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I. Gultepe and G. A. Isaac

Abstract

The vertical distribution of liquid water content (LWC) and its relationship with temperature (T) strongly affect the heat budget of the atmosphere. Some large-scale models of the atmosphere use a relationship between LWC and T to diagnostically obtain LWC from T under saturated conditions. Airborne observations conducted within clouds over northeastern North America during the 1984–93 time period are used to study the relationship between LWC and T. Observed frequency distributions of LWC are approximated by lognormal distribution curves and are best represented by median values. The median LWC values monotonically increase with warmer temperatures. However, the mean LWC reaches 0.23 g m−3 at about T = 2.5°C. LWC decreases below and above 2.5°C, except that it reaches a maximum value of 0.26 g m−3 at 22.5°C. The relationship between LWC and T from the present study is compared with that of earlier studies from the former Soviet Union. Differences can be attributed to the design and limits of the probes, natural variability in the 35 years, and the limited dataset for some temperature intervals. The LWC versus T relationship developed from observations in this study can be compared with large-scale model simulations.

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I. Gultepe and G. A. Isaac

Abstract

The purpose of this study, using in situ observations from five field projects, is to analyze cloud boundaries and averaging scales related to droplet number concentration (N d) and total aerosol number concentration (N a), and to discuss parameterizations of these variables for use in numerical weather prediction and global climate models. Here N d and N a for stratus and stratocumulus clouds are averaged over various lengths from 1 km up to 35 km. The relationships between these variables for 1-s and 200-s data are compared with current parameterizations. Comparisons between N d and N a show that N a plays an important role for activating cloud droplets. The variability in N d from 1-s data is estimated to have a standard deviation of about ±150 cm−3. Median values of N d representing scales from about 0.1 km up to approximately 35 km are found to be dependent on scale and the presence of clear patches in the clouds. The average values of N d are also dependent on the lower concentration threshold used to define the cloud boundaries. It is concluded that scale effects, including clear air regions, should be considered when developing parameterization schemes used for modeling studies of cloud systems.

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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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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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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

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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W. R. Leaitch and G. A. Isaac

Abstract

Comparisons are drawn between the aerosol cloud microphysical theory implicit in the modeling of Kaufman et al. and the cloud droplet and cloud water sulfate concentrations of Leaitch et al. for the purpose of helping to understand the effect of sulfate particles on climate through cloud modification. In terms of the range of possibilities and prospects for future climate given by Kaufman et al. for the effect of sulfur on cloud albedo, the data favor the possibility of stronger cooling. Scatter in the data makes it impossible to constrain model parameters., however, the comparisons suggest that there may not be a universal relationship, and that the uncertainties involved in trying to model this proem are large.

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G. A. Isaac and R. S. Schemenauer

Abstract

No abstract available.

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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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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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