Collisional Breakup Research Articles

Secondary ice production processes in wintertime alpine mixed-phase clouds

Abstract. Observations of orographic mixed-phase clouds (MPCs) have long shown that measured ice crystal number concentrations (ICNCs) can exceed the concentration of ice nucleating particles by orders of magnitude. Additionally, model simulations of alpine clouds are frequently found to underestimate the amount of ice compared with observations. Surface-based blowing snow, hoar frost, and secondary ice production processes have been suggested as potential causes, but their relative importance and persistence remains highly uncertain. Here we study ice production mechanisms in wintertime orographic MPCs observed during the Cloud and Aerosol Characterization Experiment (CLACE) 2014 campaign at the Jungfraujoch site in the Swiss Alps with the Weather Research and Forecasting model (WRF). Simulations suggest that droplet shattering is not a significant source of ice crystals at this specific location, but breakups upon collisions between ice particles are quite active, elevating the predicted ICNCs by up to 3 orders of magnitude, which is consistent with observations. The initiation of the ice–ice collisional breakup mechanism is primarily associated with the occurrence of seeder–feeder events from higher precipitating cloud layers. The enhanced aggregation of snowflakes is found to drive secondary ice formation in the simulated clouds, the role of which is strengthened when the large hydrometeors interact with the primary ice crystals formed in the feeder cloud. Including a constant source of cloud ice crystals from blowing snow, through the action of the breakup mechanism, can episodically enhance ICNCs. Increases in secondary ice fragment generation can be counterbalanced by enhanced orographic precipitation, which seems to prevent explosive multiplication and cloud dissipation. These findings highlight the importance of secondary ice and seeding mechanisms – primarily falling ice from above and, to a lesser degree, blowing ice from the surface – which frequently enhance primary ice and determine the phase state and properties of MPCs.

A Comparison of Convective and Stratiform Precipitation Microphysics of the Record-breaking Typhoon In-Fa (2021)

In July 2021, Typhoon In-Fa attacked eastern China and broke many records for extreme precipitation over the last century. Such an unrivaled impact results from In-Fa’s slow moving speed and long residence time due to atmospheric circulations. With the supports of 66 networked surface disdrometers over eastern China and collaborative observations from the advanced GPM satellite, we are able to reveal the unique precipitation microphysical properties of the record-breaking Typhoon In-Fa (2021). After separating the typhoon precipitation into convective and stratiform types and comparing the drop size distribution (DSD) properties of Typhoon In-Fa with other typhoons from different climate regimes, it is found that typhoon precipitation shows significant internal differences as well as regional differences in terms of DSD-related parameters, such as mass-weighted mean diameter (Dm), normalized intercept parameter (Nw), radar reflectivity (Z), rain rate (R), and intercept, shape, and slope parameters (N0, µ, Λ). Comparing different rain types inside Typhoon In-Fa, convective rain (Nw ranging from 3.80 to 3.96 mm−1 m−3) shows higher raindrop concentration than stratiform rain (Nw ranging from 3.40 to 3.50 mm−1 m−3) due to more graupels melting into liquid water while falling. Large raindrops occupy most of the region below the melting layer in convective rain due to a dominant coalescence process of small raindrops (featured by larger ZKu, Dm, and smaller N0, µ, Λ), while small raindrops account for a considerable proportion in stratiform rain, reflecting a significant collisional breakup process of large raindrops (featured by smaller ZKu, Dm, and larger N0, µ, Λ). Compared with other typhoons in Hainan and Taiwan, the convective precipitation of Typhoon In-Fa shows a larger (smaller) raindrop concentration than that of Taiwan (Hainan), while smaller raindrop diameter than both Hainan and Taiwan. Moreover, the typhoon convective precipitation measured in In-Fa is more maritime-like than precipitation in Taiwan. Based on a great number of surface disdrometer observational data, the GPM precipitation products were further validated for both rain types, and a series of native quantitative precipitation estimation relations, such as Z–R and R–Dm relations were derived to improve the typhoon rainfall retrieval for both ground-based radar and spaceborne radar.

Determination of the SmO+ bond energy by threshold photodissociation of the cryogenically cooled ion.

