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P. L. Smith Jr.
,
C. G. Myers
, and
H. D. Orville

Abstract

This paper describes and compares various methods for calculating radar reflectivity factors in numerical cloud models that use bulk methods to characterize the precipitation processes. Equations sensitive to changes in the parameters of the particle size distributions are favored because they allow simulation of phenomena causing such changes. Marshall-Palmer-type functions are established to represent hailstone size distributions because the previously available distributions lead to implausibly large reflectivity factors. Simplified equations are developed for calculating reflectivity factors for both dry and wet hail. Some examples are given of the use of the various equations in numerical cloud models.

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Oleg A. Saenko
,
Frédéric Dupont
,
Duo Yang
,
Paul G. Myers
,
Igor Yashayaev
, and
Gregory C. Smith

Abstract

Deep convection in the Labrador Sea is an important component of the global ocean ventilation. The associated loss of heat to the atmosphere from the interior of the sea is thought to be mostly supplied by mesoscale eddies, generated either remotely or as a result of convection itself—processes that are not resolved by low-resolution ocean climate models. The authors first employ a high-resolution ( °) ocean model forced with high-resolution (33 km, 3 h) atmospheric fields to further elaborate on the role of mesoscale eddies in maintaining the balance of heat and buoyancy in the Labrador Sea. In general agreement with previous studies, it is found that eddies remove heat along the coast and supply it to the interior. Some of the eddies that are generated because of the barotropic instability off the west coast of Greenland are recaptured by the boundary current. In the region of deep convection, the convergence of heat and buoyancy by eddies significantly increases with the deepening of the winter mixed layer. In addition, the vertical eddy flux plays an important part in the heat budget of the upper Labrador Sea, accounting for up to half of the heat loss to the atmosphere north of 60°N. A low-resolution (1°) model with parameterized eddies is then applied to show that it does capture, qualitatively, the general structure of eddy buoyancy advection along the Labrador Current. However, the 1° model is deficient in this regard in the most eddy active region off the west coast of Greenland, although some improvements can be made by forcing it with the high-resolution atmospheric fields.

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I. Reda
,
J. Hickey
,
C. Long
,
D. Myers
,
T. Stoffel
,
S. Wilcox
,
J. J. Michalsky
,
E. G. Dutton
, and
D. Nelson

Abstract

Thermopile pyranometers’ thermal offset has been recognized since the pyranometer’s inception. This offset is often overlooked or ignored because its magnitude is small compared to the overall solar signal at higher irradiance. With the demand of smaller uncertainty in measuring solar radiation, recent publications have described a renewed interest in this offset, its magnitude, and its effect on solar measurement networks for atmospheric science and solar energy applications. Recently, it was suggested that the magnitude of the pyranometer thermal offset is the same if the pyranometer is shaded or unshaded. Therefore, calibrating a pyranometer using a method known as the shade/unshade method would result in accurate responsivity calculations because the thermal offset error is canceled. When using the component sum method for the pyranometer calibration, the thermal offset error, which is typically negative when the sky is cloudless, does not cancel, resulting in an underestimated shortwave responsivity. Most operational pyranometers that are in use for solar radiation measuring networks are calibrated using the component sum method since it is possible to calibrate many pyranometers simultaneously. From this arises the importance of correcting the component sum method results to account for the thermal offset error.

In this article a method of using a blackbody system to calculate the net longwave responsivity of pyranometers, which is largely responsible for the offset error, is described. This longwave responsivity is then used to correct the pyranometer’s shortwave responsivity during the component sum method calibrations and thereby substantially reduces the effect of the offset error on the final pyranometer responsivity. Practical procedures for performing this calibration procedure along with its limitations and remaining uncertainties are given.

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P. A. Romashkin
,
D. F. Hurst
,
J. W. Elkins
,
G. S. Dutton
,
D. W. Fahey
,
R. E. Dunn
,
F. L. Moore
,
R. C. Myers
, and
B. D. Hall

Abstract

Detailed information on the four-channel Airborne Chromatograph for Atmospheric Trace Species (ACATS-IV), used to measure long-lived atmospheric trace gases, is presented. Since ACATS-IV was last described in the literature, the temporal resolution of some measurements was tripled during 1997–99, chromatography was significantly changed, and data processing improved. ACATS-IV presently measures CCl3F [chlorofluorocarbon (CFC)-11], CCl2FCClF2 (CFC-113), CH3CCl3 (methyl chloroform), CCl4 (carbon tetrachloride), CH4 (methane), H2 (hydrogen), and CHCl3 (chloroform) every 140 s, and N2O (nitrous oxide), CCl2F2 (CFC-12), CBrClF2 (halon-1211), and SF6 (sulfur hexafluoride) every 70 s. An in-depth description of the instrument operation, standardization, calibration, and data processing is provided, along with a discussion of precision and uncertainties of ambient air measurements for several airborne missions.

