Rodolfo G. Pereyra

A laboratory study of graupel charging

Measurements have been made of charge transfer when vapor grown ice crystals rebound from a riming target representing a graupel pellet falling in a thunderstorm. Earlier studies in the laboratory in Cordoba of charge transfer between an individual falling ice sphere and a riming target noted that the sign of the charge transfer was dependent upon temperature and effective liquid water content (EW). The new work uses a similar experimental technique; however, a cloud of ice crystals is grown in order to study multiple interactions with the riming target. The results also show charge sign dependence on temperature and EW; positive rimer charging is observed at high temperatures and for low and high values of EW at low temperature, while negative rimer charging is noted at low temperatures for intermediate values of EW. These results are similar to those obtained by Takahashi (1978) and, as has been reported before, are rather different from those obtained in Manchester by Jayaratne et al. (1983), Saunders et al. (1991), and Saunders and Peck (1998). Significant differences between the two types of data sets are attributed to the experimental techniques used in the various studies. In the present work the ice crystal cloud and the cloud of supercooled droplets responsible for riming the target are grown in separate chambers and then mixed shortly before the crystals and droplets encounter the riming target, so that the droplet cloud is not depleted by the growing ice crystals. In the Manchester experiments, the ice crystals grow in the same supercooled droplet cloud used to rime the target. It is possible that the mixing process provides an undepleted droplet cloud and a transient enhanced vapor supply that affects both the ice crystal and graupel vapor depositional growth rates, leading to the present results.

Temperature dependence of ice critical nucleus size

We present a molecular dynamics study of ice growth from supercooled water. By performing a series of simulations with different initial conditions, we have quantitative established the relationship existing between the critical nucleus size and the temperature. The results show that ice embryos containing hundreds or thousands of molecules are needed for the system to crystallize macroscopically, even at high degrees of supercooling. Our findings explain the difficulty in observing spontaneous ice nucleation in atomistic simulations and the relative ease with which water droplets can be supercooled under controlled experiments.

Charge transfer during Crystal-Graupel Collisions for two different cloud droplet size distributions

Laboratory experiments of graupel charging during ice crystal collisions reveal a charge sign dependence on temperature, liquid water concentration as well as the size distribution of the cloud droplets used for riming. The measurements were performed with an impact velocity of 8.5 m s−1, the ambient temperature was varied in the range −5 to −30°C and the effective water content up to 4 g m−3. Charge diagrams of the sign of the electric current on the graupel as a function of the ambient temperature (Ta) and the effective liquid water content (EW) for each of the two cloud droplet spectra used are presented. The results indicate that broadening the droplet spectrum leads to negative graupel charging and the implications of these findings to thunderstorm electrification are discussed.

The water supercooled regime as described by four common water models.

The temperature scale of simple water models in general does not coincide with the natural one. Therefore, in order to make a meaningful evaluation of different water models, a temperature rescaling is necessary. In this paper, we introduce a rescaling using the melting temperature and the temperature corresponding to the maximum of the heat capacity to evaluate four common water models (TIP4P-Ew, TIP4P-2005, TIP5P-Ew and Six-Sites) in the supercooled regime. Although all the models show the same general qualitative behavior, the TIP5P-Ew appears as the best representation of the supercooled regime when the rescaled temperature is used. We also analyze, using thermodynamic arguments, the critical nucleus size for ice growth. Finally, we speculate on the possible reasons why atomistic models do not usually crystalize while the coarse grained mW model do crystallize.

Charge separation in updraft of convective regions of thunderstorm

analyzed for three different velocities: 6, 8 and 11 m s 1 . The ambient temperature was varied in the range 5t o 30� C and the effective water content between 0 to 2gm 3 . Charge diagrams of the sign of the electric current on the graupel as a function of the ambient temperature and the effective liquid water content for each velocity are presented. The results indicate that increasing the velocity leads to negative particle charging during riming at higher velocity and the implications of these findings to non-severe

Ventilation coefficients for cylindrical collectors growing by riming as a function of the cloud droplet spectra

Abstract Laboratory measurements of the ventilation coefficient of ice particles growing by riming are presented in this work. The effect of the cloud droplet size spectrum after accretion on the ventilation coefficient was analyzed with droplets of mean volume diameter between 15 and 33 μm. The study was performed with cylindrical collectors of 2.8 and 4 mm diameter, the air temperature was varied from −5°C to −27°C, and three different velocities were used: 4.0, 7.0 and 8.5 m s −1 . The results show a significant dependence of the ventilation coefficient on the droplet sizes; in particular it was found that for small droplets the coefficient is increased and it can be twice its predicted theoretical value. It is suggested that this effect is produced by the different surface structure formed on the collector as a consequence of the different sizes of water droplets. The influences and effects of the cloud droplet size spectrum on the surface temperature and ventilation coefficient are discussed as a function of the Stokes number, which could be a more appropriate parameter to describe or simulate heat and mass transfer processes to accreting surfaces.

Systematic Characterization of Gas Phase Binary Pre-Nucleation Complexes Containing H2SO4 + X, [ X = NH3, (CH3)NH2, (CH3)2NH, (CH3)3N, H2O, (CH3)OH, (CH3)2O, HF, CH3F, PH3, (CH3)PH2, (CH3)2PH, (CH3)3P, H2S, (CH3)SH, (CH3)2S, HCl, (CH3)Cl)]. A Computational Study

A systematic characterization of gas phase binary prenucleation complexes between H2SO4 (SA) and other molecules present in the atmosphere (NH3, (CH3)NH2, (CH3)2NH, (CH3)3N, H2O, (CH3)OH, (CH3)2O, HF, CH3 F, PH3, (CH3)PH2, (CH3)2PH, (CH3)3P, H2S, (CH3)SH, (CH3)2S, HCl, (CH3)Cl) has been carried out using the ωB97X-D/6-311++(2d,2p) method at the DFT level of theory. A relationship between the energy gap of the SAs LUMO and the partner molecules HOMO, and the increasing number of methyl groups -CH3 in the SAs partner molecule is provided. The binding energies of the bimolecular complexes are found to be related to the electron density in the hydrogen bond critical point, the HOMO-LUMO energy gap, the nature of the hydrogen acceptor atom, and the frequencies shift of acid OH bonds. The results show how the frontier orbital compatibility determines the binding energy and that the properties of SAs OH bond which remains free of interactions are affected by the bimolecular adduct formation.