scholarly journals New investigations on homogeneous ice nucleation: the effects of water activity and water saturation formulations

2022 ◽  
Vol 22 (1) ◽  
pp. 65-91
Author(s):  
Manuel Baumgartner ◽  
Christian Rolf ◽  
Jens-Uwe Grooß ◽  
Julia Schneider ◽  
Tobias Schorr ◽  
...  
Keyword(s):  
Water Activity ◽  
Low Temperatures ◽  
Water Saturation ◽  
Aerosol Particles ◽  
Ice Nucleation ◽  
Ice Crystals ◽  
Microphysics Scheme ◽  
Cold Temperatures ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause

Abstract. Laboratory measurements at the AIDA cloud chamber and airborne in situ observations suggest that the homogeneous freezing thresholds at low temperatures are possibly higher than expected from the so-called “Koop line”. This finding is of importance, because the ice onset relative humidity affects the cirrus cloud coverage and, at the very low temperatures of the tropical tropopause layer, together with the number of ice crystals also the transport of water vapor into the stratosphere. Both the appearance of cirrus clouds and the amount of stratospheric water feed back to the radiative budget of the atmosphere. In order to explore the enhanced ice onset humidities, we re-examine the entire homogeneous ice nucleation process, ice onset, and nucleated crystal numbers, by means of a two-moment microphysics scheme embedded in the trajectory-based model (CLaMS-Ice) as follows: the well-understood and described theoretical framework of homogeneous ice nucleation includes certain formulations of the water activity of the freezing aerosol particles and the saturation vapor pressure of water with respect to liquid water. However, different formulations are available for both parameters. Here, we present extensive sensitivity simulations testing the influence of three different formulations for the water activity and four for the water saturation on homogeneous ice nucleation. We found that the number of nucleated ice crystals is almost independent of these formulations but is instead sensitive to the size distribution of the freezing aerosol particles. The ice onset humidities, also depending on the particle size, are however significantly affected by the choices of the water activity and water saturation, in particular at cold temperatures ≲205 K. From the CLaMS-Ice sensitivity simulations, we here provide combinations of water saturation and water activity formulations suitable to reproduce the new, enhanced freezing line.

scholarly journals New investigations on homogeneous ice nucleation: the effects of water activity and water saturation formulations

2021 ◽  
Author(s):  
Manuel Baumgartner ◽  
Christian Rolf ◽  
Jens-Uwe Grooß ◽  
Julia Schneider ◽  
Tobias Schorr ◽  
...  
Keyword(s):  
Water Activity ◽  
Low Temperatures ◽  
Water Saturation ◽  
Aerosol Particles ◽  
Ice Nucleation ◽  
Ice Crystals ◽  
Microphysics Scheme ◽  
Cold Temperatures ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause

Abstract. Laboratory measurements at the AIDA cloud chamber and airborne in-situ observations suggest that the homogeneous freezing thresholds at low temperatures are possibly higher than expected from the so-called “Koop-line”. This finding is of importance, because the ice onset relative humidity affects the cirrus cloud coverage and, at the very low temperatures of the tropical tropopause layer, together with the number of ice crystals also the transport of water vapor into the stratosphere. Both, the appearance of cirrus clouds and the amount of stratospheric water feed back to the radiative budget of the atmosphere. In order to explore the enhanced ice onset humidities, we re-examine the entire homogeneous ice nucleation process, ice onset and nucleated crystal numbers, by means of a two-moment microphysics scheme embedded in the trajectory based model (CLaMS-Ice) as follows: the well-understood and described theoretical framework of homogeneous ice nucleation yet includes certain formulations of the water activity of the freezing aerosol particles and the saturation vapor pressure of water with respect to liquid water. However, different formulations are available for both parameters. Here, we present extensive sensitivity simulations testing the influence of three different formulations for the water activity and four for the water saturation on homogeneous ice nucleation. We found that the number of nucleated ice crystals is almost independent of these formulations but is instead sensitive to the size distribution of the freezing aerosol particles. The ice onset humidities, also depending on the particle size, are however significantly affected by the choices of the water activity and water saturation, in particular at cold temperatures  205 K. From the CLaMS-Ice sensitivity simulations, we here provide combinations of water saturation and water activity formulations suitable to reproduce the new, enhanced freezing line.

scholarly journals Ice supersaturations exceeding 100% at the cold tropical tropopause: implications for cirrus formation and dehydration

