14-10] METEOROLOGICAL EFFECTS AT MICROWAVE FREQUENCIES 763 



rainfall rates. They often group themselves in bands of cells along a frontal 

 or squall line, building up and dissipating rapidly, only to build up in new 

 cells. Within a cell it is quite normal to have extremely high rainfall 

 gradients between the dry air just outside the leading edge of the cell and 

 the heavy rain within. This is often accompanied by high vertical shear 

 winds, updrafts predominating on the leading edges of the storm cell and 

 downdrafts in the trailing areas of the cell.^ For this reason, the most 

 dangerous turbulance, insofar as aircraft are concerned, tends to be found 

 where this rainfall gradient is the sharpest. Another distinguishing 

 characteristic of the thunderstorm is the tremendous heights they often 

 reach (as much as forty or fifty thousand feet) during their maturing stage. 

 This type of storm activity can often be identified by radar by tilting the 

 antenna upwards. Because of the preceding factors it is highly desirable 

 for the radar system to be capable of distinguishing varying levels of target 

 returns and to possess sufficient definition of display to make it possible for 

 the observer not only to identify storm cells but also to pinpoint areas 

 where high turbulence is most likely. 



Areas of general rainfall, often associated with warm fronts, do not 

 normally provide such good targets as do thundershowers, since n is so much 

 less owing to the considerably lower rainfall rates and scatterers of 

 generally smaller diameter. These regions likewise cause much less attenua- 

 tion of the microwave energy. At wavelengths of 10 cm these scatterers 

 will often be indiscernible with the exception of small cells of more intense 

 activity, which will sometimes occur within the overall rain region. At 3.2 

 cm this same weather activity will often give an area of snowy appearance 

 on the radar indicator. 



Because of the damage that hail is capable of doing on the aircraft, it is 

 only natural for the pilot to be extremely interested in a method of avoiding 

 this meteorological phenomenon through use of weather radar. Unfor- 

 tunately, although hail normally gives good radar returns thanks to the 

 large diameters T> of its scatterers as already noted, it is always associated 

 with storm activity which likewise gives strong returns, thus no accurate 

 differentiation is possible on a back-scatter basis. There is some evidence, 

 however, collected during an evaluation of airborne radar by United 

 Airlines,^ to indicate that hail is often accompanied by a distinctive hook 

 or finger extending out of the displayed parent storm cell. Sometimes the 

 display exhibits a scalloped edge. The utilization of such radar pictures for 

 hail avoidance would obviously require a great deal of operator "feel" for 

 interpreting the display. 



■'R. F. Jones, Radar Echoes from and Turbulence within Cumulus and Cumulonimbus Clouds, 

 Prof. Notes 109, Meteorological Office, Air Ministry, London, 1954. 



SH. T. Harrison and E. A. Post, Evaluation of C-Band {5.5-cm) Airborne Weather Radar, 

 United Airlines, Inc., Denver, Colorado, 1954. 



