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  • Author or Editor: William E. Johns x
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William E. Johns

Abstract

During November 1986, a 6-day record was collected from a 150 kHz Acoustic Doppler Current Profiler (ADCP) mounted in the upward-looking mode on a subsurface mooring in the Gulf Stream near Cape Hatteras. The flotation unit used for the ADCP was a newly developed streamlined float, designed to minimize the effects of drag-induced tilt and high-frequency buoy motion on the range and precision of the Doppler measurements. The overall performance of the float was found to be excellent, with a mean tilt of less than 2° in up to 2 kt of current and a high apparent stability to vortex-induced oscillations. As a result, good velocity data were obtained to within 30 m of the surface from a mean depth of 375 m. A comparison of the near-field ADCP velocity data with a conventional Aanderra current meter moored 20 m below the ADCP yielded mean and root-mean-square speed and direction differences of 1.0 ± 3.7 cm s−1 and 0.5 ± 2.9°, respectively. Also, a comparison with Pegasus velocity profiles taken within 1 n mi of the mooring site showed qualitatively good agreement, with the ADCP reproducing well the small-scale vertical structure. Significant fluctuations in the vertical component were also observed, related to diurnal migration of biological scatterers, with vertical “speeds” often in excess of 3–4 cm s−1.

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David M. Fratantoni
and
William E. Johns

Abstract

A deep-towed instrument package has been developed to study the velocity and tracer signature of abyssal overflows in the northeastern Caribbean. Primary package components include a conductivity-temperature-depth (CTD) instrument and an acoustic Doppler current profiler (ADCP), allowing for simultaneous measurement of density, watermass tracers, and absolute velocity. A description of package construction and operation is supplemented by examples from a set of 17 deployments during two oceanographic cruises in January 1991 and March 1992. A new method for determining the three-dimensional position of the instrument package is described, based on the ability of the ADCP to acquire reference velocities corresponding to its motion over the seafloor. Factors affecting ADCP data quality are discussed, particularly those stemming from the low-scatterer environment at abyssal depths and the impact of large vertical accelerations due to surface wave-induced ship heave.

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John R. Christy
,
Roy W. Spencer
,
William B. Norris
,
William D. Braswell
, and
David E. Parker

Abstract

Deep-layer temperatures derived from satellite-borne microwave sensors since 1979 are revised (version 5.0) to account for 1) a change from microwave sounding units (MSUs) to the advanced MSUs (AMSUs) and 2) an improved diurnal drift adjustment for tropospheric products. AMSU data, beginning in 1998, show characteristics indistinguishable from the earlier MSU products. MSU–AMSU error estimates are calculated through comparisons with radiosonde-simulated bulk temperatures for the low–middle troposphere (TLT), midtroposphere (TMT), and lower stratosphere (TLS.) Monthly (annual) standard errors for global mean anomalies of TLT satellite temperatures are estimated at 0.10°C (0.07°C). The TLT (TMT) trend for January 1979 to April 2002 is estimated as +0.06° (+0.02°) ±0.05°C decade–1 (95% confidence interval). Error estimates for TLS temperatures are less well characterized due to significant heterogeneities in the radiosonde data at high altitudes, though evidence is presented to suggest that since 1979 the trend is −0.51° ± 0.10°C decade–1.

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Feili Li
,
M. Susan Lozier
, and
William E. Johns

Abstract

A transbasin monitoring array from Labrador to Scotland was deployed in the summer of 2014 as part of the Overturning in the Subpolar North Atlantic Program (OSNAP). The aim of the observing system is to provide a multiyear continuous measure of the Atlantic meridional overturning circulation (AMOC) and the associated meridional heat and freshwater transports in the subpolar North Atlantic. Results from the array are expected to improve the understanding of the variability of the subpolar transports and the nature and degree of the AMOC’s latitudinal dependence. In this present work, the measurements of the OSNAP array are described and a suite of observing system simulation experiments in an eddy-permitting numerical model are used to assess how well these measurements will estimate the fluxes across the OSNAP section. The simulation experiments indicate that the OSNAP array and calculation methods will adequately capture the mean and temporal variability of the overturning circulation and of the heat and freshwater transports across the subpolar North Atlantic.

