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In the atmosphere, microphysics refers to the microscale processes that affect cloud and precipitation particles and is a key linkage among the various components of Earth's atmospheric water and energy cycles. The representation of microphysical processes in models continues to pose a major challenge leading to uncertainty in numerical weather forecasts and climate simulations. In this paper, the problem of treating microphysics in models is divided into two parts: (i) how to represent the population of cloud and precipitation particles, given the impossibility of simulating all particles individually within a cloud, and (ii) uncertainties in the microphysical process rates owing to fundamental gaps in knowledge of cloud physics. The recently developed Lagrangian particle‐based method is advocated as a way to address several conceptual and practical challenges of representing particle populations using traditional bulk and bin microphysics parameterization schemes. For addressing critical gaps in cloud physics knowledge, sustained investment for observational advances from laboratory experiments, new probe development, and next‐generation instruments in space is needed. Greater emphasis on laboratory work, which has apparently declined over the past several decades relative to other areas of cloud physics research, is argued to be an essential ingredient for improving process‐level understanding. More systematic use of natural cloud and precipitation observations to constrain microphysics schemes is also advocated. Because it is generally difficult to quantify individual microphysical process rates from these observations directly, this presents an inverse problem that can be viewed from the standpoint of Bayesian statistics. Following this idea, a probabilistic framework is proposed that combines elements from statistical and physical modeling. Besides providing rigorous constraint of schemes, there is an added benefit of quantifying uncertainty systematically. Finally, a broader hierarchical approach is proposed to accelerate improvements in microphysics schemes, leveraging the advances described in this paper related to process modeling (using Lagrangian particle‐based schemes), laboratory experimentation, cloud and precipitation observations, and statistical methods., Key Points Microphysics is an important component of weather and climate models, but its representation in current models is highly uncertainTwo critical challenges are identified: representing cloud and precipitation particle populations and knowledge gaps in cloud physicsA possible blueprint for addressing these challenges is proposed to accelerate progress in improving microphysics schemes

The present study considers final stages of in-cloud mixing in the framework of classical concept of homogeneous and extreme inhomogeneous mixing. Simple analytical relationships between basic microphysical parameters were obtained for homogeneous and extreme inhomogeneous mixing based on the adiabatic consideration. It was demonstrated that during homogeneous mixing the functional relationships between the moments of the droplets size distribution hold only during the primary stage of mixing. Subsequent random mixing between already mixed parcels and undiluted cloud parcels breaks these relationships. However, during extreme inhomogeneous mixing the functional relationships between the microphysical parameters hold both for primary and subsequent mixing. The obtained relationships can be used to identify the type of mixing from in situ observations. The effectiveness of the developed method was demonstrated using in situ data collected in convective clouds. It was found that for the specific set of in situ measurements the interaction between cloudy and entrained environments was dominated by extreme inhomogeneous mixing.

The Convective Precipitation Experiment (COPE) was a joint U.K.–U.S. field campaign held during the summer of 2013 in the southwest peninsula of England, designed to study convective clouds that produce heavy rain leading to flash floods. The clouds form along convergence lines that develop regularly as a result of the topography. Major flash floods have occurred in the past, most famously at Boscastle in 2004. It has been suggested that much of the rain was produced by warm rain processes, similar to some flash floods that have occurred in the United States. The overarching goal of COPE is to improve quantitative convective precipitation forecasting by understanding the interactions of the cloud microphysics and dynamics and thereby to improve numerical weather prediction (NWP) model skill for forecasts of flash floods. Two research aircraft, the University of Wyoming King Air and the U.K. BAe 146, obtained detailed in situ and remote sensing measurements in, around, and below storms on several days. A new fast-scanning X-band dual-polarization Doppler radar made 360° volume scans over 10 elevation angles approximately every 5 min and was augmented by two Met Office C-band radars and the Chilbolton S-band radar. Detailed aerosol measurements were made on the aircraft and on the ground. This paper i) provides an overview of the COPE field campaign and the resulting dataset, ii) presents examples of heavy convective rainfall in clouds containing ice and also in relatively shallow clouds through the warm rain process alone, and iii) explains how COPE data will be used to improve high-resolution NWP models for operational use.

