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Stewart G. Cober and Roland List

Abstract

Rigidly suspended conical graupel were grown in a wind tunnel, starting from 1-mm hexagonal plates, with liquid water content varied from 0.5 to 3.0 g m−1, velocity from 1.1 to 3.0 m s−1, ambient temperature from −4.4 to −20.9°C, cloud droplet median volume radius from 12 to 21 μm, and ambient pressure from 100 to 60 kPa. Growth conditions were chosen to simulate natural conditions in which conical graupel grow and serve as embryos for hail. Final graupel diameters ranged from 1.5 to 6 mm, with Reynolds numbers between 300 and 1500. Measurements of the mass, volume, growth height, geometric shape, and surface temperature with time were used to calculate the Nusselt and Sherwood numbers (representing the convective heat and mass transfers), bulk collection efficiency, and accretion density. The bulk collection efficiency and Nusselt number were parameterized in terms of the Stokes parameter and Reynolds number, respectively. The density and cone angle were parameterized in terms of the relative graupel-airstream velocity, the cloud-droplet median volume radius, and the surface temperature. The surface temperatures were measured remotely with an infrared radiometer to within ±0.2°C and are the first ever of growing graupel.

The bulk collection efficiency was found to be 25% lower than that for ideal smooth spheres, while the Nusselt numbers were approximately 50% higher than those of smooth cones. The enhanced heat convection and mass deposition or sublimation is attributed to the roughness of the ice surface. The parameterizations of bulk collection efficiency, Nusselt number, density, cone angle, and geometric shape obtained represent solutions to the heat and mass transfer equations for the laboratory-grown conical graupel and can be used to improve graupel growth calculations in cloud dynamical models.

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Greg M. McFarquhar and Stewart G. Cober

Abstract

In situ observations of the sizes, shapes, and phases of Arctic clouds were obtained during the First International Satellite Cloud Climatology Project Regional Experiment (FIRE) Arctic Clouds Experiment (ACE). These particle distributions were then combined with a library of single-scattering properties, calculated using Mie theory and improved geometric ray optics, to determine the corresponding single-scattering properties (single-scattering albedo ω 0, phase function, and asymmetry parameter g) at solar wavelengths. During FIRE-ACE, mixed-phase clouds, where both water and ice were detected in 30 s of flight track, corresponding to 3.0-km horizontal extent, were observed in 33% of clouds. Because supercooled water drops generally dominate mass contents of these mixed-phase clouds, there is no statistically significant difference in the distributions of single-scattering properties of mixed-phase clouds compared to liquid-phase clouds, whereas those of ice crystals differ significantly. The average g for all mixed-phase clouds at visible wavelengths is 0.855±.005, similar to 0.863±.007 computed for water clouds, but higher than 0.767±.007 computed for ice clouds. Differences in g and ω 0 between mixed- and ice-phase clouds for near-infrared bands are also noted, whereas they are similar for mixed- and liquid-phase clouds.

Single-scattering properties computed using observations of mixed-phase clouds differ by more than 10% on average from those computed using a parameterization that describes the average fraction of water and ice in mixed-phase clouds. Simulations using a plane-parallel radiative transfer model show that these differences can cause top of the atmosphere albedos to vary between 6% and 100% depending on wavelength. However, when single-scattering properties are computed from observations over all phases (mixed, ice, and liquid), and average albedos are compared against those determined using the parameterized scattering properties, there is a difference of only 2% at visible wavelengths. Since observations show that the occurrence of phases is clustered, large-scale averages may not be representative of mixed-phase cloud climatic effects.

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Stewart G. Cober and George A. Isaac

Abstract

Observations 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.

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Stewart G. Cober, Andre Tremblay, and George A. Isaac

Abstract

Comparisons have been made between in situ aircraft measurements of integrated liquid water and retrievals of integrated liquid water path (LWP) from algorithms using SSM/I brightness temperatures. The aircraft measurements were made over the North Atlantic Ocean during the winter of 1992. Six case studies are presented from which trends in the LWP algorithms are discussed. SSM/I liquid water path validation has previously only been performed through comparisons with measurements from upward-looking radiometers or with calculations from radiative transfer models. The case studies presented here reflect an alternative technique for validation.

Aircraft-derived liquid water paths ranged from 0.01 to 0.09 kg m−2 for the six cases presented. The SSM/I algorithms investigated predicted LWP to within ±0.02–0.03 kg m−2, provided one accounted for systematic biases in the retrievals. These biases were systematic in the range ±0.06 kg m−2 and were presumably caused by latitudinal and seasonal influences inherent in the algorithms. Algorithms based on radiative transfer models appeared to perform better than the statistically based algorithms.

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Stewart G. Cober, George A. Isaac, and J. W. Strapp

Abstract

Analysis of the aircraft icing environments of East Coast winter storms have been made from 3 1 flights duringthe second Canadian Atlantic Storms Program. Microphysical parameters have been summarized and are compared to common icing intensity envelopes and to other icing datasets. Cloud regions with supercooled liquid water had an average horizontal extent of 4.3 km, with average droplet concentrations of 130 μ, liquid water contents of 0.13 g m-3, and droplet median volume diameters of 18 pm. In general, the icing intensity observed was classified as light, although moderate to severe icing was observed in several common synoptic situationsand several cases are discussed. Freezing drizzle was observed on four flights, and represented the most severeicing environment encountered.

