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.
Highlights
Thunderstorms are an important source of ice particles to the upper troposphere and lower stratosphere, yet we are still learning about how these ice particles are formed and how their properties change in different locations and in different types of storms
We report on observations of frozen-drop aggregates and frozen-drop chain aggregates made in the upper regions of several midlatitude thunderstorm anvils over the continental United States (US) during the Deep Convective Clouds and Chemistry (DC3; Barth et al, 2012) experiment
Better in these clouds, as suggested by Fig. 8, we use it for our large particle data in the analysis presented below. 3.3 Ice water content in the anvils: the role of small and large particles Ice water content (IWC) is computed from the CDP and the 2DC in order to determine the relative contribution of frozen droplets versus larger frozen-drop aggregates (FDAs) to the anvil ice
Summary
Thunderstorms are an important source of ice particles to the upper troposphere and lower stratosphere, yet we are still learning about how these ice particles are formed and how their properties change in different locations and in different types of storms. Lawson et al (2003) analyzed 8600 CPI images from three different passes made through continental Colorado anvils at temperatures of −47 ◦C They reported that 2–3 % of the images were chain aggregates, 4–6 % were doublets, and 25–32 % were other aggregates (the remaining particles were other particle types, such as spheroids). They did a similar analysis of 16 600 CPI images taken at temperatures between −5 and −60 ◦C in three tropical maritime anvils that formed near Kwajalein In these cases they found 0 % chains, 0.1–2.3 % doublets, and 0.4 to 1.4 % other aggregates (the remaining particles were other particle types, such as blocky irregulars, and spheroids). After describing the experimental techniques, we analyze the ice particle morphology and microphysical characteristics of two FDAcontaining upper anvils in eastern Colorado, and present a survey identifying other storm anvils where similar FDAs were found
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