This study aimed to assess tropical cloud properties predicted by Environment and Climate Change Canada's Global Environmental Multiscale (GEM) model when run with the Milbrandt–Yau double‐moment cloud microphysical scheme and one‐way nesting that culminated at a (∼300 km)2 inner domain with 0.25 km horizontal grid spacing. The assessment utilized satellite and in situ data collected during the High Ice Water Content (HIWC) and High Altitude Ice Crystals (HAIC) projects for a mesoscale convective system on 16 May 2015 over French Guiana. Data from CloudSat's cloud‐profiling radar and GOES‐13's imager were compared to data either simulated directly by GEM or produced by operating on GEM's cloud data with both the CFMIP (Cloud Feedback Model Intercomparison Project) Observation Simulator Package (COSP) instrument simulator and a three‐dimensional Monte Carlo solar radiative transfer model. In situ observations were made from research aircraft – Canada's National Research Council Convair‐580 and the French SAFIRE Falcon‐20 – whose flight paths were aligned with CloudSat's ground‐track. Spatial and temporal shifts of clouds simulated by GEM compared well to GOES‐13 imagery. There are, however, differences between simulated and observed amounts of high and low cloud. While GEM did well at predicting ranges of ice‐water content (IWC) near 11 km altitude (Falcon‐20), it produces too much graupel and snow near 7 km (Convair‐580). This produced large differences between CloudSat's and COSP‐generated radar reflectivities and two‐way attenuations. On the other hand, CloudSat's inferred values of IWC agree well with in situ samples at both altitudes. Generally, GEM's visible reflectances exceeded GOES‐13's on account of having produced too much low‐level liquid cloud. It is expected that GEM's disproportioning of cloud hydrometeors will improve once it includes a better representation of secondary ice production.

Occurrences of jet engine power loss and damage have been associated with flight through fully glaciated deep convection at −10 to −50 °C. Power loss events commonly occur during flight through radar reflectivity (Ze) less than 20–30 dBZ and no more than moderate turbulence, often overlying moderate to heavy rain near the surface. During 2010–2012, Airbus carried out flight tests seeking to characterize the highest ice water content (IWC) in such low-Ze regions of large, cold-topped storm systems in the vicinity of Cayenne, Darwin, and Santiago. Within the highest IWC regions encountered, at typical sampling elevations (circa 11 km), the measured ice size distributions exhibit a notably narrow concentration of mass over area-equivalent diameters of 100–500 μm. Given substantial and poorly quantified measurement uncertainties, here we evaluate the consistency of the Airbus in situ measurements with ground-based profiling radar observations obtained under quasi-steady, heavy stratiform rain conditions in one of the Airbus-sampled locations. We find that profiler-observed radar reflectivities and mean Doppler velocities at Airbus sampling temperatures are generally consistent with those calculated from in situ size-distribution measurements. We also find that column simulations using the in situ size distributions as an upper boundary condition are generally consistent with observed profiles of Ze, mean Doppler velocity (MDV), and retrieved rain rate. The results of these consistency checks motivate an examination of the microphysical pathways that could be responsible for the observed size-distribution features in Ackerman et al. (2015).

Prior estimates of ice crystal size distributions derived from 2D cloud probes (2DCs) have been artificially amplified by small ice crystals generated from the shattering of large ice crystals on the probe tips. Although antishatter tips and algorithms exist, there is considerable uncertainty in their effectiveness. This paper examines differences in ice crystal size distributions from adjacent 2DCs with standard and antishatter tips, and processed with and without antishattering algorithms. The measurements were obtained from the National Research Council of Canada Convair-580 during the 2008 Indirect and Semi-Direct Aerosol Campaign (ISDAC) and the National Center for Atmospheric Research C-130 during the 2011 Instrumentation Development and Education in Airborne Science (IDEAS-2011). The 2DC size distributions are compared with those from the Holographic Detector for Clouds (HOLODEC), which has antishatter tips and allows for identification of shattering through spatial statistics.The ratio of the number concentration N of particles with maximum dimensions 125–500 μm from the 2DC with standard tips to that from the 2DC with modified tips was correlated with median mass diameter and perimeter divided by area, but not with airspeed, attack, and attitude angles. Antishatter tips and algorithms reduced N by up to a factor of 10 for IDEAS-2011 and ISDAC, but neither alone removed all artifacts. For the period with coincident data, both N from the HOLODEC and 2DC with modified tips are around 5 × 10−3 L−1 μm−1, suggesting that antishatter tips and algorithms combined remove artifacts from the 2DC for the conditions sampled during IDEAS-2011.

Large-eddy simulations of mixed-phase Arctic clouds by 11 different models are analyzed with the goal of improving understanding and model representation of processes controlling the evolution of these clouds. In a case based on observations from the Indirect and Semi-Direct Aerosol Campaign (ISDAC), it is found that ice number concentration, Ni, exerts significant influence on the cloud structure. Increasing Ni leads to a substantial reduction in liquid water path (LWP), in agreement with earlier studies. In contrast to previous intercomparison studies, all models here use the same ice particle properties (i.e., mass-size, mass-fall speed, and mass-capacitance relationships) and a common radiation parameterization. The constrained setup exposes the importance of ice particle size distributions (PSDs) in influencing cloud evolution. A clear separation in LWP and IWP predicted by models with bin and bulk microphysical treatments is documented and attributed primarily to the assumed shape of ice PSD used in bulk schemes. Compared to the bin schemes that explicitly predict the PSD, schemes assuming exponential ice PSD underestimate ice growth by vapor deposition and overestimate mass-weighted fall speed leading to an underprediction of IWP by a factor of two in the considered case. Sensitivity tests indicate LWP and IWP are much closer to the bin model simulations when a modified shape factor which is similar to that predicted by bin model simulation is used in bulk scheme. These results demonstrate the importance of representation of ice PSD in determining the partitioning of liquid and ice and the longevity of mixed-phase clouds.

