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A. A. Tsonis
and
G. A. Isaac

Abstract

Using satellite and weather radar data, a simple clustering analysis has been used in order to differentiate between raining and nonraining clouds. Based on these results, a scheme is proposed for instantaneous rain area delineation in the midlatitudes. Delineation of the rain areas will not require coextensive radar data which are only used to develop and evaluate the method. Warm season data during daylight hours were used to test the scheme. Results indicate that the proposed scheme has very good skills in delineating rain areas in the midlatitudes, resulting in an average probability of detection of about 66% and an average false alarm ratio of about 37%.

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R. A. Stuart
and
G. A. Isaac

Abstract

A simple temperature and precipitation relationship, as obtained from observed daily values for Canadian stations, has been compared with the relationship produced by the Canadian Climate Centre second-generation climate model. In the winter, over broad areas, the model and the observations agree, both showing more precipitation when the average daily “screen-level” temperature is warmer than the median daily value. The observations show that more precipitation falls on days that are cooler than normal throughout most of the country in warmer seasons, and in the lee of the Rocky Mountains year-round. However, the model only rarely predicts more precipitation with cooler temperatures, which suggests that parameterization schemes in this model could be improved. This method of comparing observations and model results using a relationship among several important variables has significant value, and it could be applied to other models.

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G. A. Isaac
and
R. A. Stuart

Abstract

Hourly data from climatological stations in the Mackenzie River valley-Beaufort Sea area of northern Canada have been examined to determine the relationships between cloud type and amount, precipitation, and surface temperatures. During all seasons, stratocumulus is the dominant cloud type for both precipitating and non-precipitating hours. More stratocumulus cloud occurs when temperatures are warmer in the winter and colder in the summer. Similarly, precipitation occurs more frequently and the total amount is greater when temperatures are warmer in the winter and colder in the summer. Overcast skies are dominant for all seasons when precipitation is falling. During the winter, during nonprecipitating hours, clear skies are most frequent. During the summer, during nonprecipitating hours, some cloud is usually present. Surface temperatures are warmer in the winter with overcast skies and warmer in the summer with clear skies. An attempt has been made to quantify the above conclusions so that comparisons can be made with global climate models.

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G. A. Isaac
and
R. A. Stuart

Abstract

The dependence of daily precipitation upon average daily temperature has been examined for all seasons using climatological data from 56 stations across Canada. For cast and west coast sites, and the north, more precipitation occurs with warm and cold temperatures during January and July, respectively. In the middle of the country, the temperature dependence tends to increase toward the Arctic, with strong dependencies in the Northwest Territories and weaker dependencies on the Prairies. Southern Ontario and Quebec show almost no dependence of precipitation upon temperature during July, but more precipitation falls during warm weather during the winter. For stations within and immediately downwind of the Rockies, for all seasons, more precipitation occurs when the temperature is colder. These temperature-precipitation relationships can provide information on precipitation formation processes, as well as assistance in weather and climate forecasting.

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G. A. Isaac
and
R. E. Douglas

Abstract

If most atmospheric ice nuclei activate by contacting or immersing themselves inside a water droplet, then ice nucleus cloud chambers may seriously underestimate the concentration of these particles. Ice nuclei diffuse toward cloud droplets due to Brownian and turbulent motion. In a natural cloud of 0.5 gm m−3 with a drop concentration of 1000 cm−3, ten times more particles 50–104 Å in radius enter the drops in 1 hr than collide in 2 min. In an ice nucleus counter with a similar cloud, a normal air sample residence time of under 2 min would not successfully model a stable atmospheric cloud, which might last several hours. In a specific time, the fraction collected by the above mechanisms increases with liquid water content and cloud drop concentration. Before ice nucleus counter measurements are used for theoretical computations, some attempt should be made to adjust the concentration to the parameters existing in the particular cloud under consideration. The relative proportion of nuclei which activate with or without collision with cloud drops should at least be estimated in the laboratory.

