George A. Isaac

The relationship between cloud droplet number concentrations and anthropogenic pollution: Observations and climatic implications

Measurements of the concentrations of sulfate and nitrate in approximately 400 cloud water samples collected during four field studies carried out since 1982 are used with coincident measurements of cloud droplet number concentrations (CDNC) and liquid water content (LWC) to examine the relationship between CDNC and anthropogenic pollution, where sulfate concentration is used as the measure of the latter. The number of samples is compressed to 92 by averaging duplicates and multiple samples at similar altitudes during any particular flight, with 85 including CDNC measurements. Positive linear regressions between log (CDNC) and log (cloud water sulfate concentration) are determined for both stratiform and cumuliform cloud. Because of the number of factors affecting the CDNC, the coefficients of determination are only 0.30 and 0.49 for the respective cloud types. The LWC is relatively invariant with the cloud water sulfate concentration. The observed range of CDNC for the study region is 20–600 cm−3 (median of 59 observations is 210 cm−3) for stratiform clouds and 170–1100 cm−3 (median of 26 observations is 400 cm−3) for cumuliform clouds. The median CDNC for all sampled clouds is 250 cm−3. CDNC are also determined for “clean-air” conditions. The latter is defined as cases for which the concentrations of both cloud water sulfate and cloud water nitrate are comparable to aerosol sulfate concentrations and aerosol nitrate plus HNO3 concentrations, respectively, as reported for remote regions of the globe. For the clean-air clouds the observed range of CDNC for the study region is 20–250 cm−3 (median of 12 observations is 120 cm−3) for stratiform clouds and 170–370 cm−3 (median of four observations is 240 cm−3) for cumuliform clouds. The median CDNC for all clean-air clouds is 160 cm−3. The median CDNC for the complete population is 56% greater than the clean-air CDNC; the hypothesis that the clean-air CDNC is not different from the median CDNC is rejected at a confidence level of >99.5%. The present-day climatic forcing due to cloud albedo change arising from increased CDNC is estimated from a rudimentary model at between −2 W m−2 and −3 W m−2 for eastern North America.

Ice particle habits in Arctic clouds

Ice crystals in atmospheric clouds have shapes, which affect their density, terminal fall velocity, growth rate and radiative properties. In calculations for climate change predictions, weather forecasting of precipitation, and remote sensing retrievals, idealized crystal shapes such as columns, needles, plates and dendrites are assumed. Using new technology imaging instrumentation with a resolution of 2.3 µm, recent observations in Arctic clouds have shown that such pristine habits only describe approximately 3% of the particles. The measurements were made from an aircraft during April 1998 and cover a temperature range of 0 °C to −45°C. Boundary layer, multi-layer and cirrus clouds were examined. The commonly observed irregularly shaped particles either consisted of faceted polycrystalline particles or sublimating (solid to vapor) ice particles with smooth curving sides and edges. Since climate warming is now predicted to be largest in the Arctic, and cloud properties significantly affect the radiation balance, it will be necessary to consider the effects of non-pristine ice particle habits in such calculations and predictions.

The Nevzorov Airborne Hot-Wire LWC-TWC Probe: Principle of Operation and Performance Characteristics

The Nevzorov liquid water content (LWC) and total water content (TWC) probe is a constant-temperature, hot-wire probe designed for aircraft measurements of the ice and liquid water content of clouds. The probe consists of two separate sensors for measurements of cloud liquid and total (ice plus liquid) water content. Each sensor consists of a collector and a reference winding. The reference sensors are shielded from impact with cloud particles, specifically to provide an automatic compensation for convective heat losses. This results in a potentially improved sensitivity over uncompensated probes such as the King LWC probe. The Nevzorov probe has been used in four Canadian field experiments on the National Research Council (NRC) Convair580 since 1994. Intercomparison of Nevzorov LWC, TWC, King, and two PMS Forward Scattering Spectrometer Probes show good agreement in liquid clouds, although the Nevzorov probe displays distinct advantages in low-LWC situations due to a more stable baseline. The sensitivity of the probe is estimated to be approximately 0.003‐ 0.005 g m23. The accuracy of LWC measurements in nonprecipitating liquid clouds is estimated as 10%‐15%. Tests at the NRC high-speed icing tunnel have provided verification of the TWC measurement for small frozen droplets to an accuracy of approximately 10%‐20%, but verification in snow and natural ice crystals has not yet been possible due to the absence of any accurate standards. The TWC measurement offers not only the possibility of direct measurements of ice content but also improved liquid water contents in drizzle situations. Airborne measurements have provided data on the baseline drift and sensitivity of the probe and have provided comparisons to other conventional instruments. Several cases have been documented that exhibit the unique capabilities of the instrument to separate the ice and liquid components of supercooled clouds.