In 2014 a new AECL (Airborne Elastic Cloud Lidar) lidar system was installed on-board the NRC Convair-580. AECL is a single wavelength elastic lidar which operates at 355nm and can supply vertical profiles of clouds and aerosols at high vertical and temporal resolution (1.5m and 0.05s). AECL is also equipped with a polarization channel and can provide information on particle phase (i.e. liquid or glaciated). The NRC AECL lidar was flown briefly on March 28, 2014 near Ottawa, Canada. In May of 2015 it was also deployed during the multi-week international HAIC (High Altitude Ice Crystals) – HIWC (High Ice Water Content) campaign near Cayenne, French Guinea. During the midlatitude flight near Ottawa, a convective cloud with cloud top extending to 4000 m was sampled by AECL. The on board in-situ cloud microphysics probes showed that the aircraft climbed through rain below 2 km reaching a mixedphase cloud above the melting layer and finally going through a supercooled layer. The lidar depolarization data from the AECL clearly identified the shallow supercooled layer near the cloud top and ice crystals with high depolarization ratio between the melting layer and the supercooled layer. In the regions of HIWC near Cayenne, the AECL laser beam was generally completely extinguished within the first 200 m. The lidar extinction coefficient, estimated using the Klett inversion technique and taken at 50 m above the aircraft showed a very good qualitative agreement with the measured in-situ extinction at flight level. The lidar extinction values had to be scaled by a factor of 5.88 to match the in-situ data. The discrepancies between the lidar estimated extinction and the direct measurements were explained, in part, by insufficient overlap correction and/or the error in the initial parameters used for the Klett inversion. In general, AECL showed promising initial results and in conjunction with other instrumentation, supplied valuable insight into the cloud optical and microphysical properties.

Aircraft in situ observations of precipitation during the Rain in Cumulus over the Ocean (RICO) field project are used to study and parameterize the effects of precipitation on cloud probes. Specifically, the effects of precipitation on the Forward Scattering Spectrometer Probe, the King cloud liquid water hot-wire probe, and the particle volume monitor are parameterized as linear functions of the precipitation water content.

Data from the new two-dimensional stereo (2D-S) probe are used to evaluate drop size distributions in rain shafts observed during the Rain in Shallow Cumulus over the Ocean (RICO) experiment. The 2D-S takes images of both precipitation drops and cloud droplets with 10-μm resolution. These are the first reported measurements of rain to include sizes smaller than 100 μm. The primary result is that there are almost no hydrometeors smaller than about 100 μm in these rain shafts. The measured low concentration of small hydrometeors implies that their rate of production is slow relative to their removal rate. Algorithms for removing the spurious effects of splashing precipitation and noisy photodiodes on 2D probes are also described.

A theoretical framework has been developed describing nonequilibrium formation and maintenance of mixed-phase clouds. The necessary and sufficient conditions required to activate liquid water within a preexisting ice cloud, and thus convert it to mixed phase, are considered for three scenarios: (i) uniform ascent, (ii) harmonic vertical oscillations, and (iii) turbulent fluctuations. The general conditions are the following:First necessary condition: The vertical velocity of an ice cloud parcel must exceed a threshold velocity to activate liquid water.Second necessary condition: The activation of liquid water within an ice cloud parcel, below water saturation, requires a vertical ascent above some threshold altitude to bring the vapor pressure of the parcel to water saturation. Only when the first and second conditions are true do these conditions become sufficient for the activation of liquid water in ice clouds. These required conditions for the generation of mixed-phase cloud are supported by parcel modeling results and analogous conditions for a harmonic oscillation concerning the amplitude and tangential velocity of the parcel motion are proposed. The authors do not assume steady-state conditions, but demonstrate that nonequilibrium evolution of cloud parcels can lead to long-term steady existence of mixed-phase cloud.

Statistical characteristics of 584 liquid water content (LWC) profiles collected mainly in midlatitude supercooled stratiform frontal clouds during five field campaigns are presented. It has been found that the majority (55%) of liquid layers have thickness less than 500 m and the depth of these layers decreases with decreasing temperature. LWC profiles for thin cloud layers are usually close to quasi-adiabatic, whereas for thick clouds they are approximately constant with an average LWC value close to 0.14 g m−3. Good agreement was observed between cumulative distributions for liquid water paths derived from in situ LWC profiles and those inferred in previous experiments from ground-based 37 GHz radiometers. Copyright © 2007 Crown in the right of Canada. Published by John Wiley & Sons, Ltd

