Investigation of supercooled cloud droplet temperatures and lifetimes in subsaturated environments with implications for ice nucleation at cloud boundaries

Roy, Puja

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https://hdl.handle.net/2142/122001 Copy

Description

Title

Investigation of supercooled cloud droplet temperatures and lifetimes in subsaturated environments with implications for ice nucleation at cloud boundaries

Author(s)

Roy, Puja

Issue Date

2023-11-30

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

• RAUBER, Robert M.
• Di Girolamo, Larry

Doctoral Committee Chair(s)

• RAUBER, Robert M.
• Di Girolamo, Larry

Committee Member(s)

• Lasher-Trapp, Sonia
• Jewett, Brian F.

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)

• Supercooled cloud droplet
• droplet temperature
• droplet radius
• evaporation
• evaporative cooling
• heat transfer
• mass transfer
• droplet lifetime
• ice nucleation
• cloud edges
• cloud-tops
• generating cells

Abstract

Cloud droplet temperature plays a critical role in influencing fundamental cloud microphysical and radiative processes. These processes include the determination of the diffusional growth and decay rates of cloud droplets, and the specification of droplet refractive indices, which in turn impact the radiative properties of clouds. The supercooled droplet temperature and lifetime affect crucial microphysical processes such as ice and precipitation formation via homogeneous freezing and activation of ice-nucleating particles (INPs) through contact and immersion freezing. While most observational and modeling studies often assume droplet temperature to be spatially uniform and equal to the ambient temperature, this assumption may not always be valid, particularly in evaporating regions of the clouds such as cloud tops and edges, where the droplets experience strong relative humidity gradients and can undergo rapid evaporation. This research aims to address the current knowledge gap in the literature by presenting two numerical frameworks designed to examine the evolution of single, evaporating supercooled cloud droplet temperatures and radii in subsaturated environments. The first approach, a simpler and idealized approximation, assumes the isolated droplet is surrounded by a prescribed environment outside the droplet. This method investigates the droplet temperature and lifetime evolution considering initial droplet radius (r0) and temperature (Tr0) and environmental relative humidity (RH), temperature (T), and pressure (P). The time (tss) required by droplets to reach a lower steady-state temperature (Tss) after sudden introduction into a new subsaturated environment, the magnitude of ΔT = T - Tss, and droplet survival time (tst) at Tss are calculated. ΔT is found to increase with T and decrease with RH and P. ΔT was typically 1-5ºC lower than T, with highest values (~10.3ºC) for very low RH, low P, and T closer to 0ºC. Results show that tss is < 0.5s over the range of initial droplet and environmental conditions considered. Larger droplets (r0 = 30 to 50 μm) can survive at Tss for about 15 seconds to over 10 minutes, depending on the subsaturation of the environment. For higher RH and larger droplets, droplet lifetimes can increase by more than 100s compared to the pure diffusion-limited evaporation approach, which ignores droplet cooling. Tss of the evaporating droplets can be approximated by the environmental thermodynamic wet-bulb temperature. Radiation was found to play a minor role in influencing droplet temperatures, except for larger droplets in environments close to saturation. The second approach relaxes the idealized assumption of a prescribed ambient environment and adopts a more advanced technique to present a first-of-its-kind quantitative investigation of evaporating droplet temperatures and lifetimes. The novelty of this analysis lies in including the impact of internal thermal gradients within the droplet as well as resolving thermal and vapor density gradients in the surrounding spatiotemporally varying ambient air domain. Using an advanced numerical model, this framework employs the finite-element method to solve the Navier-Stokes and continuity equations, coupled with heat and vapor diffusion, with appropriate boundary conditions. The simulations show for typical cloud droplet sizes (r0 = 30, 50 µm) and subsaturated environmental conditions considered, the internal thermal gradients dissipate quickly and the temperatures throughout the droplets become uniform in  0.1 s. Compared to the previous method, these results demonstrate a higher subsaturation-dependent decrease in the temperature of the droplet as well as the envelope of air surrounding the droplet surface due to droplet evaporation. For an ambient environment specified far away, with T = -5ºC, RH = 10%, 40%, and 70% the decrease in droplet temperatures due to evaporative cooling is ~ 24, 11, and 5°C, respectively. Compared to previous estimates, the evaporatively cooled droplets survive longer in this framework. For temperatures between -5°C and -10°C, for the three different subsaturated environments (RH = 10, 40, and 70%) closely studied in this analysis, droplet lifetimes typically range from ~ 11-61 s for 30 µm and ~ 33-176 s for 50 µm initial size droplets, respectively. Finally, potential implications of the evaporative cooling and increased lifetimes of supercooled cloud droplets on ice particle formation near cloud edges, such as cloud-top generating cells, are discussed. Based on the findings from the first method, using Tss instead of T in widely used parameterization schemes could lead to enhanced number concentrations of activated ice-nucleating particles (INPs), by a typical factor of 2-30, with the greatest increases (>100) coincident with low RH, low P, and T closer to 0ºC. Notably, the estimates of activated INP concentrations from the second method are even higher than previous analyses, as the droplet temperatures are much lower towards the end of their lifetimes, with several orders of magnitude increase in activated INPs for drier environments. The Fletcher Scheme predicts the greatest enhancement in activated INPs by a factor of ~10^6 for RH = 10%, T = -5°C, P = 500 hPa, while the corresponding enhancement factor values for Cooper and Demott schemes are ~10^3 and 80, respectively. One of the main motivations behind this study was to investigate a hypothesized ice nucleation enhancement mechanism from the evaporation of supercooled cloud droplets at cloud boundaries and for ambient temperatures between 0ºC and -20ºC. The findings from both methods demonstrate that the evaporating droplets can exist at lower temperatures than that of the ambient environment, with the magnitude of difference depending on the subsaturation, temperature, and pressure of the environment. The reduction in droplet temperatures can range from 1-5ºC to even ~ 20ºC for very dry, close to 0ºC, and low-pressure environments. The simulations also demonstrate that even though the droplets are evaporating in subsaturated conditions, droplets with an initial size of 30-50 µm survive for tens or hundreds of seconds to be able to potentially activate an INP, either immersed within the droplet or through external contact. Therefore, the findings suggest the plausibility of the aforementioned hypothesized ice nucleation enhancement mechanism, especially for relatively higher ambient temperatures (close to 0ºC), at evaporating regions of the cloud such as within cloud-top generating cells. The results also provide valuable insight regarding understanding, at least partially, the disparities between the number concentrations of observed ice crystals and estimated activated INPs, especially at relatively higher sub-0ºC temperatures.

Graduation Semester

2023-12

Type of Resource

Thesis

Copyright and License Information

© 2023 Puja Roy

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