2004 ◽  
Vol 4 (6) ◽  
pp. 7433-7462
Author(s):  
E. Jensen ◽  
J. B. Smith ◽  
L. Pfister ◽  
J. V. Pitman ◽  
E. M. Weinstock ◽  
...  
Keyword(s):  
Water Saturation ◽  
Accommodation Coefficient ◽  
Ice Nucleation ◽  
Cloud Formation ◽  
High Threshold ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
Accommodation Coefficients ◽  
The Tropics

Abstract. Recent in situ measurements at tropical tropopause temperatures as low as 187 K indicate supersaturations with respect to ice exceeding 100% with little or no ice present. In contrast, models used to simulate cloud formation near the tropopause assume a supersaturation threshold for ice nucleation of about 65% based on laboratory measurements of sulfate aerosol freezing. The high supersaturations reported here, along with cloud simulations assuming a plausible range of temperature histories in the sampled air mass, indicate that the vast majority of aerosols in the air sampled on this flight must have had supersaturation thresholds for ice nucleation exceeding 100% (i.e. near liquid water saturation at these temperatures). Possible explanations for this high threshold are that (1) the expressions used for calculating vapor pressure over supercooled water at low temperatures give values at least 20% too low, (2) most of the available aerosols had a composition that makes them much more resistant to ice nucleation than aerosols used in laboratory experiments, and (3) organic films on the aerosol surfaces reduce their accommodation coefficient for uptake of water, resulting in aerosols with more concentrated solutions when moderate-rapid cooling occurs and correspondingly inhibited homogeneous freezing. Simulations of in situ cloud formation in the tropical tropopause layer (TTL) throughout the tropics indicate that if these decreased accommodation coefficients and resulting high thresholds for ice nucleation prevailed throughout the tropics, then the calculated occurrence frequency and areal coverage of TTL cirrus would be significantly suppressed. However, the simulations also show that even if in situ TTL cirrus form only over a very small fraction of the tropics in the western Pacific, enough air passes through them due to rapid horizontal transport such that they can still effectively freeze-dry air entering the stratosphere.

scholarly journals Ice supersaturations exceeding 100% at the cold tropical tropopause: implications for cirrus formation and dehydration

2005 ◽  
Vol 5 (3) ◽  
pp. 851-862 ◽  
Author(s):  
E. J. Jensen ◽  
J. B. Smith ◽  
L. Pfister ◽  
J. V. Pittman ◽  
E. M. Weinstock ◽  
...  
Keyword(s):  
Water Saturation ◽  
Accommodation Coefficient ◽  
Ice Nucleation ◽  
Cloud Formation ◽  
High Threshold ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
Accommodation Coefficients ◽  
The Tropics

Abstract. Recent in situ measurements at tropical tropopause temperatures as low as 187 K indicate supersaturations with respect to ice exceeding 100% with little or no ice present. In contrast, models used to simulate cloud formation near the tropopause assume a supersaturation threshold for ice nucleation of about 65% based on laboratory measurements of aqueous aerosol freezing. The high supersaturations reported here, along with cloud simulations assuming a plausible range of temperature histories in the sampled air mass, indicate that the vast majority of aerosols in the air sampled on this flight must have had supersaturation thresholds for ice nucleation exceeding 100% (i.e. near liquid water saturation at these temperatures). Possible explanations for this high threshold are that (1) the expressions used for calculating vapor pressure over supercooled water at low temperatures give values are at least 20% too low, (2) organic films on the aerosol surfaces reduce their accommodation coefficient for uptake of water, resulting in aerosols with more concentrated solutions when moderate-rapid cooling occurs and correspondingly inhibited homogeneous freezing, and (3) if surface freezing dominates, organic coatings may increase the surface energy of the ice embryo/vapor interface resulting in suppressed ice nucleation. Simulations of in situ cloud formation in the tropical tropopause layer (TTL) throughout the tropics indicate that if decreased accommodation coefficients and resulting high thresholds for ice nucleation prevailed throughout the tropics, then the calculated occurrence frequency and areal coverage of TTL cirrus would be significantly suppressed. However, the simulations also show that even if in situ TTL cirrus form only over a very small fraction of the tropics in the western Pacific, enough air passes through them due to rapid horizontal transport such that they can still effectively freeze-dry air entering the stratosphere. The TTL cirrus simulations show that even if very large supersaturations are required for ice nucleation, these large supersaturations should occur very rarely.

