Structure and statistical analysis of the microphysical properties of the comma-head region of cold-season midlatitude cyclones

Microphysics;radar;midlatitude;extratropical;cold-season;cyclone;generating cells;fall streaks;convection;stratiform

Plummer, David

关键词: Microphysics;    radar;    midlatitude;    extratropical;    cold-season;    cyclone;    generating cells;    fall streaks;    convection;    stratiform;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/73029/David_Plummer.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This dissertation presents analyses of the microphysical structure and processes producing precipitation in the comma-head region of cold-season continental cyclones, using data collected by the W-band Wyoming Cloud Radar and in-situ instrumentation aboard the NSF/NCAR C-130 research aircraft during the 2009-10 Profiling of Winter Storms (PLOWS) field campaign. The analyses describe the characteristics both of convective generating cells at cloud top, and regions of stratiform cloud further below, in which convectively-generated fall streaks of hydrometeors were embedded. Measurements were obtained within convective generating cells occurring at cloud top in 11 cyclones at temperatures between -10 and55°C, and in regions of stratiform cloud in 14 cyclones at temperatures between4 and45°C. Detailed descriptions are presented of the generating cell characteristics in an individual cyclone, as well as statistical analyses of the entire generating cell data set, followed by a corresponding case study and large-scale statistical analysis of the full stratiform data set.Within the generating cell regions, ice particle number concentrations averaged 1.9 times larger inside generating cells compared to outside, and derived ice water contents and median mass dimensions averaged 2.2 and 1.1 times larger in cells. Supercooled water was directly measured at temperatures between31.4 and11.1°C, with the median and 95th percentile liquid water content increasing from ~0.09 to 0.12 g m 3 and 0.14 to 0.28 g m 3 over this temperature range. Liquid water was present in 26% of observations within cells and 18% of observations between cells over the same temperature range, and was nearly ubiquitous at temperatures above16°C. The larger ice particle concentrations in generating cells are consistent with greater ice production in convective updrafts. The increased mass and size of the ice particles demonstrate that generating cells provide environments favorable for enhanced particle growth. The impact of water saturation and supercooled water in the cells was evident, with rapid particle growth by diffusion and sometimes riming apparent, in addition to aggregation. Turbulent mixing lessened the observed differences between cells and surrounding regions, with supercooled water observed within and between cells, similar habits within and between cells, and rimed particles evident even in ice-phase conditions.Within the stratiform regions, ice-phase conditions predominated, with deposition and aggregation the primary growth mechanisms. Larger particle sizes and integrated mass were commonly observed relative to measurements in generating cells at similar temperatures, with the relative difference generally increasing with temperature, providing evidence that primary ice growth occurs below the generating cell level. Furthermore, while ice growth occurred both within and between convectively-generated fall streaks, ice number concentrations averaged approximately 1.9 times larger within compared to between the fall streaks, and the derived ice water contents and median mass dimensions averaged 2.3 and 1.1 times larger, respectively, similar on average to the initial enhancements observed within generating cell cores. Averaged number distributions were also generally enhanced within the fall streaks, with the enhancement for larger particles increasing (up to a factor of 3 or more near12.5°C) in most cases. As in the generating cell regions, mixing likely acted to lessen the differences between the fall streaks and the surrounding environment, with fall streaks commonly becoming more diffuse as they descended through the stratiform cloud.Combined, the analyses provide an overall representation of the structure and microphysical processes occurring within the observed cyclones, generally corresponding to a seeder-feeder mechanism. The generating cells initiated the production of precipitation by enhancing ice nucleation, as well as providing environments favorable for initial ice growth to occur. The stratiform regions below provided a deeper moisture source for subsequent ice growth after hydrometeors exited the generating cell regions. Ice growth occurred both within and between convectively-generated fall streaks, although enhanced ice concentrations and mass were typical within the fall streaks compared to the surrounding regions.