Topics in Micrometeorology. A Festschrift for Arch Dyer

151 Pages·1988·3.84 MB·English

by  B. B. Hicks (auth.)

TOPICS IN MICROMETEOROLOGY
TOPICS IN MICROMETEOROLOGY.
A FESTSCHRIFT FOR ARCH DYER.
Edited by
BRUCE B. HICKS
NOAA/ARL, Turbulence and Diffusion Division,
Oak Ridge, Tennessee, U.S.A.
Reprinted from Boundary-Layer Meteorology
Vol. 42, Nos. 1-2 (1988)
D. Reidel Publishing Company / Dordrecht / Boston
Library of Congress Cataloging in Publication Data
CIP-data appear on seperate card.
ISBN-13: 978-94-010-7822-1 e-ISBN-13: 978-94-009-2935-7
DOl: 10.1007/978-94-009-2935-7
Published by D. Reidel Publishing Company,
P.O. Box 17, 3300 AA Dordrecht, Holland.
Sold and distributed in the U.S.A. and Canada
by Kluwer Academic Publishers,
101 Philip Drive, Norwell, MA 02061, U.S.A.
In all other countries, sold and distributed
by Kluwer Academic Publishers Group,
P.O. Box 322, 3300 AH Dordrecht, Holland.
All Rights Reserved
© 1988 by D. Reidel Publishing Company, Dordrecht, Holland
Softcover reprint of the hardcover 1s t edition 1988
No part of the material protected by this copyright notice may be reproduced or
utilized in any form or by any means, electronic or mechanical
including photocopying, recording or by any information storage and
retrieval system, without written permission from the copyright owner
ARCH
Some Introductory Notes to an Issue of Boundary-Layer Meteorology Dedicated
to Arthur James Dyer
B. B. HICKS
NOAA Atmospheric Turbulence and Diffosion Division. P.O. Box 2456. Oak Ridge. TN 37831. U.S.A.
This issue is a Festschrift to Dr A. J. ('Arch') Dyer, the distinguished Australian
micrometeorologist. In these introductory notes, the intent is to summarize Arch's
contributions in micrometeorology, and especially in the development of flux-gradient
relationships, not in depth but in sufficient detail to give the unfamiliar reader an idea
of the thoroughness of his work. Arch's own review of the history of eddy flux measure
ment makes interesting reading; in it, his own role is characteristically downplayed
(Dyer, 1968).
Arthur James Dyer was born in 1925. He eventually became a student at the
University of Melbourne, after leaving an intimidating academic record behind him at
Melbourne's University High School. In 1946, Arch was awarded his B.Sc. degree in
Physics, with First Class Honours. He went on to receive his M.Sc. degree in 1948, and
a Ph.D. in Physics in 1953.
Arch's thesis was on the detection and measurement of cosmic rays. There was no
meteorology in his curriculum, but immediately upon joining the Commonwealth
Scientific and Industrial Research Organisation's Meteorological Physics Section in
1954, he was rudely introduced to meteorology and especially to the technical limitations
of meteorological sensing. At that time, the Section (later to become the Division of
Meteorological Physics, then the Division of Atmospheric Physics, and now the
Division of Atmospheric Research) was geared towards answering some specific
questions about the dependence of surface fluxes on atmospheric and surface condi
tions. The team Arch joined was powerful, headed by C. H. B. (Bill) Priestley and W. C.
(Bill) Swinbank, and with R. J. (Reg) Taylor, E. L. (Len) Deacon, I. C. (Ian) McIlroy,
and E. K. (Eric) Webb forming the core of the experimental effort. Arch took over the
responsibility for the electronics shop, which was then heavily involved in deVelopments
related to eddy correlation.
It is worth remembering the times. Much earlier, Scrase (1930) had demonstrated that
eddy correlation could be made to work, but had shown (with great emphasis) the
difficulty of the job. In Australia, Swinbank, Priestley, Taylor, and Webb had explored
the approach further, and had started intensive work on automating the analysis. The
first computerized system employed a mechanical integration device, a ball-and-disc
integrator in which one signal controls the speed of rotation of a disc and another
controls the radius at which a tangential wheel contacts the disc (Taylor and Webb,
1955). The number of turns of the contacting wheel is directly related to the integral of
Boundary-Layer Meteorology 42 (1988) 1-8.
