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J. Walter Strapp and Robert S. Schemenauer

Abstract

Wind tunnel tests have provided calibrations and intercomparisons of 14 Johnson-Williams (J–W) cloud liquid water content (LWC) measuring devices with 23 sensor heads from 10 research organizations. The absolute tunnel LWC was deduced using a rotating icing cylinder technique accurate to ∼5%.

A significant fraction of the systems arrived at the tunnel with nonfunctional shell or strut heaters, which can degrade measurements below 0°C. Several sensor heads exhibited airspeed dependencies. Switching heads sometimes produced calibration changes. At −15°C an instrument problem was discovered associated with icing of the compensating wire posts, which resulted in mild to severe measurement errors in 75% of the sensor heads at 103 m s−1.

Calibrations at −5°C revealed that J-W measurements usually varied linearly with tunnel LWC, but sometimes with a slope differing from unity, implying that the system dummy head did not always define the correct conversion from J-W output voltage to grams per cubic meter. No more than six of the 13 systems tested at −5°C agreed to within 20% of the tunnel LWC with each of their sensor heads, but at least 10 of 13 did so with one sensor head. At −15°C similar results were obtained, but most systems suffered from the aforementioned icing problem, resulting in unreliable small-scale measurements.

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Stéphane Bélair, Jocelyn Mailhot, J. Walter Strapp, and J. Ian MacPherson

Abstract

In this study, the ability of a turbulent kinetic energy (TKE)–based boundary layer scheme to reproduce the rapid evolution of the planetary boundary layer (PBL) observed during two clear convective days is examined together with the impact of including nonlocal features in the boundary layer scheme. The two cases are chosen from the Montreal-96 Experiment on Regional Mixing and Ozone (MERMOZ): one is characterized by strong buoyancy, a strong capping inversion, and weak vertical wind shear; the other displays moderate buoyancy, a weaker subsidence inversion, and significant wind shear near the PBL top. With the original local version of the turbulence scheme, the model reproduces the vertical structures and turbulent quantities observed in the well-developed boundary layer for the first case. For the second case, the model fails to reproduce the rapid evolution of the boundary layer even though the TKE and sensible heat fluxes are greatly overpredicted.

Some nonlocal aspects of the turbulence scheme are tested for these two cases. Inclusion of nonlocal (countergradient) terms in the vertical diffusivity equation has little impact on the simulated PBL. In contrast, alternative formulations of the turbulent length scales that follow the strategy proposed by Bougeault and Lacarrère have a greater influence. With the new turbulent lengths, entrainment at the top of the boundary layer is enhanced so that the depth of the well-mixed layer is much larger compared to that of the local simulations even though the turbulent sensible heat fluxes are smaller. Comparison with observations reveals, however, that the inclusion of these modifications does not improve all aspects of the simulation. To improve the performance and reduce somewhat the arbitrariness in the Bougeault–Lacarrère technique, a relationship between the two turbulent length scales (mixing and dissipation) used in the turbulence scheme is proposed. It is shown that, in addition to reducing the sensitivity of the results to the particular formulations, the simulated boundary layer agrees better with observations.

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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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J. Walter Strapp, W. R. Leaitch, and P. S. K. Liu

Abstract

Comparisons of particle-size distributions measured by Particle Measuring Systems FSSP-300 and PCASP-100X probes through a range of relative humidities reveal that the deiced PCASP-100X probe dries hydrated submicron aerosols before measurement. The FSSP-300 appears to measure the particles in their hydrated state and detects the expected growth in the particle spectrum with increasing relative humidity. Calibration changes fox refractive-index changes with hydration are not applicable to the deiced PCASP-100X probe but are for the FSSP-300. The combined use of the two probes with their differing responses to hydrated aerosols may provide information related to the chemical composition of the aerosol.

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James Abraham, J. Walter Strapp, Christopher Fogarty, and Mengistu Wolde

In order to better understand the behavior and impacts of tropical cyclones undergoing extratropical transition (ET), the Meteorological Service of Canada (MSC) conducted a test flight into Hurricane Michael. Between 16 and 19 October 2000 the transition of Hurricane Michael from a hurricane to an intense extratropical storm was investigated using a Canadian research aircraft instrumented for storm research. This paper presents the various data collected from the flight with a detailed description of the storm structure at the time when Michael was in the midst of ET.