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Stewart G. Cober, J. Walter Strapp, and George A. Isaac

Abstract

The microphysics associated with observations of supercooled drizzle drops, which formed through a condensation and collision-coalescence process, are reported and discussed. The growth environment was an 1100-m-thick stratiform cloud with cloud-base and cloud-top temperatures of −7.5° and −12°C, respectively. The cloud was characterized by a low droplet concentration of 21 cm−3 and a large droplet median volume diameter of 29 µm, with a concentration of interstitial aerosol particles of less than 15 cm−3 (larger than 0. 13 µm in diameter). The evolution of drizzle drops was traced downward from cloud top, with a maximum diameter of 500 µm observed at cloud base. The air mass was sufficiently clean to ensure only a small number of active cloud condensation nuclei. Consequently, small concentrations of cloud droplets led to concentrations of over 300 L−1 for droplets larger than 40 µm, which set up strong conditions for continued growth by collision-coalescence. Ice crystals in concentrations of 0.08 L−1 were measured simultaneously with the drizzle drops and were not effective in glaciating the cloud, even though the drizzle drops were estimated to have taken at least 1–2 h to form.

While the growth of precipitation-sized drops through collision-coalescence has been well documented, there are few measurements of this phenomena at temperatures less than 0°C. This study provides a well-documented example of such an event at subfreezing temperatures. The applicability of this measurement in terms of hazardous aircraft icing is discussed.

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Stewart G. Cober, George A. Isaac, and Alexei V. Korolev

Abstract

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 mixed phase clouds. There was no change in the RID response between liquid and mixed phase conditions, suggesting that ice crystals neither eroded ice accumulation nor accreted to the RID surface under the range of conditions experienced. During sustained icing conditions, a linear relationship between the RID signal and LWC was observed after the RID signal exceeded 400 mV above the clear-air signal level. The LWC derived from the RID was found to agree with LWC measurements from Nevzorov probes within ±50% for 92% of the data. The relationship between the RID signal and LWC was unchanged for freezing precipitation environments with drop median volume diameters >100 μm. The Ludlam limit was estimated for low LWC values and was found to agree well with theoretical calculations. The analysis provides considerable insight into the strengths and weaknesses of the instrument for operations in natural icing conditions.

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Hong Guan, Stewart G. Cober, and George A. Isaac

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In situ measurements of temperature (Ta), horizontal wind speed (V), dewpoint (Td), total water content (TWC), and cloud and supercooled cloud water (SCW) events, made during 50 flights from three research field programs, have been compared to forecasts made with the High Resolution Model Application Project version of the Global Environmental Multiscale model. The main purpose of the comparisons was to test the accuracy of the forecasts of cloud and SCW fields. The forecast accuracy for Ta, V, and Td agreed closely with the results from radiosonde–model validation experiments, implying that the aircraft–model validation methodology was equally feasible and, therefore, potentially applicable to SCW forecast verifications (which the radiosondes could not validate).

The hit rate (HR), false alarm rate (FAR), and true skill statistic (TSS) for cloud forecasts were found to be 0.52, 0.30, and 0.22, respectively, when the model data were inferred at a horizontal resolution of 1.5 km (averaging scale of the aircraft data). The corresponding values for SCW forecasts were 0.37, 0.22, and 0.15, respectively. The HRs (FARs) for cloud and SCW events are sensitive to horizontal resolution and increase to 0.76 (0.50) and 0.66 (0.53), respectively, when a horizontal resolution of 100 km is used. The model TWC was found to agree poorly with aircraft measurements, with the model generally underestimating TWC. For cases when the forecasts and observations of cloud agreed, the SCW-forecast HR, FAR, and TSS were 0.63, 0.22, and 0.41, respectively, which implies that improvement in the model cloud fields would substantially improve the SCW forecast accuracy.

The demonstrated comparison methodology will allow a quantitative comparison between different SCW and cloud algorithms. Such a comparison will provide insight into the strengths and weaknesses of these algorithms and will allow the development of more accurate cloud and SCW forecasts.

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Stewart G. Cober, George A. Isaac, and J. Walter Strapp

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 associated with warm-frontal or low pressure regions. For in-cloud measurements with temperatures less than or equal to 0°C, the relative fraction of liquid-, mixed-, and glaciated-phase conditions were 0.4, 0.4, and 0.2, respectively. For each 30-s (3 km) measurement, integrated drop spectra that spanned 1–3000 μm were determined using measurements from forward-scattering spectrometer probes and 2D-C and 2D-P probes. The integrated liquid water content (LWC) for each drop spectrum was compared with the LWC measured with a Nevzorov total water content probe and a Rosemount icing detector. The agreement was within the errors expected for such comparisons. This provides confidence in the droplet spectra measurements, particularly in the assessment of extreme conditions. The 99.9th-percentile LWC value was 0.7 g m−3, and the 99th-percentile LWC for drops greater than 50 μm in diameter was 0.2 g m−3. The 99.5th-percentile values of LWC and droplet concentrations are determined for different horizontal length scales and droplet diameter intervals, and are used to characterize the extreme icing conditions observed. The largest median volume diameters (MVD) observed were approximately 1000 μm and represent cases in which the aircraft was flown below cloud base in freezing-rain conditions. In one case, SLD was observed to form at −21°C, and the associated icing was rated as severe. Approximately 3% of the data for which SLD were observed had LWC greater than 0.2 g m−3 and MVD greater than 30 μm. Such conditions are believed to represent conditions that have the largest potential effects on aircraft performance. The analysis is presented in a format that is suitable for several applications within the aviation community, and comparisons are made to four common icing-envelope formulations. The data should be beneficial to regulatory authorities who are currently attempting to assess certification requirements for aircraft that are expected to encounter freezing-precipitation conditions.

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