The Microphysics of Ice and Precipitation Development in Tropical Cumulus Clouds

Abstract

The rapid glaciation of tropical cumulus clouds has been an enigma and has been debated in the literature for over 60 years. Possible mechanisms responsible for the rapid freezing have been postulated, but until now direct evidence has been lacking. Recent high-speed photography of electrostatically suspended supercooled drops in the laboratory has shown that freezing events produce small secondary ice particles. Aircraft observations from the Ice in Clouds Experiment-Tropical (ICE-T), strongly suggest that the drop-freezing secondary ice production mechanism is operating in strong, tropical cumulus updraft cores. The result is the production of small ice particles colliding with large supercooled drops (hundreds of microns up to millimeters in diameter), producing a cascading process that results in rapid glaciation of water drops in the updraft. The process was analyzed from data collected using state-of-the-art cloud particle probes during 54 Learjet penetrations of strong cumulus updraft cores over open ocean in a temperature range from 58 to 2208C. Repeated Learjet penetrations of an updraft core containing 3-5 g m 23 supercooled liquid showed an order-of-magnitude decrease in liquid mass concentration 3 min later at an elevation 1-1.5 km higher in the cloud. The aircraft observations were simulated using a one-dimensional cloud model with explicit bin microphysics. The model was initialized with drop and ice particle size distributions observed prior to rapid glaciation. Simulations show that the model can explain the observed rapid glaciation by the drop-freezing secondary ice production process and subsequent riming, which results when large supercooled drops collide with ice particles.

Investigating the Impacts of Saharan Dust on Tropical Deep Convection Using Spectral Bin Microphysics, Part 1: Ice Formation and Cloud Properties

“…As activation numbers for the individual ice formation schemes are tracked by the model, we will verify this reduction in a subsequent figure and table. The profound loss of liquid mass following initial ice formation at temperatures warmer than -10°C is similar to the rapid glaciation observed in strong 20 updrafts in the study of Lawson et al (2015), which was suggested to occur due to the cascading formation of small crystals from drop freezing ice multiplication processes and subsequent riming.…”

“…While ice multiplication from the Hallett-Mossop scheme is active in both cases, removing the depositional-condensation nucleation is sufficient to reduce graupel formation and delay the glaciation of the clouds until temperatures near the homogeneous freezing level 25 where immersion freezing is most active. The observations/modeling studies of Lawson et al (2015) and Ackerman et al (2015) both suggest that this effect on cloud glaciation can be explained by sufficiently fast ice multiplication at warmer temperatures, creating small ice crystals similar to the effects of significantly large deposition-condensation freezing, although it should be noted that primary ice formation was not the focus of those studies and ice nucleation was treated simply, leaving a possible gap in the understanding of the interactions between the ice nucleation and ice multiplication 30 processes. In our simulation the removal of the deposition-condensation freezing greatly hinders the formation of midlevel ice, despite an unchanged IN concentration and active ice multiplication, showing the importance of even small number of midlevel ice formation to the overall vertical hydrometeor distribution.…”

“…While ice multiplication is active in the D1.2b case, it does not 20 compensate for the loss of small ice formed by the deposition-condensation scheme, subsequently delaying glaciation and reducing graupel formation until immersion freezing converts the large drops to graupel. Lawson et al (2015) suggests that Hallett-Mossop ice multiplication is too limited to produce sufficient ice crystals to initiate rapid glaciation as opposed to the more vigorous drop shattering ice multiplication method used in that study. This may explain the reduced ice and graupel content in the D1.2b case despite the presence of other ice formation mechanisms.…”

“…As noted in Meyers et al (1992) it is difficult to distinguish the relative contributions of depositional and condensational freezing in a parcel, since both form similarly sized small ice crystals, despite the different mechanisms of vapor to ice in the former and condensation followed immediately by freezing in the latter case. However, studies suggest that small ice formed in the temperature range covered by this scheme can have a large impact on subsequent ice formation at higher altitudes (Ackerman et al 2015;Hiron and 25 Flossman, 2015;Lawson et al, 2015).To link depositional/condensational freezing with aerosols, we follow the implementation of van den Heever et al (2006), updated from the Meyers et al (1992) version. The number of ice crystals generated by depositional-condensational nucleation (N hen ) is proportional to the IN number concentration (N IN ; l -1 ) by Eq.…”

Characteristics of vertical air motion in isolated convective clouds

“…The liquid fraction is correlated to the vertical velocity between −3 and −8 • C, possibly because the Hallet-Mossop process is more significant in weaker updrafts (Heymsfield and Willis, 2014). Lawson et al (2015) show that the existence of millimeter drops in the convective clouds can result in fast ice initiation, and the significant latent heat released during the ice initiation process can strengthen the updrafts. In ICE-T and COPE, we also observe many millimeter drops, which may strongly interact with vertical velocity through a fast ice initiation process.…”