Hurricane Michael was moving rapidly to the northeast, approximately 300 km southeast of Nova Scotia, Canada, during the time of the aircraft mission. A period of rapid intensification had also occurred during this time as the system moved north of the warm Gulf Stream waters and merged with a baroclinic low pressure system moving offshore of Nova Scotia. Consequently, the hurricane was sampled near the period of its lowest surface pressure and maximum surface winds. It is estimated that the aircraft passed approximately 10 km south of the estimated 42.7°N, 59.7°W position of the surface low pressure center at about 1645 UTC 19 October. Sixteen dropsondes were deployed in a single traverse from northwest to east of the storm center, and then back westbound south of the center. Winds were found to be highest on the southeast side of the hurricane where the storm movement adds to the hurricane rotational flow. A southwesterly jet with winds exceeding 70 m s−1 was observed between 500 and 2000 m approximately 85 km southeast of the center. This low-level jet was much deeper than the usual lowlevel maximum winds found in hurricanes. Michael was observed to have an elevated warm core similar to purely tropical systems, but low-altitude humidity appeared to be eroded by entrainment of dry midlatitude air surrounding the storm, which is typically observed during the ET process.

A cloud-profiling 35-GHz radar provided data on the distribution of precipitation across the system, and cloud microphysical probes measured cloud water contents, particle phases, and spectra. Although a wide variety of liquid, mixed phase, and deep glaciated clouds were observed, the glaciated cloud encountered on the northwest side of the center, associated with the most significant precipitation area, was relatively stratiform in nature, with a broad area of high ice water content reaching 1.5 g m−3, and very high concentrations of small ice particles.

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

Abstract

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 concentrations observed. Glaciated cloud conditions always had FSSP-measured median volume diameters greater than 30 μm and particle concentrations less than 15 cm−3, whereas similar measurements in entirely liquid-phase clouds were observed less than 4% of the time. Images of drops greater than or equal to 125 μm in diameter, which were collected in warm clouds greater than 0°C, were used to calibrate geometric criteria, which were, in turn, used to segregate 2D images into circular and noncircular categories. It is shown that, on average, between 5% and 40% of ice crystals greater than or equal to 125 μm in diameter will be classified as circular, depending on the particle size, with the percentage decreasing with increasing particle size. In liquid-phase clouds, between 85% and 95% of the 2D images will be correctly classified as circular for all particle sizes. At temperatures less than −4°C, a Rosemount icing-detector threshold of 2 mV s−1, corresponding to a maximum LWC of 0.002 g m−3, was used to help to identify glaciated and nonglaciated conditions. A methodology for segregating liquid, mixed, and glaciated cloud regions, based on instrument responses to liquid and ice hydrometeors, was developed and applied to the CFDE dataset. The results were used to determine the relative frequency of liquid, mixed, and glaciated cloud conditions for the data collected during the two field projects. Approximately 40% of the in-cloud observations at temperatures less than 0°C were assessed as liquid phase. The fractions of mixed-phase and glaciated-phase conditions were 26% and 34% for CFDE I and 46% and 14% for CFDE III, respectively. Because the ice-crystal responses for each instrument depend on the aircraft sampling speed and the ice-crystal sizes and concentrations, the results may be limited to conditions similar to those in clouds in midlatitude winter storms. Regardless, the results may have application to several fields, including development of parameterizations for numerical modeling, precipitation formation, remote sensing, ice multiplication, radiative transfer, and aircraft icing investigations. Important implications for aircraft icing investigations are discussed.

