Lewis O. Grant

The characteristics and distribution of cloud water over the mountains of northern Colorado during wintertime storms. Part II: spatial distribution and microphysical characteristics.

Abstract The Physical and microphysical structure of the supercooled water fields in wintertime storms over the Park Range of the northern Colorado Rocky Mountains is examined using aircraft and ground-based measurements. Cloud top, cloud base, and zones of strong orographic lift are identified as regions in stratiform systems where supercooled water production can occur. Cloud systems over Colorados Park Range were found to have low droplet concentrations (≪300 cm−3). In clouds with the lowest droplet concentrations (<100 cm−3), broad droplet spectra were consistently observed. Significant numbers of large (<20 μm) droplets were present in these cases. The data presented here and in Part I are used to construct conceptual models of the structure and evolution of the liquid water fields in 1) shallow cloud systems with warm cloud tops, 2) deep stratiform clouds with cold tops, and 3) deep convective regions.

The Characteristics and Distribution of Cloud Water over the Mountains of Northern Colorado during Wintertime Storms. Part I: Temporal Variations

Abstract The spatial and temporal evolution of supercooled water fields in ten wintertime storm systems occurring over the northern Colorado Rocky Mountain region have been examined using data collected by the recently developed scanning dual-channel microwave radiometer. These data were supported by several independent datasets including vertically pointing radar data, mountaintop liquid water content measurements, low and high altitude measurements of crystal rime characteristic rawinsonde data and precipitation intensity measurements. The ten case studies discussed in this paper represent various stages in the synoptic scale evolution of storms that affect the northern Colorado Rockies. Liquid water was found to occur in nearly all stages of most of these storms. The temporal variations in the magnitude of the liquid water content were significant. Three common features concerning the evolution of the liquid water field were observed in the prefrontal cloud systems: 1) an inverse relationship between p...

A Statistical Reanalysis of the Replicated Climax I and II Wintertime Orographic Cloud Seeding Experiments

Abstract A reanalysis of the Climax I and II experiments is described. The concern prompting this reanalysis is a suggestion arising from Colorado State University analyses of extended area effects. Those analyses suggested a regionwide pattern of precipitation that, by chance, may have favored the randomly selected seeding days for some of the important meteorological partitions used in earlier analyses. In order to address this concern, this reanalysis employs excellent covariate relationships developed before the initiation of Climax II and which account for over half of the target variability for most meteorological partitions of major interest (e.g., warm 500 mb temperatures and southwest 700 mb wind directions). The statistical evidence of seeding-induced increases for this reanalysis is generally much stronger than in the previous analysis, which did not utilize covariates. For example, the joint one-sided Wilcoxon test statistic P-value for testing the null hypothesis that seeding did not induce a...

The Cloud Seeding Temperature Window

Abstract The greatest potential for seeding with artificial ice nuclei to augment precipitation should occur with cloud summit temperatures in the range from about −10C to about −25C. This is the temperature region where there may be a deficiency of natural ice-forming nuclei. This cloud-top temperature range therefore constitutes a “temperature window” for seeding effectiveness. This article considers the results from a number of cloud seeding experiments reported in the literature with respect to this temperature window. The analysis of seven randomized experiments and references to four other experiments indicates that there is a window in the cloud-top temperature range for which precipitation increases are indicated. This extends from about −10C to about −24C for seeding conducted in the modes employed on these projects. At the coldest cloud-top temperatures, generally less than about −30C, decreases in precipitation are indicated. There are variations among the samples which appear to be explainable...

An Independent Replication of the Climax Wintertime Orographic Cloud Seeding Experiment

Abstract An orographic cloud seeding experiment conducted in the vicinity of Climax, Colo., has been continued for five additional wintertime periods from 1965–70. A comparison of this new independent information is made with previously discussed wintertime operations of the experiment from 1960–65. As a whole, agreement between these independent data sets is good. In particular, the agreement in temperature and wind partitions is consistent with a previously reported model which describes seeding effects under various physically defined conditions. These comparisons have been made using pooled groups of precipitation sensors having similar elevations and locations.

