THE INTERPRETATION OF MICROWAVE REFLECTIONS FROM RAINFALL

Abstract

The theory of the display of rainfall patterns on radar oscilloscopes is reviewed, and the origin and nature of distortions in the resulting echo pattern is considered, with emphasis on those distortions caused by range and rainfall attenuation. It is shown that (1) the echo patterns of intense rainstorms faithfully represent the corresponding rainfall patterns only at wavelengths greater than approximately 7 cm, and then only when the range to the storm is greater than approximately twice the maximum depth of the storm; (2) at wavelengths less than about 7 cm, the echo patterns of storms viewed through extensive areas of rainfall may bear little resemblance to the actual structure of those storms; (3) at such wavelengths, the pattern of intense non-symmetrical storms changes markedly with the direction of observation. Distortions due to range attenuation are manifested by (1) a displacement toward the radar of the position of the echo maximum from that of the rainfall maximum, and (2) a packing of the power contours, with greater packing on the near side of the storm. These effects decrease with increasing range, and are negligible when the range to the storm is much greater than its depth. Distortions due to rainfall attenuation take the form of ?shadows? or indentations, which occur radially outward from the radar along the directions of maximum intervening rainfall, as well as the displacement and contour-crowding effects associated with range attenuation. The distortions due to rainfall attenuation increase with increasing magnitude and gradient of rainfall intensity, and with decreasing wavelength; at wavelengths greater than approximately 7 cm they are negligible, except for rare, extremely intense storms.