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David W. Reynolds

Abstract

A combination of rawinsonde balloon ascent rates, low-elevation aircraft, and ground-based tracer sampling measurements are presented. These data indicate that mountain-induced gravity waves have a significant impact on the transport of ice crystals produced by the release of liquid propane from high-altitude dispensers along the crest of the northern Sierra Nevada in California. Special rawinsonde launches were made just downwind of the main Sierra Nevada crest. Balloon ascent rates show a very well defined mountain lee wave present during most precipitation events. Strong descent to the lee of the Sierra will thus have a detrimental effect on the growth of particles generated on the crest. The tracer SF6 (sulfur hexaflouride) is used to simulate the transport and dispersion of propane-generated ice crystals. Sulfur hexaflouride was released from two propane dispenser sites as a proxy for seeded ice crystals. Aircraft measurements of SF6 indicated that at the normal flight altitudes of 2500 m over the downwind valley and 2800 m over the downwind ridge the aircraft was flying near the top of the plumes. When the aircraft was able to fly below cloud base, near the release altitude of 2200 m, substantial SF6 was observed. The lower portion of the plume was also observed to descend into the valley some 700 m below the release altitude. A simple two-dimensional model is used to determine the impact that these gravity waves have on particle trajectories. Model output is presented for one well-documented seeding case to determine how well such models might be used operationally to predict particle trajectories downwind of the Sierra.

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David W. Reynolds

Abstract

During the months of May, June, July and August, 1978, a record number of damaging hailstorms, causing losses upward of $100 million, struck along the High Plains and Front Range regions of New Mexico, Colorado, Wyoming and Montana. Nine of these storms were observed from the GOES-E geostationary satellite with the digital visible and infrared data recorded at the CSU Direct Readout Satellite Groundstation. The digital, navigated imagery were processed on an interactive image processing system for detection of hail signatures.

In all but one case of reported hail, the coldest cloud-top temperature of the storm system located nearest the hailfall was from 1 to 8°C colder than the environmental tropopause temperature during at least a portion of its lifetime. In most cases this occurred coincident with the best estimate of the onset of hail. Also, the imagery showed each of these storm complexes having long lifetimes (2–5 h), with some exhibiting temperatures colder than the tropopause temperature for this length of time. Through analysis of the 22 June 1976 NHRE storm complex, it was determined that hailfall occurred at close to the maximum growth rate of the storm. This paper thus begins to identify a potential technique for identifying damaging hailstorms through proper enhancement of digital GOES infrared imagery.

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Terry Deshler and David W. Reynolds

Abstract

A single case-study of a winter orographic cloud over the central Sierra Nevada is presented in which the effects of aerial seeding with silver iodide, an AgI NH4C1O4 mixture burned in acetone, were observed to persist for over 90 min after seeding and 100 km downwind of the seedline. A research aircraft was able to locate and track the line source of AgI using an ice nucleus counter. High ice crystal concentrations due to seeding were not apparent until more than one hour after seeding. This may have been partially due to the high natural concentrations of ice, but post-mission analysis revealed that most sampling passes during the first hour following seeding were made below the AgI seeded volume. Ice nucleus measurements confirmed sampling of the seedline from 1–1.5 h after seeding, with associated increases in ice crystal concentrations. The effectiveness of the seeding material in the field was higher than laboratory measurements would suggest.

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K. L. Coulson and David W. Reynolds

Abstract

The amount of solar energy reflected from various soils and types of vegetation has been measured as a function of sun elevation in six different wavelength ranges in the ultraviolet, visible and near-infrared regions of the spectrum. It is shown that there is a significant dependence of reflectance on both wavelength and elevation of the sun for all surfaces for which measurements were made.

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David W. Reynolds and Arunas P. Kuciauskas

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 in cloud top temperature, rainbands, and decreasing echo height, associated with the passage of the upper jet and accompanying front, can be identified with satellite and radar several hours before affecting the Sierra Nevada. thus providing a prediction for the onset or increase in SLW. These relationships have application to wintertime cloud modification programs over the central Sierra.