© 1988 by D. Reidel Publishing Company.
2 B. B. HICKS
the product of the two signals. Using three of these systems, it is possible to solve the
eddy correlation problem: one system is used to integrate the product of vertical velocity
and temperature, say, and two other systems are used to integrate the individual signals
(see Swinbank, 1951, 1955). The problem that most severely constrained this delightfully
direct approach was the same as that which has kept cropping up ever since - the need
for rapid response of the data analysis system as well as of the sensors.
Arch and his team contributed a major improvement: the first fast-response electronic
product integrator (Dyer, 1958). This was a modified Watt-hour meter, in which a disc
is rotated in response to induced eddy currents in the presence of external magnetic and
electrical fields. The number of rotations of the disc is then directly related to the product
of the magnetic and electrical field strengths, both of which are modulated by input
signals. This innovation formed the foundation for work on eddy correlation until the
development of electronic integrators in the mid-1960's. The 'Evapotron' was a field
portable instrument that used three of these integrators in the same way as the ball-and
disc devices that had been used previously (Taylor, 1956; Dyer and Taylor, 1959; Dyer,
1961; Dyer and Maher, 1965).
The team of Taylor and Dyer contributed in the development of sensors, as well. This
was well before the era of sonic anemometry, although sonic techniques were among
the many potential sensing systems that were investigated. At that time, electronics were
not yet well enough developed to make sonic anemometers feasible (likewise for
pressure-sensing devices - the other approach that was extensively explored without
great profit), and hot-wire technology was the best available. Taylor and Dyer invested
enormous energy into the improvement of hot-wire technology; a highly refined heated
wake anemometer was the result (Dyer, 1960). This provided one-second response
times, with excellent cosine response.
A major limitation was never overcome with complete satisfaction: as with all hot
wire anemometers, the devices did not work well in light winds and, therefore, could
not be configured to measure u and w directly; instead, they were arranged to measure
(u + w) and (u - w), from which signals proportional to u and w were then extracted.
(As usual, u is the longitudinal wind component, w the vertical.) Electronic noise arising
in either of the original inputs would then appear in both of the derived velocity
components, obviously correlated, and clearly confounding any attempt to derive
estimates of momentum covariances. The Evapotron was never fully successful as a
means for measuring u *' but it worked well for sensible and latent heat fluxes.
With a working data-reduction and analysis system and a suitable anemometer for
measuring fluctuations in the vertical velocity component, it was then necessary to
measure temperature and water vapor. Arch and his team developed a method for
standardizing fine-wire thermometers, so that these sensitive elements could be replaced
quickly in the field (Dyer and Maher, 1965). The selection of nickel wire was unusual
in micrometeorological research, but half-hardened nickel worked well, was easy to
handle, and offered a large temperature coefficient. It had an additional advantage of
wetability; an element constructed of cotton wound with the nickel wire as an integral
element of the thread formed an excellent fast-response wet bulb, which could then be
INTRODUcrORY NOTES TO AN ISSUE DEDICATED TO A. J. DYER 3
fine-tuned to precisely the same temperature coefficients as the dry bulb. The Evapotron
used such wet and dry bulb systems. When operated with a wet bulb, the Evapotron
measured H + LE, and with a dry bulb it measured H. Using both, it was possible to
quantify both H and LE using a field-portable instrument.
The complexity of the sensors and the computing routines used to analyze their
outputs created a healthy respect for the problem of error propagation. Fundamental
tests were devised to generate confidence in the measurements that were made in the
field. The matter of sensitivity to sensor orientation was addressed directly: two sensors
were operated side by side, and one was rotated relative to the other. Sensors were
operated at different heights to find out what errors would arise if the instruments were
operated too close to the surface. Studies were also conducted on the role of fetch, to
quantify the error margin associated with operations over fields of limited extent (Dyer
and Crawford, 1965). This last study was accompanied by an extensive theoretical
analysis of the leading-edge question (Dyer, 1963).
In all such studies, a key consideration was the need to satisfy the constraints of
energy balance at the surface. On more than one occasion, it was found that the eddy
fluxes were not agreeing well with measurements of net radiation and ground heat
(~~.",.~.
-···
7
G
_-:,M".:
--"
- -
'i ~ :.
Fig.1. Arch Dyer, and some memories.
