Sierra Cooperative Pilot Project Research Articles

On a Natural Environment for Glaciogenic Cloud Seeding

AbstractA “climatology” of supercooled cloud tops is presented for southeastern Australia and the western United States, where historic glaciogenic cloud-seeding trials have been located. The climatology finds that supercooled cloud tops are common over the mountainous region of southeastern Australia and Tasmania (SEAT). Regions where cloud-seeding trials reported positive results coincide with a higher likelihood of observing supercooled cloud tops. Maximum absolute frequencies (AFs) occur ∼40% of the time during winter. There is a relationship between the underlying orography and the likelihood of observing supercooled liquid water (SLW)-topped clouds. Regions of the United States that have been the subject of cloud-seeding trials show lower AFs of SLW-topped clouds. The maximum is located over the Sierra Nevada and occurs ∼20% of the time during winter (Sierra Cooperative Pilot Project). These sites are on mountains with peaks higher than any found in SEAT (>3000 m). For the Sierra Nevada, the AF of SLW-topped clouds decreases as the elevation increases, with glaciation occurring at the higher elevations. The remote sensing of supercooled cloud tops is not proof of a region’s amenability for glaciogenic cloud seeding. This study simply highlights the significant environmental differences between historical cloud-seeding regions in the United States and Australia, suggesting that it is not reasonable to extrapolate results from one region to another. Without in situ cloud microphysical measurements, in-depth knowledge of the timing and duration of potentially seedable events, or knowledge of the synoptic forcing of such events, it is not possible to categorize a region’s potential for precipitation augmentation operations.

Quantitative Precipitation Forecasting of Wintertime Storms in the Sierra Nevada: Sensitivity to the Microphysical Parameterization and Horizontal Resolution

Abstract The skill of a mesoscale model in predicting orographic precipitation during high-impact precipitation events in the Sierra Nevada, and the sensitivity of that skill to the choice of the microphysical parameterization and horizontal resolution, are examined. The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) and four bulk microphysical parameterization schemes examined are the Dudhia ice scheme, and the Schultz, GSFC, and Reisner2 mixed-phase schemes. The verification dataset consists of ground precipitation measurements from a selected number of wintertime heavy precipitation events documented during the Sierra Cooperative Pilot Project in the 1980s. At high horizontal resolutions, the predicted spatial precipitation patterns on the upwind Sierra Nevada slopes were found to have filamentary structure, with precipitation amounts over the transverse upwind ridges exceeding severalfold those over the nearby deep river valleys. The verification results show that all four tested bulk microphysical schemes in MM5 produce overprediction of precipitation on both the windward and lee slopes of the Sierra Nevada. The examined accuracy measures indicate that the Reisner2 scheme displays the best overall performance on both sides of the mountain range. The examined statistical skill scores on the other hand reveal that, regardless of the microphysical scheme used, the skill of the MM5 model in predicting the observed spatial distribution of the Sierra Nevada orographic precipitation is fairly low, that this skill is not improved by increasing the horizontal resolution of the model simulations, and that on average the quantitative precipitation forecasting (QPF) skill is better on the windward than on the lee side. Furthermore, a significance test shows that differences in skill scores obtained with the four microphysical schemes are not statistically significant.

Validation and Sensitivities of Dynamic Precipitation Simulation for Winter Events over the Folsom Lake Watershed: 1964–99

Abstract A total of 62 winter-storm events in the period 1964–99 over the Folsom Lake watershed located at the windward slope of the Sierra Nevada were simulated with a 9-km resolution using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Mean areal precipitation (MAP) over the entire watershed and each of four subbasins was estimated based on gridded simulated precipitation. The simulated MAP was verified with MAP estimated (a) by the California Nevada River Forecast Center (CNRFC) for the four subbasins based on eight operational precipitation stations, and (b) for the period from 1980 to 1986, on the basis of a denser precipitation observing network deployed by the Sierra Cooperative Pilot Project (SCPP). A number of sensitivity runs were performed to understand the dependence of model precipitation on boundary and initial fields, cold versus warm start, and microphysical parameterization. The principal findings of the validation analysis are that (a) MM5 achieves a good percentage bias score of 103% in simulating Folsom basin MAP when compared to MAP derived from dense precipitation gauge networks; (b) spatial grid resolution higher than 9 km is necessary to reproduce the spatial MAP pattern among subbasins of the Folsom basin; and (c) the model performs better for heavy than for light and moderate precipitation. The analysis also showed significant simulation dependence on the spatial resolution of the boundary and initial fields and on the microphysical scheme used.

