Examining Ice Microphysical Evolution in a Quasi-Idealized Simulation of a Squall Line using an Ice Crystal Trajectory Growth (ICTG) Model
Authors: Laurencin Chelsey , Didlake, Jr. Anthony , Harrington Jerry , Jensen Anders 
Published: June 2021

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&lt;p&gt;&lt;span&gt;Squall lines have a large impact on the annual rainfall in the mid-latitudes, yet there are still unanswered questions on the mechanisms generating their precipitation structure.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;P&lt;/span&gt;&lt;span&gt;recipitation processes&amp;#160;&lt;/span&gt;&lt;span&gt;have a&amp;#160;&lt;/span&gt;&lt;span&gt;large&lt;/span&gt;&lt;span&gt;&amp;#160;impact on the storm&amp;#8217;s intensity, lifetime&lt;/span&gt;&lt;span&gt;, and&amp;#160;&lt;/span&gt;&lt;span&gt;flooding&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;impacts.&lt;/span&gt;&lt;span&gt;&amp;#160;I&lt;/span&gt;&lt;span&gt;ce crystals are involved in&amp;#160;&lt;/span&gt;&lt;span&gt;modulating&lt;/span&gt;&lt;span&gt;&amp;#160;the distribution of latent heating&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;throughout&lt;/span&gt;&lt;span&gt;&amp;#160;the storm during phase change&lt;/span&gt;&lt;span&gt;&amp;#160;and th&lt;/span&gt;&lt;span&gt;u&lt;/span&gt;&lt;span&gt;s play a role in de&lt;/span&gt;&lt;span&gt;termining the thermodynamic and kinematic structure of the storm&lt;/span&gt;&lt;span&gt;.&amp;#160;&lt;/span&gt;&lt;span&gt;The current study&lt;/span&gt;&lt;span&gt;&amp;#160;uses&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;a&amp;#160;&lt;/span&gt;&lt;span&gt;novel ice crystal trajectory growth (ICTG) model&lt;/span&gt;&lt;span&gt;&amp;#160;to&amp;#160;&lt;/span&gt;&lt;span&gt;examine&amp;#160;&lt;/span&gt;&lt;span&gt;the&amp;#160;&lt;/span&gt;&lt;span&gt;physical&amp;#160;&lt;/span&gt;&lt;span&gt;and kinematic mec&lt;/span&gt;&lt;span&gt;hanisms contributing to the ice&lt;/span&gt;&lt;span&gt;&amp;#160;structure of a&lt;/span&gt;&lt;span&gt;&amp;#160;quasi-idealized&amp;#160;&lt;/span&gt;&lt;span&gt;3D&amp;#160;&lt;/span&gt;&lt;span&gt;simulation of a leading-line, trailing-stratiform squall line&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;span&gt;&amp;#160;This analysis is important for scientific understanding and accurate representation of&amp;#160;&lt;/span&gt;&lt;span&gt;microphysical&lt;/span&gt;&lt;span&gt;&amp;#160;processes in numerical models.&amp;#160;&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span&gt;Ice crystals are initialized in the leading convective line, and they are advected by the storm-relative winds while simultaneously growing by vapor deposition and riming (or sublimating).&amp;#160;&lt;/span&gt;&lt;span&gt;To&amp;#160;&lt;/span&gt;&lt;span&gt;account for variations in the ice particle size distribution, m&lt;/span&gt;&lt;span&gt;ultiple&amp;#160;&lt;/span&gt;&lt;span&gt;trajectory simulations are conducted for crystals&amp;#160;&lt;/span&gt;&lt;span&gt;with varying&lt;/span&gt;&lt;span&gt;&amp;#160;initial diameters&lt;/span&gt;&lt;span&gt;.&amp;#160;&lt;/span&gt;&lt;span&gt;The ICTG model produced a spatial distribution of ice crystals consistent with the precipitation and reflectivity structure of the simulated squall line above the melting level. Trajectory simulations using&amp;#160;&lt;/span&gt;&lt;span&gt;initially small&amp;#160;&lt;/span&gt;&lt;span&gt;crystals&amp;#160;&lt;/span&gt;&lt;span&gt;(&lt; 0.0&lt;/span&gt;&lt;span&gt;5&lt;/span&gt;&lt;span&gt;&amp;#160;mm in diameter)&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;result in&amp;#160;&lt;/span&gt;&lt;span&gt;the&amp;#160;&lt;/span&gt;&lt;span&gt;crystals being transported to the forward and rear anvils and the stratiform region. There&amp;#160;&lt;/span&gt;&lt;span&gt;a&lt;/span&gt;&lt;span&gt;re two main paths to the stratiform region: one&amp;#160;&lt;/span&gt;&lt;span&gt;i&lt;/span&gt;&lt;span&gt;s characterized by sustained interaction with the mesoscale updraft, dendritic growth, and a large final particle size; the other&amp;#160;&lt;/span&gt;&lt;span&gt;i&lt;/span&gt;&lt;span&gt;s characterized by lack of interaction with the mesoscale updraft, prolonged sublimation or suppressed growth, and a small final particle size. Simulations using&lt;/span&gt;&lt;span&gt;&amp;#160;initially&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;large&lt;/span&gt;&lt;span&gt;&amp;#160;crystals&amp;#160;&lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;0&lt;/span&gt;&lt;span&gt;.5&lt;/span&gt;&lt;span&gt;&amp;#160;mm in diameter)&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;result&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;in graupel-like particles that f&lt;/span&gt;&lt;span&gt;a&lt;/span&gt;&lt;span&gt;ll out primarily in the leading convective line. These findings support past studies identifying ice particle size sorting in squall lines which result in a local precipitation rate minimum occurring between the convective leading line and trailing stratiform region. The simulated patterns of ice crystal growth and trajectories also hold implications for the latent heating structure and evolution of the overall storm.&amp;#160;&lt;/span&gt;&lt;/p&gt;
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