Abstract
Abstract. In deep convective clouds, heavy rain is often formed involving the ice phase. Simulations were performed using the 3-D cloud resolving model COSMO-SPECS with detailed spectral microphysics including parameterizations of homogeneous and three heterogeneous freezing modes. The initial conditions were selected to result in a deep convective cloud reaching 14 km of altitude with strong updrafts up to 40 m s−1. At such altitudes with corresponding temperatures below −40 ∘C the major fraction of liquid drops freezes homogeneously. The goal of the present model simulations was to investigate how additional heterogeneous freezing will affect ice formation and precipitation although its contribution to total ice formation may be rather low. In such a situation small perturbations that do not show significant effects at first sight may trigger cloud microphysical responses. Effects of the following small perturbations were studied: (1) additional ice formation via immersion, contact, and deposition modes in comparison to solely homogeneous freezing, (2) contact and deposition freezing in comparison to immersion freezing, and (3) small fractions of biological ice nucleating particles (INPs) in comparison to higher fractions of mineral dust INP. The results indicate that the modification of precipitation proceeds via the formation of larger ice particles, which may be supported by direct freezing of larger drops, the growth of pristine ice particles by riming, and by nucleation of larger drops by collisions with pristine ice particles. In comparison to the reference case with homogeneous freezing only, such small perturbations due to additional heterogeneous freezing rather affect the total precipitation amount. It is more likely that the temporal development and the local distribution of precipitation are affected by such perturbations. This results in a gradual increase in precipitation at early cloud stages instead of a strong increase at later cloud stages coupled with approximately 50 % more precipitation in the cloud center. The modifications depend on the active freezing modes, the fractions of active INP, and the composition of the internal mixtures in the drops.
Highlights
Deep convective clouds may cover a wide temperature range from +20 ◦C at ground level down to −40 ◦C at altitudes of 14 km
The condensed water is present in the form of liquid droplets, but after passing the zero degree level, mixed-phase conditions are established in which ice particles and supercooled liquid drops are present simultaneously (e.g., Rosenfeld and Woodley, 2000)
This study demonstrated the formation of a deep convective cloud in which the cloud top reached 14 km of altitude with temperatures of −50 ◦C
Summary
Deep convective clouds may cover a wide temperature range from +20 ◦C at ground level down to −40 ◦C at altitudes of 14 km. The present model simulations address the question of how additional heterogeneous freezing will affect ice formation and precipitation its contribution to total ice formation may be rather low This situation may create socalled “small trigger effects”; i.e., small perturbations that do not show significant effects on first sight may trigger cloud microphysical responses. A small number of ice particles is formed by a small amount of ice nucleating particles They grow further by the deposition of water vapor, including the effects of the Bergeron–Findeisen process, i.e., at the expense of liquid drops. Even small additional contributions from contact and deposition freezing may alter eventually precipitation Another situation with small perturbations is the composition of ice nucleating particles (INPs). For more details see the INUIT website: http://www.ice-nuclei.de (INUIT Research group, 2018)
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