Coherent scattering of microwaves by particles: Evidence from clouds and smoke

Many radar measurements of the atmosphere can be explained in terms of two scattering mechanisms: incoherent scattering from particles, and coherent s cattering from variations in the refractive index of the air, commonly call ed clear-air or Bragg scattering. Spatial variations in the liquid water co ntent of clouds may also give a coherent contribution to the radar return, but it is commonly believed that this coherent scattering from the droplets is insignificant because variations in humidity have a much larger influen ce on the refractive index than equal variations in liquid water content. I t is argued that the fluctuations in water vapor mixing ratio in clouds can be much smaller than those in liquid water mixing ratio. In this article an expression for the strength of the coherent scattering f rom particles will be derived for fluctuations caused by turbulent mixing w ith clean (i. e., particle-free) air, where it will be assumed that the par ticles follow the flow, that is, their inertia is neglected. It will be sho wn that the coherent contribution adds to the incoherent contribution, the latter always being present. The coherent particle scattering can be strong er than the incoherent scattering, especially at longer wavelengths and hig h particle concentrations. Recently published dual-frequency measurements of developing cumulus clouds and smoke show a correlation for which no explanation has been found in te rms of incoherent particle scattering and coherent air scattering. Scatterp lots of the reflectivity factors at both frequencies show a clustering of p oints in between the values that correspond to pure clear-air and pure inco herent scattering. Those differences in the radar reflectivity factors coul d be due to a mixture of Bragg scattering and incoherent particle scatterin g, but then no correlation is expected, because the origin of the scatterin g mechanism that dominates at each wavelength is different. However, coherent scattering from the particles can cause the radar reflect ivities of dual-wavelength radar measurements to become correlated with eac h other. It may explain the slopes and the differences seen in the scatterp lots of the radar reflectivities of cloud and smoke measurements, with reas onable values of the parameters involved. However, the correlation between the radar reflectivities is very tight near the cloud top and seems to be p resent in adiabatic cores as well. This is an indication that, apart from m ixing with environmental air, the inertia of the droplets could also be imp ortant for the creation of small-scale fluctuations in droplet concentratio n.