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William T. Scott

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William T. Scott

Abstract

The Telford statistical approach to cloud droplet growth by coalescence and the kinetic-equation approach are shown to give identical results when the Telford assumptions are used to linearize the latter. It is concluded that in this case the kinetic equation contains all relevant statistical information. A brief demonstration is given that the nonlinear kinetic equation also is statistically complete.

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William T. Scott

Abstract

The kinetic equation for the pure growth-by-coalescence process is solved exactly for three types of overall collection probability: proportional to the sum of droplet volumes, proportional to the product of droplet volumes, and constant. Series and asymptotic expressions are given for a variety of initial conditions, and methods indicated for use of arbitrary initial functions. Calculations are presented for initial volume distributions equivalent to Gaussian distributions in radius with σ/ = 0.37, O.25, 0.15 and O.123, and several stages of real time from 69 to 1600 sec. We assume 1 gm m−8 water content, a mean volume radius of 10 μ, and normalization of the collection efficiency formulas to fit those of Shafrir and Neiburger for a 30–10 μ collision.

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William T. Scott

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William T. Scott

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William T. Scott
and
Chong-Yuan Chen

Abstract

Two approximate interpolating formulas suitable for rapid computation are given that fit a combined version of the Shafrir-Neiburger and Davis-Sartor collision efficiencies for unchanged spherical cloud droplets. The method of fitting is indicated for use in approximating future improvements in the theory.

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Chong-Yuan Chen
and
William T. Scott

Abstract

A fast computational method for evaluating Telford's simplified stochastic model of the coalescence process of cloud droplet growth is shown to be useful for approximating the spectrum development in the early stages of precipitation, particularly for cases of narrow droplet size spectra. The method uses a saddle-point integration of the Laplace transform of the growth probability function, and turns out to be at least two orders of magnitude faster than the usual kinetic equation computations. Simple, rough approximations for large and small time intervals are developed.

Instead of setting the collection efficiency E = 1 as in Telford (1955), realistic formulas for E taken from the Scott-Chen (1970) formula, the Neiburger-Lee-Lobl-Rodriguez (Lee, 1975) formula, or a Lagrange interpolation to fit the Davis-Sartor and Schafrir-Neiburger theories are used, yielding considerably different conclusions from those of Telford. Davies' and Beard and Pruppacher's terminal velocity formulas were both used, with little difference found between them.

A simple modification of our method was developed to treat the continuous collection model for a single particle failing in a uniform cloud of droplets. The results show that the continuous and discrete models have nearly the same growth rate when the growing droplets are larger than 50 µm in radius; the probability curves of the two models have nearly the same behavior. Consequently, the growth rate calculated by the continuous model is used for droplets from 50 to 500 µm. A typical result shows that 0.01% of the droplets with an initial size of 30 µm can grow to a size of 400 µm within 30.77 min.

Three examples which consider a single large drop size and either a single or narrow discrete spectrum of smaller droplet sizes are presented for the comparison of Telford's model with a kinetic equation calculation, arranged to treat volume categories which are integral multiples of a smallest size. An estimate of the range of validity of Telford's model as an approximation to the calculation of the kinetic equation is discussed. Under the same initial conditions, the calculation of the kinetic equation in FORTRAN needs about 50 s of computer time but the modified Telford method needs only 0.2 s in BASIC.

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Paul T. Scott
,
William G. Finnegan
, and
Peter C. Sinclair

Abstract

A new ice nucleant aerosol was produced by combustion of a 2% AgI-0.5 mole % Bil3-NH4I-acetone-water solution. The ice nucleating effectiveness of this aerosol is an order of magnitude greater than AgI alone at −10°C. An X-ray powder analysis identified the aerosol as the hexagonal crystal form of AgI having the closest match to ice ever reported for a nucleant of this type.