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John M. Frank
,
William J. Massman
,
Edward Swiatek
,
Herb A. Zimmerman
, and
Brent E. Ewers

Abstract

Sonic anemometry is fundamental to all eddy-covariance studies of surface energy and ecosystem carbon and water balance. Recent studies have shown that some nonorthogonal anemometers underestimate vertical wind. Here it is hypothesized that this is due to a lack of transducer and structural shadowing correction. This is tested with a replicated intercomparison experiment between orthogonal (K-probe, Applied Technologies, Inc.) and nonorthogonal (A-probe, Applied Technologies, Inc.; and CSAT3 and CSAT3V, Campbell Scientific, Inc.) anemometer designs. For each of the 12 weeks, five randomly selected and located anemometers were mounted both vertically and horizontally. Bayesian analysis was used to test differences between half-hourly anemometer measurements of the standard deviation of wind (σ u , συ, and σ w ) and temperature, turbulent kinetic energy (TKE), the ratio between vertical/horizontal TKE (VHTKE), and sensible heat flux (H). Datasets were analyzed with various applications of transducer shadow correction. Using the manufacturer’s current recommendations, orthogonal anemometers partitioned higher VHTKE and measured about 8%–9% higher σ w and ~10% higher H. This difference can be mitigated by adding shadow correction to nonorthogonal anemometers. The horizontal manipulation challenged each anemometer to measure the three dimensions consistently, which allowed for testing two hypotheses explaining the underestimate in vertical wind. While measurements were essentially unchanged when the orthogonal anemometers were mounted sideways, the nonorthogonal anemometers changed substantially and confirmed the lack of shadow correction. Considering the ubiquity of nonorthogonal anemometers, these results are consequential across flux networks and could potentially explain half of the ~20% missing energy that is typical at most flux sites.

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John E. Yorks
,
Dennis L. Hlavka
,
William D. Hart
, and
Matthew J. McGill

Abstract

Accurate knowledge of cloud optical properties, such as extinction-to-backscatter ratio and depolarization ratio, can have a significant impact on the quality of cloud extinction retrievals from lidar systems because parameterizations of these variables are often used in nonideal conditions to determine cloud phase and optical depth. Statistics and trends of these optical parameters are analyzed for 4 yr (2003–07) of cloud physics lidar data during five projects that occurred in varying geographic locations and meteorological seasons. Extinction-to-backscatter ratios (also called lidar ratios) are derived at 532 nm by calculating the transmission loss through the cloud layer and then applying it to the attenuated backscatter profile in the layer, while volume depolarization ratios are computed using the ratio of the parallel and perpendicular polarized 1064-nm channels. The majority of the cloud layers yields a lidar ratio between 10 and 40 sr, with the lidar ratio frequency distribution centered at 25 sr for ice clouds and 16 sr for altocumulus clouds. On average, for ice clouds the lidar ratio slightly decreases with decreasing temperature, while the volume depolarization ratio increases significantly as temperatures decrease. Trends for liquid water clouds (altocumulus clouds) are also observed. Ultimately, these observed trends in optical properties, as functions of temperature and geographic location, should help to improve current parameterizations of extinction-to-backscatter ratio, which in turn should yield increased accuracy in cloud optical depth and radiative forcing estimates.

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John E. Yorks
,
Matthew J. McGill
,
V. Stanley Scott
,
Shane W. Wake
,
Andrew Kupchock
,
Dennis L. Hlavka
,
William D. Hart
, and
Patrick A. Selmer

