High Supersaturation in the Wake of Falling Hydrometeors: Implications for Cloud Invigoration and Ice Nucleation

Abstract

Aerosol particles, cloud droplets, and ice crystals, coupled through the supersaturation field, play an important role in the buoyancy and life cycle of convective clouds. This letter reports laboratory observations of copious cloud droplets and ice crystals formed in the wake of a warm, falling water drop, which is a laboratory surrogate for a relatively warm hydrometeor in atmospheric clouds, such as a graupel particle in the wet growth regime. Aerosols were activated in the regions of very high supersaturation due to mixing in the wake. A mechanism is explored for attaining very high supersaturations capable of activating significant fractions of the interstitial aerosols within the lifetime of a convective cloud. The latent heat released from the activation of interstitial aerosols and subsequent growth may provide an additional source of buoyancy for cloud invigoration and may lead to larger concentrations of ice crystals. Plain Language SummaryHail or other large icy hydrometeors like graupel fall through clouds, leaving regions of disturbed turbulent air in their wake. Because graupel particles are very likely to be warm or cold, relative to their surroundings, numerous new cloud droplets and even ice crystals can form in the disturbed air. The heat associated with the condensation of water vapor onto these newly formed droplets or crystals could provide a significant boost to the cloud's buoyancy. Calculations suggest that this mechanism could expose regions of convective clouds to high supersaturations in tens of minutes, providing a source of invigoration and higher concentrations of ice crystals.

Review of experimental studies of secondary ice production

“…where D is the droplet diameter, and G( T ) is the rate of ice growth at water supercooling T . The growth rate G( T ) was studied by many research groups (e.g., Lindenmeyer et al, 1959;Hallett, 1964;Pruppacher, 1967a;Feuillebois et al, 1995;Shibkov et al, 2003Shibkov et al, , 2005and others). It was found that the velocity of ice growth along the c axis, G c , is considerably smaller than that along the a axis, G a (e.g., Macklin…”

Supersaturation in the Wake of a Precipitating Hydrometeor and Its Impact on Aerosol Activation

“…In the case of a liquid hydrometeor of density 10 3 kg m −3 , these Re corresponds to hydrometeors with diameter between 3 × 10 −4 m and 1.03 × 10 −3 m falling with terminal velocities between 1.21 and 4.03 m s −1 . The ambient relative humidity RH ∞ is varied from nearly saturated ( RH ∞ ∼ 100 % ) within the cloud (Siebert & Shaw, 2017) to a highly subsaturated condition in the open atmosphere (see also Prabhakaran et al, 2020). The supersaturation is computed with respect to the water phase when T > 0°C and with respect to the ice phase when T ≤ 0°C by using the following empirical Equations and for liquid and frozen hydrometeors, respectively (Huang, 2018).…”

“…Physical processes contributing to the production of secondary ice particles (Field et al, 2017) within a mature cloud cannot explain the observed discrepancies between the measured activation and the observed hydrometeor population, which is several orders of magnitude higher than expected (Huang et al, 2017; Pruppacher & Klett, 2010). One possible explanation of this riddle might be the wake‐induced supersaturation (Chouippe et al, 2019; Fukuta & Lee, 1986; Gagin, 1972) and activation of aerosols behind large precipitating hydrometeors, that is, heterogeneous wake‐induced nucleation (Prabhakaran et al, 2020). The experimental investigation by Prabhakaran et al (2017) of falling drops (diameter of mm) in near critical point conditions of pressurized sulfur hexafluoride showed the evidences of homogeneous nucleation in the wake.…”

“…Prabhakaran et al (2020) conducted a follow‐up experiment on heterogeneous nucleation using sodium chloride and silver iodide aerosols under atmosphere‐like conditions. Warm droplets with a diameter of ∼2 mm were able to induce the activation of aerosols as water droplets and ice crystals in their wake when precipitating through a subsaturated colder environment.…”

Supersaturation in the Wake of a Precipitating Hydrometeor and its Impact on Aerosol Activation

“…Physical processes contributing to the production of secondary ice particles (Field et al, 2017) within a mature cloud cannot explain the observed discrepancies between the measured activation and the observed hydrometeor population, which is several orders of magnitude higher than expected (Huang et al, 2017;Pruppacher & Klett, 2010). One possible explanation of this riddle might be the wake-induced supersaturation (Chouippe et al, 2019;Fukuta & Lee, 1986;Gagin, 1972) and activation of aerosols behind large precipitating hydrometeors, that is, heterogeneous wake-induced nucleation (Prabhakaran et al, 2020). The experimental investigation by Prabhakaran et al (2017) of falling drops (diameter of (1) mm) in near critical point conditions of pressurized sulfur hexafluoride showed the evidences of homogeneous nucleation in the wake.…”