Supercooled Large Drops Research Articles

Improving Supercooled Water Forecast Through Real‐Time Aerosol Input: A Case Study

AbstractAccurately simulating and forecasting supercooled water has been crucial for addressing the threat of aircraft icing. This study utilized the Weather Research and Forecast model and Aerosol‐Aware Thompson‐Eidhammer microphysics (TE14) scheme to simulate supercooled water properties. Three high‐resolution experiments were conducted to explore the impact of increased model resolution and different aerosol initial conditions. The synoptic‐scale characteristics and cloud extent were well‐matched between observations and the model simulation, which established a background for supercooled water comparisons. Microphysical characteristics of supercooled droplets and the formation of supercooled large drops demonstrated the TE14 scheme's reasonable simulation of supercooled water microphysics under different aerosol loadings. In situ aircraft measurements were used to validate the simulated supercooled water properties. Results highlighted the sensitivity of supercooled water simulation to aerosol number concentration via droplet activation in the scheme. The choice for the aerosol initial condition could result in a significant underestimation or overestimation of supercooled water number concentration, consequently leading to biased estimations of droplet size. Incorporating real‐time forecast aerosol data improved the simulation by increasing cloud droplet number concentration and reducing droplet size. These findings underscored the importance of aerosols via droplet activation in accurately simulating cloud water properties and emphasized their role in forecasting aircraft icing hazards.

Particle size distribution measurements under supercooled large drop conditions with Cloud Combination Probe in CARDC icing wind tunnel

Particle size distribution (PSD) of the icing cloud is one of the most important icing parameters for icing wind tunnels. However, accurate measurements of the PSD are challenging for supercooled large drop (SLD) conditions due to the wide droplet size spectrum and low number density of large droplets. In this study, PSD measurements with the Cloud Combination Probe (CCP) are comprehensively investigated in the CARDC icing wind tunnel. A method based on the CCP is developed for composite PSD calculation, and an icing cloud measurement test with 79 test conditions covering Appendix C and freezing drizzle conditions, is conducted for accuracy assessment of the CCP and reliability validation of the PSD calculation method. Based on the test, the effects of spurious particle images are examined first, followed by a quantitative assessment of the influences of particle size calculation and sample volume calculation on the PSD, medium volume diameter (MVD), and liquid water content (LWC). The results show that all PSD measurements are adversely impacted by spurious particle images, primarily caused by coincidence errors. For five typical particle size parameters, the effects of particle size calculation on the PSD are significant, with maximum relative deviations of about 110% for MVD and 63% for LWC. The influences of Depth-of-Field factors are not noticeable except for the manufacturer factor. Additionally, the dead time also dramatically impacts the PSD, resulting in a decrease in the MVD and LWC with maximum decreases of about 60% and 43%, respectively. Finally, the LWCs determined by composite PSDs show good agreement with the reference LWCs obtained by the SEA multi-element sensor, where the relative deviation is within ±10% for the majority of measured points. This demonstrates the CCP is suitable for wind tunnel measurements of PSDs under Appendix C and freezing drizzle conditions employing the PSD calculation method. To accurately calculate the PSD, spurious particle image removal and dead-time correction should be considered, and out-of-focus correction diameter and in-house factor are preferred.

The Prediction of Supercooled Large Drops by a Microphysics and a Machine Learning Model for the ICICLE Field Campaign

Abstract The prediction of supercooled large drops (SLD) from the Thompson–Eidhammer (TE) microphysics scheme—run as part of the High-Resolution Rapid Refresh (HRRR) model—is evaluated with observations from the In-Cloud Icing and Large drop Experiment (ICICLE) field campaign. These observations are also used to train a random forest machine learning (ML) model, which is then used to predict SLD from several variables derived from HRRR model output. Results provide insight on the limitations and benefits of each model. Generally, the ML model results in an increase in the probability of detection (POD) and false alarm rate (FAR) of SLD compared to prediction from TE microphysics. Additionally, the POD of SLD increases with increasing forecast lead time for both models, likely since clouds and precipitation have more time to develop as forecast length increases. Since SLD take time to develop in TE microphysics and may be poorly represented in short-term (<3 h) forecasts, the ML model can provide improved short-term guidance on supercooled large-drop icing conditions. Results also show that TE microphysics predicts a frequency of SLD in cold (<−10°C) or high ice water content (IWC) environments that is too low compared to observations, whereas the ML model better captures the relative frequency of SLD in these environments.

