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George A. Isaac

Abstract

Any observing program studying summer cumulus clouds should attempt to measure cloud lifetime. This parameter is important for determining whether a cloud will last long enough for precipitation to form by either natural or artificially stimulated mechanisms. When reporting cloud lifetime, the definition used and the method of calculation should be clearly specified. In North America, after a summer cumulus cloud has been identified and selected, lifetimes, at temperatures below –5°C, of approximately 10 to 12 min are being reported. This lifetime must be considered marginal for static mode seeding to produce precipitation by artificial ice nucleants.

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Alexei Korolev and George A. Isaac

Abstract

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.

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Alexei Korolev and George A. Isaac

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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.

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Alexei V. Korolev and George A. Isaac

Abstract

A new conceptual model is proposed for enhanced cloud droplet growth during condensation. Rapid droplet growth may occur in zones of high supersaturation resulting from isobaric mixing of saturated volumes with different temperatures. Cloud volumes having a temperature different from the general cloud environment may form due to turbulent vertical motions in a temperature lapse rate that is not pseudoadiabatic. This mechanism is most effective in the vicinity of cloud-top inversions. It is also shown that the isobaric mixing of saturated and dry volumes with different temperatures may also lead to high supersaturations. The high supersaturations are associated with zones of molecular mixing, and they have a characteristic size of the order of millimeters with a characteristic lifetime near tenths of a second. Some small proportion of cloud droplets, over many supersaturation events, may grow large enough to grow effectively through collision–coalescence. This hypothesis of isobaric mixing may help explain freezing and warm drizzle formation.

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Faisal Boudala, George A. Isaac, and Di Wu

Abstract

Light (LGT) to moderate (MOD) aircraft icing (AI) is frequently reported at Cold Lake, Alberta, but forecasting AI has been a big challenge. The purpose of this study is to investigate and understand the weather conditions associated with AI based on observations in order to improve the icing forecast. To achieve this goal, Environment and Climate Change Canada in cooperation with the Department of National Defence deployed a number of ground-based instruments that include a microwave radiometer, a ceilometer, disdrometers, and conventional present weather sensors at the Cold Lake airport (CYOD). A number of pilot reports (PIREPs) of icing at Cold Lake during the 2016/17 winter period and associated observation data are examined. Most of the AI events were LGT (76%) followed by MOD (20%) and occurred during landing and takeoff at relatively warm temperatures. Two AI intensity algorithms have been tested based on an ice accumulation rate (IAR) assuming a cylindrical shape moving with airspeed υ a of 60 and 89.4 m s−1, and the Canadian numerical weather prediction model forecasts. It was found that the algorithms IAR2 with υ a = 89.4 m s−1 and IAR1 with υ a = 60 m s−1 underestimated (overestimated) the LGT (MOD) icing events, respectively. The algorithm IAR2 with υ a = 60 m s−1 appeared to be more suitable for forecasting LGT icing. Over all, the hit rate score was 0.33 for the 1200 UTC model run and 0.6 for 0000 UTC run for both algorithms, but based on the individual icing intensity scores, the IAR2 did better than IAR1 for forecasting LGT icing events.

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

Abstract

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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