scholarly journals Simulating the influence of primary biological aerosol particles on clouds by heterogeneous ice nucleation

2018 ◽  
Vol 18 (20) ◽  
pp. 15437-15450 ◽  
Author(s):  
Matthias Hummel ◽  
Corinna Hoose ◽  
Bernhard Pummer ◽  
Caroline Schaupp ◽  
Janine Fröhlich-Nowoisky ◽  
...  
Keyword(s):  
Pseudomonas Syringae ◽  
Aerosol Particles ◽  
Fungal Spores ◽  
Ice Nucleation ◽  
Ice Crystals ◽  
Atmospheric Measurements ◽  
Cloud Ice ◽  
Biological Aerosol Particles ◽  
Ice Phase ◽  
Heterogeneous Ice Nucleation

Abstract. Primary ice formation, which is an important process for mixed-phase clouds with an impact on their lifetime, radiative balance, and hence the climate, strongly depends on the availability of ice-nucleating particles (INPs). Supercooled droplets within these clouds remain liquid until an INP immersed in or colliding with the droplet reaches its activation temperature. Only a few aerosol particles are acting as INPs and the freezing efficiency varies among them. Thus, the fraction of supercooled water in the cloud depends on the specific properties and concentrations of the INPs. Primary biological aerosol particles (PBAPs) have been identified as very efficient INPs at high subzero temperatures, but their very low atmospheric concentrations make it difficult to quantify their impact on clouds. Here we use the regional atmospheric model COSMO–ART to simulate the heterogeneous ice nucleation by PBAPs during a 1-week case study on a domain covering Europe. We focus on three highly ice-nucleation-active PBAP species, Pseudomonas syringae bacteria cells and spores from the fungi Cladosporium sp. and Mortierella alpina. PBAP emissions are parameterized in order to represent the entirety of bacteria and fungal spores in the atmosphere. Thus, only parts of the simulated PBAPs are assumed to act as INPs. The ice nucleation parameterizations are specific for the three selected species and are based on a deterministic approach. The PBAP concentrations simulated in this study are within the range of previously reported results from other modeling studies and atmospheric measurements. Two regimes of PBAP INP concentrations are identified: a temperature-limited and a PBAP-limited regime, which occur at temperatures above and below a maximal concentration at around −10 ∘C, respectively. In an ensemble of control and disturbed simulations, the change in the average ice crystal concentration by biological INPs is not statistically significant, suggesting that PBAPs have no significant influence on the average state of the cloud ice phase. However, if the cloud top temperature is below −15 ∘C, PBAP can influence the cloud ice phase and produce ice crystals in the absence of other INPs. Nevertheless, the number of produced ice crystals is very low and it has no influence on the modeled number of cloud droplets and hence the cloud structure.

scholarly journals Cold trap dehydration in the Tropical Tropopause Layer characterized by SOWER chilled-mirror hygrometer network data in the Tropical Pacific

2012 ◽  
Vol 12 (9) ◽  
pp. 25833-25885 ◽  
Author(s):  
F. Hasebe ◽  
Y. Inai ◽  
M. Shiotani ◽  
M. Fujiwara ◽  
H. Vömel ◽  
...  
Keyword(s):  
Ice Nucleation ◽  
Tropical Pacific ◽  
Cold Trap ◽  
Mixing Ratio ◽  
Back Trajectories ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
Scatter Plots ◽  
Cold Point ◽  
The Tropical Pacific

Abstract. A network of balloon-born radiosonde observations employing chilled-mirror hygrometers for water and electrochemical concentration cells for ozone has been operated since late 1990s in the Tropical Pacific trying to capture the progress of dehydration for the air parcels advected horizontally in the Tropical Tropopause Layer (TTL). The analyses of this dataset are made on isentropes taking advantage of the conservative properties of tracers in adiabatic motion. The existence of ice particles is diagnosed by lidars simultaneously operated with sonde flights. Characteristics of the TTL dehydration are presented on the basis of individual soundings and statistical features. Supersaturations close to 80% in the relative humidity with respect to ice (RHice) have been observed in subvisible cirrus clouds located near the cold point tropopause at extremely low temperatures around 180 K. Further observational evidence is needed to confirm the credibility of such high values of RHice. The progress of TTL dehydration is reflected in isentropic scatter plots between the sonde-observed mixing ratio (OMR) and the minimum saturation mixing ratio (SMRmin) along the back trajectories associated with the observed air mass. The supersaturation exceeding the critical value of the homogeneous ice nucleation (OMR > 1.6 × SMRmin) is frequently observed on 360 and 365 K surfaces indicating that the cold trap dehydration is under progress in the TTL. The near correspondence between the two (OMR ~ SMRmin) on 380 K on the other hand implies that this surface is not significantly cold for the advected air parcels to be dehydrated. Above 380 K, the cold trap dehydration would scarcely function while some moistening in turn occurs before the air parcels reach the lowermost stratosphere at around 400 K where OMR is generally smaller than SMRmin.

