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A statistical approach to evaluate the parametrisation of turbulence in convection-permitting models using radar-retrieved eddy dissipation rates

Feist, M. (2019) A statistical approach to evaluate the parametrisation of turbulence in convection-permitting models using radar-retrieved eddy dissipation rates. PhD thesis, University of Reading

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Abstract/Summary

The effects of turbulence on the evolution of convective clouds remains uncertain both in observations and in numerical weather prediction (NWP) models. Turbulent processes remain parametrised in convection-permitting models (CPMs), and simulated clouds remain highly sensitive to the configuration of sub-filter turbulence schemes. It remains unclear whether assumptions implicit in these schemes are valid for CPMs, indicating the need for thorough evaluation of their performance using observations; the primary aim of this thesis. Eddy dissipation rates ε, are retrieved in radar data by applying a comprehensive method to infer the turbulent component of the Doppler spectrum variance. Hydrometeor fall-speed variances are shown to be negligible when sampling at elevations lower than 11.5°. Shear is calculated directly by applying a linear velocity model to Doppler velocities. New equations are presented to account for variance from azimuthal shear – an unseen dimension in rangeheight scans. Resulting values of ε are insensitive to the scale over which shear is calculated. A thorough statistical analysis of ε in observed clouds suitable for model evaluation is presented for the first time. Retrievals of ε were analysed for two contrasting case studies; shallow “shower” clouds and more vigorous “deep” clouds. Values of ε range from 10−3 − 10−1 m2 s −3 in shower clouds and from 10−3 − 1 m2 s −3 in deep clouds. Turbulent intensity increases with height in deep clouds while remaining constant in shower clouds. In both cases, significant positive correlations are demonstrated between ε and many cloud characteristics. The strongest correlations are found between the velocity and horizontal shear in updrafts. Coherent features of ε are found to have typical spatial scales of 0.5 – 1 km. Results are compared with equivalent statistics derived in 100-m and 55-m grid-length Met Office Unified Model simulations of the observed cases to evaluate the SmagorinskyLilly sub-grid mixing scheme. Simulated turbulence is characterised by small, intense regions of ε more strongly co-located with shear around updrafts than observed. The 95th and 99th percentiles of model ε are one and two orders of magnitude larger than observations, respectively, with similar median values. Values of ε increase consistently with the mixing length and appear insensitive to grid-length suggesting 100-m was sufficient to resolve an inertial sub-range.

Item Type:Thesis (PhD)
Thesis Supervisor:Westbrook, C.
Thesis/Report Department:School of Mathematical, Physical and Computational Sciences
Identification Number/DOI:
Divisions:Faculty of Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
ID Code:85241
Date on Title Page:2018

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