MIT Lincoln Laboratory is tasked by the U.S. Federal Aviation Administration to investigate the use of the NEXRAD polarimetric radars* for the remote sensing of icing conditions hazardous to aircraft. A critical aspect of the investigation concerns validation that has relied upon commercial airline icing pilot reports and a dedicated campaign of in situ flights in winter storms. During the month of February in 2012 and 2013, the Convair-580 aircraft operated by the National Research Council of Canada was used for in situ validation of snowstorm characteristics under simultaneous observation by NEXRAD radars in Cleveland, Ohio and Buffalo, New York. The most anisotropic and easily distinguished winter targets to dual pol radar are ice crystals. Accordingly, laboratory diffusion chamber measurements in a tightly-controlled parameter space of temperature and humidity provide the linkage between shape and the expectation for the presence/absence of water saturation conditions necessary for icing hazard in situ. In agreement with the laboratory measurements pertaining to dendritic and hexagonal flat plate crystals, the aircraft measurements have verified the presence of supercooled water in mainly low concentrations coincident with regions showing layered anomalies of positive differential reflectivity (ZDR) by ground-based radar, otherwise known as +ZDR ‘bright bands’. Extreme values of ZDR (up to +8 dB) have also been found to be coincident with hexagonal flat plate crystals and intermittent supercooled water, also consistent with laboratory measurements. The icing conditions found with the anisotropic description are considered non-classical (condensation/collision-coalescence) and require the ascent of air and availability of ice nuclei. A modest ascent rate (, SAE 2015 International Conference on Icing of Aircraft, Engines, and Structures, June 22-25, 2015, Prague, Czech Republic

The results of in situ observations of the relative humidity in liquid, mixed, and ice clouds typically stratiform in nature and associated with mesoscale frontal systems at temperatures −45°C < Ta < −5°C are presented. The data were collected with the help of instrumentation deployed on the National Research Council (NRC) Convair-580. The length of sampled in-cloud space is approximately 23 × 103 km. The liquid sensor was calibrated in liquid clouds with the assumption that the air in liquid clouds is saturated with respect to water. It was found that the relative humidity in mixed-phase clouds is close to saturation over water in the temperature range from −5° to −35°C for an averaging scale of 100 m. In ice clouds the relative humidity over ice is not necessarily equal to 100%, and it may be either lower or higher than saturation over ice, but it is always lower than saturation over water. On average the relative humidity in ice clouds increases with a decrease of temperature. At −40°C the relative humidity over ice is midway between saturation over ice and liquid. A parameterization for the relative humidity in ice clouds is suggested. A large fraction of ice clouds was found to be undersaturated with respect to ice. The fraction of ice clouds undersaturated with respect to ice increases toward warmer temperatures.

The data on cloud particle sizes and concentrations collected with the help of aircraft imaging probes [optical array probes OAP-2DC, OAP-2DP, and the High Volume Precipitation Spectrometer (HVPS)] are widely used for cloud parameterization and validation of remote sensing. The goal of the present work is to study the effect of shattering of ice particles during sampling. The shattering of ice particles may occur due to 1) mechanical impact with the probe arms prior to their entering the sample volume, and 2) fragmentation due to interaction with turbulence and wind shear generated by the probe housing. The effect of shattering is characterized by the shattering efficiency that is equal to the ratio of counts of disintegrated particles, to all counts. The shattering efficiency depends on the habit, size, and density of ice particles, probe inlet design, and airspeed. For the case of aggregates, the shattering efficiency may reach 10% or even more. The shattering of ice particles results in an overcounting of small particles and an undercounting of large ones. The number of fragments in the images of shattered particles may reach several hundreds. It was found that particles as small as 600 μm may shatter after impact with the probe arms. The effect of particle shattering should be taken into account during data analysis and carefully considered in future designs of airborne cloud particle size spectrometers.

The frequency of occurrence of the aspect ratio and roundness of particles in ice clouds from aircraft observations have been examined. Images of cloud particles were measured by a cloud particle imager (CPI) at 2.3-μm resolution, installed on the National Research Council (NRC) of Canada Convair-580. Data were collected in winter midlatitude and polar stratiform clouds associated with frontal systems during three field projects in the Canadian and U.S. Arctic and over the Great Lakes. Approximately 106 images of particles measured in ice clouds were included in the statistics. The frequency of occurrence of the aspect ratio and roundness were calculated in eight 5° temperature intervals from −40°C to 0°C. In each temperature interval, the distributions were calculated for 12 size intervals in the range from 20 μm to 1 mm. It was found that the roundness is a function of particle size and within each size interval it does not depend significantly on temperature. However, the aspect ratio of parti...

SUMMARY A detailed study of mixed-phase clouds associated with frontal systems obtained from a large dataset collected by the Convair 580 aircraft of the National Research Council (NRC) of Canada is presented. The total length of analysed in-cloud legs having total-water content (TWC) > 0.01g m i3 was about 44 £ 10 3 km. The ice‐water fraction (π3 D ice-water content/TWC) had a minimum in the range 0:1 0:9). The concentration of particles in glaciated clouds was found to be nearly constant at 2‐5 cm i3 for temperatures i35 ± C < T < 0 ± C. The concentration of droplets in liquid clouds decreased with decreasing temperature. The mean volume diameter of particles in ice clouds varied between 20 πm and 35 πm, and in liquid clouds between 10 πm and 12 πm. Both ice- and liquid-water content decreased with decreasing temperature. The results of this study may be used for validation of remote-sensing retrievals, and for weather- and climate-models.