The Secondary Production of Ice in Cumulus Experiment (SPICULE)

Abstract The secondary ice process (SIP) is a major microphysical process, which can result in rapid enhancement of ice particle concentration in the presence of preexisting ice. SPICULE was conducted to further investigate the effect of collision–coalescence on the rate of the fragmentation of freezing drop (FFD) SIP mechanism in cumulus congestus clouds. Measurements were conducted over the Great Plains and central United States from two coordinated aircraft, the NSF Gulfstream V (GV) and SPEC Learjet 35A, both equipped with state-of-the-art microphysical instrumentation and vertically pointing W- and Ka-band radars, respectively. The GV primarily targeted measurements of subcloud aerosols with subsequent sampling in warm cloud. Simultaneously, the Learjet performed multiple penetrations of the ascending cumulus congestus (CuCg) cloud top. First primary ice was typically detected at temperatures colder than −10°C, consistent with measured ice nucleating particles. Subsequent production of ice via FFD SIP was strongly related to the concentration of supercooled large drops (SLDs), with diameters from about 0.2 to a few millimeters. The concentration of SLDs is directly linked to the rate of collision–coalescence, which depends primarily on the subcloud aerosol size distribution and cloud-base temperature. SPICULE supports previous observational results showing that FFD SIP efficiency could be deduced from the product of cloud-base temperature and maximum diameter of drops measured ∼300 m above cloud base. However, new measurements with higher concentrations of aerosol and total cloud-base drop concentrations show an attenuating effect on the rate of coalescence. The SPICULE dataset provides rich material for validation of numerical schemes of collision–coalescence and SIP to improve weather prediction simulations

Experimental investigation on the performances of a valve-based and on-demand droplet generator producing droplets in a wide size range

Many aspects of our daily lives are affected by the generation of water droplets, and it is important to controllably produce droplets with a wide size range in various applications. In this paper, we describe an on-demand droplet generating system based on a high-speed two-way solenoid valve. A nozzle made of stainless steel tubing is fit into one port of the valve, and the other port is connected to the fluid reservoir by which the pressure can be controlled via a pressure regulator. When the liquid is pressurized and the valve is opened with a short pulse voltage, trace amounts of liquid will be ejected from the nozzle to form a droplet. Droplet generation is captured using a high-speed camera to measure the dimension and velocity and to evaluate the performances of the generator, such as repeatability and stability. We demonstrate the influences of applied pressure and pulse width of driving voltage on droplet generation. It is shown that the droplet generator is capable of producing droplets in a wide size range for a given nozzle (e.g., about 0.7–2.2 mm for a 1.0 mm diameter nozzle). A single droplet is stably generated at Z = 268.1, obviously higher than the published data and the Weber number of a fluid jet (Wej) ranging from 2.1 to 5.6. The droplet generator presented here will be useful for research related to large droplets, such as freezing rain in atmospheric science and supercooled large drops in aircraft icing.

A Mesh-Free Formalism for Ice Accretion Prediction Due to Supercooled Large Drop Impingement

A Mesh-Free Formalism for Ice Accretion Prediction Due to Supercooled Large Drop Impingement

Aircraft Observations of Cumulus Microphysics Ranging from the Tropics to Midlatitudes: Implications for a “New” Secondary Ice Process

AbstractIn situ data collected by three research aircraft in four geographical locations are analyzed to determine the relationship between cloud-base temperature, drop size distribution, and the development of supercooled water drops and ice in strong updraft cores of convective clouds. Data were collected in towering cumulus and feeder cells in the Caribbean, over the Gulf of Mexico, over land near the Gulf Coast, over land in the southeastern United States, and the high plains in Colorado and Wyoming. Convective clouds in the Caribbean, over the Gulf of Mexico and its coast, and over the southeastern United States all develop millimeter-diameter supercooled drops in updraft cores. Clouds over the high plains do not generate supercooled large drops, and rarely are drops >70 μm observed in updraft cores. Commensurate with the production of supercooled large drops, ice is generated and rapidly glaciates updraft cores through a hypothesized secondary ice process that is based on laboratory observations of large drops freezing and emitting tiny ice particles. Clouds over the high plains do not experience the secondary ice process and significant concentrations of supercooled liquid in the form of small drops are carried much higher (up to −35.5°C) in the updraft cores. An empirical relationship that estimates the maximum level to which supercooled liquid water will be transported, based on cloud-base drop size distribution and temperature, is developed. Implications have applications for modeling the transport of water vapor and particles into the upper troposphere and hygroscopic seeding of cumulus clouds.

