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Rick Damiani
,
Gabor Vali
, and
Samuel Haimov

Abstract

A newly developed technique for airborne dual-Doppler observations with the Wyoming Cloud Radar is used to characterize the velocity fields in vertical planes across cumulus turrets. The clouds sampled were continental in nature, with high bases (near 0°C) and with depths of 2–3 km. Clear evidence was found that the clouds evolved through sequences of bubbles, or thermals, with well-defined toroidal circulations, or vortex rings. The ring core and tube diameters were about 200–600 m, leading to turret sizes of 1–2 km in the horizontal. The largest updraft speeds were observed in the ring centers, but regions of turbulent, ascending air extended behind the thermals to distances comparable with the toroid sizes. Vertical shear of ambient winds, when present, led to a tilting of the updrafts and toroids. Patterns in the reflectivity and velocity fields indicated regions of major intrusions into the thermals, accompanied by entrainment of ambient air, or recycling of larger hydrometeors, depending on their location. In addition, at the upper cloud/environment interface, instability nodes contributed to further entrapment of cloud-free air. The observations presented in this paper constitute clear demonstrations and quantitative characterization of vortical circulations in growing cumulus turrets; they should provide a more reliable basis for the assessment of simulations and of model parameterizations.

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Gabor Vali
and
Russell C. Schnell

Abstract

An overview is given of the path of research that led from asking how hailstones originate to the discovery that ice nucleation can be initiated by bacteria and other microorganisms at temperatures as high as −2°C. The major steps along that path were finding exceptionally effective ice nucleators in soils with a high content of decayed vegetative matter, then in decaying tree leaves, and then in plankton-laden ocean water. Eventually, it was shown that Pseudomonas syringae bacteria were responsible for most of the observed activity. That identification coincided with the demonstration that the same bacteria cause frost damage on plants. Ice nucleation by bacteria meant an unexpected turn in the understanding of ice nucleation and of ice formation in the atmosphere. Subsequent research confirmed the unique effectiveness of ice nucleating particles (INP) of biological origin, referred to as bio-INPs, so that bio-INPs are now considered to be important elements of lower-tropospheric cloud processes. Nonetheless, some of the questions which originally motivated the research are still unresolved, so that revisiting the early work may be helpful to current endeavors. Part I of this manuscript summarizes how the discovery progressed. Part II (Schnell and Vali) shows the relationship between bio-INPs in soils and in precipitation with climate and other findings. The online supplemental material contains a bibliography of recent work about bio-INPs.

Open access
Russell C. Schnell
and
Gabor Vali

Abstract

In Part I, we described the discoveries we and our associates made in the 1960s and 1970s about biological ice nucleating particles (bio-INPs). The bio-INPs are far more effective than mineral INPs at temperatures above −10°C. The bio-INPs were found in decayed vegetation and in ocean water, and then, bacteria were identified as being the most active source for this remarkable activity. In this Part II, we recount how, within a few years, the worldwide distribution of bio-INP sources was shown to correlate with climate zones, as was the abundance of INPs in precipitation. Oceanic sources were further studied, and the presence of bio-INPs in fog diagnosed. The potential for release of bio-INPs from the ground to the atmosphere was demonstrated. Bacterial INPs were found to play a crucial role in a plant’s frost resistance. These and other early developments of biological INPs are described. A bibliography of related recent literature is presented in the online supplemental material (https://doi.org/10.1175/BAMS-D-23-0114.s1).

Open access
David C. Rogers
,
Darrel Baumgardner
, and
Gabor Vali

Abstract

A ground-based technique is described for determining the liquid water content of supercooled clouds orfog by measuring the mass rate of rime accumulation on a small rotating wire. Development of the techniqueis described, examples of the data are presented, and comparisons are made with two conventional methodsof liquid water measurement. The comparisons were quite favorable for the winter orographic clouds studied.

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Gabor Vali
,
L. R. Koenig
, and
T. C. Yoksas

Abstract

No abstract available.

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Gabor Vali
,
L. Randall Koenig
, and
Thomas C. Yoksas

Abstract

Field investigations for the Precipitation Enhancement Project (PEP) were undertaken during the winter months of 1979–1981 in the upper Duero River Basin of Spain. The purpose of these studies was to examine what potential might exist for the enhancement of precipitation from different cloud types via ice-nucleus seeding of clouds. This paper describes procedures for estimating that potential. Specific regions within natural clouds were qualified as potentially seedable on the basis of observations by instrumented aircraft of persistent zones of supercooled water content. The observed “regions of potential” are described, and precipitation increases that might result from seeding the regions are estimated using two relatively simple models. Summed over all cloud types, and expressed as averages over the 100-km radius project area, increases of 10% and 23% are estimated with the two models for the days of seeding. For the February–May season as a whole, the increases are 0.75% and 1.8% of the normal precipitation (160 mm) for the season. The major contributions to these increases come from cumulus mediocris and cumulus congestus, and from shallow stratiform and clouds.