The glaciation time of a mixed-phase cloud due to the Wegener-Bergeron-Findeisen mechanism is calculated using an adiabatic one-dimensional numerical model for the cases of zero, ascending, descending and oscillating vertical velocities. The characteristic values of the glaciation time are obtained for different concentrations of ice particles and liquid-water content. Steady state is not possible for the ice-water content/total water content ratio in a uniformly vertically moving mixed-phase parcel. The vertical oscillation of a cloud parcel may result in a periodic evaporation and activation of liquid droplets in the presence of ice particles during infinite time. After a certain time, the average ice-water content and liquid-water content reach a steady state. This phenomenon may explain the existence of long-lived mixed-phase stratiform layers. The obtained results are important for understanding the mechanisms of formation and life cycle of mixed-phase clouds.

The impact of clouds on weather and climate is determined by their microphysical properties (i.e., the size distribution, water content, optical properties, and shape). Understanding how these properties evolve requires detailed studies with in situ measurements using a suite of different sensors. This article gives an overview of the different techniques that are used to determine these cloud characteristics and examples of some of the types of information that can be extracted from these data.

International audience; Most atmospheric motions of different spatial scales and precipitation are closely related to phase transitions in clouds. The continuously increasing resolution of large-scale and mesoscale atmospheric models makes it feasible to treat the evolution of individual clouds. The explicit treatment of clouds requires the simulation of cloud microphysics. Two main approaches describing cloud microphysical properties and processes have been developed in the past four and a half decades: bulk microphysics parameterization and spectral (bin) microphysics (SBM). The development and utilization of both represent an important step forward in cloud modeling. This study presents a detailed survey of the physical basis and the applications of both bulk microphysics parameterization and SBM. The results obtained from simulations of a wide range of atmospheric phenomena, from tropical cyclones through Arctic clouds using these two approaches are compared. Advantages and disadvantages, as well as lines of future development for these methods are discussed.

Evolution of droplet size distribution (DSD) due to the water vapor diffusion in a vertically moving adiabatic parcels is investigated. Analytical expressions for height dependences of the main DSD parameters and DSD moments are obtained. The asymptotic behavior of the DSD parameters at large heights above cloud base is determined. It is shown that during diffusion growth, the width and the relative dispersion of the DSD decrease with height as z− 1/3 and z− 2/3, respectively. The paper presents examples of DSD evolution in cases DSD forms on aerosols with a three-mode lognormal distribution. The aerosol distribution parameters used in the study correspond to four aerosol types: “Marine,” “Clean continental,” “Background,” and extremely polluted “Urban.” The vertical profiles of DSD parameters are compared with the asymptotic profiles. It is shown that in case of polydisperse DSD evolution, the vertical profile of supersaturation within several hundred meters above the cloud base can be approximated by a supersaturation profile corresponding to the “equivalent” monodisperse DSD. The initial radius of this equivalent DSD is equal to the mean radius of polydisperse DSD (haze size distribution) at cloud base, which is estimated using the Kohler theory. This result of the relation between the polydisperse and monodisperse solutions is universal. A new equation for estimation of supersaturation maximum for polydisperse case is obtained. The obtained analytical expressions and numerical results are useful for understanding the mechanisms of DSD formation in clouds and for parameterization of warm microphysical processes in cloud models.

In situ microphysics measurements made during the First and Third Canadian Freezing Drizzle Experiments (CFDE I and III, respectively) have been used to assess the relative responses to ice and liquid hydrometeors for several common instruments. These included the Rosemount icing detector, 2D-C monoscale and 2D-C grayscale probes, forward-scattering spectrometer probes (FSSP) on three measurement ranges, Nevzorov liquid water content (LWC) and total water content probes, and King LWC probes. The Nevzorov LWC and King LWC probes responded to between 5% and 30% of the ice water content, with an average response of approximately 20%. The average FSSP measurements of droplet spectra were dominated by ice particles for sizes greater than 35 μm, independent of the measurement range used, when the ice-crystal concentrations exceeded approximately 1 L−1. In contrast, the FSSP measurements of the droplet spectra less than 30 μm appeared free of ice-crystal contamination, independent of the ice-crystal con...

In situ measurements of microphysics conditions, obtained during 38 research flights into winter storms, have been used to characterize the performance of a Rosemount Icing Detector (RID). Characteristics of the RID were determined under a wide range of cloud environments, which included icing conditions within mixed phase, freezing rain, and freezing drizzle environments. Cloud conditions observed included temperatures between 0° and −29°C and liquid water contents (LWCs) up to 0.7 g m−3. The detection threshold for LWC was found to be 0.007 ± 0.010 g m−3 for the RID operated at an air speed of 97 ± 10 m s−1, which agrees well with theoretical predictions. A signal level of 0 ± 2 mV s−1 accounted for 99.6% of the measurements in clear air and 98.5% of the measurements in glaciated clouds, when the data were averaged over 30-s intervals. No significant response to glaciated clouds was found during any of the research flights, implying that the instrument can be used to segregate glaciated and mix...

