Influence of microscale turbulent droplet clustering on radar cloud observations

Keigo Matsuda*, Ryo Onishi, Masaaki Hirahara, Ryoichi Kurose, Keiko Takahashi, Satoru Komori

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

16 Citations (Scopus)

Abstract

This study investigates the influence of microscale turbulent clustering of cloud droplets on the radar reflectivity factor and proposes a new parameterization to account for it. A three-dimensional direct numerical simulation of particle-laden isotropic turbulence is performed to obtain turbulent clustering data. The clustering data are then used to calculate the power spectra of droplet number density fluctuations, which show a dependence on the Taylor microscale-based Reynolds number (Reλ) and the Stokes number (St). First, the Reynolds number dependency of the turbulent clustering influence is investigated for 127 < Reλ < 531. The spectra for this wide range of Reλ values reveal that Reλ = 204 is sufficiently large to be representative of the whole wavenumber range relevant for radar observations of atmospheric clouds. The authors then investigate the Stokes number dependency for Reλ = 204 and propose an empiricalmodel for the turbulent clustering influence assuming power laws for the number density spectrum. For Stokes numbers less than 2, the proposed model can estimate the influence of turbulence on the spectrum with an RMS error less than 1 dB when calculated over the wave number range relevant for radar observations. For larger Stokes number droplets, the model estimate has larger errors, but the influence of turbulence is likely negligible in typical clouds. Applications of the proposed model to two idealized cloud observing scenarios reveal that microscale turbulent clustering can cause a significant error in estimating cloud droplet amounts from radar observations with microwave frequencies less than 13.8 GHz.

Original languageEnglish
Pages (from-to)3569-3582
Number of pages14
JournalJournals of the Atmospheric Sciences
Volume71
Issue number10
DOIs
Publication statusPublished - 2014
Externally publishedYes

Keywords

  • Cloud droplets
  • Cloud microphysics
  • Radars/Radar observations
  • Subgrid-scale processes

ASJC Scopus subject areas

  • Atmospheric Science

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