The low values obtained for the estimated increases indicate that a major augmentation of total seasonal precipitation in the Duero Basin is not likely to result from seeding the “regions of potential”. Consequently, within a 5-yr period envisaged for the project, it would be difficult to discern a seeding effect in terms of area-averaged precipitation. Useful increases in precipitation may possibly be produced by seeding the regions of potential, but demonstration of the seeding effects would have to be based on evaluations which are focused (in time and space) on the treated clouds or cloud regions.

Since no seeding was actually carried out during, or subsequent to these field studies, the validity of the criteria employed for defining the regions of potential, and the derived estimates, remain unverified. The criteria which define the regions of potential, and the methods of estimation developed in this study, can also be applied, in principle, to other situations, where precipitation enhancement is sought via seeding with ice nuclei.

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Andrew Pazmany
,
James Mead
,
Robert McIntosh
,
Mark Hervig
,
Robert Kelly
, and
Gabor Vali

Abstract

The use of millimeter-wavelength radars for cloud microphysical research was investigated in experiments at the Elk Mountain Observatory near Laramie, Wyoming, between April 1990 and March 1992. The 95-GHz polarimetric radar used in these experiments is a portable, high-power, dual-polarization radar capable of characterizing the complex scattering matrix in two pulses. The scatterer's polarimetric response is characterized in terms of the Mueller matrix, a form that is seen to be convenient for computing the response of a scatterer for any arbitrary combination of transmit and receive antenna polarizations.

This paper summarizes the results of recent experiments carried out at the Elk Mountain Observatory. Polarimetric data from orographic cap clouds are found to be sensitive to ice-particle orientation and composition. Comparison of radar-observed reflectivities with those computed from in situ images shows good agreement if the volume fraction of ice in ice-air mixtures is taken into account.

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WANTED: MORE Ph.D.s

Graduate Enrollments in the Atmospheric Sciences

Gabor Vali
,
Richard Anthes
,
Dennis Thomson
,
David Houghton
,
Jack Fellows
, and
Susan Friberg

A recent survey of UCAR member institutions shows reason for concern about a possible decline in the number and quality of applicants entering graduate studies in the atmospheric and related sciences. If reported trends continue, a shortage of qualified Ph.D. graduates in the field in the next decade may be faced.

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Gabor Vali
,
Robert D. Kelly
,
Jeffrey French
,
Samuel Haimov
,
David Leon
,
Robert E. McIntosh
, and
Andrew Pazmany

Abstract

Observations were made of unbroken marine stratus off the coast of Oregon using the combined capabilities of in situ probes and a 95-GHz radar mounted on an aircraft. Reflectivity and Doppler velocity measurements were obtained in vertical and horizontal planes that extend from the flight lines. Data from three consecutive days were used to examine echo structure and microphysics characteristics. The clouds appeared horizontally homogeneous and light drizzle reached the surface in all three cases.

Radar reflectivity is dominated by drizzle drops over the lower two-thirds to four-fifths of the clouds and by cloud droplets above that. Cells with above-average drizzle concentrations exist in all cases and exhibit a large range of sizes. The cells have irregular horizontal cross sections but occur with a dominant spacing that is roughly 1.2–1.5 times the depth of the cloud layer. Doppler velocities in the vertical are downward in all but a very small fraction of the cloud volumes. The cross correlation between reflectivity and vertical Doppler velocity changes sign at or below the midpoint of the cloud, indicating that in the upper parts of the clouds above-average reflectivities are associated with smaller downward velocities. This correlation and related observations are interpreted as the combined results of upward transport of drizzle drops and of downward motion of regions diluted by entrainment. The in situ measurements support these conclusions.

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Bjorn Stevens
,
Gabor Vali
,
Kimberly Comstock
,
Robert Wood
,
Margreet C. van Zanten
,
Philip H. Austin
,
Christopher S. Bretherton
, and
Donald H. Lenschow

Data from recent field studies in the northeast and southeast Pacific are used to investigate pockets of open cells (POCs) that are embedded in otherwise uniform stratocumulus. The cellular structure within a POC resembles broader regions of open cellular convection typically found further offshore. In both regions, cells are composed of precipitating cell walls and cell interiors with depleted cloud water and even clearing. POCs are long lived and embedded in broader regions of stratocumulus where average droplet sizes are relatively large. In contrast, stratiform, or unbroken, cloud formations tend to be accompanied by less, or no, drizzle, suggesting that precipitation is necessary for the sustenance of the open cellular structure. Because, by definition, open cells are associated with a reduction in cloud cover these observations provide direct evidence of a connection between cloudiness and precipitation—a linchpin of hypotheses that posit a connection between changes in the atmospheric aerosol and climate.

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