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Terry Deshler, David W. Reynolds, and Arlen W. Huggins

Abstract

Cloud seeding experiments devoted to physical measurements of the effects of seeding shallow stable winter orographic clouds have been conducted in the central Sierra Nevada of California from 1984 to 1986. Seeding was done by aircraft using either dry ice or silver iodide at temperatures between −6° and −14°C. Aircraft, radar, and surface instruments were used to measure the effects. A trajectory model was used to target seeded precipitation to the ground where the surface instruments were deployed. Results from these experiments are presented in two case studies and a summary analysis of all 36 experiments. Observations from the various measurement platforms conformed with results expected from seeding in 35 percent of the seedlines sampled with a research aircraft, 4 percent of those observed with radar, and 17 percent of these which passed over the surface instrumentation; however, the complete seeding chain was believed to be documented in only 2 of 36 experiments. The failures result from difficult technical and logistical problems, and from the variability of even simple cloud systems, particularly in the spatial and temporal distributions of liquid water and in the natural fluctuations in ice crystal concentrations. Based on the difficulty of these experiments and the magnitude of seeding effects observed, a statistical experiment would be a formidable undertaking.

During the two experiments when seeding effects were detected by all measurement platforms the following effects were observed. A high concentration, 50–100 L−1, of small compact ice crystals formed quickly along the seedline. Although aggregation was seldom observed, riming often began 5–10 min after seeding. The seeded ice crystals dispersed at 1 m s−1 and cloud liquid-water evaporated in regions corresponding to the seedlines. Seeding in a non-echoing region occasionally produced echoes of 3–10 dBZ in portions of the seedlines. At the surface seeding effects arrived 35 to 60 min after seeding, 20–30 km downwind. Snow crystal concentrations increased, snow crystal habits changed to small rimed particles, and precipitation rates increased by 0.1–1.0 mm h −1. The duration of these effects was short, <10 min per seedline. Changes in ice particle development induced by seeding were similar when seeding with either dry ice or silver iodide. This was found to be the case even at temperatures as warm as −6°C using AgI NH4I NH4ClO4 burned in an acetone solution.

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Ming Ji, Richard W. Reynolds, and David W. Behringer

Abstract

In this study, two sets of Pacific Ocean analyses for 1993–96 were analyzed. Both analyses were produced with the assimilation of subsurface temperature data from expendable bathythermographs and tropical atmosphere–ocean moorings. In addition one analysis also assimilated sea level data from TOPEX/Poseidon. Sea level variability in the two analyses agreed well with each other, and both agree with tide gauge and altimetry data for 1993–95. However, beginning in late 1995 through 1996, large sea level differences of 5–8 cm were found in the tropical western Pacific between the two analyses. Furthermore, large sea level discrepancies were also found between dynamic height estimated from TAO temperatures and tide gauge–altimetry observations in the same region during 1996. These discrepancies are consistent with the sea level differences between the two model based analyses.

Historical conductivity–temperature–depth data along 165°E near the equator were also analyzed and it was found that salinity variability on interannual timescale can result in a sea level variability of at least −5 dyn cm to +6 dyn cm. These results suggest that the sea level discrepancy in 1996 is likely due to inadequate salinity information both in estimating dynamic height from TAO temperature and in the data assimilation system used here, which corrects only temperature field.

The sea level error that resulted from inadequate salinity variability has a significant projection onto the second sea level anomaly EOF, which is linked to the onset phase of ENSO. This suggests that the error in the ocean initial conditions due to underestimate of interannual salinity variations in 1996 could impact the accuracy of ENSO prediction. Results from a twin experiment that uses two summer 1996 ocean initial conditions to hindcast for winter 1996/97 equatorial Pacific SST anomalies appear to support this hypothesis.

The results also pointed to a weakness of the present univariate assimilation system, which corrects only temperature. The improved sea level variability comes at the expense of reduced accuracy in temperature. A better solution would be a bivariate data assimilation system, which corrects both salinity and temperature, producing more accurate and consistent ocean initial conditions for ENSO prediction.

