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Radio Science

A fresh look at the radar reflectivity of clouds


  • E. E. Gossard


There is now considerable evidence that long-wavelength radars measure backscatter from clouds better than would be predicted from the classical incoherent Rayleigh scatter theory. This could be explained by substantial coherence (in the Bragg sense) in the scattering process. Such scattering could arise from cloud-enhanced fluctuations in the dielectric constant of the gaseous medium or from significant spatial correlation in the drop-size/drop-number density distribution within clouds. Recent cloud modeling and computer simulation suggest that important sources of variance in both the liquid water concentration and the dielectric fluctuations exist in the thermodynamic cloud processes through their irreversible coupling with the cloud dynamics (Clark and Hall, 1979). In this paper these variances are compared with estimates of the conventional dynamical variance resulting from clear-air turbulence and also with that due to spatial correlation in the droplet distribution. It is concluded that gaseous water vapor fluctuations are about 30 times as important as fluctuations in cloud liquid water if the power (variance) in both spatial spectra are the same. It is found that the variance in water vapor resulting from thermodynamic processes in the cloud is at least comparable with typical water vapor variances generated dynamically in the clear atmosphere. Temperature-humidity covariance resulting from thermodynamic processes in clouds can be negative and may thus lead to significantly enhanced radar reflectivity.

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