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Michael D. King
,
W. Paul Menzel
,
Patrick S. Grant
,
Jeffrey S. Myers
,
G. Thomas Arnold
,
Steven E. Platnick
,
Liam E. Gumley
,
Si-Chee Tsay
,
Christopher C. Moeller
,
Michael Fitzgerald
,
Kenneth S. Brown
, and
Fred G. Osterwisch

Abstract

An airborne scanning spectrometer was developed for measuring reflected solar and emitted thermal radiation in 50 narrowband channels between 0.55 and 14.2 µm. The instrument provides multispectral images of outgoing radiation for purposes of developing and validating algorithms for the remote sensing of cloud, aerosol, water vapor, and surface properties from space. The spectrometer scans a swath width of 37 km, perpendicular to the aircraft flight track, with a 2.5-mrad instantaneous field of view. Images are thereby produced with a spatial resolution of 50 m at nadir from a nominal aircraft altitude of 20 km. Nineteen of the spectral bands correspond closely to comparable bands on the Moderate Resolution Imaging Spectroradiometer (MODIS), a facility instrument being developed for the Earth Observing System to be launched in the late 1990s. This paper describes the optical, mechanical, electrical, and data acquisition system design of the MODIS Airborne Simulator and presents some early results obtained from measurements acquired aboard the National Aeronautics and Space Administration ER-2 aircraft that illustrate the performance and quality of the data produced by this instrument.

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Sara H. Knox
,
Robert B. Jackson
,
Benjamin Poulter
,
Gavin McNicol
,
Etienne Fluet-Chouinard
,
Zhen Zhang
,
Gustaf Hugelius
,
Philippe Bousquet
,
Josep G. Canadell
,
Marielle Saunois
,
Dario Papale
,
Housen Chu
,
Trevor F. Keenan
,
Dennis Baldocchi
,
Margaret S. Torn
,
Ivan Mammarella
,
Carlo Trotta
,
Mika Aurela
,
Gil Bohrer
,
David I. Campbell
,
Alessandro Cescatti
,
Samuel Chamberlain
,
Jiquan Chen
,
Weinan Chen
,
Sigrid Dengel
,
Ankur R. Desai
,
Eugenie Euskirchen
,
Thomas Friborg
,
Daniele Gasbarra
,
Ignacio Goded
,
Mathias Goeckede
,
Martin Heimann
,
Manuel Helbig
,
Takashi Hirano
,
David Y. Hollinger
,
Hiroki Iwata
,
Minseok Kang
,
Janina Klatt
,
Ken W. Krauss
,
Lars Kutzbach
,
Annalea Lohila
,
Bhaskar Mitra
,
Timothy H. Morin
,
Mats B. Nilsson
,
Shuli Niu
,
Asko Noormets
,
Walter C. Oechel
,
Matthias Peichl
,
Olli Peltola
,
Michele L. Reba
,
Andrew D. Richardson
,
Benjamin R. K. Runkle
,
Youngryel Ryu
,
Torsten Sachs
,
Karina V. R. Schäfer
,
Hans Peter Schmid
,
Narasinha Shurpali
,
Oliver Sonnentag
,
Angela C. I. Tang
,
Masahito Ueyama
,
Rodrigo Vargas
,
Timo Vesala
,
Eric J. Ward
,
Lisamarie Windham-Myers
,
Georg Wohlfahrt
, and
Donatella Zona

Abstract

This paper describes the formation of, and initial results for, a new FLUXNET coordination network for ecosystem-scale methane (CH4) measurements at 60 sites globally, organized by the Global Carbon Project in partnership with other initiatives and regional flux tower networks. The objectives of the effort are presented along with an overview of the coverage of eddy covariance (EC) CH4 flux measurements globally, initial results comparing CH4 fluxes across the sites, and future research directions and needs. Annual estimates of net CH4 fluxes across sites ranged from −0.2 ± 0.02 g C m–2 yr–1 for an upland forest site to 114.9 ± 13.4 g C m–2 yr–1 for an estuarine freshwater marsh, with fluxes exceeding 40 g C m–2 yr–1 at multiple sites. Average annual soil and air temperatures were found to be the strongest predictor of annual CH4 flux across wetland sites globally. Water table position was positively correlated with annual CH4 emissions, although only for wetland sites that were not consistently inundated throughout the year. The ratio of annual CH4 fluxes to ecosystem respiration increased significantly with mean site temperature. Uncertainties in annual CH4 estimates due to gap-filling and random errors were on average ±1.6 g C m–2 yr–1 at 95% confidence, with the relative error decreasing exponentially with increasing flux magnitude across sites. Through the analysis and synthesis of a growing EC CH4 flux database, the controls on ecosystem CH4 fluxes can be better understood, used to inform and validate Earth system models, and reconcile differences between land surface model- and atmospheric-based estimates of CH4 emissions.

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