Microstructures and precipitation development in cumulus and small cumulonimbus clouds over the warm pool of the tropical Pacific Ocean

Abstract

SUMMARYIn situ airborne measurements obtained in convective clouds in the vicinity of the Marshall Islands on 15 days in July and August 1999 are used to determine the microphysical structures and precipitation-producing mechanisms in these clouds. The liquid water contents of the clouds were generally well below adiabatic values, even in newly risen turrets. This is attributed to the entrainment of ambient air and to the very efficient removal of cloud water by the collision and coalescence of drops. The formation of raindrops began when the concentration of droplets with radii >15 µm exceeded ∼3 cm −3 or, equivalently, when the effective cloud drop radius reached ∼13 µm. Clouds rained when their depth exceeded ∼1.5 km. Due to rainout, the effective radius of cloud droplets began decreasing ∼2-4 km above cloud base.Extremely high concentrations of ice particles (often >500 litre −1 ) formed very rapidly at temperatures between −4 and −10 • C. High-resolution imagery of these particles, which were primarily sheaths, needles, frozen drops and irregular ice fragments, indicates that the high concentrations of ice were initiated by the freezing of individual drops, some of which fragmented upon freezing, accompanied by ice splinter production during riming. However, for ice particles <100 µm in maximum dimension, frozen drops and ice fragments were much more numerous than columnar ice crystals, the latter being indicative of ice splinter production. Narrow (10-50 m wide) streamers of precipitation containing large (>3 mm diameter) solid and liquid particles were often encountered in growing clouds. It is proposed that the particles in these streamers grew rapidly by riming in water-rich zones and, under appropriate conditions, the ice particles produced by both drop fragmentation and riming resulted in exceptionally high localized concentrations of ice. Convective clouds consisting entirely of liquid water produced rain rates that were similar to those from deep convective clouds containing ice.Since the concentration of drizzle and raindrops within a few kilometres of cloud base was highly correlated with cloud depth, and therefore cloud top temperature, the concentration of raindrops in cumulus and cumulonimbus clouds over the warm pool of the tropical Pacific Ocean can be inferred from satellite measurements of cloud top temperatures.

Observations of cloud microphysics and ice formation during COPE

“…Chisnell and Latham, 1976;Harris-Hobbs and Cooper, 1987;Blyth and Latham, 1993;Rangno and Hobbs, 2005) and that by providing the initial graupel the freezing of supercooled drizzle/raindrops formed by the warmrain process can advance the onset of H-M (Phillips et al, 2001;Huang et al, 2008Huang et al, , 2011. The recent work by Lawson et al (2015) suggests secondary ice production associated with droplet freezing may also be an important mechanism for cloud glaciation, but we are not able to fully quantify how much influence it had on the clouds in this study based on our observations alone.…”

“…Several studies have observed such drops in the early stages of convective clouds (e.g. Koenig, 1963;Blyth and Latham, 1993;Rangno and Hobbs, 2005;Lawson et al, 2015), particularly in environments with a warmer cloud base. It is not clear if the supercooled raindrops are the first particles to freeze, or if they collide with primary ice particles produced from frozen cloud drops (e.g.…”

Fast Dynamics of Water Droplets Freezing from the Outside In

“…This can explain (some of) the shapes of the larger ice particles in clouds [14][15][16] and of hailstone embryos [17,18]. It is believed that the fragmentation of freezing water droplets can play a role in the rapid glaciation of supercooled clouds and the development of precipitation [15,19,20]. Ice drop bursting has also recently been observed for condensation droplets formed on chilled super-hydrophobic surfaces, where the thrown out ice fragments can speed up frost formation [21].…”

“…This is consistent with previous lab experiments [7,[9][10][11]24] in which a sharp decrease in the drop bursting probability is observed around this droplet size. Also field observations on natural clouds indicate that there exists such a size threshold [15,19,35].…”

Distribution and origin of aerosol and its transform relationship with CCN derived from the spring multi-aircraft measurements of Beijing Cloud Experiment (BCE)

“…The first step is to distinguish the in-cloud and out-of-cloud data from the overall dataset. In fact, there is no generally acknowledged criterion to determine the appearance of clouds from the data, so that scientists use their own criterions in their researches [22,[35][36][37][38][39]. In addition, there is no comparison study to evaluate influences of using different criterions.…”