4 B. B. HICKS
transfer. The entire experimental team packed up, and went home. The lessons learned
were strong, and remain a part of good micrometeorological field practice. Any
researcher who steps into the field with sensors and recording apparatus which he
expects to work well, and does not take steps to reduce data on the spot for real-time
inspection and quality control, is setting himself up for some shocks when the analysis
is finally completed.
A parallel concern centered around the need to assure that the measured eddy fluxes
were correct. This worry generated a series of comparisons against different techniques,
such as weighed lysimeters for testing measurements of evaporation rate, and Bowen
ratio methods for testing the heat fluxes. The critical measurements were made at a field
site near the Aspendale location of the Division of Meteorological Physics (which it was,
by this time), at Edithvale. This was a field site suitable for testing and improving
equipment, but significantly affected by mobile roughness elements (bovine) over which
little control was possible. More satisfactory field sites were sought for studies of
flux-gradient relationships, the elucidation of which was the final goal of these develop
ments. There followed the sequence of field studies that blazed the trail for micro
meteorological experiments to follow: Deniliquin (western New South Wales) in 1961;
Kerang (northern Victoria) three times in 1962; back to Kerang in 1963, and a trial study
at Hay (north of Deniliquin, in NSW); Hay once again in 1964, and several times in
1965; and finally to Gurley in northern New South Wales.
Fig. 2. The 1965 site at Hay, N.S.W., showing a few of the instrument towers that were used.
INTRODUCTORY NOTES TO AN ISSUE DEDICATED TO A. 1. DYER 5
In these experiments, site quality was a basic requirement, to be assessed by studies
of the wind profile in neutral stability, and by measurement of horizontal gradients of
wind, temperature, and humidity. (A photograph ofthe site used at Hay, in 1965, is given
as Figure 2. Instruments were distributed over a large expanse of very flat grazing land,
typically supporting one or two sheep per hectare.) Stationarity was also a fundamental
requirement; measurements were not made when conditions were changing with time,
such as near dawn and dusk. In all these early studies, it was assumed that the value
Karman
of the von constant was 0.4, until measurements of momentum flux improved
sufficiently to cause this estimate to be refined to 0.41 (but see the paper by J. A. Businger
in this issue). The series of experiments is summarized by Swinbank and Dyer (1968).
In a typical field experiment, three sets of eddy flux instruments would be set up near
each other, one each for sensible heat, latent heat of evaporation, and momentum flux.
The three-headed monster was affectionately known as 'Cerberus', after the mythical
creature guarding the entrance to Hades. The three operators were the same through
most of these experiments: Arch Dyer, Guntis Grauze, and Bruce Hicks, the present
author. In later years, Peter Hyson joined this team.
The data would be collected and checked as they came off the machines, reduced,
scrutinized, and tabulated. Each night, there would be a wake as the day's products were
evaluated. Points would be carefully drawn on a plot of dimensionless gradients against
stability. It was in some motel room at some stage through these expeditions that a first
cautious curve was drawn through a set of points describing the average relationship
between eddy fluxes and gradients. The first question was whether Kh = Km; the
answer was clearly negative. The second question was whether K w was more like Kh
or Km ; the data pointed to Kh. The last question concerned the best formulation of each
of these eddy diffusivities in terms of the friction velocity, height, and stability. Dyer
(1968) and Dyer and Hicks (1970) summarized the results.
During this sequence of field studies the advantages of signal filtering became a center
of attention. The subject coincided with the development of the helicoid propeller
anemometer as a w sensor. The 'Fluxatron' was developed and was tested in 1965 (Dyer
et al., 1967). In this device, the average vertical velocity signal was driven to zero with
a time constant that was variously set between 20 and 200 s (this was found to be not
critical for unstable conditions, within the accuracy limitations of the available data).
The temperature signal was not treated so well; a crude average was removed using a
lagged temperature element in a second arm of the temperature bridge circuit. The
desired product was obtained by driving the temperature bridge with a signal pro
portional to w' (the departure from the mean of the vertical velocity). The Fluxatron
measured sensible heat only. Shortly thereafter, the development of precise analog
multipliers enabled the Fluxatron approach to be generalized, so that running means
could be removed from any signal, and permitting computation of any of the many
covariances of interest. This generalized covariance computer was the backbone offield
studies until modern digital computing techniques took over, in the mid 1970's. The
sensing head of the covariance system used in field studies during the late 1960's and
early 1970's is seen in the foreground of Figure 3. In the background is a wind profile
6 B. B. HICKS
Fig. 3. A 'Fluxatron' sensing head, as used in field experiments of the late 1960's and early 1970's. In the
background is a wind profile mast.
mast as was used in all of this series of micrometeorological field experiments.