Roundness and Aspect Ratio of Particles in Ice Clouds

Abstract 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...

Assessing Cloud-Phase Conditions

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 con...

Shattering during Sampling by OAPs and HVPS. Part I: Snow Particles

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 ove...

Physical and chemical observations in marine stratus during the 1993 North Atlantic Regional Experiment: Factors controlling cloud droplet number concentrations

Airborne observations from 14 flights in marine stratus over the Gulf of Maine and Bay of Fundy in August and September of 1993 are examined for the relationships among the cloud droplet number concentrations (N d ), the out-of-cloud aerosol particle number concentrations (N a ), the major ion concentrations in the cloud water, and turbulence in cloud. There was a wide range of aerosol concentrations, but when low stratus and the main anthropogenic plume from eastern North America were in the same area the plume overrode the cloud. The N d increased with increasing N a and cloud water sulfate concentration (cwSO 4 = ), but the relationships were very weak. The separation of the data between smooth and lightly turbulent air substantially improved the ability to explain the variance in the N d by either of these two quantities. Also, the relative increase in N d for increases in N a and cwSO 4 = was greater for lightly turbulent air than for smooth air. The estimated minimum size of particles activated in these clouds ranged from 0.14 μm to 0.31 μm, corresponding to average supersaturations of <0.1%. The minimum size tended to be lower for lightly turbulent air and smaller N a . The results for lightly turbulent air agree well with previously reported parameterizations of the impact of aerosols on N d , but the results for smooth air do not agree. In general, more knowledge of the physical factors controlling the N d in stratiform clouds, such as turbulence, is needed to improve not only our ability to represent N d but also to increase our understanding of the impact of the aerosol particles on the N d and climate.

Evaluation of the Accuracy of PMS Optical Array Probes

This paper considers the theory of diffraction image formation of spherical particles and peculiarities of particle sizing by discrete imaging probes. The diffraction images of spherical water droplets are approximated by Fresnel diffraction by an opaque disc. The approach developed in the paper is applicable to all types of array and matrix imaging probes. The analysis measurement accuracy is performed for the PMS Optical Array Prove (OAP)-2D-C and OAP-2Dgray probes. It is shown that a 25-mm resolution PMS OAP-2D-C probe can both oversize and undersize droplets smaller than approximately 100 mm in diameter, and oversize droplets larger than approximately 100 mm. The errors in droplet sizing increase with decreasing size. The discrete manner of particle image registration also leads to losses of particles with sizes smaller than 100 mm. For the ideal case with zero photodiode response time, these losses reach 70% for 25-mm droplets. A nonzero response time will increase these losses. These findings help explain discrepancies observed in the overlap region of the PMS FSSP and OAP droplet spectra. A variety of calculated digital images for PMS OAP-2D-C and OAP-2Dgray probes is presented. Different methods of particle image sizing are discussed. Several methods of size correction of individual droplets and droplet ensembles are suggested. Correction algorithms for these effects are derived, and distortion and correction retrieval matrices are calculated. Several examples of actual and measured size distributions are presented.

Characterizations of Aircraft Icing Environments that Include Supercooled Large Drops

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

Aircraft Icing Measurements in East Coast Winter Storms

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.