studies of rainstorm events have been carried out in many different climatic areas. An example for

Hilo Rainstorm
Figure 4.10 Relation between rainfall intensity and duration for Milwaukee, USA, during three months in 1973. Source: US Environmental Data Service (1974). Courtesy US Environmental Data Service.

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