Supercooled Liquid Water Structure of a Shallow Orographic Cloud System in Southern Utah

Abstract A case study of an orographic cloud system that developed over the mountains or southern Utah is presented. The storm system contained supercooled liquid water over several hours, and produced almost no precipitation. Because of the high liquid water content, low ice particle concentrations, minimal precipitation, and long duration, the storm appears to have been a good candidate for seeding to augment precipitation. A preliminary analysis of the climatological frequency of orographic cloud systems over these mountains is discussed.

Application of Ice Nucleation Kinetics in Orographic Clouds

Abstract Ice nucleation by silver iodide-sodium iodide aerosol particles has been characterized in the Colorado State University isothermal cloud chamber using the techniques of chemical kinetics. Two separate mechanisms of condensation-freezing ice nucleation have been observed. One mechanism occurs at water saturation and is a characteristically slow process, with a half-life of the order of 10–30 min. The other mechanism occurs when the environment is supersaturated with respect to liquid water. This mechanism is characteristically fast, requires less than a minute for completion, and results in a higher yield of ice crystals than the slow mechanism. The mechanism, rate and yield data obtained in the laboratory investigations are applied to an orographic cloud particle trajectory model to assess the ice nucleation characteristics of silver iodide-sodium iodide aerosol particles in the temporal and spatial scale of an orographic cloud. The importance of nucleation mechanism, rate and yield are investiga...

Elevation and Spatial Variation Effects of Wintertime Orographic Cloud Seeding

Abstract This study is concerned with the elevation and spatial variation effects of wintertime orographic cloud seeding over an area encompassing Fremont, Hoosier and Vail mountain passes in the central Colorado mountains during a period from 1960 to 1965. The observation network consisted of 65 precipitation stations distributed over the three passes. Depending on the grouping of precipitation stations used to represent the prime target area of the study, the average daily precipitation for all 120 seeded days was from 6 to 11% greater than the average daily precipitation for all 131 non-seeded days. There is a high probability that these differences could have occurred by chance alone. Analyses have also been made according to physically defined stratifications based on a model which describes the seeding effects ascribed to the various strata. Statistically significant increases (decreases) were observed over much of the area for the seeded periods in comparison with the non-seeded periods when 500 mb...

Cloud Seeding Effects on Precipitation Intensity and Duration of Wintertime Orographic Clouds

Abstract The nature of precipitation changes resulting from seeding cold orographic clouds is examined by separating the observed total precipitation change into duration and intensity change components. The total precipitation change and its two components are then evaluated as functions of cloud temperature using precipitation data recorded in the primary target area during the cloud seeding experiment conducted near Climax, Colo. The results show that the total change in observed precipitation is mainly controlled by changes in precipitation duration, rather than intensity. The main effects of seeding appear to be the initiation of a precipitation release for the warmer clouds during many hours when it would not have occurred naturally, and the suppression of precipitation for the coldest clouds during some hours when it would have occurred naturally. These results are consistent with the concepts of cloud microstability and cloud over-seeding.

Distributions of liquid, Vapor, and ice in an orographic cloud from field observations

Abstract The phase distribution of the water mass of a cold orographic cloud into vapor, liquid, and ice is calculated from measurements made from an instrumented aircraft. The vapor values are calculated from thermodynamic measurements, and the liquid is measured directly with a Johnson-Williams hot-wire device. Ice mass is calculated from particle size spectra obtained with a two-dimensional optical array cloud probe (2-D probe) and a knowledge of crystal habit based on decelerator measurements and cloud temperatures. Maximum vapor mass in the cloud is 2.0 g m−3, which is comparable with maximum ice mass in the cloud of 1.5 G m−3. Maximum liquid mass is approximately one order of magnitude lower at 0.15 g m−3 and appears to be a small remainder between the vapor and the ice as they compete for the major portion of the cloud water mass. In the cloud upwind of the mountain, liquid + vapor + ice is nearly constant, suggesting that precipitation does not deplete the water mass at the levels studied by the a...