scholarly journals Modeling upper tropospheric and lower stratospheric water vapor anomalies

2013 ◽  
Vol 13 (4) ◽  
pp. 9653-9679 ◽  
Author(s):  
M. R. Schoeberl ◽  
A. E. Dessler ◽  
T. Wang
Keyword(s):  
Water Vapor ◽  
Lower Stratosphere ◽  
Calculation Model ◽  
Vapor Concentration ◽  
Cold Temperatures ◽  
Tropical Tropopause Layer ◽  
Stratospheric Water Vapor ◽  
Tropical Tropopause ◽  
Minimum Displacement ◽  
Asian Monsoons

Abstract. The domain-filling, forward trajectory calculation model developed by Schoeberl and Dessler (2011) is used to further investigate processes that produce upper tropospheric and lower stratospheric water vapor anomalies. We examine the pathways parcels take from the base of the tropical tropopause layer (TTL) to the lower stratosphere. Most parcels found in the lower stratosphere arise from East Asia, the Tropical West Pacific (TWP) and the Central/South America. The belt of TTL parcel origins is very wide compared to the final dehydration zones near the top of the TTL. This is due to the convergence of rising air as a result of the stronger diabatic heating near the tropopause relative to levels above and below. The observed water vapor anomalies – both wet and dry – correspond to regions where parcels have minimal displacement from their initialization. These minimum displacement regions include the winter TWP and the Asian and American monsoons. To better understand the stratospheric water vapor concentration we introduce the water vapor spectrum and investigate the source of the wettest and driest components of the spectrum. We find that the driest air parcels that originate below the TWP, moving upward to dehydrate in the TWP cold upper troposphere. The wettest air parcels originate at the edges of the TWP as well as the summer American and Asian monsoons. The wet air parcels are important since they skew the mean stratospheric water vapor distribution toward higher values. Both TWP cold temperatures that produce dry parcels as well as extra-TWP processes that control the wet parcels determine stratospheric water vapor.

scholarly journals Tropical tropopause ice clouds: a dynamic approach to the mystery of low crystal numbers

2013 ◽  
Vol 13 (19) ◽  
pp. 9801-9818 ◽  
Author(s):  
P. Spichtinger ◽  
M. Krämer
Keyword(s):  
Gravity Waves ◽  
Large Scale ◽  
Full Range ◽  
Ice Nucleation ◽  
Ice Formation ◽  
Ice Crystal ◽  
Ice Clouds ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
Homogeneous Freezing

Abstract. The occurrence of high, persistent ice supersaturation inside and outside cold cirrus in the tropical tropopause layer (TTL) remains an enigma that is intensely debated as the "ice supersaturation puzzle". However, it was recently confirmed that observed supersaturations are consistent with very low ice crystal concentrations, which is incompatible with the idea that homogeneous freezing is the major method of ice formation in the TTL. Thus, the tropical tropopause "ice supersaturation puzzle" has become an "ice nucleation puzzle". To explain the low ice crystal concentrations, a number of mainly heterogeneous freezing methods have been proposed. Here, we reproduce in situ measurements of frequencies of occurrence of ice crystal concentrations by extensive model simulations, driven by the special dynamic conditions in the TTL, namely the superposition of slow large-scale updraughts with high-frequency short waves. From the simulations, it follows that the full range of observed ice crystal concentrations can be explained when the model results are composed from scenarios with consecutive heterogeneous and homogeneous ice formation and scenarios with pure homogeneous ice formation occurring in very slow (< 1 cm s−1) and faster (> 1 cm s−1) large-scale updraughts, respectively. This statistical analysis shows that about 80% of TTL cirrus can be explained by "classical" homogeneous ice nucleation, while the remaining 20% stem from heterogeneous and homogeneous freezing occurring within the same environment. The mechanism limiting ice crystal production via homogeneous freezing in an environment full of gravity waves is the shortness of the gravity waves, which stalls freezing events before a higher ice crystal concentration can be formed.