Icing test capabilities at DGA Aero-engine Testing

Since the 1960s, DGA Aero-engine Testing has built facilities to test and certify parts and equipment of aircraft and rotorcraft in icing conditions. For aerofoil or blade testing, a 2D test set-up has been developed. For engine tests, a free jet configuration is used. Until recently, the standard conditions for icing were associated with the presence of clouds composed of supercooled small droplets. Nevertheless, recent accidents and incidents have highlighted the danger of other icing conditions such as supercooled large drops (SLD), ice crystals and mixed phase which is composed of ice crystals and small droplets. So, current and future challenges include the ability to simulate these conditions. To generate droplets larger than 100 micrometres, a new atomiser has been developed and calibration methods have been modified in consequence. The next planned step is the generation of ice crystals and mixed phase.

Study on Airworthiness Problems of Operating in Supercooled Large Drops Icing Conditions for Transport Category Airplanes

FAA has issued a Notice of Proposed Rulemaking (NPRM) on supercooled large drops (SLD) icing conditions[1]. Based on following up and studying of the newest SLD icing materials, the paper gives a detailed analysis of SLD icing conditions for transport category airplanes airworthiness problems. The proposed Part 25, Appendix O about SLD icing conditions are analyzed and compared with Part 25, Appendix C, including icing clouds extend or range, altitude-temperature envelop, liquid water content (LWC) and drop diameter distribution. The changes of icing envelopes used by IPS design and certification are detailed analyzed. The proposed SLD amendment related regulations are summarized, and the proposed new §25.1420 are deeply analyzed, including the requirements of each subsection, and the requirements of the three SLD icing certification options. Moreover, the certification and compliance of the proposed §25.1420(a) directly referring to the three certification options are discussed, respectively.

Reply to “Comments on ‘Characterization of Aircraft Icing Environments with Supercooled Large Drops for Application to Commercial Aircraft Certification'”

Cober et al. (2009) and Cober and Isaac (2012) characterized aircraft icing environments that contained supercooledlargedrops (SLD).100mm indiameteron the basis of data collected during 134 research flights into winter storms in three geographic regions of North America. The characterization was designed to be supplemental to existing aircraft icing certification envelopes, given that existing envelopes do not explicitly incorporate SLD conditions. The U.S. Federal Aviation Administration is considering using the results from Cober et al. (2009) and Cober and Isaac (2012) as the basis for new certification regulations. Marwitz (2013) has suggested that the results of Cober and Isaac (2012) are deficient with respect to their application to the proposed new certification regulations, and he provided some criticisms. The purpose of this response is to address these criticisms. Marwitz (2013) suggested that the use of median volume diameter (MVD) and maximum diameter (Dmax) by Cober and Isaac (2012) as part of their characterization method is invalid because MVD and Dmax are not related to performance measurement. Cober and Isaac (2012) made no attempt to measure performance degradation of the research aircraft. Rather the intent was to collect and analyze a sufficiently large dataset to characterize icing environments that included SLD at the 99% and 99.9% levels (i.e., extreme values) so that manufacturers could subsequently determine the associated performance degradation on their instruments, aircraft components, or engines. A justification for using MVD and Dmax follows. MVD is the 50% mass diameter, and hence large MVD values (i.e., .40mm) specifically identify icing conditions that are outside the existing certifications standards and for which the majority of the mass is incorporated in drops that have higher collision efficiency (and corresponding performance degradation) with aircraft surfaces. Aircraft, instrument, and engine manufacturers use different combinations of MVD (or other drop diameter characteristics), temperature, and liquid water content (LWC) to evaluate performance losses primarily through their use in wind-tunnel, naturalicing, and numerical simulation experiments. Aircraft icing envelopes have been based in part on MVD since their inception in the 1950s. It is an industry standard to correlate the analysis of icing conditions against performance degradation, and MVD is one of the primary