The SmO+ bond energy has been measured by monitoring the threshold for photodissociation of the cryogenically cooled ion. The action spectrum features a very sharp onset, indicating a bond energy of 5.596 ± 0.004eV. This value, when combined with the literature value of the samarium ionization energy, indicates that the chemi-ionization reaction of atomic Sm with atomic oxygen is endothermic by 0.048 ± 0.004eV, which has important implications on the reactivity of Sm atoms released into the upper atmosphere. The SmO+ ion was prepared by electrospray ionization followed by collisional breakup of two different precursors and characterized by the vibrational spectrum of the He-tagged ion. The UV photodissociation threshold is similar for the 10K bare ion and the He tagged ion, which rules out the possible role of metastable electronically excited states. Reanalysis and remeasurement of previous reaction kinetics experiments that are dependent on D0(SmO+) are included, bringing all experimental results in accord.

Sensitivity of precipitation formation to secondary ice production in winter orographic mixed-phase clouds

Abstract. The discrepancy between the observed concentration of ice nucleating particles (INPs) and the ice crystal number concentration (ICNC) remains unresolved and limits our understanding of ice formation and, hence, precipitation amount, location and intensity. Enhanced ice formation through secondary ice production (SIP) could account for this discrepancy. Here, in a region over the eastern Swiss Alps, we perform sensitivity studies of additional simulated SIP processes on precipitation formation and surface precipitation intensity. The SIP processes considered include rime splintering, droplet shattering during freezing and breakup through ice–graupel collisions. We simulated the passage of a cold front at Gotschnagrat, a peak at 2281 m a.s.l. (above sea level), on 7 March 2019 with the Consortium for Small-scale Modeling (COSMO), at a 1 km horizontal grid spacing, as part of the RACLETS (Role of Aerosols and CLouds Enhanced by Topography and Snow) field campaign in the Davos region in Switzerland. The largest simulated difference in the ICNC at the surface originated from the breakup simulations. Indeed, breakup caused a 1 to 3 orders of magnitude increase in the ICNC compared to SIP from rime splintering or without SIP processes in the control simulation. The ICNCs from the collisional breakup simulations at Gotschnagrat were in best agreement with the ICNCs measured on a gondola near the surface. However, these simulations were not able to reproduce the ice crystal habits near the surface. Enhanced ICNCs from collisional breakup reduced localized regions of higher precipitation and, thereby, improved the model performance in terms of surface precipitation over the domain.

Microphysical Origin of Raindrop Size Distributions During the Indian Monsoon

AbstractSurface raindrop size distributions over a rain shadow region during the Indian summer monsoon are clustered using the k‐means algorithm. The rainfall for five dominant clusters has distinct vertical features in polarimetric radar and micro rain radar. The deep convection with low cloud bases and high liquid water is associated with sharply increasing radar reflectivity at low levels and the largest drops at the surface. The large drops, originated by ice processes, break while falling below the melting layer, causing a peak in raindrops smaller than 0.5 mm diameter near the cloud base, where the falling raindrops rapidly grow by collision‐coalescence without breaking. The number concentration in the stratiform rain is relatively moderate; hence a gradual raindrop growth, and uniform vertical reflectivity. However, few raindrops in stratiform rain grow to a larger diameter than in convective rain, for the same rain rate due to the absence of collisional breakup.

Impacts of secondary ice production on Arctic mixed-phase clouds based on ARM observations and CAM6 single-column model simulations

Abstract. For decades, measured ice crystal number concentrations have been found to be orders of magnitude higher than measured ice-nucleating particle number concentrations in moderately cold clouds. This observed discrepancy reveals the existence of secondary ice production (SIP) in addition to the primary ice nucleation. However, the importance of SIP relative to primary ice nucleation remains highly unclear. Furthermore, most weather and climate models do not represent SIP processes well, leading to large biases in simulated cloud properties. This study demonstrates a first attempt to represent different SIP mechanisms (frozen raindrop shattering, ice–ice collisional breakup, and rime splintering) in a global climate model (GCM). The model is run in the single column mode to facilitate comparisons with the Department of Energy (DOE)'s Atmospheric Radiation Measurement (ARM) Mixed-Phase Arctic Cloud Experiment (M-PACE) observations. We show the important role of SIP in four types of clouds during M-PACE (i.e., multilayer, single-layer stratus, transition, and frontal clouds), with the maximum enhancement in ice crystal number concentrations up to 4 orders of magnitude in moderately supercooled clouds. We reveal that SIP is the dominant source of ice crystals near the cloud base for the long-lived Arctic single-layer mixed-phase clouds. The model with SIP improves the occurrence and phase partitioning of the mixed-phase clouds, reverses the vertical distribution pattern of ice number concentrations, and provides a better agreement with observations. The findings of this study highlight the importance of considering SIP in GCMs.