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John V. Cortinas Jr., Ben C. Bernstein, Christopher C. Robbins, and J. Walter Strapp

Abstract

A comprehensive analysis of freezing rain, freezing drizzle, and ice pellets was conducted using data from surface observations across the United States and Canada. This study complements other studies of freezing precipitation in the United States and Canada, and provides additional information about the temporal characteristics of the distribution. In particular, it was found that during this period 1) spatial variability in the annual frequency of freezing precipitation and ice pellets is large across the United States and Canada, and these precipitation types occur most frequently across the central and eastern portions of the United States and Canada, much of Alaska, and the northern shores of Canada; 2) freezing precipitation and ice pellets occur most often from December to March, except in northern Canada and Alaska where it occurs during the warm season, as well; 3) freezing rain and freezing drizzle appear to be influenced by the diurnal solar cycle; 4) freezing precipitation is often short lived; 5) most freezing rain and freezing drizzle are not mixed with other precipitation types, whereas most reports of ice pellets included other types of precipitation; 6) freezing precipitation and ice pellets occur most frequently with a surface (2 m) temperature slightly less than 0°C; and 7) following most freezing rain events, the surface temperature remains at or below freezing for up to 10 h, and for up to 25 h for freezing drizzle.

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J. Walter Strapp, Frank Albers, Andreas Reuter, A. V. Korolev, Uwe Maixner, E. Rashke, and Z. Vukovic

Abstract

Laboratory measurements of the response of the Particle Measuring Systems, Inc., 2DC probe have been conducted to characterize counting and sizing errors of the probe for spherical particles. Measurements of the shadow threshold intensity of a Meteorological Service of Canada (MSC) 2DC probe varied from approximately 30% to 51%, depending on the photodiode, and averaged 46% for the central 16 photodiodes. Depth-of-field and sizing measurements are quite sensitive to this threshold, which is nominally considered as 50% for the 2DC probe. Response times also varied significantly, from 0.44 to 0.90 μs. Measurements of the depth of field for known particle sizes at low velocity agreed well with published calculations at zero velocity. For particles smaller than 100 μm, the depth of field decreased significantly with increasing airspeed due to the nonzero response time of the sensing photodiodes. The average particle size also decreased with increasing airspeed but did so in such a manner as to counteract oversizing due to out-of-focus images. At 100 m s−1, the average measured sizing error of a 100-μm particle was close to negligible, rising to approximately 5% at 500 μm. The application of measured depth-of-field values and sizing calibrations at specific sizes to improve 2DC size distribution accuracy is nontrivial because measurement errors cause particles to be redistributed to other sizes in a complicated manner. However, when hypothetical true particle distributions were redistributed according to a distortion matrix approximated by the results of this study, the average error of uncorrected size distributions measured by the MSC 2DC probe, expressed as a sizing error, was found to be ±10% for particles larger than 125 μm. Although these results are not strictly transferable to other 2DC probes, the methods described can be used to derive similar results for other probes.

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Julie A. Haggerty, Allyson Rugg, Rodney Potts, Alain Protat, J. Walter Strapp, Thomas Ratvasky, Kristopher Bedka, and Alice Grandin

Abstract

This paper describes development of a method for discriminating high ice water content (HIWC) conditions that can disrupt jet-engine performance in commuter and large transport aircraft. Using input data from satellites, numerical weather prediction models, and ground-based radar, this effort employs machine learning to determine optimal combinations of available information using fuzzy logic. Airborne in situ measurements of ice water content (IWC) from a series of field experiments that sampled HIWC conditions serve as training data in the machine-learning process. The resulting method, known as the Algorithm for Prediction of HIWC Areas (ALPHA), estimates the likelihood of HIWC conditions over a three-dimensional domain. Performance statistics calculated from an independent subset of data reserved for verification indicate that the ALPHA has skill for detecting HIWC conditions, albeit with significant false alarm rates. Probability of detection (POD), probability of false detection (POFD), and false alarm ratio (FAR) are 86%, 29% (60% when IWC below 0.1 g m−3 are omitted), and 51%, respectively, for one set of detection thresholds using in situ measurements. Corresponding receiver operating characteristic (ROC) curves give an area under the curve of 0.85 when considering all data and 0.69 for only points with IWC of at least 0.1 g m−3. Monte Carlo simulations suggest that aircraft sampling biases resulted in a positive POD bias and the actual probability of detection is between 78.5% and 83.1% (95% confidence interval). Analysis of individual case studies shows that the ALPHA output product generally tracks variation in the measured IWC.

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