A linear relationship between vertical velocity and condensation processes in deep convection

Vertical velocities and microphysical processes within deep convection are intricately linked, having wide-ranging impacts on water and mass vertical transport, severe weather, extreme precipitation, and the global circulation. The goal of this research is to investigate the functional form of the relationship between vertical velocity, w, and microphysical processes that convert water vapor into condensed water, M, in deep convection. We examine an ensemble of high-resolution simulations spanning a range of tropical and midlatitude environments, a variety of convective organizational modes, and different model platforms and microphysics schemes. The results demonstrate that the relationship between w and M is robustly linear, with the slope of the linear fit being primarily a function of temperature and secondarily a function of supersaturation. The R2 of the linear fit is generally above 0.6 except near the freezing and homogeneous freezing levels. The linear fit is examined both as a function of local in-cloud temperature and environmental temperature. The results for in-cloud temperature are more consistent across the simulation suite, although environmental temperatures are more useful when considering potential observational applications. The linear relationship between w and M is substituted into the condensate tendency equation and rearranged to form a diagnostic equation for w. The performance of the diagnostic equation is tested in several simulations, and it is found to diagnose the storm-scale updraft speeds to within 1 m s−1 throughout the upper half of the clouds. Potential applications of the linear relationship between w and M and the diagnostic w equation are discussed.

High homogeneous freezing onsets of sulfuric acid aerosol at cirrus temperatures

Homogeneous freezing of aqueous solution aerosol particles is an important process for cloud ice formation in the upper troposphere. There the air temperature is low, the ice supersaturation can be high and the concentration of ice-nucleating particles is too low to initiate and dominate cirrus cloud formation by heterogeneous ice nucleation processes. The most common description to quantify homogeneous freezing processes is based on the water activity criterion (WAC) as proposed by Koop et al. (2000). The WAC describes the homogeneous nucleation rate coefficients only as a function of the water activity, which makes this approach well applicable in numerical models. In this study, we investigate the homogeneous freezing behavior of aqueous sulfuric acid aerosol particles by means of a comprehensive collection of laboratory-based homogeneous freezing experiments conducted at the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud simulation chamber, which were conducted as part of 17 measurement campaigns since 2007. The most recent experiments were conducted during October 2020 with special emphasis on temperatures below 200 K. Aqueous sulfuric acid aerosol particles of high purity were generated by particle nucleation in a gas flow composed of clean synthetic air and sulfuric acid vapor, which was added to the AIDA chamber. The resulting chamber aerosol had number concentrations from 30 cm−3 up to several thousand per cubic centimeter with particle diameters ranging from about 30 nm to 1.1 µm. Homogeneous freezing of the aerosol particles was measured at simulated cirrus formation conditions in a wide range of temperatures between 185 and 230 K with a steady increase of relative humidity during each experiment. At temperatures between about 205 K and about 230 K, the AIDA results agree well with the WAC-based predictions of homogeneous freezing onsets. At lower temperatures, however, the AIDA results show an increasing deviation from the WAC-based predictions towards higher freezing onsets. For temperatures between 185 and 205 K, the WAC-based ice saturation ratios for homogeneous freezing onsets increase from about 1.6 to 1.7, whereas the AIDA measurements show an increase from about 1.7 to 2.0 in the same temperature range. Based on the experimental results of our direct measurements, we suggest a new fit line to formulate the onset conditions of homogeneous freezing of sulfuric acid aerosol particles as an isoline for nucleation rate coefficients between 5×108 and 1013 cm−3 s−1. The potential significant impacts of the higher homogeneous freezing thresholds, as directly observed in the AIDA experiments under simulated cirrus formation conditions, on the model prediction of cirrus cloud occurrence and related cloud radiative effects are discussed.