Evidence for early fragmentation-reassembly of ordinary chondrite (H, L, and LL) parent bodies from REE-in-two-pyroxene thermometry

Ordinary chondrites (OCs) are variably thermally metamorphosed meteorites thought to originate from at least three different parent bodies (H, L, and LL) in the Main Belt of asteroids. The thermal evolutions of OC parent bodies are frequently explained by the onion shell model; however, a competing hypothesis is the fragmentation-reassembly model. The onion shell model proposes undisrupted, internally heated parent bodies with concentrically stratified thermal structure, and posits that OC petrologic types (i.e., 3–6) develop with increasing temperature and burial depth. In this model, petrologic types are inversely correlated with cooling rate in the parent body. The alternative fragmentation-reassembly model invokes catastrophic collisional disruption of parent bodies that initially possessed onion shell structures, followed by rapid reaccretion of hot fragments, forming rubble pile bodies. Fragmentation would result in fast cooling (quenching) of collisional fragments from the temperature experienced by the parent body at the time of collision. Discrimination between these two models may be possible via investigation of the thermal histories of OCs by application of geothermometry and geospeedometry, which are used to constrain the temperatures and rates through which igneous and metamorphic rock samples cool. Most published cooling rate data for OC parent bodies are based on methods that record rates through low closure temperatures (∼500–200 °C) rather than from peak metamorphic temperatures. Recently, a rare earth element (REE)-in-two-pyroxene thermometer has been shown to establish peak or magmatic temperatures (TREE; Liang et al. [2013]) for rocks that cooled at moderate to fast geologic rates. We applied the REE-in-two-pyroxene method to determine peak temperatures for 18 OC samples (mostly type 6), in conjunction with conventional two-pyroxene thermometry (TBKN; Brey and Köhler [1990]) and Ca-in-olivine thermometry (TCa-Ol; Köhler and Brey [1990]), to determine closure temperatures and estimate cooling rates for OC parent bodies. Inconsistent with slow cooling rates expected within an onion shell structure, we obtain fast cooling at rates ≳0.3 °C/y from peak temperatures of ∼900 °C. Corroborating the TREE and TBKN measurements, TCa-Ol suggests that the OCs cooled through TCa-Ol closure temperatures (∼700 to 800 °C) at ∼10−2 to 10−1 °C/y. These cooling rates are three to six orders of magnitude faster than rates determined using methods sensitive to low temperature (≤500 °C) cooling (e.g., metallography, 40Ar–39Ar ages, 244Pu fission track). We developed a novel numerical thermal model that incorporates fragmentation of an initial onion shell body and reassembly into a rubble pile body that reproduces both the fast cooling from high temperatures and the slow cooling through low temperatures observed in chondritic meteorites. We hypothesize that OC parent bodies initially possessed onion shell thermal structures, but later experienced collisional breakup, then reaccreted rapidly to form thermally stable rubble-pile asteroids.

Hurricane Dorian Outer Rain Band Observations and 1D Particle Model Simulations: A Case Study

The availability of high quality surface observations of precipitation and volume observations by polarimetric operational radars make it possible to constrain, evaluate, and validate numerical models with a wide variety of microphysical schemes. In this article, a novel particle-based Monte-Carlo microphysical model (called McSnow) is used to simulate the outer rain bands of Hurricane Dorian which traversed the densely instrumented precipitation research facility operated by NASA at Wallops Island, Virginia. The rain bands showed steady stratiform vertical profiles with radar signature of dendritic growth layers near −15 °C and peak reflectivity in the bright band of 55 dBZ along with polarimetric signatures of wet snow with sizes inferred to exceed 15 mm. A 2D-video disdrometer measured frequent occurrences of large drops >5 mm and combined with an optical array probe the drop size distribution was well-documented in spite of uncertainty for drops <0.5 mm due to high wind gusts and turbulence. The 1D McSnow control run and four numerical experiments were conducted and compared with observations. One of the main findings is that even at the moderate rain rate of 10 mm/h collisional breakup is essential for the shape of the drop size distribution.