Ice crystals in clouds in the atmosphere have shapes that relate to their density, terminal fall velocity, growth rate and radiative properties. In calculations for climate-change predictions, forecasting of precipitation, and remote-sensing retrievals, idealized crystal shapes such as columns, needles, plates and dendrites are often assumed. The objective of this work is to study the frequency of occurrence of different habits of ice particles in natural clouds from aircraft observations. Images of cloud particles were measured by a PMS Optical Array Probe-2DC at 25 μm resolution installed on the National Research Council Convair-580. The processing of particle images was conducted with a newly developed algorithm for pattern recognition. Data were collected during four field projects in the Canadian and US Arctic over the North Atlantic near Newfoundland, and over the Great Lakes. Approximately 5 × 106 images of cloud particles having a size larger than 125 μm were analysed. The cloud particles were classified into four categories; spheres, irregulars, needles/columns and dendrites. The habit classification of particles was done for three different size ranges: > 125 μm, >250 μm. and >500 μm. The frequency of occurrence of different habits was found for each 5 degC temperature interval in the range −45°C < T < 0°C. It was concluded that the majority of ice particles in natural clouds were of irregular shape. The frequency of occurrence of irregular ice decreases with increasing particle size. On average, the concentration of particles larger than 125 μm was approximately constant down to −35°C, whereas the concentration of particles larger than 500 μm decreased at temperatures below −15°C. Since the data were collected in different climatic zones within many cloud lypes. and covered a significant cloud path length (3.6 × 104 km), the conclusions are applicable to most stratiform clouds containing ice.

[1] Two types of Arctic mixed‐phase clouds observed during the ISDAC and M‐PACE field campaigns are simulated using a 3‐dimensional cloud‐resolving model (CRM) with size‐resolved cloud microphysics. The modeled cloud properties agree reasonably well with aircraft measurements and surface‐based retrievals. Cloud properties such as the probability density function (PDF) of vertical velocity (w), cloud liquid and ice, regimes of cloud particle growth, including the Wegener–Bergeron–Findeisen (WBF) process, and the relationships among properties/processes in mixed‐phase clouds are examined to gain insights for improving their representation in General Circulation Models (GCMs). The PDF of the simulated w is well represented by a Gaussian function, validating, at least for arctic clouds, the subgrid treatment used in GCMs. The PDFs of liquid and ice water contents can be approximated by Gamma functions, and a Gaussian function can describe the total water distribution, but a fixed variance assumption should be avoided in both cases. The CRM results support the assumption frequently used in GCMs that mixed phase clouds maintain water vapor near liquid saturation. Thus, ice continues to grow throughout the stratiform cloud but the WBF process occurs in about 50% of cloud volume where liquid and ice co‐exist, predominantly in downdrafts. In updrafts, liquid and ice particles grow simultaneously. The relationship between the ice depositional growth rate and cloud ice strongly depends on the capacitance of ice particles. The simplified size‐independent capacitance of ice particles used in GCMs could lead to large deviations in ice depositional growth.

The evidence for crystal bouncing and small ice article shattering is presented from wind tunnel studies and aircraft measurements using modified probe geometries. Direct experimental evidence for the shattering hypothesis is obtained with high-speed video recording led by the NASA Glenn Research Center, with support from Environment Canada (EC) at the Cox and Co. wind tunnel. The results show that contamination of particle size distribution caused by shattering of ice particles on probe tips and inlets is a significant problem for airborne microphysical characterization of ice clouds. Large ice particles are found to produce a higher level of contamination with their artifacts than small ice particles. The results also show that by modifying the probe inlets and by applying interarrival time algorithms, the effects of shattering can be significantly reduced.

An overview is presented of airborne systems for in situ measurements of aerosol particles, clouds and radiation that are currently in use on research aircraft around the world. Description of the technology is at a level sufficient for introducing the basic principles of operation and an extensive list of references for further reading is given. A number of newer instruments that implement emerging technology are described and the review concludes with a description of some of the most important measurement challenges that remain. This overview is a synthesis of material from a reference book that is currently in preparation and that will be published in 2012 by Wiley.

