Observations of the microphysical evolution of convective clouds in southwest United Kingdom

Abstract

<p><strong>Abstract.</strong> The COnvective Precipitation Experiment (COPE) was designed to investigate the origins of heavy convective precipitation over the South Western UK, a region that experiences flash flooding due to heavy precipitation from slow-moving convective systems. In this study, the microphysical and dynamical characteristics of developing turrets during four days in July and August, 2013 are analyzed. In situ cloud microphysical measurements from the University of Wyoming King Air and vertically pointing W-band radar measurements from Wyoming Cloud Radar are examined, together with data from the ground-based NXPol radar. <br><br> The four days presented here cover a range of environmental conditions in terms of wind shear and instability, resulting in a similarly wide variability in observed ice crystal concentrations, both across days as well as between clouds on individual days. The highest concentration of ice was observed on the days in which there was an active warm rain process supplying precipitation-sized liquid drops. The high ice concentrations observed (> 100 L<sup>−1</sup>) are consistent with the production of secondary ice particles through the Hallett-Mossop process. Turrets that ascended through remnant cloud layers above the 0 °C level had higher ice particle concentrations, suggesting that entrainment of ice particles from older clouds or previous thermals may have acted to aid in the production of secondary ice through the Hallett-Mossop process. Other mechanisms such as the shattering of frozen drops may be more important for producing ice in more isolated clouds.</p>

On Different Microphysical Pathways to Convective Rainfall

“…Maximum cloud-base droplet number concentrations measured by the aircraft were 550-600 cm 23 on 2 August and 300-350 cm 23 on 3 August. At comparable sampling temperatures within 1 km of cloud top, Jackson et al (2018) found generally fewer hydrometeors exceeding 300-mm diameter in the 2 August clouds than on 3 August, and often many fewer irregular particles indicative of ice. They also reported an abundance of ice crystals (.100 L 21 ) in the temperature range from 238 to 288C on 3 August, consistent with ice multiplication by the rime-splintering process.…”

“…1). The 50th and 75th percentiles of aircraft-sampled updraft speeds over similar temperature ranges were approximately 1 and 4 m s 21 greater on 2 August than on 3 August (Jackson et al 2018).…”

“…In time, however, both cases developed much more ice, particularly within the rime-splintering zone (shaded), but the shallower 3 August clouds developed more ice (maximum of 190 L 21 ) than did the deeper 2 August clouds (maximum of 80 L 21 ). Particles exceeding 100 mm in diameter measured in the rime-splintering zone by the aircraft on these two days numbered from 10 to 300 L 21 and were mostly ice on 3 August but still mostly liquid on 2 August (Jackson et al 2018). The progression of microphysical processes leading to these differences in ice formation, and its effects upon surface rainfall, are explored in the following sections.…”

“…The cloud condensation nuclei (CCN) in the local air (ranging from 100 to 500 cm 23 at 0.9% supersaturation), as well as prevalent giant and ultragiant aerosol particles (Taylor et al 2016a) did often result in a productive warm rain process in the stronger convection (Leon et al 2016). Supercooled raindrops were often observed in the updrafts by radar and aircraft (Plummer et al 2018;Jackson et al 2018). Although quantification of icenucleating particles from aircraft measurements was not available, in situ aircraft measurements frequently showed ice number concentrations exceeding hundreds per liter, far greater than the number of icenucleating particles typically active at the sampling temperatures (from 08 to 2148C).…”

“…Although quantification of icenucleating particles from aircraft measurements was not available, in situ aircraft measurements frequently showed ice number concentrations exceeding hundreds per liter, far greater than the number of icenucleating particles typically active at the sampling temperatures (from 08 to 2148C). Rime splintering was implicated as a source of these large numbers of ice crystals in some cases (Taylor et al 2016b;Jackson et al 2018). However, determining the dominant microphysical pathway is very difficult from the observations alone.…”