Raindrop size distribution and microphysical characteristics of a great rainstorm in 2016 in Beijing, China

This paper analyzes the characteristics of rain drop size distributions (DSDs) and vertical profiles of hydrometeors for a severe rainstorm event that occurred in Beijing, northern China, from 19 to 21 July 2016 using in situ measurements from two disdrometers (OTT-Parsivel2 and 2D-Video-Distrometer) and remote sensing data collected by a second-generation Micro Rain Radar (MRR2). This rainstorm was characterized by stratiform precipitation with embedded strong convection on 19 July and widespread, intense mixed convective-stratified rainfall resulting in persistent rainfall between 20 and 21 July. The temporal and spatial variations in rainfall rate R, liquid water content W, reflectivity Z, DSDs and the gamma DSD parameters in different precipitation episodes of this event are derived and compared between two meteorological stations in Beijing. Rainfall throughout the entire event was characterized by large concentrations of small- to medium-sized raindrops (diameters between 2 and 4 mm) and few larger particles (D > 4 mm), resulting in small values of reflectivity Z (<40 dBZ) and liquid water content W (<1.0 g m−3). In addition, the log10Nw-Dm scatter pairs recorded during the convective phase of this rainstorm indicate maritime-like convection. The time evolution of DSDs shows that most single peak DSDs occurred during stratiform rainfall, and bimodal DSDs were observed when the convective rainfall cells passed by and collisional breakups occurred. The gamma DSD model with a single peak in the stratiform phase of the storm and the bimodal DSD in the convective episode of the storm are quantitatively evaluated. In addition, in each precipitation episode, the differences in exponent b and coefficient A of the Z-R relationship between the two stations are smaller for stratiform rainfall than for convective rainfall due to the more evident variability of drop size and number concentration in convective rain. The physical nature of this relationship and the possible mechanisms leading to diverse characteristics in different phases of this heavy rainstorm are discussed.

A Modeling Study of the Effects of Vertical Wind Shear on the Raindrop Size Distribution in Typhoon Nida (2016)

AbstractIn this study, the effects of vertical wind shear (VWS) on the raindrop size distribution in tropical cyclone are investigated, based on the theoretical analyses that intense VWS, which commonly appears in the lower layers of tropical cyclones, can enhance the collisional breakup of raindrops. This is achieved by comparing the numerical sensitivity experiments of Typhoon Nida (2016) using Weather Research and Forecasting model against the polarimetric radar and disdrometer observations. In the control run with the default Morrison microphysics, unrealistic large raindrops are produced with excessively large differential reflectivity and heavier precipitation. An obvious decrease of raindrop size is present when the constant value of cloud droplet number concentration reduces from 250 to 30 cm−3 (NC30), leading to more comparable microphysics characteristics with the observations due to the enhancement of autoconversion rate from cloud droplets to raindrops; however, some unrealistic large‐sized raindrops still exist. A semiempirical raindrop collection/breakup parameterization is further proposed in NC30_WS run by modifying the threshold diameter of raindrops at which breakup occurs as a function of VWS. The certain improvements of simulating raindrop size distribution and precipitation in Typhoon Nida are present in NC30_WS run, owing to the efficient collisional breakup and the induced stronger raindrop evaporation cooling. Our results indicate that the combined effects of reasonable cloud droplet numbers as well as reliable raindrop breakup parameterization are pronounced and highlight the impacts of VWS on raindrop size distribution, which should be involved in current microphysical schemes to forecast the TCs more accurately.