[1] Reflectivities recorded by the W-band Cloud Profiling Radar (CPR) aboard NASA's CloudSat satellite and some of CloudSat's retrieval products are compared to Ka-band radar reflectivities and in situ cloud properties gathered by instrumentation on the NRC's Convair-580 aircraft. On 20 February 2007, the Convair flew several transects along a 60 nautical mile stretch of CloudSat's afternoon ground track over southern Quebec. On one of the transects it was well within CloudSat's radar's footprint while in situ sampling a mixed phase boundary layer cloud. A cirrus cloud was also sampled before and after overpass. Air temperature and humidity profiles from ECMWF reanalyses, as employed in CloudSat's retrieval stream, agree very well with those measured by the Convair. The boundary layer cloud was clearly visible, to the eye and lidar, and dominated the region's solar radiation budget. It was, however, often below or near the Ka-band's distance-dependent minimum detectable signal. In situ samples at overpass revealed it to be composed primarily of small, supercooled droplets at the south end and increasingly intermixed with ice northward. Convair and CloudSat CPR reflectivities for the low cloud agree well, but while CloudSat properly ascribed it as overcast, mixed phase, and mostly liquid near the south end, its estimates of liquid water content LWC (and visible extinction coefficient κ) and droplet effective radii are too small and large, respectively. The cirrus consisted largely of irregular crystals with typical effective radii ∼150 μm. While both CPR reflectivities agree nicely, CloudSat's estimates of crystal number concentrations are too large by a factor of 5. Nevertheless, distributions of ice water content and κ deduced from in situ data agree quite well with values retrieved from CloudSat algorithms.

During the course of AIRS 1.5 and AIRS II inflight icing flight research operations, the NRC Convair conducted aerodynamic characterization manoeuvres, following and during icing accretion in a wide range of environmental conditions of altitude, air temperature, LWC and droplet spectra. The paper describe the correlation of the effects of inflight icing accretion on the lift and drag of the NRC Convair, a turbo-propellor powered transport aeroplane, with full droplet spectral parameters, for which the small (FSSP) and large (2D) portions of droplet spectra have been combined. The changes to profile drag, CDo, lift-dependent drag, k, and lift-curve slope, a, were analysed initially for correlation with Ta, LWC and FSSP MVD. With the airframe IPS ON, ΔCDo increases did not correlate with SLW exposure. Maximum Δk and Δa degradations correlated with each other, and with air temperature. With airframe IPS OFF, maximum Δa degradations correlated with exposure to SLW, whereas maximum ΔCDo and Δk correlated with exposure to small droplet conditions. Following 2D data archiving by Environment Canada, a detailed natural icing with atmospheric conditions analysis was conducted, in particular for correlation with LWC and the full-spectra droplet spectral parameters of 50VD (or MVD, when defined as median volumetric diameter), 80VD, 95VD and spectral width, approximated as maximum droplet diameter. When combined with LWC, the parameters of 95VD and maximum droplet diameter provided the better correlation of CDo, k and a icing-induced effects, essentially limiting canyons in the contour cross-plot of effects. The analysis also highlighted the need to include the full droplet size distribution for the determination of the most detrimental icing accretion environmental regimes.

The ARM Climate Research Facility’s (ACRF) Aerial Vehicle Program (AVP) will deploy an intensive cloud and aerosol observing system to the ARM North Slope of Alaska (NSA) locale for a five week Indirect and Semi-Direct Aerosol Campaign (ISDAC) during period 29 March through 30 April 2008. The deployment period is within the International Polar Year, thus contributing to and benefiting from the many ancillary observing systems collecting data synergistically. We will deploy the Canadian National Research Council Convair 580 aircraft to measure temperature, humidity, total particle number, aerosol size distribution, single particle composition, concentrations of cloud condensation nuclei and ice nuclei, optical scattering and absorption, updraft velocity, cloud liquid water and ice contents, cloud droplet and crystal size distributions, cloud particle shape, and cloud extinction. In addition to these aircraft measurements, ISDAC will deploy two instruments at the ARM site in Barrow: a spectroradiometer to retrieve cloud optical depth and effective radius, and a tandem differential mobility analyzer to measure the aerosol size distribution and hygroscopicity. By using many of the same instruments used during Mixed-Phase Arctic Cloud Experiment (M-PACE), conducted in October 2004, we will be able to contrast the arctic aerosol and cloud properties during the fall andmore » spring transitions. The aerosol measurements can be used in cloud models driven by objectively analyzed boundary conditions to test whether the cloud models can simulate the aerosol influence on the clouds. The influence of aerosol and boundary conditions on the simulated clouds can be separated by running the cloud models with all four combinations of M-PACE and ISDAC aerosol and boundary conditions: M-PACE aerosol and boundary conditions, M-PACE aerosol and ISDAC boundary conditions, ISDAC aerosol and M-PACE boundary conditions, and ISDAC aerosol and boundary conditions. ISDAC and M-PACE boundary conditions are likely to be very different because of the much more extensive ocean water during M-PACE. The uniformity of the surface conditions during ISDAC greatly simplifies the objective analysis (surface fluxes and precipitation are very weak), so that it can largely rely on the European Centre for Medium-Range Weather Forecasts analysis. The aerosol measurements can also be used as input to the cloud models and to evaluate the aerosol retrievals. By running the cloud models with and without solar absorption by the aerosols, we can determine the semidirect effect of the aerosol on the clouds.« less

