Charles A. Knight and L. J. Miller
National Center for Atmospheric Research, Boulder Colorado
Bulletin American Meteorological Society, 74, 179-188
In attempting to use centimeter-wavelength radars to investigate the early stage of precipitation formation in clouds, "mantle echoes" are rediscovered and shown to come mostly from scattering by small-scale variations in refractive index, a Bragg kind of scattering mechanism. This limits the usefulness of single-wavelength radar for studies of hydrometeor growth, according to data on summer cumulus clouds in North Dakota, Hawaii, and Florida, to values of reflectivity factor above about 10 dBZe with 10-cm radar, 0 dBZe with 5-cm radar, and -10 dBZe with 3-cm radar. These are limits at or above which the backscattered radar signal from the kinds of clouds observed can be assumed to be almost entirely from hydrometeors or (rarely) other particulate material such as insects. Dual-wave-length radar data can provide the desired information about hydrometeors at very low reflectivity levels if assumptions can be made about the inhomogeneities responsible for the Bragg scattering.
The Bragg scattering signal itself probably will be a useful way to prove inhomogeneities one-half the radar wavelength in scale for studying cloud entrainment and mixing processes. However, this use is possible only before scattering from hydrometeors dominates the radar return.
First-echo studies of clouds for the purpose of investigating precipitation processes are complicated by interference from a Bragg-scattering mechanism of radar return when carried below a sensitivity threshold that is a function of radar wavelength. It appears that conservative, safe limits may be 10 dBZe at 10 cm, 0 dBZe at 5 cm, and -10 dBZe at 3 cm. Dual-wavelength early-echo studies show promise of extending the interpretation of hydrometeor echo to lower reflectivity values using the different wavelength dependence of Rayleigh and Bragg scattering, but some ambiguities remain to be resolved, and more attention to instrumental problems is also needed.
While this may be a relatively minor point, it is worth noting that studies of the placement and evolution of the first precipitation growth within cloud will be immensely facilitated by having the Bragg echo pattern to provide the cloud outline in three dimensions. In some respects, this removes the need for coordinated photography, and in fact it provides much more information than photographs can. The Bragg echo also can provide the scatterers for Doppler studies of small clouds; and again the cloud outline is a valuable context for the deduced wind fields.