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Geoffrey E. Hill

Abstract

Aircraft icing reports from northern Utah and southeastern Idaho along with radiosonde and precipitation data for six winter seasons are utilized in an analysis of precipitation augmentation potential in winter orographic clouds. According to this analysis clouds with top temperatures warmer than a critical value are primarily composed of supercooled water and colder clouds are primarily composed of ice. This critical cloud-top temperature varies from ∼−20°C at zero to 10 m s−1 cross-barrier wind speeds (measured near the altitude of the barrier crest) to about −26°C at cross-barrier wind speeds between 10 and 20 m s−1. Deep convective clouds are excluded from the analysis.

It is concluded that the critical cloud-top temperature is governed both by the temperature dependence of active ice nuclei, and the time available for glaciation, which is directly related to the cross-barrier wind speed. For cloud-top temperatures warmer than the critical value, the time available for glaciation is of secondary importance. The primary factor in this case is the cross-barrier wind speed, which determines 1) the critical cloud top temperature, 2) the amount of supercooled water, and 3) the cross-barrier flow of supercooled water. Thus, the precipitation augmentation potential depends approximately upon the square of the cross-barrier wind speed when the cloud-top temperature is warmer than a critical value which itself depends upon the cross-barrier wind speed. A cloud-seeding potential exists within the warm cloud-top temperature category at cross-barrier wind speeds ≳10 m s−1. Out of the total number of cases, 21% fall in this high-yield category.

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Geoffrey E. Hill

Abstract

The effect of the Great Salt Lake on the frequency and geographical extent of wintertime fog is analyzed by use of fog reports, precipitation, and temperature records over a 25-yr period, during which time the size of the lake has more then doubled. Fog reports at Salt Lake City on days when precipitation other than snow grains did not occur are analyzed to find a relationship between lake size and fog frequency. While a large winter-to-winter Variability is found, there is also a strong relationship with lake size.

To analyze the geographical effect of the lake on fog, the daily range of temperature is first related to the occurrence of persistent fog; it is shown that when fog is present throughout a day the difference between the maximum and minimum temperature will be low. Analysis of the geographical distribution of these persistent fog days shows the strong effect of the lake. Also, the occurrence of persistent fog was much less when the lake was small compared to when the lake was large.

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Geoffrey E. Hill

Abstract

Analysis is made of two randomized winter orographic cloud seeding experiments conducted in the northern Wasatch Mountains. In the first experiment seeding material was released from airborne pyrotechnics and in the second experiment, from three mountain-top generators. Precipitation was measured by a network of remotely interrogated precipitation gages. Interpretation of the results are aided in particular by the use of storm type classifications, precipitation estimators based on upper level data, vertical incidence radar and aircraft icing reports.

An a priori hypothesis, that seeding of clouds would increase precipitation when the 500 mb temperature is warmer than −22°C, is rejected. Also, precipitation increases are not found in orographic clouds when the cloud-top temperature is warmer than −29°C.

An a posteriori analysis fails to show increases in precipitation when the cloud-top temperature is warmer than −24°C. On the other hand, stratification of experimental events according to degree of aircraft icing, as a measure of supercooled water concentration, indicates that marked seeding effects may be present.

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Geoffrey E. Hill

Abstract

Evaluation of an operational-type winter cloud seeding project in Utah is made by developing meteorological predictors of target precipitation. Predictors (covariates) are developed by matching 1200 GMT rawinsonde data and 24 h precipitation amounts. These predictors and precipitations are summed over seven unseeded seasons to form a seasonal predictor-predictand relationship, for which the correlation is 0.975 when the average precipitation for all stations is used, and 0.879 when only the two highest altitude stations are used. Then, the predictor is found for each of the seeded seasons, and based upon the unseeded predictor-predictand relationship, the predicted precipitation is obtained. Differences between predicted and observed precipitation in seeded years are compared and tested for seeding effects. Application of the method to the first two years of the project indicates a substantial chance that little or no effect of seeding occurred. It is concluded that the method offers a promising approach to the evaluation of winter cloud seeding projects.

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Geoffrey E. Hill

Abstract

Detailed measurements of supercooled cloud water, precipitation, cloud-top temperature and vertical air motion in winter orographic clouds are used to develop criteria for the seedability of those clouds. Winter orographic clouds over the upwind mountain base with cloud-top temperatures between 0 and −22°C are found to be primarily composed of supercooled water and are therefore seedable. The supercooled water concentration is empirically found to depend on the updraft velocity. The potential precipitation yield is dependent on the flux of supercooled water over the barrier. Because the updraft velocity is approximately proportional to the cross-barrier wind, the potential precipitation yield is approximately proportional to the square of the cross-barrier wind, provided that the cloud-top temperature is in the seedable range of temperatures.