Three‐Moment Representation of Rain in a Bulk Microphysics Model

AbstractA bulk three‐moment representation for rain microphysics is developed and implemented in the Predicted Particle Properties (P3) microphysics scheme. In addition, a new parameterization for rain self‐collection and collisional breakup (RSCB) is presented using a lookup table approach, based on the Spectral‐Bin Model (SBM). To quantify the impacts of sedimentation, evaporation, and RSCB on drop size distributions (DSDs), a rain shaft model is applied to a wide range of atmospheric scenarios (i.e., initial conditions and regimes) and compared against results from the SBM. DSD shapes are mainly determined by both sedimentation and evaporation, except in heavy rain where the impact of RSCB on DSD shape becomes more important than evaporation. The new parameterization for RSCB has a considerable impact on the mean drop size, improving the agreement between P3 and SBM. Only 4% of the original two‐moment rainshaft simulations have mean drop sizes and rain rates within ±20% of the SBM results, but this increases to more than 95% agreement when the three‐moment rain representation is used together with the new parameterization for RSCB. Generally, the improvement is more significant for heavy rain than for light drizzle. Remaining differences between bin and bulk model are attributable to treatments of evaporation, and the restriction to gamma DSDs in P3.

The effect of secondary ice production parameterization on the simulation of a cold frontal rainband

Abstract. Secondary ice production via processes like rime splintering, frozen droplet shattering, and breakup upon ice hydrometeor collision have been proposed to explain discrepancies between in-cloud ice crystal and ice-nucleating particle numbers. To understand the impact of this additional ice crystal generation on surface precipitation, we present one of the first studies to implement frozen droplet shattering and ice–ice collisional breakup parameterizations in a mesoscale model. We simulate a cold frontal rainband from the Aerosol Properties, PRocesses, And InfluenceS on the Earth's Climate campaign and investigate the impact of the new parameterizations on the simulated ice crystal number concentrations (ICNC) and precipitation. Near the convective regions of the rainband, contributions to ICNC can be as large from secondary production as from primary nucleation, but ICNCs greater than 50 L−1 remain underestimated by the model. The addition of the secondary production parameterizations also clearly intensifies the differences in both accumulated precipitation and precipitation rate between the convective towers and non-convective gap regions. We suggest, then, that secondary ice production parameterizations be included in large-scale models on the basis of large hydrometeor concentration and convective activity criteria.

A representation of the collisional ice break-up process in the two-moment microphysics LIMA v1.0 scheme of Meso-NH

Abstract. The paper describes a switchable parameterization of collisional ice break-up (CIBU), an ice multiplication process that fits in with the two-moment microphysical Liquid Ice Multiple Aerosols (LIMA) scheme. The LIMA scheme with three ice types (pristine cloud ice crystals, snow aggregates, and graupel hail) was developed in the cloud-resolving mesoscale model (Meso-NH). Here, the CIBU parameterization assumes that collisional break-up is mostly efficient for the small and fragile snow aggregate class of particles when they are hit by large, dense graupel particles. The increase of cloud ice number concentration depends on a prescribed number (or a random number) of fragments being produced per collision. This point is discussed and analytical expressions of the newly contributing CIBU terms in LIMA are given. The scheme is run in the cloud-resolving mesoscale model (Meso-NH) to simulate a first case of a three-dimensional deep convective event with heavy production of graupel. The consequence of dramatically changing the number of fragments produced per collision is investigated by examining the rainfall rates and the changes in small ice concentrations and mass mixing ratios. Many budgets of the ice phase are shown and the sensitivity of CIBU to the initial concentration of freezing nuclei is explored. The scheme is then tested for another deep convective case where, additionally, the convective available potential energy (CAPE) is varied. The results confirm the strong impact of CIBU with up to a 1000-fold increase in small ice concentrations, a reduction of the rainfall or precipitating area, and an invigoration of the convection with higher cloud tops. Finally, it is concluded that the efficiency of the ice crystal fragmentation needs to be tuned carefully. The proposed parameterization of CIBU is easy to implement in any two-moment microphysics scheme. It could be used in this form to simulate deep tropical cloud systems where anomalously high concentrations of small ice crystals are suspected.

On the age of the Nele asteroid family

The Nele group, formerly known as the Iannini family, is one of the youngest asteroid families in the main belt. Previously, it has been noted that the pericenter longitudes $\varpi$ and nodal longitudes $\Omega$ of its largest member asteroids are clustered at the present time, therefore suggesting that the collisional breakup of parent body must have happened recently. Here we verify this conclusion by detailed orbit-propagation of a synthetic Nele family and show that the current level of clustering of secular angles of the largest Nele family members requires an approximate age limit of $4.5$ Myr. Additionally, we make use of an updated and largely extended Nele membership to obtain, for the first time, an age estimate of this family using the Backward Integration Method (BIM). Convergence of the secular angles in a purely gravitational model and in a model including the non-gravitational forces caused by the Yarkovsky effect are both compatible with an age younger than $7$ Myr. More accurate determination of the Nele family age would require additional data about the spin state of its members.