A Comparison of Seeded and Nonseeded Orographic Cloud Simulations with an Explicit Cloud Model

Abstract Ice initiation by specific cloud seeding aerosols, quantified in laboratory studies, has been formulated for use in mesoscale numerical cloud models. This detailed approach, which explicitly represents artificial ice nuclei activation, is unique for mesoscale simulators of cloud seeding. This new scheme was applied in the simulation of an orographic precipitation event seeded with the specific aerosols on 18 December 1986 from the Sierra Cooperative Pilot Project using the Regional Atmospheric Modeling System (RAMS). Total ice concentrations formed following seeding agreed well with observations. RAMS's three-dimensional results showed that the new seeding parameterization impacted the microphysical fields producing increased pristine ice crystal, aggregate, and graupel mass downstream of the seeded regions. Pristine ice concentration also increased as much as an order of magnitude in some locations due to seeding. Precipitation augmentation due to the seeding was 0.1–0.7 mm, similar to values in...

Microphysical Structure and Evolution of a Central Sierra Nevada Orographic Cloud System

This paper describes complex microphysical interactions observed within an orographic cloud system over the central Sierra Nevada of California. Measurements of the physical structure and microphysical characteristics of this cloud system were made during the Sierra Cooperative Pilot Project (SCPP) with a 5-cm scanning radar, an instrumented aircraft, two rawinsonde stations, and a network of precipitation gauges across the mountain. The cloud system extended 130 km upwind of the Sierra Nevada crestline, was 4 km deep, had a base temperature of +5°C, and a top temperature of approximately −15°C. Cloud droplets rising from cloud base grew to sizes >50 μm and first encountered dendritic ice particles descending from cloud top over the middle elevations of the Sierra Nevada. Within this region, rapid removal of the large droplets by riming occurred, and secondary ice-crystal production due to rime splintering was observed. Needles produced by secondary ice-particle production were incorporated into dendritic aggregates falling from cloud top farther toward the mountain crest. The nature of the microphysical interactions within this cloud system and their impact on the distribution, amount, and type of precipitation across the mountain range are discussed. Changes in the distribution of precipitation from the orographic cloud system due to the approach of a short-wave trough are also presented.

Evaluation of a Two-Dimensional Kinematic Cloud Model Using Data from a Central Sierra Nevada Orographic Cloud System

Abstract A two-dimensional kinematic cloud model was developed to include snowflake growth by aggregation as well as riming and vapor deposition. The model includes new parameterizations of ice crystals and aggregates of ice crystals. The model was run and evaluated using a dataset gathered by the Sierra Cooperative Pilot Project (SCPP). The model was tested for its sensitivity to two model parameters, a constant relating the mean mass of collected ice crystals to the mean mass of all ice crystals in the cloud, and an ice nucleation rate constant. Varying the collection constant by a factor of 2.5 had only minor effects on the model behavior, but varying the ice nucleation rate by a factor of 50 had significant effects on the relative importance of riming and vapor deposition growth. Two problems that did develop in the model were the result of the model's bulk microphysical assumptions relating graupel and raindrop size to the magnitude of the mixing ratio, so that small concentrations behaved as cloud p...

The Characteristics and Evolution of Supercooled Water in Wintertime Storms over the Sierra Nevada: A Summary of Microwave Radiometric Measurements Taken during the Sierra Cooperative Pilot Project

A comprehensive examination of the evolution and vertical distribution of supercooled liquid water in Sierra Nevada winter storms was completed as part of the Sierra Cooperative Pilot Project. Measurements of supercooled water in 63 storms, which affected the area from the 1983/84 through the 1986/87 winter field seasons, were made with a dual-channel microwave radiometer located near the Sierra Nevada crestline. The analyses were carried out in two parts. In the first part, winter storms were grouped into two general categories according to the prevailing flow and resultant storm trajectory. The two storm types consisted of those propagating in zonal flow and those in meridional flow. A complete description of the storm typing is provided. Storms in zonal flow were either developing, occluding, or dissipating during the time they affected the Sierra Nevada. The stage of evolution of the storm dictated the characteristics of supercooled water observed during the storm's passage over the Sierra Nevada. The presence of supercooled water in meridional storms was influenced mainly by the trajectory of the storm. Results indicated that zonal storms had the most sustained periods of supecooled water in the postfrontal region, while sustained supercooled water measurements in meridional storms were usually greatest within the prefrontal portions of the storm. These periods of supercooled water were associated primarily with orographic clouds. Studies of the vertical supercooled water distribution showed that saturation often existed within 1 km of the local terrain when supercooled water was measured by the radiometer. Fifty percent of the rawinsondes launched through supercooled water-bearing clouds measured water-saturated layers deeper than 0.5 km. Saturated layers were found to exist most often between −8° and and −10°C, although they were often present over a much wider temperature range. Conditions favorable for ice crystal growth were common, which supports the pursuit of precipitation enhancement as a means of supplementing regional water supplies.

