Turbulent enhancement of radar reflectivity factor for polydisperse cloud droplets
<p>The radar reflectivity factor is important for estimating cloud microphysical properties; thus, in this study, we determine the quantitative influence of microscale turbulent clustering of polydisperse droplets on the radar reflectivity factor. The theoretical solution for particulate Bragg...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/19/1785/2019/acp-19-1785-2019.pdf |
| Summary: | <p>The radar reflectivity factor is important for estimating cloud
microphysical properties; thus, in this study, we determine the quantitative
influence of microscale turbulent clustering of polydisperse droplets on the
radar reflectivity factor. The theoretical solution for particulate Bragg
scattering is obtained without assuming monodisperse droplet sizes. The
scattering intensity is given by an integral function including the cross
spectrum of number density fluctuations for two different droplet sizes. We
calculate the cross spectrum based on turbulent clustering data, which are
obtained by the direct numerical simulation (DNS) of particle-laden
homogeneous isotropic turbulence. The results show that the coherence of the
cross spectrum is close to unity for small wave numbers and decreases almost
exponentially with increasing wave number. This decreasing trend is dependent
on the combination of Stokes numbers. A critical wave number is introduced to
characterize the exponential decrease of the coherence and parameterized using
the Stokes number difference. Comparison with DNS results confirms that the
proposed model can reproduce the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>r</mi><mi mathvariant="normal">p</mi><mn mathvariant="normal">3</mn></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="12pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="2faf24fba3b2758095fe0eaeb23475a2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-1785-2019-ie00001.svg" width="12pt" height="17pt" src="acp-19-1785-2019-ie00001.png"/></svg:svg></span></span>-weighted power spectrum, which
is proportional to the clustering influence on the radar reflectivity factor
to a sufficiently high accuracy. Furthermore, the proposed model is extended
to incorporate the gravitational settling influence by modifying the critical
wave number based on the analytical equation derived for the bidisperse radial
distribution function. The estimate of the modified model also shows good
agreement with the DNS results for the case with gravitational droplet
settling. The model is then applied to high-resolution cloud-simulation data
obtained from a spectral-bin cloud simulation. The result shows that the
influence of turbulent clustering can be significant inside turbulent clouds.
The large influence is observed at the near-top of the clouds, where the
liquid water content and the energy dissipation rate are sufficiently large.</p> |
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| ISSN: | 1680-7316 1680-7324 |