Initiation of secondary ice production in clouds

Abstract. Disparities between the measured concentrations of ice-nucleating particles (INPs) and in-cloud ice crystal number concentrations (ICNCs) have led to the hypothesis that mechanisms other than primary nucleation form ice in the atmosphere. Here, we model three of these secondary production mechanisms – rime splintering, frozen droplet shattering, and ice–ice collisional breakup – with a six-hydrometeor-class parcel model. We perform three sets of simulations to understand temporal evolution of ice hydrometeor number (Nice), thermodynamic limitations, and the impact of parametric uncertainty when secondary production is active. Output is assessed in terms of the number of primarily nucleated ice crystals that must exist before secondary production initiates (NINP(lim)) as well as the ICNC enhancement from secondary production and the timing of a 100-fold enhancement. Nice evolution can be understood in terms of collision-based nonlinearity and the “phasedness” of the process, i.e., whether it involves ice hydrometeors, liquid ones, or both. Ice–ice collisional breakup is the only process for which a meaningful NINP(lim) exists (0.002 up to 0.15 L−1). For droplet shattering and rime splintering, a warm enough cloud base temperature and modest updraft are the more important criteria for initiation. The low values of NINP(lim) here suggest that, under appropriate thermodynamic conditions for secondary ice production, perturbations in cloud concentration nuclei concentrations are more influential in mixed-phase partitioning than those in INP concentrations.

Ice Multiplication by Breakup in Ice–Ice Collisions. Part II: Numerical Simulations

AbstractIn Part I of this two-part paper, a formulation was developed to treat fragmentation in ice–ice collisions. In the present Part II, the formulation is implemented in two microphysically advanced cloud models simulating a convective line observed over the U.S. high plains. One model is 2D with a spectral bin microphysics scheme. The other has a hybrid bin–two-moment bulk microphysics scheme in 3D. The case consists of cumulonimbus cells with cold cloud bases (near 0°C) in a dry troposphere.Only with breakup included in the simulation are aircraft observations of particles with maximum dimensions &gt;0.2 mm in the storm adequately predicted by both models. In fact, breakup in ice–ice collisions is by far the most prolific process of ice initiation in the simulated clouds (95%–98% of all nonhomogeneous ice), apart from homogeneous freezing of droplets. Inclusion of breakup in the cloud-resolving model (CRM) simulations increased, by between about one and two orders of magnitude, the average concentration of ice between about 0° and −30°C. Most of the breakup is due to collisions of snow with graupel/hail. It is broadly consistent with the theoretical result in Part I about an explosive tendency for ice multiplication.Breakup in collisions of snow (crystals &gt;~1 mm and aggregates) with denser graupel/hail was the main pathway for collisional breakup and initiated about 60%–90% of all ice particles not from homogeneous freezing, in the simulations by both models. Breakup is predicted to reduce accumulated surface precipitation in the simulated storm by about 20%–40%.

Wind Effects on the Shape of Raindrop Size Distribution

AbstractThe wind effects on the shape of drop size distribution (DSD) and the driving microphysical processes for the DSD shape evolution were investigated using the dataset from the Midlatitude Continental Convective Clouds Experiment (MC3E). The quality-controlled DSD spectra from MC3E were grouped for each of the rainfall events by considering the precipitation type (stratiform vs convective) and liquid water content for the analysis. The DSD parameters (e.g., mass-weighted mean diameter) and the fitted DSD slopes for these grouped spectra showed statistically significant trends with varying wind speed. Increasing wind speeds were observed to modify the DSD shapes by increasing the number of small drops and decreasing the number of large drops, indicating that the raindrop breakup process governs the DSD shape evolution. Both spontaneous and collisional raindrop breakup modes were analyzed to elucidate the process responsible for the DSD shape evolution with varying wind speed. The analysis revealed that the collisional breakup process controls the wind-induced DSD shape. The findings of this study are of importance in DSD parameterizations that are essential to a wide variety of applications such as radar rainfall retrievals and hydrologic models.