A Diagnostic Technique for Targeting during Airborne Seeding Experiments in Wintertime Storms over the Sierra Nevada

Abstract A diagnostic technique for targeting during airborne seeding experiments has been developed for the Sierra Cooperative Pilot Project (SCPP). This technique was used operationally during SCPP for real-time guidance to aircraft, providing 1) the location and orientation of the seeding line required to target ice particles created by seeding to a specified ground location and 2) an estimate of the areal coverage of the seeding effect on the ground. Procedures to use this technique as a real-time guidance tool during seeding operations in Sierra wintertime storms are discussed. Three evaluation studies of the targeting method are presented. These include 1) comparisons of diagnosed wind fields with those measured by aircraft; 2) comparisons of ice particle growth rates and habits within seeded cloud regions with those used in the targeting computations; and 3) comparison of radar echo evolution within seeded cloud regions with calculated particle trajectories.

Winter Diurnal Trends of Sierra Nevada Supercooled Liquid Water and Precipitation

Ten-year climatologies from the Sierra Cooperative Pilot Project (SCPP) show diurnal variations of clouds, precipitation, supercooled liquid water, stability and temperature. Diurnal variations were generally more pronounced in March than in January and February. March, in particular, developed a pronounced maximum of afternoon convection due to strong modulation of spring air masses by surface heating. Supercooled liquid water was most abundant between midnight and dawn and least abundant at midday; this trend grew stronger from January through March. Two effects help to explain the diurnal trend of supercooled liquid water at Squaw Peak, a mountain top location. First, when the freezing level stayed above Squaw Peak during the day but descended below it at night, water cloud was only supercooled at the site at night. Second, low-level water cloud often dissipated as surface heating raised the temperature of overlying air above its dewpoint; water cloud formed at night through a reverse process.

Remote and In Situ Observations of Sierra Nevada Winter Mountain Clouds: Relationships between Mesoscale Structure, Precipitation and Liquid Water

Abstract A small subset of midlatitude, midwinter precipitation events affecting the central Sierra Nevada are analyzed. The examples given are representative of 60% of the storm types documented during the past 4 yr of the Sierra Cooperative Pilot Project (SCPP). The structure of thee frontal systems is consistent with those observed in the United States Pacific Northwest and the British Isles. Combining information from a vertically pointing microwave radiometer, conventional radar, satellite imagery, and detailed time cross sections of rawinsonde data, relationships are developed between these remote sensing devices and the onset of supercooled liquid water (SLW). For the storms described. the highest concentration of SLW occurs after passage of an upper jet with accompanying upper-level front or surface cold ana- and/or katafront. Thee frontal passages lead to decreasing cloud thickness, warming cloud tops, decreasing precipitation rate, and shallow embedded convection over the Sierra. Discontinuities...

Seasonal and Interannual Trends of Sierra Nevada Clouds and Precipitation

Abstract Seasonal and interannual variations in Sierra Nevada winter storms are discussed with reference to precipitation augmentation. Seasonal variations occur with respect to freezing level, storm type, vertical cloud distribution, mesoscale precipitation systems, snowmelt and runoff. Statistical results from a previous operational program by Mooney and Lunn suggest that “cold westerly” storms yield increased precipitation from cloud seeding. Case studies from the Sierra Cooperative Pilot Project have shown that postfrontal conditions, which closely correspond to cold westerly storms, are characterized by high supercooled liquid water contents. Eight years of data from the American River Basin have been analyzed here which show that cold westerly storms 1) are more frequent in late winter and spring than earlier in the precipitation season; 2) contribute greater precipitation in seasons of normal and below-normal precipitation than in above-normal seasons. Hydrological factors make these storms attract...

Field Evaluation of a Dual-Channel Microwave Radiometer Designed for Measurements of Integrated Water Vapor and Cloud Liquid Water in the Atmosphere

Abstract The dual-channel microwave radiometer is evaluated in regard to the measurement of integrated water vapor and supercooled liquid water. The study includes comparisons of integrated vapor content measured by the radiometer and rawinsondes as part of the Sierra Cooperative Pilot Project in the central Sierra Nevada. In addition, two radiometers with virtually identical characteristics were brought together on the Colorado Orographic Seeding Experiment at Steamboat Springs, Colorado, in order to study the stability and comparability of integrated vapor and liquid measurements. Comparison of vapor measurements by the radiometer and by rawinsonde yielded a correlation coefficient of 0.94, and a rms difference of 0.08 cm. There were no significant differences between results for paired data gathered in storms in the presence of supercooled liquid water and for paired data gathered on clear days under more ideal conditions. The collocated-radiometer experiment showed slightly closer agreement than did t...