Vertical profiles of liquid water in supercooled frontal stratiform clouds have been studied in order to estimate the potential rate of ice accretion at different levels within the cloud and to develop recommendations for escape strategies to avoid severe in-flight icing. The vertical soundings of the supercooled liquid clouds were obtained using the National Research Council of Canada Convair-580 equipped by Environment Canada for cloud microphysical measurements. The data were collected during five flight campaigns (CFDE 1, CFDE 3, AIRS 1, AIRS 1.5 and AIRS 2). In total 584 vertical LWC profiles were analyzed. A statistical summary has been prepared from the profiles of the potential thickness of accreted ice, liquid water content, temperature, and cloud depths. The maximum potential accreted thickness of ice does not exceed 2cm for a transit with a 3 degree glide slope thoughout the cloud depth. Based on the statistics, in order to avoid severe icing once significant icing is encountered, it is suggested that a climb or descent should be initiated. The aircraft should not stay at the same altitude within the cloud layer. I. Introduction T is difficult for pilots to decide to climb, descend, or stay at the same level when encountering in-flight icing, especially in icing events associated with drizzle. During these periods the airplane may have to traverse one or several supercooled cloud layers and as a result experience in-flight icing. To estimate the potential consequences of in-flight icing, liquid water content (LWC) profiles in supercooled cloud layers associated with frontal systems were analyzed. The vertical distribution of liquid water and temperature inside supercooled clouds is important for estimation of icing severity during any traverse of such layers. The statistical characteristics of supercooled stratiform frontal clouds collected during five aircraft field campaigns are presented here. Effort was focused on an analysis of the statistics of cloud depths, vertical distribution of liquid water and temperature inside clouds, and the potential thickness of accreted ice. The results provide an overview of conditions that pilots may encounter in mid and high latitude winter clouds. Based on these results suggestions have been made to minimize in-flight icing and avoid severe icing during climb and descent.

During the course of the Alliance Icing Research Study (AIRS) II inflight icing research flight programme, a freezing drizzle (FZL) icing accretion encounter occurred whilst on the base and final approach track legs in the radar-vectored Instrument Landing System (ILS) pattern, and configuring progressively into the landing configuration. Aerodynamic performance data has been derived from recorded inertial and air data, sliced into time intervals of varying length from 4 to 20 second duration. In particular, the changes in profile and induced drag coefficients ∆CDo and ∆k, respectively, and in lift-curve slope, ∆a, have been derived for each time slice, and examined in the time-domain. Useful drag component data has been extracted from the timeslices, indicating that non-substantial changes to ∂CD/∂CL 2 occurred, but approximately 380% rise in CDo occurred. The CDo rise correlated reasonably with 80VD, but not with the MVD, when the droplet size parameters were derived for the combined small (FSSP probes) and large droplet (2DC probe) spectra. The drag rise was much larger than that measured in FZL, with wing flaps UP. It would equate to an approximate loss of climb gradient of 8.7%, very substantial when compared with landing-climb gradient and go-around obstacle-clearance climb-out path airworthiness and operational requirements, respectively. Icing accretion completely obscured the front windscreen panels. Post-flight, a runback icing accretion band was observed well aft of the heated leading edge sections, on the left wing upper surface.

Microphysical properties of clouds in thunderstorm outflow regions, derived from measurements of a specially instrumented test aircraft at approximately 8.8 km and -28°C are presented. Although the heaviest echo regions were not penetrated, measurements in areas adjacent to the red-echo region of the pilot’s radar showed clouds with maximum ice water contents in excess of 1.3 gmM3 , and often sustained ice water contents in excess of 0.5 gmm3. It was concluded that at most trace amounts of supercooled liquid water content were present. Although ice particles of up to 10 mm were observed, mass was concentrated at small sizes and number concentrations were surprisingly high, with average median mass diameter and concentration of 43 pm and 90 cm” respectively for a 47 minute case study. Some unexpected conflicting indications from conventional aircraft cloud physics probes were experienced, and are also discussed.

An Airport Vicinity Icing and Snow Advisor (AVISA) was tested using observations from the Alliance Icing Research Study II (AIRS II), conducted in the Ottawa-Mirabel area from 3 November 2003 to 12 February 2004. The AVISA Nowcasting system is designed to help provide decision makers (airport authorities, airline dispatch, ground de-icing crews, pilots, etc) with real-time, accurate, and up-to-date weather information to help alleviate problems and to increase safety. This system uses numerical model data, pilot reports, ground sensor data (precipitation, ceiling, visibility, winds, etc) as well as remote sensing (satellite, radar, radiometer) information to provide the necessary Nowcasts out to approximately 6 hours. The AIRS II project involved 5 research aircraft and several airport Nowcasting systems installed at Mirabel. The limitations and strengths of some of the AVISA component inputs (e.g. model data, radar, radiometric, precipitation rate) are discussed using experiences from AIRS II. Illustrations are given of some of the displays currently being developed. Plans for future work are described.