An increased abundance of micrometeorites on Earth owing to vesicular parachutes

AbstractMicrometeorites (MMs) are extraterrestrial dust particles that survive atmospheric entry and can be recovered from sedimentary rocks. Fossil MMs allow events beyond the Earth, such as the collisional breakup of asteroids, to be identified. Here the effects of vesicle formation during melting of dust are investigated through numerical modeling and observations of Antarctic MMs. Vesicle formation is shown to cause a parachute effect that causes rapid deceleration, decreasing peak temperature. Vesicular parachuting enhances the abundance of melted MMs formed from phyllosilicate‐bearing C‐type asteroid dust on the Earth surface by a factor of 2. Micrometeorites recovered from the geological record, therefore, are biased toward breakup events involving hydrated C‐type asteroids, whilst those involving phyllosilicate‐poor particles are diluted by the enhanced background flux of hydrous dust. The parachute effect is also likely to increase the delivery of 3He to ocean sediments by C‐type asteroid dust.

Optimization of the Nano-Dust Analyzer (NDA) for operation under solar UV illumination

The performance of the Nano-Dust Analyzer (NDA) instrument is analyzed for close pointing to the Sun, finding the optimal field-of-view (FOV), arrangement of internal baffles and measurement requirements. The laboratory version of the NDA instrument was recently developed (O׳Brien et al., 2014) for the detection and elemental composition analysis of nano-dust particles. These particles are generated near the Sun by the collisional breakup of interplanetary dust particles (IDP), and delivered to Earth׳s orbit through interaction with the magnetic field of the expanding solar wind plasma. NDA is operating on the basis of impact ionization of the particle and collecting the generated ions in a time-of-flight fashion. The challenge in the measurement is that nano-dust particles arrive from a direction close to that of the Sun and thus the instrument is exposed to intense ultraviolet (UV) radiation. The performed optical ray-tracing analysis shows that it is possible to suppress the number of UV photons scattering into NDA׳s ion detector to levels that allow both high signal-to-noise ratio measurements, and long-term instrument operation. Analysis results show that by avoiding direct illumination of the target, the photon flux reaching the detector is reduced by a factor of about 103. Furthermore, by avoiding the target and also implementing a low-reflective coating, as well as an optimized instrument geometry consisting of an internal baffle system and a conical detector housing, the photon flux can be reduced by a factor of 106, bringing it well below the operation requirement. The instrument׳s FOV is optimized for the detection of nano-dust particles, while excluding the Sun. With the Sun in the FOV, the instrument can operate with reduced sensitivity and for a limited duration. The NDA instrument is suitable for future space missions to provide the unambiguous detection of nano-dust particles, to understand the conditions in the inner heliosphere and its temporal variability, and to constrain the chemical differentiation and processing of IDPs.

Identification and Analysis of Collisional Breakup in Natural Rain

Abstract Numerous laboratory and numerical studies have been dedicated to understanding collisional breakup as one of the most important processes in rain formation. The present study aims to identify when, in natural rain, collisional breakup is dominant and thus able to modify the shape of the raindrop size distribution (DSD), up to the equilibrium DSD. To this end, an automated objective algorithm has been developed and applied to a total of more than 6000 two-minute-averaged DSDs. Since breakup is mostly observed in heavy precipitation, the method was applied to the DSDs where rain rate was above 5 mm h−1. The selected breakup DSDs had good agreement with those predicted to be the equilibrium DSD by different theoretical models. The equilibrium DSD was found in a variable fraction of the total samples (0%–7%), confirming that the onset of equilibrium is a rare event in natural rain. The occurrence of a DSD in which breakup is dominant and modifies the DSD but the equilibrium DSD is not reached is higher (15%–47%). The gamma distribution, which has been widely used in the parameterization of observed size spectra, had a poor fitting in breakup-induced DSD, especially in the 1.0–2.6-mm-diameter interval. This can impact applications for which the parameterization of DSD is needed, such as in the retrieval of a DSD integral parameter (such as rain rate) from active remote sensor data.