A Review of the Sierra Cooperative Pilot Project

The Sierra Cooperative Pilot Project (SCPP) is an investigation of cloud seeding as a means of increasing winter precipitation on the Sierra Nevada. It is a concerted effort in the development of a physically sound cloud-seeding technology. It involves the use of remote-sensing devices, in situ observations, and the application of a numerical targeting model in randomized seeding experiments. The results have led SCPP scientists to believe that shallow but widespread orographic clouds provide the best opportunity for glaciogenic seeding in the central Sierra Nevada.

An Airborne Comparison of Three PMS Probes

Abstract The 1977–78 Sierra Cooperative Pilot Project (SCPP) field season offered an opportunity for comparing several Particle Measuring Systems (PMS) probes. During that winter the University of Wyoming's King Air aircraft was instrumented with 1D cloud, 2D cloud and 2D precipitation probes. Data from the three probes were analyzed from several flights in stable oregraphic storms. Various temperature levels (and hence, hydrometeor habits) above and below 0°C were flown in order to compare the data for water drops and for different ice crystal habits. It was found that all three probes agree quite well when sampling water drops of any size; however, when sampling ice crystals the probes agreed only at sizes larger than about 1 mm.

Supercooled Liquid Water and Ice Crystal Distributions Within Sierra Nevada Winter Storms

Abstract Cloud physics data measured by aircraft during two successive winter field seasons (1978–79 and 1979–80) of the Sierra Cooperative Pilot Project operating over the Sierra Nevada Range have been examined in order to determine the distributions of supercooled liquid water and ice crystals. Results indicate that convective clouds provide the greatest likelihood of significant supercooled water. The Sierra barrier appears to optimize these conditions 40 to 90 km upwind of the crest within pockets of horizontal extent up to 64 km, although these conditions were greatly reduced at temperatures less than −10°C. The dominance of liquid water content over ice crystal concentration was maximized 7–10 h after the 700 mb trough passage. Area-wide and banded clouds, which make up the remaining precipitation events, showed only small amounts of supercooled water and general abundance of ice crystals. The largest liquid water contents were observed at the greatest temperatures, usually 0° to −5°C. Such climatol...

The Use of Microwave Radiometry to Determine a Cloud Seeding Opportunity

Abstract A ground-based combination microwave radiometer-satellite receiver operating at 28.5 GHz (wavelength = 1.05 cm) was employed to measure liquid water in clouds during the 1979-80 field season of the Sierra Cooperative Pilot Project (SCPP) in northern California. We report upon the ability of the instrument to detect small amounts (∼0.1 g m−3) of liquid water in non-precipitating clouds which may not be observed by other remote sensing systems. A successful cloud seeding experiment, performed after cloud liquid was detected by the instrument, is described.

Some Seeding Signatures in Sierra Storms

Abstract Airborne seeding experiments were conducted over the Sierra Nevada Mountains in essentially ice-free convective clouds on two days in March 1979 as part of the Sierra Cooperative Pilot Project. On 18 March towering cumuli which extended above a stratiform layer of clouds were seeded, while on 21 March individual towering cumuli were seeded as they developed and moved over the windward side of the mountains. Each cloud was seeded with a vertical curtain oriented perpendicular to the winds during a single pass through the cloud top. The seeding mode was either a low (∼0.1 g m−1) or high (∼1 g m−1) CO2 rate or AgI flares (one 20-gram flare per 250 m). The seeded curtains were penetrated a number of times by the University of Wyoming King Air. The high CO2 rate apparently overseeded the cloud in that the liquid water was depleted and the cloud dissipated in ∼35 min. Even though much of the liquid water was depleted in the other seeded clouds, they persisted and precipitated for over an hour because a...

Contract to analyze cloud systems

The Bureau of Reclamation has awarded a $1.1 million contract to Colorado State University (CSU) to analyze cloud systems by satellite in support of weather modification research, Secretary of Interior Cecil Andrus announced recently. ‘The use of satellite imagery holds great promise in helping us understand the processes that produce water from the atmosphere,’ Secretary Andrus said.The 5‐year study will support two Bureau of Reclamation research projects: the High Plains Cooperative Program (Hiplex) in growing season rainfall management and the Sierra Cooperative Pilot Project in winter snowfall enhancement.