Comments on “Characterization of Aircraft Icing Environments with Supercooled Large Drops for Application to Commercial Aircraft Certification”

Comments on “Characterization of Aircraft Icing Environments with Supercooled Large Drops for Application to Commercial Aircraft Certification”

Characterization of Aircraft Icing Environments with Supercooled Large Drops for Application to Commercial Aircraft Certification

AbstractObservations of aircraft icing environments that included supercooled large drops (SLD) greater than 100 μm in diameter have been analyzed. The observations were collected by instrumented research aircraft from 134 flights during six field programs in three different geographic regions of North America. The research aircraft were specifically instrumented to accurately measure the microphysics characteristics of SLD conditions. In total 2444 SLD icing environments were observed at 3-km resolution. Each observation had an average liquid water content (LWC) > 0.005 g m−3, drops > 100 μm in diameter, ice crystal concentrations <1 L−1, and an average static temperature ≤0°C. SLD conditions were observed approximately 5% of the in-flight time. The SLD observations were segregated into four subsets, which included conditions with maximum drop sizes <500 μm and >500 μm in diameter, each with median drop volume diameters <40 μm and >40 μm. For each SLD subset, the observations were used to develop envelopes of maximum LWC values as a function of horizontal extent and temperature. In addition, characteristic drop size distributions were developed for each SLD subset. The maximum LWC values physically represent either the 99% or 99.9% LWC values, as determined from an extreme value analysis of the data. The analysis is sufficient for simulation of SLD environments with either numerical icing accretion models or wind-tunnel icing simulations. The SLD envelopes are similar in structure and supplemental to existing aircraft icing envelopes, the difference being that the existing envelopes did not explicitly incorporate SLD conditions.

An Inferred Climatology of Icing Conditions Aloft, Including Supercooled Large Drops. Part II: Europe, Asia, and the Globe

Abstract Because of a lack of regular, direct measurements, limited information is available about the frequency and the spatial and temporal distribution of icing conditions aloft, including supercooled large drops (SLD). Research aircraft provide in situ observations of these conditions, but the sample set is small and can be biased. Surface observations of freezing fog and freezing precipitation provide additional insight, but cannot be used alone to assess the presence of icing aloft. Climatologies based solely on such observations can underestimate their presence in areas where subfreezing temperatures are uncommon. Other techniques can be used in an effort to reduce some of these biases and limitations. Expanding upon results for North America reported in Part I, the frequencies of icing and SLD over Europe and Asia are inferred here using 1) surface weather observations in conjunction with vertical profiles of temperature and moisture and 2) model reanalyses of temperature and moisture. Icing maxima and minima are found to migrate seasonally, both geographically and in the vertical, and to vary in their intensity. They are linked to the location of common storm tracks and other forcing, such as that associated with sloped terrain. After establishing reasonable consistency between the methods over data rich regions, the model analyses are used to examine icing frequencies over the remainder of the globe.

An Investigation of the Influence of Droplet Number Concentration and Giant Aerosol Particles upon Supercooled Large Drop Formation in Wintertime Stratiform Clouds

Abstract Supercooled large drops (SLD) can be a significant hazard for aviation. Past studies have shown that warm-rain processes are prevalent, or even dominant, in stratiform clouds containing SLD, but the primary factors that control SLD production are still not well understood. Giant aerosol particles have been shown to accelerate the formation of the first drizzle drops in some clouds and thus are a viable source of SLD, but observational support for testing their effectiveness in supercooled stratiform clouds has been lacking. In this study, new observations collected during six research flights from the Alliance Icing Research Study II (AIRS II) are analyzed to assess the factors that may be relevant to SLD formation, with a particular emphasis on the importance of giant aerosol particles. An initial comparison of observed giant aerosol particle number concentrations with the observed SLD suggests that they were present in sufficient numbers to be the source of the SLD. However, microphysical calculations within an adiabatic parcel model, initialized with the observed aerosol distributions and cloud properties, suggest that the giant aerosol particles were only a limited source of SLD. More SLD was produced in the modeled clouds with low droplet concentrations, simply by an efficient warm-rain process acting at temperatures below 0°C. For cases in which the warm-rain process is limited by a higher droplet concentration and small cloud depth/liquid water content, the giant aerosol particles were then the only source of SLD. The modeling results are consistent with the observed trends in SLD across the six AIRS II cases.

