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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

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

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

A study of the morphology of large sporadic enhancements of F-region electron density in the darkened portions of the polar cap reveals a close connection between “sporadic F” and ionospheric current systems. This connection arises from electric fields which are set up by these currents and then transferred to the F-region along an essentially vertical magnetic field. Under the influence of the electric and magnetic fields, a horizontal drift of ionization may become sufficiently strong to cause the development of sporadic F. Drift velocities obtained from synoptic analyses range between 2000 and 5000 kilometers per hour and agree with theoretical estimates. Criteria are presented for the development of sporadic F events, and the general diurnal and seasonal variations in the frequency of occurrence are inferred.

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Geoffrey E. Hill

Abstract

Two cases of deep convection are numerically simulated in a two-dimensional time-dependent model and compared with observations. Various aspects of the simulated convection include the cloud size, height, spacing, lifetime, vertical motion, time of occurrence and precipitation. Maximum cloud-top heights based upon quasi-uniform surface temperature changes, in accordance with the observed, are compared with numerical model results obtained by using various type and size perturbations.

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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

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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

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

Observational evidence is presented which identifies the occurrence of strong precipitation-forced circulations during winter orographic storms. It is shown by analysis of several case studies that precipitation in the mountains is greatly reduced following the development of such circulations.

The development of a precipitation-forced circulation occurs first at low levels and expands upward to near mountain-top levels. Both the original updraft and the downdraft beneath are subsequently weakened; at that time there is a rapid reduction of precipitation. In most instances, a weak low-level updraft and a secondary maximum of precipitation redevelop.

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