Cloud Liquid Water and Radar Reflectivity of Nonprecipitating Cumulus Clouds

View More View Less
  • 1 National Center for Atmospheric Research, Boulder, Colorado
© Get Permissions
Full access

Abstract

Measurements of cloud droplet size spectra from a forward scattering spectrometer probe (FSSP) in developing cumulus show an excellent correlation between the calculated radar reflectivity factor and cloud liquid water content at constant altitude, if the data are restricted to the early stage of cumulus development before coalescence growth becomes significant. The correlation is examined using droplet data from the FSSP both with and without the correction for the nonuniformities in the laser beam field of the FSSP. The close correlation between liquid water content and reflectivity factor suggests that sensitive, relatively low wavelength radar can be useful for mapping liquid water content in the early stages of cumulus development. The analysis reveals several ambiguities that require further study, including the altitude-dependent parameter in the empirical correlation, the value of which appears anomalous. The relation between liquid water content and reflectivity factor provides indirect evidence that aircraft samples collected over 100-m distances represent averages of nonuniform mixtures of cloud and entrained air, and the observed relation can be explained in terms of a simple model of a mixed region that is locally homogeneous but nonuniformly mixed over 100-m aircraft sampling distances. An “equivalent mixed interface” 50–100 m long, with linearly increasing mixing proportions, can reproduce the observed liquid water-reflectivity factor relationship.

Abstract

Measurements of cloud droplet size spectra from a forward scattering spectrometer probe (FSSP) in developing cumulus show an excellent correlation between the calculated radar reflectivity factor and cloud liquid water content at constant altitude, if the data are restricted to the early stage of cumulus development before coalescence growth becomes significant. The correlation is examined using droplet data from the FSSP both with and without the correction for the nonuniformities in the laser beam field of the FSSP. The close correlation between liquid water content and reflectivity factor suggests that sensitive, relatively low wavelength radar can be useful for mapping liquid water content in the early stages of cumulus development. The analysis reveals several ambiguities that require further study, including the altitude-dependent parameter in the empirical correlation, the value of which appears anomalous. The relation between liquid water content and reflectivity factor provides indirect evidence that aircraft samples collected over 100-m distances represent averages of nonuniform mixtures of cloud and entrained air, and the observed relation can be explained in terms of a simple model of a mixed region that is locally homogeneous but nonuniformly mixed over 100-m aircraft sampling distances. An “equivalent mixed interface” 50–100 m long, with linearly increasing mixing proportions, can reproduce the observed liquid water-reflectivity factor relationship.

Save