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 D_m 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 D_m and $N0*$ for both microphysics schemes can impact the evaporation rate, which is systematically underestimated in the model.

This study investigates how multifrequency cloud radar observations can be used to evaluate the representation of rain microphysics in the WRF Model using two bulk microphysics schemes. A squall line observed over Oklahoma on 12 June 2011 is used as a case study. A recently developed retrieval technique combining observations of two vertically pointing cloud radars provides quantitative description of the drop size distribution (DSD) properties of the transition and stratiform regions of the squall-line system. For the first time, the results of this multifrequency cloud radar retrieval are compared to more conventional retrievals from a nearby polarimetric radar, and a supplementary result of this work is that this new methodology provides a much more detailed description of the DSD vertical and temporal variations. While the extent and evolution of the squall line is well reproduced by the model, the 1-h low-reflectivity transition region is not. In the stratiform region, simulations with both schemes are able to reproduce the observed downdraft and the associated significative subsaturation below the melting level, but with a slight overestimation of the relative humidity. Under this subsaturated air, the simulated rain mixing ratio continuously decreases toward the ground, in agreement with the observations. Conversely, the profiles of the mean volume diameter and the concentration parameter of the DSDs are not well reproduced. These discrepancies pinpoint at an issue in the representation of rain microphysics. The companion paper, investigates the sources of the biases in the microphysics processes in the rain layer by performing numerical sensitivity studies.

Simulations of an idealized stratiform cloud system are performed with the 3D cloud-scale model DESCAM-3D (DEtailed SCAvenging Model) using a bin-resolved microphysics scheme for ice, water and aerosol particles. This idealized case corresponds to numerical simulations of moist airflow over a narrow isolated mountain and a gentle slope in order to obtain a stratiform situation where radar bright-band phenomena are often visible. For this dynamical situation the impact of a detailed time-dependent melting scheme has been studied. This scheme allows the calculation of a water/ice ratio within each mass bin for the melting ice crystals. The results obtained with this detailed melting process are compared with corresponding results from simulations involving instantaneous melting at the 0 °C isotherm level. The new detailed melting scheme allows a penetration of the ice phase into the positive temperature areas. Most of the mass of falling ice particles melts over a distance of a few hundred metres. The non-instantaneous melting scheme also produces a deeper layer of latent cooling that slightly modifies the dynamical field. With the detailed melting scheme, a simple backscattering scheme is implemented in order to consider the wetted ice particles and simulate the bright-band phenomenon. It supposes that the backscattering proportions of the wetted ice particles are similar to those of raindrops. The association of the non-instantaneous melting process with this backscattering scheme permits us to reproduce the radar bright-band phenomenon. This study will enable a better understanding of the complex microphysics inside the melting layer and then will help to reduce the radar uncertainties in the precipitation estimation at the ground.

Volcanic emissions can significantly affect the Earth's radiation budget by emitting aerosol particles and gas‐phase species that can result in the new particle formation (NPF). These particles can scatter solar radiation or modify cloud properties, with consequences on health, weather, and climate. To our knowledge, this is the first dedicated study detailing how gas‐phase precursors emitted from volcanic plumes can influence the NPF. A series of airborne measurements were performed around the Etna and Stromboli volcanoes within the framework of the CLerVolc and STRAP projects. The ATR‐42 aircraft was equipped with a range of instrumentation allowing the measurement of particle number concentration in diameter range above 2.5 nm and gaseous species to investigate the aerosol dynamics and the processes governing the NPF and their growth within the volcanic plumes. We demonstrate that NPF occurs within the volcanic plumes in the free troposphere (FT) and boundary layer (BL). Typically, the NPF events were more pronounced in the FT, where the condensational sink was up to two orders of magnitude smaller and the temperature was ~20 °C lower than in the BL. Within the passive volcanic plume, the concentration of sulfur dioxide, sulfuric acid, and N_2.5 were as high as 92 ppbV, 5.65 × 10⁸ and 2.4 × 10⁵ cm⁻³, respectively. Using these measurements, we propose a new parameterization for NPF rate (J_2.5) within the passive volcanic plume in the FT. These results can be incorporated into mesoscale models to better assess the impact of the particle formed by natural processes, that is, volcanic plumes, on climate.