These findings are strongly substantiated by systematic use of aircraft icing reports over a full winter season (November–March 1978–79). It is shown that a cloud-top temperature of about −22°C separates clouds with a precipitation enhancement potential from those without such a potential. It is found that aircraft icing is approximately proportional to the cross-barrier wind, and that the flux of supercooled water over the barrier for cloud-top temperatures warmer than −22°C is (as derived from the research data) approximately proportional to the square of the cross-barrier wind.

About 20% of cloud episodes over the mountains of northern Utah may be expected to have a high modification potential.

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Geoffrey E. Hill

Abstract

This article is a review of work on the subject of seedability of winter orographic clouds for increasing precipitation. Various aspects of seedability are examined in the review, including definitions, distribution of supercooled liquid water, related meteorological factors, relationship of supercooled liquid water to storm stage, factors governing seedability, and the use of seeding criteria.

Of particular interest is the conclusion that seedability is greatest when supercooled liquid water concentrations are large and at the same time precipitation rates are small. Such a combination of conditions is favored if the cloud-top temperature is warmer than a limiting value and as the cross-barrier wind speed at mountaintop levels increases.

It is also suggested that cloud seeding is best initiated in accordance with direct measurements of supercooled liquid water, precipitation, and cross-barrier wind speed. However, in forecasting these conditions or in continuation of seeding previously initiated, the cloud-top temperature and cross-barrier wind speed are the most useful quantities.

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Geoffrey E. Hill

Abstract

Two flight plans were used for a seeding aircraft and a measurement aircraft in the study of silver iodide plume dispersion. An instantaneous line source and a continuous point source plume generation were simulated approximately by the seeding aircraft. Plume interceptions were made about an hour later by another aircraft equipped with an acoustical type ice nucleus counter.

Results of the measurements are that both the vertical and horizontal diffusion of silver iodide released from airborne generators in the northern Wasatch Mountains during winter orographic storms are much lower than that desired for effective seeding. Plume-edge outward velocities are only about 0.5–1 m s−1 in the horizontal and about 0.1 m s−1 in the vertical directions, respectively.

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Geoffrey E. Hill

Abstract

Simultaneous measurements of supercooled liquid water by an instrumented aircraft and a dual-frequency microwave radiometer were made at Lake Ontario, New York, during wintertime. The geographic location and typical meteorological conditions for making the measurements were specifically selected to facilitate the comparisons. Flight paths from below cloud base to above cloud tops were made over the radiometer site. Seven flights were made; supercooled liquid water was measured by a calibrated Rosemount icing meter.

The primary finding is that when the temperature of the atmosphere in the viewing path of the radiometer is below the melting point of ice, the airborne liquid-water measurements are in general agreement with the radiometric measurements. When an inversion with the temperature above the melting point is present, the radiometric readings of liquid water are much larger than the values found from the aircraft. Also, the, possibility is raised that in very heavy snowfall with large ice particles the amount of supercooled liquid water will appear too large according to the radiometer.

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Geoffrey E. Hill

Abstract

Simultaneous measurements of supercooled liquid water were made by an instrumented aircraft and a microwave radiometer at Muskegon, Michigan during wintertime. The purpose was to confirm recent findings that there is good agreement between the two measuring systems. The flight paths were over the radiometer site from cloud base through the cloud top and back down. Ten flights were made; supercooled liquid water was measured by a calibrated Rosemount icing meter.

It is found that when the temperature in the entire viewing path of the radiometer is below 0°C the radiometric measurements generally agree with those of the airborne measurements. It is concluded that under these conditions the liquid water measured by the radiometer will be a valid measurement of supercooled liquid water.

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Geoffrey E. Hill

Abstract

Factors controlling the size and spacing of cumulus clouds are determined by the numerical integration of a compressible set of hydrodynamic equations for an atmosphere with saturation processes and cloud microphysics included. Sets of integrations, each carried out for several hours of cloud simulation, produce fields of numerical clouds that pass through various stages of development. The characteristics of clouds in the initiation, growth, and fully-developed stages are examined. In addition, the development of group structures and cloud merging are also found. Comparison of numerical results with a variety of observations shows that the model gives realistic cloud characteristics.

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