On the Realism of the Rain Microphysics Representation of a Squall Line in the WRF Model. Part II: Sensitivity Studies on the Rain Drop Size Distributions

Abstract

A comparison between retrieved properties of the rain drop size distributions (DSDs) from multifrequency cloud radar observations and WRF Model results using either the Morrison or the Thompson bulk microphysics scheme is performed in order to evaluate the model’s ability to predict the rain microphysics. This comparison reveals discrepancies in the vertical profile of the rain DSDs for the stratiform region of the squall-line system observed on 12 June 2011 over Oklahoma. Based on numerical sensitivity analyses, this study addresses the bias at the top of the rain layer and the vertical evolution of the DSD properties (i.e., of Dm and N0*). In this way, the Thompson scheme is used to explore the sensitivity to the melting process. Moreover, using the Thompson and Morrison schemes, the sensitivity of the DSD vertical evolution to different breakup and self-collection parameterizations is studied. Results show that the DSDs are strongly dependent on the representation of the melting process in the Thompson scheme. In the Morrison scheme, the simulations with more efficient breakup reproduce the DSD properties with better fidelity. This study highlights how the inaccuracies in simulated Dm and N0* for both microphysics schemes can impact the evaporation rate, which is systematically underestimated in the model.

The sensitivity of intense rainfall to aerosol particle loading – a comparison of bin-resolved microphysics modelling with observations of heavy precipitation from HyMeX IOP7a

“…Radar reflectivity in Fig. 5a is calculated from the results of simulation HymRef for the innermost domain, using the sixth moment of the modelled hydrometeor size distribution to determine the radar reflectivity factor Z dBZ (Planche et al, 2010). Here Z dBZ at 07:50 UTC was selected, as it reflects the predominant orientation and the spatial distribution of the precipitating cells over the northern Cévennes and the Vivarais well.…”

“…Another technique to simulate clouds is the use of socalled bin microphysics schemes. Until now, only a few models have been available to simulate, in a size-resolved manner, the spectra of drops and ice particles from a few micrometres to several millimetres (Geresdi, 1998;Khain et al, 2004;Lynn et al, 2005;Planche et al, 2010) in a 3D mesoscale context due to the often prohibitive computational costs of memory and CPU (central processing units). This detailed methodology, however, allows better insights into the evolution of cloud-specific processes such as phase changes and collisional processes and describes more closely the interactions with the field of water vapour and temperature and its feedback with cloud dynamics.…”

On the Realism of the Rain Microphysics Representation of a Squall Line in the WRF Model. Part I: Evaluation with Multifrequency Cloud Radar Doppler Spectra Observations

“…Section 4 outlines the WRF model set-up, verifies that the general dynamics and thermodynamics conditions are well reproduced in the simulations, and highlight some discrepancies in the rain microphysics properties that will be further investigated in the companion paper, Part II (Planche et al 2019). Results are summarized in Section 5 while the appendix details the radar retrieval principle and its validation.…”

“…In fact, this leads to a bimodal distribution of D m values and, despite an average close to the observations, a significant number of unrealistic D m profiles are present. As it will be further discussed in the companion paper (Planche et al 2019), such limit is indeed used in the Thompson scheme in order to control the size sorting of raindrops.…”

“…More recently, techniques exploiting the non-Rayleigh scattering signatures obtained with high frequency cloud radars have been developed (Kollias et al 2002;Giangrande et al 2012). A novel technique formulated in Tridon andBattaglia (2015) andvalidated in Tridon et al (2017a) shows great potential in deriving profiles of DSDs at high resolution from the combination of radar Doppler spectra obtained at K a (35 GHz) and W-band (94 GHz).…”