An Inferred Climatology of Icing Conditions Aloft, Including Supercooled Large Drops. Part I: Canada and the Continental United States

Abstract Because of a lack of regular, direct measurements, little information is available about the frequency and spatial and temporal distribution of icing conditions aloft, including supercooled large drops (SLD). Research aircraft provide in situ observations of these conditions, but the sample set is small and can be biased. Other techniques must be used to create a more unbiased climatology. The presence and absence of icing and SLD aloft can be inferred using surface weather observations in conjunction with vertical profiles of temperature and moisture. In this study, such a climatology was created using 14 yr of coincident, 12-hourly Canadian and continental U.S. surface weather reports and balloonborne soundings. The conditions were found to be most common along the Pacific Coast from Alaska to Oregon, and in a large swath from the Canadian Maritimes to the Midwest. Prime locations migrated seasonally. Most SLD events appeared to occur below 4 km, were less than 1 km deep, and were formed via the collision–coalescence process.

Aircraft icing research flights in embedded convection

Results from in-cloud measurements with an instrumented aircraft from an icing research campaign in Southern Germany in March 1997 are presented. Measurements with conventional optical cloud probes and of the ice accretion on a cylinder exposed to the flow show the existence of supercooled large drops (SLD) in the size range up to 300 µm simultaneously with severe icing with ice-accretion rates of up to 3.5 mm min−1. Nearly all periods with icing, including the ones with severe icing, occurred in mixed-phase convective cells embedded in surrounding stratus clouds. The spatial scales of SLD occurrence, respectively severe icing, ranged between several hundred meters and some kilometers and correspond to the length of the transects through the embedded cells. SLD formed through the coalescence process and were found through the whole cloud depth pointing to a source region near cloud top, in line with the arguments of Rauber and Tokay (1991). No indication of ice-multiplication by the Hallet-Mossop process was found, despite of the favorable temperatures for that process. Comparisons of the measured amount of accreted ice with the observed cloud-particle size distributions quite surprisingly indicate that ice accretion is mostly caused by 10–30 µm sized drops rather than by SLD. The latter, therefore, appear to be a by-product of a hypothesized liquid water accumulation zone near cloud top which is also the primary cause of the observed severe ice accretion. The results confirm the importance of embedded convection and of mixed phase clouds with high amounts of liquid water and simultaneously occurring SLD.

Characterizations of Aircraft Icing Environments that Include Supercooled Large Drops

Abstract Measurements of aircraft icing environments that include supercooled large drops (SLD) greater than 50 μm in diameter have been made during 38 research flights. These flights were conducted during the First and Third Canadian Freezing Drizzle Experiments. A primary objective of each project was the collection of in situ microphysics data in order to characterize aircraft icing environments associated with SLD. In total there were 2793 30-s averages obtained in clouds with temperatures less than or equal to 0°C, maximum droplet sizes greater than or equal to 50 μm, and ice crystal concentrations less than 1 L−1. The data include measurements from 12 distinct environments in which SLD were formed through melting of ice crystals followed by supercooling in a lower cold layer and from 27 distinct environments in which SLD were formed through a condensation and collision–coalescence process. The majority of the data were collected at temperatures between 0° and −14°C, in stratiform winter clouds assoc...

U.S. government advisory organizations publications on microfiche

FAA has issued a Notice of Proposed Rulemaking (NPRM) on supercooled large drops (SLD) icing conditions[1]. Based on following up and studying of the newest SLD icing materials, the paper gives a detailed analysis of SLD icing conditions for transport category airplanes airworthiness problems. The proposed Part 25, Appendix O about SLD icing conditions are analyzed and compared with Part 25, Appendix C, including icing clouds extend or range, altitude-temperature envelop, liquid water content (LWC) and drop diameter distribution. The changes of icing envelopes used by IPS design and certification are detailed analyzed. The proposed SLD amendment related regulations are summarized, and the proposed new §25.1420 are deeply analyzed, including the requirements of each subsection, and the requirements of the three SLD icing certification options. Moreover, the certification and compliance of the proposed §25.1420(a) directly referring to the three certification options are discussed, respectively.