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Robert A. Maddox and David W. Reynolds

Abstract

Geostationary Operational Environmental Satellite (GOES) enhanced infrared (IR) imagery depicted very cold temperatures over Colorado on the morning of 8 December 1978. The situation was unusual because skies were clear and the cold temperatures were not associated with high cloud tops. Instead, satellite data mapped large areas that were experiencing extremely cold surface temperatures. The GOES data were also examined using the Colorado State University interactive data processing system and it was found that the cold IR readings corresponded well with early morning low temperatures over the state. GOES data can be of use in monitoring surface temperatures and can, in certain situations, provide detailed spatial and temporal information over regions experiencing extreme temperatures.

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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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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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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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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 W. Reynolds, Thomas H. Vonder Haar, and Lewis O. Grant

During the past several years, many weather modification programs have been incorporating meteorological satellite data into both the operations and the analysis phase of these projects. This has occurred because of the advancement of the satellite as a mesoscale measurement platform, both temporally and spatially, and as the availability of high quality data has increased. This paper surveys the applications of meteorological satellite data to both summer and winter weather modification programs. A description of the types of observations needed by the programs is given, and an assessment of how accurately satellites can determine these necessary parameters is made.

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David W. Reynolds, Thomas B. McKee, and Kelvin S. Danielson

Abstract

The relationship between a cumulus clouds' brightness, horizontal dimension and internal microphysical structure are investigated. Cumulus clouds located over the South Park region of Colorado are observed by the SMS-2 satellite and their brightness and size are determined. Aircraft observations were made in-cloud to obtain the drop size distributions and liquid water content (LWC) of the cloud. A Monte Carlo cloud model is used to imitate the sun-satellite-cloud geometry in an effort to understand the role of cloud size and microphysical structure in affecting cloud brightness.

Results show that for clouds of optical thickness between 20 and 60 (i.e., LWC of 0.037 gm−3 and 0.11 gm−3 for a 2 km deep cloud), information about a cloud's LWC may be obtained through monitoring cloud brightness for clouds of uniform depth and variable width. Theoretical results using this Monte Carlo method approximate very closely the relative brightness changes of clouds of the size and depth monitored by the SMS-2 satellite for these few days. Theory and observation both conclude that a cloud having a width to depth ratio of approximately 10:1 (and constant optical thickness) is nearly reaching its maximum brightness. Theory predicts that geometric factors affect cloud brightness more than microphysical changes.

It is also discussed that the previously reported work on the cloud height-cloud brightness relationship may indeed be seeing increasing brightness with increasing horizontal size changes (size being related to height) with finite small perturbations on top of the growing cloud slowing its approach to maximum brightness.

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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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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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David W. Reynolds, Thomas H. Vonder Haar, and Stephen K. Cox

Abstract

Data from an experiment to measure the upward and downward components of solar radiation from aircraft during the Barbados Oceanographic and Meteorological Experiment (BOMEX) have been analyzed in the present study. Two groups of results were found.

In the cloud-free tropical troposphere: 1) Absorption of solar radiation in the entire troposphere can be twice as large as previous estimates of 13% given by Manabe and Strickler. Comparison of observed heating rates to calculations shows that the increase in attenuation may be due to non-gaseous constituents in the atmosphere. 2) The vertical profile of solar radiative heating was particularly variable in the lowest layers while in the mean, the profile suggests a slight maximum near 700 mb. 3) In using the solar radiation observations of this study in an energy budget of the tropics a hypothesis regarding a nighttime maximum of precipitation in the tropical regions was formed.

Findings from a radiative study of certain cloud type cases show (i) selective vertical absorption in stratocumulus acts to destabilize the clouds' environment; (ii) large cumulus or cumulonimbus clouds occasionally decrease the solar insolation reaching the surface to only 3% of that incoming at cloud top; and (iii) cirrus clouds are possible stabilizing mechanisms in the tropical environment since they act to warm the local environment at high altitudes while suppressing solar warming from cloud base to surface.

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