Ice nucleation on surrogates of boreal forest SOA particles: effect of water content and oxidative age

We investigate the effect of water content and oxidative age on ice nucleation using 100 nm monodisperse particles of boreal forest secondary organic aerosol (SOA) surrogates. Ice nucleation experiments are conducted in the temperature range between 210 and 240 K and from ice to water saturation using the Spectrometer for Ice Nuclei (SPIN). The effect of the particle water content on the ice nucleation process is tested by preconditioning α-pinene SOA at different humidities (40 %, 10 % and <1 % RHW). The influence of the particle oxidative age is tested by varying their O:C ratio (oxygen-to-carbon ratio, O:C ∼0.45, 0.8, 1.1). To assess the suitability of α-pinene as a model compound to study the ice nucleation properties of boreal forest SOA and to confirm the atmospheric relevance of our findings, we compare them to measurements of SOA using pine-needle oil or Scots pine tree emissions as precursors. The ice nucleation measurements show that surrogates of boreal forest SOA particles promote only homogeneous ice formation. An effect of preconditioning humidity on homogeneous ice nucleation could be observed. Contrary to the expected behavior, homogeneous freezing is suppressed for SOA particles with high water content (preconditioned at 40 % RHW) and was only observed for SOA preconditioned at low RHW (≤10 %). No dependence of homogeneous freezing on the SOA oxidative age was observed. The results can be explained by a significant change of particulate water diffusivity as a function of humidity (from 10 % to 40 % RHW) at 293 K, where the aerosol is preconditioned. The measurements suggest that at low temperatures, water diffusion into dry SOA particles is slow enough to form a core-shell morphology. The liquid outer layer can equilibrate within the timescale of the experiment and freeze homogeneously. On SOA particles with higher water content, water diffuses faster into the particle, delaying equilibration at the particle surface and preventing the formation of a diluted shell, which can delay homogeneous freezing. We propose that the partial water vapor pressure to which the particles are exposed prior to an experiment can serve as an indicator of whether a core-shell structure is developing.

High Homogeneous Freezing Onsets of Sulfuric Acid Aerosol at Cirrus Temperatures

Homogeneous freezing of aqueous solution aerosol particles is an important process for cloud ice formation in theupper troposphere. There the air temperature is low, the ice supersaturation can be high, and the concentration of ice-nucleating particles is too low to initiate and dominate cirrus cloud formation by heterogeneous ice nucleation processes. The most common description to quantify homogeneous freezing processes is based on the water activity criterion (WAC) as proposed by Koop et al. (2000). The WAC describes the homogeneous nucleation rate coefficients only as a function of the water activity, temperature and size of the aqueous aerosol particles, which makes this approach well applicable in numerical models. In this study, we investigate the homogeneous freezing behaviour of aqueous sulfuric acid aerosol particles by means of a comprehensive collection of laboratory homogeneous freezing experiments conducted at the AIDA (Aerosol Interaction and Dynamicsin the Atmosphere) cloud simulation chamber, which were conducted as part of 17 measurement campaigns since 2007. The most recent experiments were conducted during October 2020 with special emphasis on temperatures below 200 K. Aqueous sulfuric acid aerosol particles were generated at high purity by particle nucleation in a gas flow composed of clean synthetic air and sulfuric acid vapor, which was added to the AIDA chamber. The resulting chamber aerosol had number concentrations from 30 cm−3 up to several 1000 cm−3 with particle diameters ranging from about 30 nm to 1.1 μm. Homogeneous freezing of the aerosol particles was measured at simulated cirrus formation conditions in a wide range of temperatures between 185 K and 230 K with a steady increase of relative humidity during each experiment. At temperatures between about 205 K and about 230 K, the AIDA results agree well with the WAC-based predictions of homogeneous freezing onsets. At lower temperatures, however, the AIDA results show an increasing deviation from the WAC-based predictions towards higher freezing onsets. For temperatures between 185 K and 205 K, the WAC-based ice saturation ratios for homogeneous freezing onsets increase from about 1.6 to 1.7, whereas the AIDA measurements show an increase from about 1.7 to 2.0 in the same temperature range. Based on our experimental results, we suggest a new fit line as a parameterization for the onset conditions of homogeneous freezing of sulfuric acid aerosol particles. As a next step, we propose the new parameterization to be implemented in atmospheric models as an improved version of the WAC-based parameterization from Koop et al. (2000). The new homogeneous freezing thresholds may have significant impacts on the prediction of cirrus cloud occurrence and related cloud radiative effects.

