the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Evolution of Cloud Droplet Temperature and Lifetime in Spatiotemporally Varying Subsaturated Environments with Implications for Ice Nucleation at Cloud Edges
Abstract. Ice formation mechanisms in generating cells near stratiform cloud-tops, where mixing and entrainment occurs in the presence of supercooled water droplets, remain poorly understood. Supercooled cloud droplet temperature and lifetime may impact heterogeneous ice nucleation through contact and immersion freezing; however, modeling studies normally assume droplet temperature to be spatially uniform and equal to the ambient temperature. Here, we present a first-of-its-kind quantitative investigation of the temperature and lifetime of evaporating droplets, considering internal thermal gradients within the droplet as well as thermal and vapor density gradients in the surrounding air. Our approach employs solving the Navier-Stokes and continuity equations, coupled with heat and vapor transport, using an advanced numerical model. For typical ranges of cloud droplet sizes and environmental conditions, the droplet internal thermal gradients dissipate quickly (≤ 0.3 s) when droplets are introduced to new subsaturated environments. However, the magnitude of droplet cooling is much greater than estimated from past studies of droplet evaporation, especially for drier environments. For example, for an environment with pressure of 500 hPa, and ambient temperature far from the droplet of -5 °C, the droplet temperature reduction can be as high as 24, 11, and 5 °C for initial ambient relative humidities of 10 %, 40 %, and 70 % respectively. Droplet lifetimes are found to be tens of seconds longer compared to previous estimates due to weaker evaporation rates because of lower droplet surface temperatures. Using these new end-of-lifetime droplet temperatures, the enhancement in activation of ice-nucleating particles predicted by current ice nucleation parameterization schemes is discussed.
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RC1: 'Comment on egusphere-2024-526', Anonymous Referee #1, 14 Mar 2024
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AC1: 'Reply on RC1', Puja Roy, 13 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-526/egusphere-2024-526-AC1-supplement.pdf
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AC2: 'Reply on RC1', Puja Roy, 13 Jun 2024
Publisher’s note: this comment is a copy of AC1 and its content was therefore removed.
Citation: https://doi.org/10.5194/egusphere-2024-526-AC2
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AC1: 'Reply on RC1', Puja Roy, 13 Jun 2024
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RC2: 'Comment on egusphere-2024-526', Anonymous Referee #2, 24 Mar 2024
This work quantitatively investigated the evolution of an evaporating droplet and its surrounding environment utilizing an idealized numerical model. With different settings of ambient temperature, relative humidity, pressure, and initial droplet radius, this work examines the evolution of droplet temperature and lifetime and their dependences with these environment factors. The findings confirmed the previous literature that assumes steady-state droplet temperature, and the main novelty is finding that droplets can be much colder and last longer, due to the cooling of the adjacent air and the temperature gradient in the immediate environment surrounding the droplets. The results of this study is particularly of interest to the modeling community, which has been struggling with the underestimation of INPs for a long time. I think this manuscript is generally well written and recommend its publication in ACP, with some comments listed below.
Detailed comments:
- Line 31: what do you mean by “cells”? do you mean a grid in numerical models?
- Line 70-72: this sentence has too many sub-sentences, suggesting rewrite it.
- Line 151: I am curious if there are any differences between using cylindrical coordinates and spheral coordinates in the model, as the droplet volume, surface curvature and water tension may be calculated differently in the two coordinates.
- Line 170-171: Can you add a few sentences describing why it uses different meshes in and out of the droplet? any pros and cons for this setting?
- Line 190-191: maybe change the temperature unit from K to C for easier read. Same as the figures and tables.
- Line 211-213: related to my comment #3, maybe this is something can be used to explain that using cylindrical coordinates is appropriate.
- Line 252: give a number to the equations.
- Line 257: μ should have a value, what is the number?
- Line 268: k should be a constant or function depending on T and p, what is the number? And is it different in the droplet and in the environment air?
- Line 276: one factor that impacts the final temperature drop at the droplet surface is the difference of water diffusivity and heat diffusivity of the environment air. I am wondering how large are the diffusivity uncertainties of water and heat, and how this will impact the temperature drop.
- Line 292: T∞ should be in the unit of K when multiplied with R.
- Line 309: again, T should be in the unit of K here.
- Line 396: the number of mean cooling rate (K/s) is huge but does not mean anything, it is just an initial model spinup. Maybe just remove it.
- Line 399-401: These numbers are different from the numbers in Figure 4.
- Figure 4: again, I suggest using C instead of K for the unit of temperature. This makes y axis cleaner.
- Section 4.4: The presentation in this section needs to be improved. The authors list many numbers for different conditions, easily making readers get lost which quantity is in comparison (e.g., Section 4.4.2). I strongly suggest the authors simplify the text. For example, saying that "For environment with RH=10%, T=273K, P=500hPa, the lifetimes of 10, 30, 50 um diameter droplet are 1.1s, 1.4s, 32.8s, respectively." (well, the effect of droplet size to lifetime is obvious, maybe section 4.4.2 can be removed or modified).
- Table 1: again, using C instead of K makes it easier to read.
