Sunday, 10 August 2003
Mixed-phase cloud retrievals using Doppler radar spectra
Doppler radar spectra can, in certain circumstances, be used to estimate the microphysical properties of both phases of mixed-phase clouds. This distinction is possible when the cloud properties and vertical air motion combine to produce a clear bi-modal Doppler velocity spectrum. Under this condition, the Doppler moments of the distinct liquid and ice spectral modes may be computed independently and used to quantitatively derive properties of the liquid droplet and ice particle size distributions. The ability of radars to derive ice properties in mixed-phase clouds is not conceptually new, however, the ability to derive the coincident liquid properties is unprecedented. A mixed-phase cloud case study from the NASA CRYSTAL-FACE experiment in Southern Florida is used to illustrate this new mixed-phase retrieval method. The case of interest occurred on 29 July 2002 when a super-cooled liquid cloud layer (T=-8 C) at 5 km AGL advected over a ground measurement site. High concentrations of Saharan dust (an ice forming nucleus) during this time period activated ice crystal formation from this super-cooled liquid layer. Ground-based measurements from collocated 35- and 93-GHz radars both revealed clear bi-modal Doppler velocity spectra within this cloud layer. Profiles of radar reflectivity were computed independently from the liquid and ice spectral modes of the 35-GHz velocity spectrum. Empirical reflectivity relationships, based on in situ measurements, were then used to derive profiles of both liquid and ice microphysical parameters, such as water content and size. Although the ice crystals extended down to a height of 4 km, the radar measurements were able to distinguish the base of the cloud liquid at 5 km, in good agreement with cloud base measurements from a collocated micropulse lidar. Furthermore, radar-derived cloud liquid water paths were in excellent agreement with liquid water paths derived from a collocated microwave radiometer. These results demonstrate the ability of the radar to both identify and quantify the presence of liquid in some mixed-phase clouds. Retrievals of this nature may be particularly useful for application to frequent mixed-phase clouds observed in the Arctic and to mid-level mixed-phase clouds at mid-latitudes.