Australian patent number 407427 is held by Arch Dyer. It refers to 'Flux measuring
apparatus', and was issued on November 7, 1970.
The developments of new instruments were paralleled by intercomparison studies.
The first such study with multinational overtones was conducted at Hay in 1965
(Businger et al., 1967). Arch later played a leading role in the Soviet turbulence sensor
comparison study at Tsimlyansk, near the Black Sea, in 1970. (He is fondly remembered
by us all for a moving fairwell address he delivered at the final dinner. The speech was
delivered flawlessly, in Russian.) As his grand finale to the international experiment
scene, Arch organized and hosted the 1976 International Turbulence Comparison
Experiment, back in the familiar territory of west em NSW, at Con argo, near Deniliquin.
Along the way, Arch detoured into air-sea interaction, and worked extensively on the
refinement of high-frequency turbulence intensity methods for determining eddy fluxes
at sea. In addition, he was a leading figure among the small cadre of researchers studying
radioactive fallout during the 1950's and 1960's. This last item constituted more than
just an additional interest. It grew into a second major scientific effort of Arch's,
changing with time into a strong interest in stratospheric diffusion and finally evolving
INTRODUcrORY NOTES TO AN ISSUE DEDICATED TO A. J. DYER 7
into considerations of possible climate change. A review of Arch's publications reveals
about 50 papers on micrometeorology, and about 20 on his more global interests. His
publications earned him a D.Sc. degree, awarded in 1968 by the University of
Melbourne. His papers on flux-gradient relations earned him the Royal Meteorological
Society's Buchan prize (also in 1968), shared with W. C. Swinbank. In 1971, he was
the recipient of Melbourne University's David Syme research prize for Physics.
In the later years of his meteorological career, Arch was Assistant Chief of the CSIRO
Division of Atmospheric Research. He was a Fellow of the Royal Meteorological
Society (and was the prime mover behind the setting up of an Australian Branch, of
which he became Chairman in 1974), and was a foundation member and Fellow of the
Australian Institute of Physics. He was a hard-working member of the Editorial Board
of Boundary-Layer Meteorology.
The papers collected in this volume are selected for their relevance to Arch's micro
meteorological interests. The paper by Businger, for example, addresses directly the
matter of the difference between the Australian flux-profile relationships and the North
American results based on observations at Kansas. The question of sensor interferences
with the flows that are meant to be measured by it is addressed in the paper by
Wyngaard; Arch spent a considerable time reflecting on this matter, both experimentally
and theoretically (see Dyer, 1981). The results of a recent field experiment that follows
the lines of the Australian sequence are presented in the paper by Hogstrum. Other
papers show typical applications of the flux gradient relations. Finally, three papers are
included to show the way in which the initial work by Dyer and his colleagues on flux
measurement techniques has evolved recently, as related to aircraft and towers.
As a result of a bout with ill-health, Arch has been forced to withdraw from the
micrometeorological community. He is now concentrating his energies on his artistic
interests. Those of us who remember him as a leader of the international community
of experimental meteorologists can only regret that he is no longer an active member
of our fraternity, but in the same breath we must marvel that any person should have
the multidisciplinary talent to enable him to leave one profession and pick up in another,
without pause.
Arch's musical talent has always been obvious to those who have been fortunate
enough to know him well. Jack Fina's 'Bumble Boogie' version of Rimsky-Korsakov's
'Flight of the Bumble Bee' has a special significance. At some time during the many field
experiments in which he participated, Arch Dyer was usually pursuaded to belt out his
rendition of this old favorite on whatever poorly tuned piano was then available. At clubs
in outback Australian towns such as Kerang in northern Victoria, and Deniliquin and
Hay in the western semi-desert of New South Wales, bars would go silent as Arch would
monopolize everybody's attention, friends and strangers alike, with his rendition at the
keyboard.
A personal note is surely appropriate. The editor of this issue was a young,
inexperienced university graduate when Arch took him and guided him into his present
career. We all thank you, Arch, but lowe you more than most.