- Table 2: I would not put lifetime difference (tL - tLC) in the table, or just use a relative difference (percentage change), which is more relevant to the modeling application.
- Line 592-605: This paragragh may need to be re-organized or re-stated. It currently reads like saying the previous assumption of steady-state droplet temperature is imperfect and this study improves it. However, this study verified that the steady-state droplet temperature assumption is valid, with the main novelty to be considering the gradient of adjacent environment, which was not considered in previous studies.
- Line 623: I am curious whether the RH=10% is realistic in real word. In another word, do we really have a droplet ~25 K colder than we thought?
- Line 633: it also includes 10um droplet
Citation: https://doi.org/10.5194/egusphere-2024-526-RC2 -
AC3: 'Reply on RC2', Puja Roy, 13 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-526/egusphere-2024-526-AC3-supplement.pdf
Status: closed
-
RC1: 'Comment on egusphere-2024-526', Anonymous Referee #1, 14 Mar 2024
-
AC1: 'Reply on RC1', Puja Roy, 13 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-526/egusphere-2024-526-AC1-supplement.pdf
-
AC2: 'Reply on RC1', Puja Roy, 13 Jun 2024
Publisher’s note: this comment is a copy of AC1 and its content was therefore removed.
Citation: https://doi.org/10.5194/egusphere-2024-526-AC2
-
AC1: 'Reply on RC1', Puja Roy, 13 Jun 2024
-
RC2: 'Comment on egusphere-2024-526', Anonymous Referee #2, 24 Mar 2024
This work quantitatively investigated the evolution of an evaporating droplet and its surrounding environment utilizing an idealized numerical model. With different settings of ambient temperature, relative humidity, pressure, and initial droplet radius, this work examines the evolution of droplet temperature and lifetime and their dependences with these environment factors. The findings confirmed the previous literature that assumes steady-state droplet temperature, and the main novelty is finding that droplets can be much colder and last longer, due to the cooling of the adjacent air and the temperature gradient in the immediate environment surrounding the droplets. The results of this study is particularly of interest to the modeling community, which has been struggling with the underestimation of INPs for a long time. I think this manuscript is generally well written and recommend its publication in ACP, with some comments listed below.
Detailed comments:
- Line 31: what do you mean by “cells”? do you mean a grid in numerical models?
- Line 70-72: this sentence has too many sub-sentences, suggesting rewrite it.
- Line 151: I am curious if there are any differences between using cylindrical coordinates and spheral coordinates in the model, as the droplet volume, surface curvature and water tension may be calculated differently in the two coordinates.
- Line 170-171: Can you add a few sentences describing why it uses different meshes in and out of the droplet? any pros and cons for this setting?
- Line 190-191: maybe change the temperature unit from K to C for easier read. Same as the figures and tables.
- Line 211-213: related to my comment #3, maybe this is something can be used to explain that using cylindrical coordinates is appropriate.
- Line 252: give a number to the equations.
- Line 257: μ should have a value, what is the number?
- Line 268: k should be a constant or function depending on T and p, what is the number? And is it different in the droplet and in the environment air?
- Line 276: one factor that impacts the final temperature drop at the droplet surface is the difference of water diffusivity and heat diffusivity of the environment air. I am wondering how large are the diffusivity uncertainties of water and heat, and how this will impact the temperature drop.
- Line 292: T∞ should be in the unit of K when multiplied with R.
- Line 309: again, T should be in the unit of K here.
- Line 396: the number of mean cooling rate (K/s) is huge but does not mean anything, it is just an initial model spinup. Maybe just remove it.
- Line 399-401: These numbers are different from the numbers in Figure 4.
- Figure 4: again, I suggest using C instead of K for the unit of temperature. This makes y axis cleaner.
- Section 4.4: The presentation in this section needs to be improved. The authors list many numbers for different conditions, easily making readers get lost which quantity is in comparison (e.g., Section 4.4.2). I strongly suggest the authors simplify the text. For example, saying that "For environment with RH=10%, T=273K, P=500hPa, the lifetimes of 10, 30, 50 um diameter droplet are 1.1s, 1.4s, 32.8s, respectively." (well, the effect of droplet size to lifetime is obvious, maybe section 4.4.2 can be removed or modified).
- Table 1: again, using C instead of K makes it easier to read.
- Table 2: I would not put lifetime difference (tL - tLC) in the table, or just use a relative difference (percentage change), which is more relevant to the modeling application.
- Line 592-605: This paragragh may need to be re-organized or re-stated. It currently reads like saying the previous assumption of steady-state droplet temperature is imperfect and this study improves it. However, this study verified that the steady-state droplet temperature assumption is valid, with the main novelty to be considering the gradient of adjacent environment, which was not considered in previous studies.
- Line 623: I am curious whether the RH=10% is realistic in real word. In another word, do we really have a droplet ~25 K colder than we thought?
- Line 633: it also includes 10um droplet
Citation: https://doi.org/10.5194/egusphere-2024-526-RC2 -
AC3: 'Reply on RC2', Puja Roy, 13 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-526/egusphere-2024-526-AC3-supplement.pdf
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