Vertical air motion and raindrop size distributions in convective systems using mm-wavelength radar observations

This study develops and demonstrates a new method for using a 94-GHz Doppler radar to measure vertical drafts and drop size distributions (DSD) in precipitating clouds. These parameters are of fundamental importance in understanding the dynamical and microphysical processes in clouds. The interaction of the short wavelength radiation (lambda = 3.2 mm) from the 94 GHz radar with the raindrops is described by Mie scattering. Using the resonant nature of the backscattering cross-section as a function of the raindrop size, and the excellent temporal and spatial resolution of the 94-GHz Doppler radar, the inverse problem of retrieving the air motion and DSD from Doppler spectra is well constrained. The retrievals from this technique provide vertical air motions to an accuracy of 0.1 ms-1 with temporal resolutions of 2--4 s, far superior to the accuracy and resolution of any other remote sensing technique.; The power of this technique was demonstrated by observing both convective and stratiform precipitation using the 94-GHz Doppler radar supplemented by observations from a 915-MHz wind profiler and the Miami WSR-88D. For the first time, Doppler vertical air motions in the low-levels of convective storms were observed. Despite the attenuation at 94 GHz, high-resolution structures are revealed and the connection with the microphysics is documented. In convective rain, the interaction of vertical drafts and raindrops was documented for first time. The vertical motion clearly causes horizontal and vertical sorting of the raindrops, thus contributing to the evolution and the DSD and the modification of the precipitation field. In stratiform rain, the small-scale variability of the vertical motion was quantified for the first time. The variance of the vertical motion is a maximum near the surface and decreases with height. The estimated decorrelation time scales of the parameters used to parameterize DSDs in stratiform rain were 3--5 min compared with a decorrelation time of 0.5 min for the vertical velocity near the surface and 5 min at higher altitude. The range and variability of the DSD parameters in stratiform rain do not vary substantially from those observed in convective rain.; The results clearly demonstrate substantial potential for using 94-GHz Doppler radars for precipitation studies. Potential applications using a 94-GHz Doppler radar in airborne operations and for defining the vertical air velocities within 30 m of the surface are discussed.