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William D. Collins
,
Cecilia M. Bitz
,
Maurice L. Blackmon
,
Gordon B. Bonan
,
Christopher S. Bretherton
,
James A. Carton
,
Ping Chang
,
Scott C. Doney
,
James J. Hack
,
Thomas B. Henderson
,
Jeffrey T. Kiehl
,
William G. Large
,
Daniel S. McKenna
,
Benjamin D. Santer
, and
Richard D. Smith

Abstract

The Community Climate System Model version 3 (CCSM3) has recently been developed and released to the climate community. CCSM3 is a coupled climate model with components representing the atmosphere, ocean, sea ice, and land surface connected by a flux coupler. CCSM3 is designed to produce realistic simulations over a wide range of spatial resolutions, enabling inexpensive simulations lasting several millennia or detailed studies of continental-scale dynamics, variability, and climate change. This paper will show results from the configuration used for climate-change simulations with a T85 grid for the atmosphere and land and a grid with approximately 1° resolution for the ocean and sea ice. The new system incorporates several significant improvements in the physical parameterizations. The enhancements in the model physics are designed to reduce or eliminate several systematic biases in the mean climate produced by previous editions of CCSM. These include new treatments of cloud processes, aerosol radiative forcing, land–atmosphere fluxes, ocean mixed layer processes, and sea ice dynamics. There are significant improvements in the sea ice thickness, polar radiation budgets, tropical sea surface temperatures, and cloud radiative effects. CCSM3 can produce stable climate simulations of millennial duration without ad hoc adjustments to the fluxes exchanged among the component models. Nonetheless, there are still systematic biases in the ocean–atmosphere fluxes in coastal regions west of continents, the spectrum of ENSO variability, the spatial distribution of precipitation in the tropical oceans, and continental precipitation and surface air temperatures. Work is under way to extend CCSM to a more accurate and comprehensive model of the earth's climate system.

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Julie K. Lundquist
,
James M. Wilczak
,
Ryan Ashton
,
Laura Bianco
,
W. Alan Brewer
,
Aditya Choukulkar
,
Andrew Clifton
,
Mithu Debnath
,
Ruben Delgado
,
Katja Friedrich
,
Scott Gunter
,
Armita Hamidi
,
Giacomo Valerio Iungo
,
Aleya Kaushik
,
Branko Kosović
,
Patrick Langan
,
Adam Lass
,
Evan Lavin
,
Joseph C.-Y. Lee
,
Katherine L. McCaffrey
,
Rob K. Newsom
,
David C. Noone
,
Steven P. Oncley
,
Paul T. Quelet
,
Scott P. Sandberg
,
John L. Schroeder
,
William J. Shaw
,
Lynn Sparling
,
Clara St. Martin
,
Alexandra St. Pe
,
Edward Strobach
,
Ken Tay
,
Brian J. Vanderwende
,
Ann Weickmann
,
Daniel Wolfe
, and
Rochelle Worsnop

Abstract

To assess current capabilities for measuring flow within the atmospheric boundary layer, including within wind farms, the U.S. Department of Energy sponsored the eXperimental Planetary boundary layer Instrumentation Assessment (XPIA) campaign at the Boulder Atmospheric Observatory (BAO) in spring 2015. Herein, we summarize the XPIA field experiment, highlight novel measurement approaches, and quantify uncertainties associated with these measurement methods. Line-of-sight velocities measured by scanning lidars and radars exhibit close agreement with tower measurements, despite differences in measurement volumes. Virtual towers of wind measurements, from multiple lidars or radars, also agree well with tower and profiling lidar measurements. Estimates of winds over volumes from scanning lidars and radars are in close agreement, enabling the assessment of spatial variability. Strengths of the radar systems used here include high scan rates, large domain coverage, and availability during most precipitation events, but they struggle at times to provide data during periods with limited atmospheric scatterers. In contrast, for the deployment geometry tested here, the lidars have slower scan rates and less range but provide more data during nonprecipitating atmospheric conditions. Microwave radiometers provide temperature profiles with approximately the same uncertainty as radio acoustic sounding systems (RASS). Using a motion platform, we assess motion-compensation algorithms for lidars to be mounted on offshore platforms. Finally, we highlight cases for validation of mesoscale or large-eddy simulations, providing information on accessing the archived dataset. We conclude that modern remote sensing systems provide a generational improvement in observational capabilities, enabling the resolution of finescale processes critical to understanding inhomogeneous boundary layer flows.

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