A kinematic modeling study of the re-organization of snowfall between cloud top generating cells and low-level snowbands in midlatitude winter storms

Janiszeski, Andrew R

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https://hdl.handle.net/2142/121938 Copy

Description

Title

A kinematic modeling study of the re-organization of snowfall between cloud top generating cells and low-level snowbands in midlatitude winter storms

Author(s)

Janiszeski, Andrew R

Issue Date

2023-09-05

Director of Research (if dissertation) or Advisor (if thesis)

Rauber, Robert M

Doctoral Committee Chair(s)

Rauber, Robert M

Committee Member(s)

• Nesbitt, Stephen
• Lasher-Trapp, Sonia
• McFarquhar, Greg M

Department of Study

Atmospheric Sciences

Discipline

Atmospheric Sciences

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• midlatitude winter storms
• snowbands
• cloud top generating cells

Abstract

Radar observations of the comma head region of wintertime midlatitude cyclones over the central and eastern United States present two different perspectives on snowfall organization. The first perspective, provided by airborne and ground-based vertically-pointing radars, is that precipitation often originates in cloud-top generating cells with precipitation fall streaks emerging from the cells. The second perspective, provided by weather surveillance radars, is that zones of heavier snowfall are often organized in quasi-linear banded structures characterized by enhanced regions of radar reflectivity factor with quasi-linear shapes. The objective of this dissertation is to provide insight on how cloud top generating cells and near-surface banded features are related by kinematic flows alone re-organizing ice particles into bands beneath cloud top generating cells. The first part of this research examined the potential role of deformation flow in re-organization particles beneath cloud top generating cells using an idealized kinematic model. A set of experiments that had a constant deformation depth and ice particle fall velocity profile, representing a more extreme case of long particle residence time, were used to shed light into the role of deformation in particle re-organization in a highly idealized framework with a 12 km deep, constant with altitude, deformation layer and constant particle fall velocities for particles falling from cloud top to the surface. These experiments, testing both an initial linear and randomly-placed ice particle cluster arrangement, showed that in one or both initial arrangements, the greatest stretching along the axis of dilatation occurred with the strongest deformation of 2.5  10-4 s-1 and a largest particle residence time of 15000 s. This shows that under conditions of strong deformation with a large particle fall depth and particle fall velocities typical of unrimed ice crystals, ice particles can be organized by the horizontal wind alone into linear structures of similar shape to low-level banding features observed by surveillance radars. These experiments also showed that spacing between cell clusters is most crucial to surface band shape. Of all these experiments, the linear experiments with 5 km initial cluster row spacing and randomly-placed experiments with no cell cluster spacing exhibited the most coherent band-like structure at the surface. Another set of experiments quantified the magnitude of stretching of an initial field of ice particles falling through deformation flow fields of various magnitudes and depths typically found in the comma head of midlatitude winter storms. Those experiments used a vertical profile of particle fall velocity that was parameterized as decreasing from -0.8 m s-1 at cloud top of 10 km to -1.2 m s-1 at the surface to account for microphysical particle growth. Findings from those experiments showed that for deformation layers shallower than 4 km, the stretching of the surface ice particle field was less than double that of the original field for all deformation magnitudes. The results indicated that in the comma head region of winter cyclones, layers of deformation flow having a depth of 4 km or less are likely to produce minimal particle re-organization along the axis of dilatation. The second part of this research conducted experiments to explore whether particles falling beneath cloud top from uniformly-spaced generating cells at terminal velocity within observed two-dimensional convergent wind fields can be re-organized consistent with the presence of single and multi-banded structures present on WSR-88D radars in the three Northeast U.S. winter storms. The results demonstrated that the greater the residence time that falling ice particles were subject to the two-dimensional flow field beneath cloud top in each of the three storms, the greater the particle re-organization, resulting in larger concentrations of particles in the vicinity of observed precipitation bands. This suggests that for a large particle fall depth, slowly decreasing particle fall velocities with decreasing altitude, and regions of convergent flow, kinematics alone can assist in band formation by locally increasing particle concentrations. The third part of this research conducted experiments to test the role of three-dimensional horizontal kinematic flow in re-organizing ice particles falling beneath cloud top generating cells atop a stratiform cloud with fall velocities characteristic of typical midaltitude winter storm environments using the same three winter storms. These experiments investigated particle re-organization in the horizontal flow by arranging particles uniformly spaced at cloud top altitudes within the comma head, based on HRRR model relative humidity and local sounding analyses, and subjecting particles to fall velocities characteristic of such storms within the full three-dimensional horizontal kinematic flow until they reached 1 km altitude. Particle concentrations were then compared to WSR-88D reflectivity at 3, 2, and 1 km to determine if particles can be reorganized consistent with low-level bands and related to regions of convergence or divergence and frontogenesis. The results from the first two storms, 16–17 Dec 2020 and 29-30 Jan 2022, including two particle fall velocity profiles and three initial particle release altitudes, found that ice particles falling through the comma head starting from either 9, 8, or 7 km altitude, were first transported to the north or northwest by a 4 - 5 km deep southeasterly, south southeasterly, or southerly flow, and then back toward the southwest at lower levels, with higher concentrations arriving on the north or northwest half of the main observed low-level snowbands. Particle concentrations in those locations were increased by convergent horizontal flow with the greatest particle concentration enhancement factor of 2.32 – 3.84 for the 16-17 Dec 2020 storm and 1.76 – 2.32 for the 29 January 2022 storm respectively, collocated with the highest observed reflectivity in each band. However, no ice particles from the comma head arrived within the south or southeastern half of the main bands. Further analysis of low-level HRRR model relative humidity and horizontal flows suggests that the source of snowfall in those regions was not from comma head but rather from particle transport by moist low-level flows off the Atlantic Ocean interacting with low-level fronts in each storm. These experiments indicated that for the heavy banded snowfall in both storms, the snowfall had two source regions: 1) on the north or northwestern side, from ice particles falling from the comma head and, 2) on the southeastern side, for particles forming at or below 4 km altitude and transported by strong low-level flows off the Atlantic Ocean. The third storm on 4 February 2022 featured a shallower stratiform cloud with weak multi-banded features observed by radar across a broad region over Upstate New York. With shallow clouds having cloud tops around 5 km altitude, and weak kinematics, particle concentration enhancements with magnitudes of 1.36 – 1.40 and 1.14 - 1.26 were organized into a multi-banded structure aligned with regions of the observed multi-banded reflectivity. This indicated that weak kinematic flows in shallow cloud environments with multi-banded structure contribute to small increases of particle concentrations in the vicinity of low-level bands.

Graduation Semester

2023-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Andrew Janiszeski

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