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C. M. R. Platt, David W. Reynolds, and N. L. Abshire

Abstract

Radiometric data from the SMS-2 and GOES-1 geostationary satellites together with ground-based lidar scans have been combined to determine the visible albedo, infrared emittance and visible optical depth of cirrus clouds. The combined observations were made on an area of cirrus of about 10 km by 10 km square at Boulder, Colorado during two days.

A method of analysis was developed to separate out the cloud albedo from surface albedo effects, to allow for possible anisotropy in the bi-directional reflectance of solar radiation from the clouds, and to compare the data with results of theoretical calculations.

Relations between the visible albedo and the infrared emittance, which were derived from satellite data, and the visible optical depth, which was derived from lidar measurements, were compared with theoretical relations derived from two models of cloud particle scattering. The first model assumes that the cloud is composed of water (or ice) spheres and the second that it is composed of long ice cylinders. It was found that the observational data agree best with the latter model, although there are still some discrepancies.

The infrared emittances varied between 0.2 and 0.95, the corresponding albedos between 0.10 and 0.32 and the visible optical depths between 0.5 and 3.5.

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David Halpern, Ming Ji, Ants Leetmaa, and Richard W. Reynolds

Abstract

Equatorial Pacific current and temperature fields were simulated with and without assimilation of subsurface temperature measurements for April 1992–March 1995 and compared with moored buoy and research vessel current measurements. Data assimilation intensified the mean east–west slope of the thermocline along the equator in the eastern Pacific, shifted eastward the longitude of the mean Equatorial Undercurrent (EUC) maximum speed 800 km to 125°W, and produced a 25% stronger mean EUC core speed in the eastern Pacific. In the eastern Pacific the mean EUC core speed simulated with data assimilation was slightly more representative of observations compared to that computed without data assimilated; in the western Pacific the data assimilation had no impact on mean EUC simulations.

Data assimilation intensified the north–south slope of the thermocline south of the equator in the western Pacific to produce a thicker and more intense westward-flowing South Equatorial Current (SEC) in the western Pacific. In the western Pacific the mean SEC transport per unit width simulated with data assimilation was more representative of observations compared to that computed without data assimilation. However, large differences remained between the observed SEC transport per unit width and that simulated with data assimilation. In the eastern Pacific, the data assimilation had no impact on mean SEC simulations.

The temporal variability of monthly mean EUC core speeds and SEC transports per unit width were increased significantly by data assimilation. It also increased the representativeness of monthly mean SEC transports per unit width to the observations. However, the data representativeness of monthly mean EUC core speeds was decreased. Results could be explained by the coupling between zonal gradient of temperature and EUC and between meridional gradient of temperature and SEC. Longitudinal variations along the Pacific equator of the impact of data assimilation on the EUC and SEC precludes the choice of a single site to evaluate the effectiveness of data assimilation schemes.

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Christophe Maes, David Behringer, Richard W. Reynolds, and Ming Ji

Abstract

Empirical orthogonal functions of the combined variability of temperature and salinity have been used as basis functions for the indirect reconstruction of salinity from observations of temperature alone. The method employs a weighted least squares procedure that minimizes the misfit between the reconstructed temperature and the observed temperature, but also constrains the variability of the reconstructed salinity to remain within specified bounds.

The method has been tested by fitting to temperature profiles from the Tropical Atmosphere Ocean array along 165°E in the western equatorial Pacific Ocean (8°N–8°S) for the 1986–97 period. Comparisons of the reconstructed salinity field with sea surface salinity and conductivity–temperature–depth data and of the reconstructed dynamic height with TOPEX/Poseidon observations of sea level demonstrate the reliability of the method. The reconstructed data successfully capture the upper-ocean variability at annual to ENSO timescales. The impact of neglecting salinity variability on the dynamic height anomaly in the western tropical Pacific Ocean is addressed.

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