Evidence of in Situ Cirrus Formation in the Tropical Tropopause Layer over the Southwestern Indian Ocean

&lt;p&gt;A nascent in situ cirrus was observed on 11 January 2019 in the tropical tropopause layer (TTL) over the southwestern Indian Ocean, with the use of balloon-borne instruments. Data from CFH (Cryogenic Frost Point Hygrometer) and COBALD (Compact Optical Backscatter and AerosoL Detector) instruments were used to characterize the cirrus and its environment. Optical modeling was employed to estimate the cirrus microphysical&lt;br&gt;properties from the COBALD backscatter measurements. Newly-formed ice crystals with radius &lt;1 &amp;#956;m and concentration &amp;#8764;500 L &lt;sup&gt;&amp;#8722;1&lt;/sup&gt; were reported at the tropopause. The relatively low concentration and CFH ice supersaturation (1.5) suggests a homogeneous freezing event stalled by a high-frequency gravity wave. The observed vertical wind speed and temperature anomalies that triggered the cirrus formation were due to a 1.5-km vertical-&lt;br&gt;scale wave, as shown by a spectral analysis. This cirrus observation shortly after nucleation is beyond remote sensing capabilities and presents a type of cirrus never reported before.&lt;/p&gt;

A microphysical scheme for secondary ice in ICON – evaluation and case studies

&lt;p&gt;Several physical mechanisms of secondary ice production are proposed and studied in laboratory experiments and observational measurements. We implemented a selection of empirical parameterisations for rime splintering, frozen droplet fragmentation and ice-ice collisional break-up in the two-moment microphysics ice modes scheme within the atmosphere model ICON.&lt;/p&gt;&lt;p&gt;The newly developed ice modes scheme distinguishes between different ice modes of origin including homogeneous nucleation, deposition freezing, immersion freezing, homogeneous freezing of water droplets and secondary ice production respectively. Each ice mode is described by its own size distribution, prognostic moments and unique formation mechanism while still interacting with all other ice modes and microphysical classes&lt;br&gt;like cloud droplets, rain and rimed cloud particles. This allows to evaluate the contribution of each ice formation mechanism, especially&lt;br&gt;secondary ice, to the total ice content.&lt;/p&gt;&lt;p&gt;Using this set-up we investigated the sensitivity and behavior of rime splintering, frozen droplet fragmentation and ice-ice collisional break-up for various parameterisations, coefficients and environmental conditions. We will present findings from idealized convection simulations as well as synoptic simulations of Europe and the North Atlantic.&lt;/p&gt;

Homogeneous Freezing of Water Using Microfluidics

The homogeneous freezing of water is important in the formation of ice in clouds, but there remains a great deal of variability in the representation of the homogeneous freezing of water in the literature. The development of new instrumentation, such as droplet microfluidic platforms, may help to constrain our understanding of the kinetics of homogeneous freezing via the analysis of monodisperse, size-selected water droplets in temporally and spatially controlled environments. Here, we evaluate droplet freezing data obtained using the Lab-on-a-Chip Nucleation by Immersed Particle Instrument (LOC-NIPI), in which droplets are generated and frozen in continuous flow. This high-throughput method was used to analyse over 16,000 water droplets (86 μm diameter) across three experimental runs, generating data with high precision and reproducibility that has largely been unrepresented in the microfluidic literature. Using this data, a new LOC-NIPI parameterisation of the volume nucleation rate coefficient (JV(T)) was determined in the temperature region of −35.1 to −36.9 °C, covering a greater JV(T) compared to most other microfluidic techniques thanks to the number of droplets analysed. Comparison to recent theory suggests inconsistencies in the theoretical representation, further implying that microfluidics could be used to inform on changes to parameterisations. By applying classical nucleation theory (CNT) to our JV(T) data, we have gone a step further than other microfluidic homogeneous freezing examples by calculating the stacking-disordered ice–supercooled water interfacial energy, estimated to be 22.5 ± 0.7 mJ m−2, again finding inconsistencies when compared to theoretical predictions. Further, we briefly review and compile all available microfluidic homogeneous freezing data in the literature, finding that the LOC-NIPI and other microfluidically generated data compare well with commonly used non-microfluidic datasets, but have generally been obtained with greater ease and with higher numbers of monodisperse droplets.

