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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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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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Melanie A. Wetzel
,
Steven K. Chai
,
Marcin J. Szumowski
,
William T. Thompson
,
Tracy Haack
,
Gabor Vali
, and
Robert Kelly

Abstract

A field project was carried out offshore of central Oregon during August 1999 to evaluate mesoscale model simulations of coastal stratiform cloud layers. Procedures for mapping cloud physical parameters such as cloud optical depth, droplet effective radius, and liquid water path retrieved from Geostationary Operational Environmental Satellite (GOES) Imager multichannel data were developed and implemented. Aircraft measurements by the University of Wyoming provided in situ verification for the satellite retrieval parameters and for the forecast model simulations of the U.S. Navy's nonhydrostatic mesoscale prediction system, the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). Case studies show that the satellite retrieval methods are valid within the range of uncertainty associated with aircraft measurements of the microphysical parameters and demonstrate how the gridded cloud parameters retrieved from satellite data can be utilized for mesoscale model verification. Satellite-derived products with applications to forecasting, such as temporal trends and composites of droplet size and liquid water path, are also discussed.

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Paul J. DeMott
,
Ottmar Möhler
,
Olaf Stetzer
,
Gabor Vali
,
Zev Levin
,
Markus D. Petters
,
Masataka Murakami
,
Thomas Leisner
,
Ulrich Bundke
,
Holger Klein
,
Zamin A. Kanji
,
Richard Cotton
,
Hazel Jones
,
Stefan Benz
,
Maren Brinkmann
,
Daniel Rzesanke
,
Harald Saathoff
,
Mathieu Nicolet
,
Atsushi Saito
,
Bjorn Nillius
,
Heinz Bingemer
,
Jonathan Abbatt
,
Karin Ardon
,
Eli Ganor
,
Dimitrios G. Georgakopoulos
, and
Clive Saunders

Understanding cloud and precipitation responses to variations in atmospheric aerosols remains an important research topic for improving the prediction of climate. Knowledge is most uncertain, and the potential impact on climate is largest with regard to how aerosols impact ice formation in clouds. In this paper, we show that research on atmospheric ice nucleation, including the development of new measurement systems, is occurring at a renewed and historically unparalleled level. A historical perspective is provided on the methods and challenges of measuring ice nuclei, and the various factors that led to a lull in research efforts during a nearly 20-yr period centered about 30 yr ago. Workshops played a major role in defining critical needs for improving measurements at that time and helped to guide renewed efforts. Workshops were recently revived for evaluating present research progress. We argue that encouraging progress has been made in the consistency of measurements using the present generation of ice nucleation instruments. Through comparison to laboratory cloud simulations, these ice nuclei measurements have provided increased confidence in our ability to quantify primary ice formation by atmospheric aerosols.

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Bjorn Stevens
,
Donald H. Lenschow
,
Gabor Vali
,
Hermann Gerber
,
A. Bandy
,
B. Blomquist
,
J. -L. Brenguier
,
C. S. Bretherton
,
F. Burnet
,
T. Campos
,
S. Chai
,
I. Faloona
,
D. Friesen
,
S. Haimov
,
K. Laursen
,
D. K. Lilly
,
S. M. Loehrer
,
Szymon P. Malinowski
,
B. Morley
,
M. D. Petters
,
D. C. Rogers
,
L. Russell
,
V. Savic-Jovcic
,
J. R. Snider
,
D. Straub
,
Marcin J. Szumowski
,
H. Takagi
,
D. C. Thornton
,
M. Tschudi
,
C. Twohy
,
M. Wetzel
, and
M. C. van Zanten

The second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study is described. The field program consisted of nine flights in marine stratocumulus west-southwest of San Diego, California. The objective of the program was to better understand the physics a n d dynamics of marine stratocumulus. Toward this end special flight strategies, including predominantly nocturnal flights, were employed to optimize estimates of entrainment velocities at cloud-top, large-scale divergence within the boundary layer, drizzle processes in the cloud, cloud microstructure, and aerosol–cloud interactions. Cloud conditions during DYCOMS-II were excellent with almost every flight having uniformly overcast clouds topping a well-mixed boundary layer. Although the emphasis of the manuscript is on the goals and methodologies of DYCOMS-II, some preliminary findings are also presented—the most significant being that the cloud layers appear to entrain less and drizzle more than previous theoretical work led investigators to expect.

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