scholarly journals Aerosol observations and growth rates in the tropical tropopause layer

2012 ◽  
Vol 12 (1) ◽  
pp. 2355-2394
Author(s):  
D. A. Waddicor ◽  
G. Vaughan ◽  
T. W. Choularton ◽  
K. N. Bower ◽  
H. Coe ◽  
...  
Keyword(s):  
Sulphuric Acid ◽  
Satellite Images ◽  
Size Range ◽  
Aerosol Particles ◽  
Current Theory ◽  
Particle Nucleation ◽  
Back Trajectories ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause

Abstract. We present a case study of Aitken and accumulation mode aerosol observed downwind of the anvils of deep tropical thunderstorms. The measurements were made by condensation nuclei counters flown on the Egrett high-altitude aircraft from Darwin during the ACTIVE campaign, in monsoon conditions producing widespread convection over land and ocean. Maximum measured concentrations of aerosol in the size range 10–100 nm were 25 000 cm−3 STP. By calculating back-trajectories from the observations, and projecting on to infrared satellite images, the time since the air exited cloud was estimated. In this way a time scale of ~ 3–4 h was derived for the 10–100 nm aerosol concentration to reach its peak. We examine the hypothesis that the growth in aerosol concentrations can be explained by production of sulphuric acid from SO2 followed by particle nucleation and coagulation. Estimates of the sulphuric acid production rate show that the observations are only consistent with this hypothesis if the particles coagulate to sizes > 10 nm much more quickly than is suggested by current theory. Alternatively, other condensible gases (possibly organic) drive the growth of aerosol particles in the TTL.

scholarly journals Ice nucleation and dehydration in the Tropical Tropopause Layer

2013 ◽  
Vol 110 (6) ◽  
pp. 2041-2046 ◽  
Author(s):  
E. J. Jensen ◽  
G. Diskin ◽  
R. P. Lawson ◽  
S. Lance ◽  
T. P. Bui ◽  
...  
Keyword(s):  
Ice Nucleation ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause

Effects of Aerosol Solubility and Regeneration on Mixed-Phase Orographic Clouds and Precipitation

2012 ◽  
Vol 69 (6) ◽  
pp. 1994-2010 ◽  
Author(s):  
Lulin Xue ◽  
Amit Teller ◽  
Roy Rasmussen ◽  
Istvan Geresdi ◽  
Zaitao Pan ◽  
...  
Keyword(s):  
Aerosol Particles ◽  
Precipitation Amount ◽  
Forecast Model ◽  
Ice Nucleation ◽  
Mixed Phase ◽  
Two Dimensional ◽  
Microphysics Scheme ◽  
Orographic Clouds ◽  
Mixed Phase Clouds ◽  
Aerosol Properties

Abstract A detailed bin aerosol-microphysics scheme has been implemented into the Weather Research and Forecast Model to investigate the effects of aerosol solubility and regeneration on mixed-phase orographic clouds and precipitation. Two-dimensional simulations of idealized moist flow over two identical bell-shaped mountains were carried out using different combinations of aerosol regeneration, solubility, loading, ice nucleation parameterizations, and humidity. The results showed the following. 1) Pollution and regenerated aerosols suppress the riming process in mixed-phase clouds by narrowing the drop spectrum. In general, the lower the aerosol solubility, the broader the drop spectrum and thus the higher the riming rate. When the solubility of initial aerosol increases with an increasing size of aerosol particles, the modified solubility of regenerated aerosols reduces precipitation. 2) The qualitative effects of aerosol solubility and regeneration on mixed-phase orographic clouds and precipitation are not affected by different ice nucleation parameterizations. 3) The impacts of aerosol properties on rain are similar in both warm- and mixed-phase clouds. Aerosols exert weaker impact on snow and stronger impact on graupel compared to rain as graupel production is strongly affected by riming. 4) Precipitation of both warm- and mixed-phase clouds is most sensitive to aerosol regeneration, then to aerosol solubility, and last to modified solubility of regenerated aerosol; however, the precipitation amount is mainly controlled by humidity and aerosol loading.

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