Fragmentation in graupel snow collisions

Abstract

Aircraft observations of precipitating clouds with cloud top temperatures higher than -38°C have revealed that Secondary Ice Production (SIP) is responsible for presence of majority of ice particles. One such SIP mechanism is fragmentation via collisions between ice particles. The central theme of this study is to understand the dependencies of this SIP mechanism and improve its existing numerical and theoretical predictions, through field-based observations.

This study is motivated by the only field experiment to observe this type of mechanism, by Vardiman in 1978, who built a probe to sample falling ice precipitation outdoors. We modify aspects of that study by building our own portable chamber with knowledge from more recent publications and advances in technology. Fragmentation of individual snowflakes falling into it was recorded with high-speed video cameras. An array of 126 ice spheres were fixed to the base of the chamber and each was assumed to be representative of graupel. With this chamber, fragments from each collision between a falling snowflake and an ice sphere could be counted and sized from inspection of video recordings, after sampling outdoors.

There was a field trip to sample naturally falling snow particles in the Svartberget forest in Vindeln, in the north of Sweden, about 650 km south of the Arctic Circle on 24 February 2022, around midnight UTC. It was a snowfall lasting about 4 hours from orographic stratiform cloud (with precipitation rate of about 7 mm/hr) with a mixed-phase cloud top of about -20°C and a cloud base of -2.6°C about 100m above the ground (elevation 270 metres MSL). Simultaneously the mass-size relationship parameters for the falling snow particles were measured, which enabled the mass of each snow particle in the chamber to be estimated from its size before collision.

The results for the average size distribution of fragments, the coincident mass-size parameters, fall speed–size relation, and dependencies on Collision Kinetic Energy (CKE) correspond well with previously reported studies for dendritic snow. From the observed number of fragments, we refitted the theoretical formulation for this type of fragmentation in graupel snow collisions. For this formulation, a new form of the dependence of rime fraction on size is inferred from the coincident measurements of axial ratio. This refitting yielded an improved value of the asperity-fragility coefficient, C, of about 3.86×10^4 J-1 for dendritic snow colliding with graupel. Our field observations suggests that fragmentation in graupel-snow collisions is even more profound (about 3 times higher) than the previous estimations from the original version of the formulation. And a new revised version of the formulation is proposed for use in atmospheric cloud models.

Popular Abstract

Cloud systems form an essential part of ecosystems sustaining life on Earth . They provide life at the Earth’s surface with water in the form of precipitation and protect us from incoming solar radiation. Precipitation in the form of rain occurs when water vapour condenses onto droplets, which further grows until the raindrops are heavy enough to descend from the clouds. At temperatures lowers than 0°C, supercooled rain drops undergoes freezing, which later precipitates as snow, graupel (small hail particles), hail etc. This is the primary ice formation process in clouds. Riming is a process where ice crystals grow by collecting supercooled water droplets on its surface. However, radar-based aircraft observations of cold precipitating clouds have found higher concentration of ice crystals than what was expected from primary ice formation process only. This led to the acceptance of the presence of different Secondary Ice Production (SIP) mechanisms. Study of one such SIP mechanism, namely fragmentation due to collision between ice particles, is the central theme of this report. Previously only one field experiment was done in 1978, by Larry Vardiman to study the fragmentation due to collision of ice particles.

There was a field trip to sample naturally falling snow particles in the Svartberget forest in Vindeln, in the north of Sweden, about 650 km south of the Arctic Circle. An experimental setup, motivated from Vardiman’s 1978 field experiment, was designed to record the fragmentation process due to collisions between snow and graupel particles. The recordings were done during a snowfall lasting about 4 hours, on 24 February 2022, around midnight UTC. From the recordings, the number of fragments produced in each collision, size of the fragments and fall speed of snow particles were measured. The mass of the snowflakes was also measured separately by collecting naturally falling snowflakes inside a small plastic container and weighing the container afterwards. Further, the Collision Kinetic Energy (CKE) of the snowflakes was also estimated from the mass and fall speed information.

The results for the average size distribution of fragments, mass-size relation, fall speed–size relation and dependencies on CKE correspond well with previously reported studies for dendritic snow particles. The information on size, mass and fall speed obtained from the video recordings, were used as an input to an established theoretical formulation for predicting the number of fragments produced due to graupel-snow collisions. This gave a new form of dependence of riming of the colliding snowflakes on the number of fragments predicted, by the formulation. Refitting of the formulation was done, and a correction parameter inside the formulation was updated. This updated formulation, obtained through field-based observations, can be applied to cloud models for a better representation of the SIP mechanism, studied in this report.

• author: Gautam, Martanda
• supervisor: Vaughan Phillips; Marcin Jackowicz-Korczynski
• organization: Dept of Physical Geography and Ecosystem Science
• course: NGEM01 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Ecosystem Analysis, Clouds, Snow, Graupel, SIP, Fragmentation, CKE, Rime Fraction
• publication/series: Student thesis series INES
• report number: 571
• language: English
• additional info: This thesis work is an attempt to improve the reperesentation of one of the Secondary Ice Production mechanism (fragmentation due to collisions of ice particles) in cloud models with the help of experimental observations.
• id: 9087233
• date added to LUP: 2022-06-13 09:58:55
• date last changed: 2022-08-15 12:09:54