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Robert S. Schemenauer
and
G. A. Isaac

Abstract

The microphysical and dynamical characteristics of 156 natural summer cumulus clouds have been documented for three locations in North America: Yellowknife, Northwest Territories; Thunder Bay, Ontario; and Miles City, Montana. The measurements (469 aircraft penetrations) were made in six consecutive years from 1975 to 1980 using state-of-the-art cloud physics instrumentation. All measurements discussed were obtained near −7°C. Yellowknife clouds had low liquid water contents (0.3 g m−3) and high large (>70 μm) particle concentrations (0.9 L−1). Thunder Bay clouds had higher liquid water contents (1 g m−3) and low large particle concentrations (0.04 L−1). Miles City clouds, which were similar in dimensions to those near Yellowknife, had low liquid water contents (0.3 g m−3) and low large particle concentrations (0.1 L−1). Yellowknife and Thunder Bay clouds produced precipitation through the warm and cold rain processes but the observed Miles City clouds did not precipitate naturally. Measurements of cloud top lifetime appear to be useful in explaining the differences between locations. Cloud top lifetime is defined in this paper in terms of the persistence of cloud liquid water at the penetration altitude near −7°C. Lifetime was found to increase with cloud width in each location but did not appear closely related to initial LWC, cloud depth, cloud base temperature, inside-outside cloud temperature difference, environmental humidity, turbulent energy dissipation rate, energy flux, heat flux nor wind shear.

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G. A. Isaac
and
R. S. Schemenauer

Abstract

Measurements have been made of the concentration and phase of large particles (>70 μm) within the supercooled regions of northern Canadian cumulus clouds. During June and July, for the years 1975 and 1976, a total of 58 cumulus clouds near Yellowknife, N.W.T., were examined with a specially equipped Twin Otter aircraft. The cumulus clouds studied were mainly 1–3 km deep with most of the 130 cloud penetrations being made within 300 m of cloud top, at temperature levels between −1 and −11°C. The median penetration average (Johnson-Williams) liquid water content was 0.3 g m−3. The median penetration average concentration of particles >70 μm and >350 μm was 0.9 l −1 and 0.015 l −1, respectively. The concentration of large particles was not well correlated with J-W liquid water content or temperature, and considering all the clouds, no consistent change in the concentration was observed in successive cloud penetrations. These large particles were predominantly water drops. Ice was only found in clouds with summit temperatures colder than −8°C. Clouds containing ice had significantly higher concentrations of large particles than did all-water clouds. The data suggest that both cold and warm rain precipitation formation mechanisms were present in some of these clouds.

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G. A. Isaac
and
R. H. Douglas

Abstract

Increases in surface ice nucleus concentrations at −20C (by a factor of 10 or higher) have been measured during precipitation from hailing and non-hailing convective storms; these increases are associated with the storm downdrafts. The intensity of the ice nucleus concentration fluctuations and the concentration at −20C are similar in both Alberta and Quebec storms. One of the seven convective storms for which measurements are presented was seeded with AgI released from an aircraft; the seeding material was subsequently detected at the surface.

If the observed higher ice nucleus concentration in the downdraft mixes with a storm updraft of 10 m sec−1, simple calculations indicate that no dramatic change would occur in the ice content of the updraft: to produce 5 gm m−3 of ice by −20C, in such an updraft, 105 times the normal background concentration of ice nuclei would be required.

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J. I. MacPherson
and
G. A. Isaac

Abstract

The turbulent characteristics of 17 Canadian cumulus clouds have been documented using the measurements from a specially instrumented T-33 aircraft. Most of the 33 cloud penetrations were made through the tops of cumuli 1–4.5 km in depth. Turbulent energy spectra over a range of wavelengths from 15 to 2500 m have been obtained for the two horizontal and the vertical gust velocities. Mean flow characteristics, especially any expected updrafts, tended to be obscured by turbulent fluctuations. The modal root-mean-square gust velocity was 1.7 m s−1 and the calculated modal turbulent energy dissipation rate was 160 cm2 s−3. Based on measured accelerations, estimates were made of expected vertical forces on several aircraft with a wide range of wing loadings. Cumulus clouds similar to those studied do not pose a safety hazard to these aircraft, and crew and passengers can easily tolerate the turbulence levels.

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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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