Ice particles in the upper anvil regions of midlatitude continental thunderstorms: the case for frozen-drop aggregates

Abstract

Abstract. This study examines the occurrence and morphology of frozen-drop aggregates in thunderstorm anvils from the United States Midwest and describes the environmental conditions where they are found. In situ airborne data collected in anvils using several particle imaging and sizing probes and bulk total water instrumentation during the 2012 Deep Convective Clouds and Chemistry experiment are examined for the presence of frozen-drop aggregates. Chains of frozen drops have been only rarely reported before and are hypothesized to aggregate due to electrical forces in the clouds. They were identified in nine of the anvil cases examined to date, suggesting that they are common features in these Midwestern anvils. High concentrations of individual frozen droplets occurred on the tops and edges of one particular set of anvils, while regions closer to the center and bottom of these anvils exhibited fewer frozen drops and more frozen-drop aggregates. Bulk ice water content measurements across these anvils could only be explained by contributions from both small particles (frozen droplets) and large particles (large aggregates of frozen droplets). Dual Doppler radar analysis confirmed the presence of deep and strong (> 15 m s −1 ) updrafts in the parent cloud of one of the anvils. These features contrast with previous anvil measurements in tropical/maritime anvils that evidently do not exhibit the same frequency of frozen-drop aggregates.

PHIPS-HALO: the airborne particle habit imaging and polar scattering probe – Part 2: Characterization and first results

“…The magnification of the zoom lenses can be manually set in the range from 1.4 × to 9 ×, which corresponds to field of view dimensions ranging from 6.27 × 4.72 to 0.98 × 0.73 mm 2 , respectively. This allows image acquisition either at the same or at two different fields of view, which is advantageous to avoid a complete failure of the imaging system in the case of alignment drifts during flight and for cloud situations when small and large ice particles are present at the same time, e.g., in the case of small frozen droplets and large ice crystal aggregates in deep convective outflows (Stith et al, 2014). Consequently, the optical resolution limit is dependent on the magnification setting and ranges between 7.2 and 2.35 µm for the 1.4 × and 9 × magnification, respectively.…”

Using cloud ice flux to parametrise large-scale lightning

“…However, development is on-going with satellite and field measurements helping to form a picture of the current distributions of cloud ice Stith et al, 2014). The objective of this study is to test the usefulness of the current state of cloud ice modelling within a lightning parametrisation.…”

Microphysical Characteristics of Frozen Droplet Aggregates from Deep Convective Clouds

“…Another unique feature of ice crystals in deep convective clouds is the high concentration of 15 small frozen droplets Baran et al, 2012;Stith et al, 2014). These are no doubt generated from the freezing of supercooled droplets that has been observed at temperatures as low as -37.5 °C in deep continental convective clouds (Rosenfeld and Woodely, 2000).…”

“…Gayet et al (2012) Deep Convective Clouds and Chemistry (DC3) experiment (Barth et al, 2012;Stith et al, 2014). Based on these observations, the occurrence of single frozen droplet and FDAs as a function of position within the anvil cloud and of updraft velocity was determined (Stith et al, 2014).…”

“…Gayet et al (2012) Deep Convective Clouds and Chemistry (DC3) experiment (Barth et al, 2012;Stith et al, 2014). Based on these observations, the occurrence of single frozen droplet and FDAs as a function of position within the anvil cloud and of updraft velocity was determined (Stith et al, 2014). A positive correlation between the frequency of occurrence of FDAs and the level of nitrogen oxide (NOx) produced by lightning suggested that a strong electric force in the updrafts was playing a role in the formation of the FDAs 5 (Stith et al, 2016).…”