Ice nucleation on surrogates of boreal forest SOA particles: effect of water content and oxidative age

α-pinene is an abundant volatile organic compound (VOC) emitted by boreal forests and a source of atmospheric Secondary Organic Aerosol (SOA). This precursor is commonly used as a model compound for SOA studies representing boreal forest emissions. α-pinene SOA particles can have a highly viscous solid or semi-solid phase state depending on water content, temperature and oxidative age. The phase state (or viscosity) of SOA particles has multiple effects on the chemical and physical processes in which SOA particles are involved; one of the affected processes is ice formation. We investigate the effect of water content and oxidative age on ice nucleation using 100 nm quasi-monodisperse particles of boreal forest SOA surrogates . Ice nucleation experiments are conducted in the temperature range between 210 and 240 K and from ice to water saturation using the Spectrometer for Ice Nuclei (SPIN). The effect of the particle water content on the ice nucleation process is tested by preconditioning α-pinene SOA at different humidity (40 %, 10 % and <1 % RHw). The influence of the particle oxidative age is tested by varying their O : C ratio (oxygen-to-carbon ratios, O : C ∼ 0.45, 0.8, 1.1). To assess the suitability of α-pinene as a model compound to study the ice nucleation properties of boreal forest SOA and to confirm the atmospheric relevance of our findings, we compare them to measurements of SOA using pine-needle oil or Scots pine tree emissions as precursors. The ice nucleation measurements show that surrogates of boreal forest SOA particles promote only homogeneous ice formation. An effect of preconditioning humidity on homogeneous ice nucleation could be observed. Contrary to the expected behavior, homogeneous freezing is suppressed for SOA particles with high water content (preconditioned at 40 % RHw) and was only observed for SOA preconditioned at low RHw (≤ 10 %). No dependence of homogeneous freezing on the SOA oxidative age was observed. The results can be explained by a significant change of particulate water diffusivity as a function of humidity (from 10 % to 40 % RHw) at 293 K, where the aerosol is preconditioned. On dry SOA particles, water diffusion into the particle is slow enough to form a core-shell morphology with an outer layer that can equilibrate within the timescale of the experiment and freeze homogeneously. On SOA particles with higher water content, water diffuses faster into the particle, delaying equilibration at the particle surface and preventing the formation of a diluted shell, which can delay homogeneous freezing. To predict if a core-shell develops, we propose that the partial water vapor pressure particles are exposed to prior to an experiment can serve as an indicator.

Mineral and biological ice-nucleating particles above the South East of the British Isles

A small fraction of aerosol particles known as Ice-Nucleating Particles (INPs) have the potential to trigger ice formation in cloud droplets at higher temperatures than homogeneous freezing. INPs can strongly...

A New Method for Ice–Ice Aggregation in the Adaptive Habit Model

A novel methodology for modeling ice–ice aggregation is presented. This methodology combines a modified hydrodynamic collection algorithm with bulk aggregate characteristic information from an offline simulator that collects ice particles, namely, the Ice Particle and Aggregate Simulator, and has been implemented into the Adaptive Habit Microphysics scheme in the Weather Research and Forecasting Model. Aggregates, or snow, are formed via collection of cloud ice particles, where initial ice characteristics and the resulting geometry determine aggregate characteristics. Upon implementation, idealized squall-line simulations are performed to examine the new methodology in comparison with commonly used bulk microphysics schemes. It is found that the adaptive habit aggregation parameterization develops snow and reduces ice mass and number concentrations compared to other schemes. The development of aggregates through the new methodology cascades into other interesting effects, including enhancements in ice and snow growth, as well as homogeneous freezing. Further microphysical analyses reveal varying sensitivities, where snow processes are most sensitive to the new parameterization, followed by ice, then cloud, rain, and graupel processes. Further, the new scheme results in enhancements in surface precipitation due to the persistence of snow at lower altitudes. This persistence is a result of shape-dependent melting and sublimation, increasing the residence time. Moreover, these low-level enhancements are reflected in increases in radar reflectivity at the surface and its spatial distribution. Finally, the ability to predict snow shape and density allows for the simulation of polarimetric radar quantities, resulting in signature enhancements compared to schemes that do not consider spatial and temporal variations in snow shape and density.