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E. P. Lozowski

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E. P. Lozowski

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B. Kochtubajda and E. P. Lozowski

Abstract

A numerical model has been developed to investigate the sublimation rate of cylindrical dry ice pellets in clear and cloudy air. Experiments conducted in the University of Alberta FROST icing-wind tunnel were used to make comparisons with the model predictions of the sublimation rate. These experiments were carried out in both cold and warm environments. Furthermore, some of the experiments were conducted with the sprays operating in order to determine the effect of a simulated “cloudy” environment on the sublimation rate.

The principal conclusions are: a) despite the use of several simplifying assumptions, the cylindrical model predict the sublimation rates of dry ice pellets to within 20%, when compared with wind tunnel observations, and b) cloudy and saturated conditions at warm temperatures enhance the sublimation rate of dry ice, but cloudy and saturated conditions at cold temperatures do not have an appreciable effect on the sublimation rate of dry ice.

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E. P. Lozowski and R. D'Amours

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A model of spherical hailstone growth thermodynamics is presented, and used to examine the validity of the continuous growth and heat balance assumptions frequently employed in the “classical” hail growth models. The model is similar to the spherically symmetric model formulated by Macklin and Payne (1969), but solutions to the model equations are obtained by means of finite-difference numerical methods. In the model, we do not try to simulate the discrete accretion process of individual drops. Instead, we attempt to identify the implications of the discrete, time-dependent nature of the icing process, by examining the accretion of a thin uniform layer of supercooled water over the entire surface of the sphere. The heat transfer equations both with the air and within the hailstone axe then solved assuming radial symmetry. By the addition of several such layers, the finite growth of a spherical hailstone can be simulated. In the present paper, only growth in constant ambient conditions is considered. It is shown that there are large internal heat fluxes during the interval between the accretion of successive layers (typically ≲1 s), which cause the temperatures near the surface to oscillate several degrees above and below their time-mean value. Nevertheless, the time-averaged temperature over an accretion cycle is almost uniform throughout the hailstone and, when the environmental conditions are constant, is approximately equal to the equilibrium surface temperature predicted by the “classical” models. As the hailstone grows under constant environmental conditions, it continually adapts to the classical equilibrium temperature, warming up almost uniformly throughout. The time scale for this adjustment to a quasi-equilibrium state is found to be of the order of the internal diffusive time scale R 2/k. It is speculated therefore that if the environmental conditions change slowly (over time scales large compared with R 2/k) the hailstone thermodynamics will be adequately described by the classics equilibrium theories. However, if conditions change rapidly, internal heat conduction and time-dependent (non-equilibrium) effects may have to be taken into account.

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E. P. Lozowski and G. S. Strong

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A model of the vertical collision of a sphere with a hailpad predicts that the dent volume is proportional to the impact kinetic energy, and gives a relationship between dent diameter and sphere diameter for ice spheres. Laboratory calibration experiments confirm the essence of the theory but cast some doubt on the validity of the assumption of a constant resistance pressure. Further experiments simulating windblown ice spheres show that for the conditions we considered, the horizontal partition of energy has a small effect on the minor axis diamter of the dent. Consequently, if the wind speed is unknown, no more than a 10% error may occur if the sphere diameter is determined using the no-wind relation. Finally, field calibration of the hailpads with hailstones falling in a natural hail shaft tend to support both the laboratory calibrations and the model predictions.

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E. P. Lozowski, J. R. Stallabrass, and P. F. Hearty

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An experimental investigation of icing on non-rotating cylinders, under both wet and dry conditions was undertaken. Airspeeds of 30, 61 and 122 m s−1 appropriate to aircraft icing, liquid water contents of 0.4, 0.8 and 1.2 g m−3 and temperatures of − 15, − 8 and − 5°C, were explored. Dry accretions were lenticular or “spearhead” shapes, while wet accretions tended to develop “horns” and stagnation line depressions as the result of the runback of unfrozen water away from the stagnation line and its subsequent freezing further around the perimeter of the cylinder. Comparisons were made between the experimental accretion shapes and those predicted by the model described in Part I. The model performed best under dry growth conditions. Under wet conditions, the model behavior, while qualitatively correct, was unable to exactly duplicate the details of the accretion profiles. Nevertheless, under both dry and wet conditions, the model predictions of the accretion cross-sectional areas, were quite accurate.

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E. P. Lozowski, J. R. Stallabrass, and P. F. Hearty

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A model is described which simulates icing on an unheated, non-rotating cylinder. Both rime and glaze ice can be accounted for. The model computes the thermodynamic conditions and the initial icing rate as a function of angle around the upstream face of the cylinder. Although the model is not time-dependent, the initial icing rate can be used to compute local ice thickness after a specified time interval, and these in turn allow one to plot the ice accretion profile in either a single-step or multi-step fashion. Thus it is possible to predict total ice accretion cross-sectional area and mass for ice grown under varying conditions of airspeed, air temperature and pressure, cloud liquid water content, droplet size distribution, and cylinder size. Results are presented on the stagnation line growth rate as a function of liquid water content and airspeed, and examples of accretion profiles over a range of environmental conditions are provided. Although the model may be applied quite generally, the model results presented here are applicable to aircraft icing conditions.

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E. P. Lozowski, K. J. Finstad, and E. M. Gates

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E. P. Lozowski, R. B. Charlton, C. D. Nguyen, and J. D. Wilson

Abstract

The Edmonton monthly mean temperature record has been examined using the concept of the cumulative high frequency monthly mean temperature anomaly, I. The time sequence of I is shown to exhibit bounded, oscillatory, nonperiodic behavior.

At times features such as annual and quasi-triennial cycles and sudden reversals are exhibited. Some implications of these observations for interannual climate modeling and forecasting are discussed.

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