Abstract

The Airborne Cloud–Aerosol Transport System (ACATS) is a Doppler wind lidar system that has recently been developed for atmospheric science capabilities at the NASA Goddard Space Flight Center (GSFC). ACATS is also a high-spectral-resolution lidar (HSRL), uniquely capable of directly resolving backscatter and extinction properties of a particle from a high-altitude aircraft. Thus, ACATS simultaneously measures optical properties and motion of cloud and aerosol layers. ACATS has flown on the NASA ER-2 during test flights over California in June 2012 and science flights during the Wallops Airborne Vegetation Experiment (WAVE) in September 2012. This paper provides an overview of the ACATS method and instrument design, describes the ACATS HSRL retrieval algorithms for cloud and aerosol properties, and demonstrates the data products that will be derived from the ACATS data using initial results from the WAVE project. The HSRL retrieval algorithms developed for ACATS have direct application to future spaceborne missions, such as the Cloud–Aerosol Transport System (CATS) to be installed on the International Space Station (ISS). Furthermore, the direct extinction and particle wind velocity retrieved from the ACATS data can be used for science applications such as dust or smoke transport and convective outflow in anvil cirrus clouds.

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Joseph G. Alfieri
,
William P. Kustas
,
John H. Prueger
,
Lawrence E. Hipps
,
José L. Chávez
,
Andrew N. French
, and
Steven R. Evett

Abstract

Land–atmosphere interactions play a critical role in regulating numerous meteorological, hydrological, and environmental processes. Investigating these processes often requires multiple measurement sites representing a range of surface conditions. Before these measurements can be compared, however, it is imperative that the differences among the instrumentation systems are fully characterized. Using data collected as a part of the 2008 Bushland Evapotranspiration and Agricultural Remote Sensing Experiment (BEAREX08), measurements from nine collocated eddy covariance (EC) systems were compared with the twofold objective of 1) characterizing the interinstrument variation in the measurements, and 2) quantifying the measurement uncertainty associated with each system. Focusing on the three turbulent fluxes (heat, water vapor, and carbon dioxide), this study evaluated the measurement uncertainty using multiple techniques. The results of the analyses indicated that there could be substantial variability in the uncertainty estimates because of the advective conditions that characterized the study site during the afternoon and evening hours. However, when the analysis was limited to nonadvective, quasi-normal conditions, the response of the nine EC stations were remarkably similar. For the daytime period, both the method of Hollinger and Richardson and the method of Mann and Lenschow indicated that the uncertainty in the measurements of sensible heat, latent heat, and carbon dioxide flux were approximately 13 W m−2, 27 W m−2, and 0.10 mg m−2 s−1, respectively. Based on the results of this study, it is clear that advection can greatly increase the uncertainty associated with EC flux measurements. Since these conditions, as well as other phenomena that could impact the measurement uncertainty, are often intermittent, it may be beneficial to conduct uncertainty analyses on an ongoing basis.

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William B. Willis
,
William E. Eichinger
,
John H. Prueger
,
Cathleen J. Hapeman
,
Hong Li
,
Michael D. Buser
,
Jerry L. Hatfield
,
John D. Wanjura
,
Gregory A. Holt
,
Alba Torrents
,
Sean J. Plenner
,
Warren Clarida
,
Stephen D. Browne
,
Peter M. Downey
, and
Qi Yao

Abstract

Pollutant emissions to the atmosphere commonly derive from nonpoint sources that are extended in space. Such sources may contain area, volume, line, or a combination of emission types. Currently, point measurements, often combined with models, are the primary means by which atmospheric emission rates are estimated from extended sources. Point measurement arrays often lack in spatial and temporal resolution and accuracy. In recent years, lidar has supplemented point measurements in agricultural research by sampling spatial ensembles nearly instantaneously. Here, a methodology using backscatter data from an elastic scanning lidar is presented to estimate emission rates from extended sources. To demonstrate the approach, a known amount of particulate matter was released upwind of a vegetative environmental buffer, a barrier designed to intercept emissions from animal production facilities. The emission rate was estimated downwind of the buffer, and the buffer capture efficiency (percentage of particles captured) was calculated. Efficiencies ranged from 21% to 74% and agree with the ranges previously published. A comprehensive uncertainty analysis of the lidar methodology was performed, revealing an uncertainty of 20% in the emission rate estimate; suggestions for significantly reducing this uncertainty in future studies are made. The methodology introduced here is demonstrated by estimating the efficiency of a vegetative buffer, but it can also be applied to any extended emission source for which point samples are inadequate, such as roads, animal feedlots, and cotton gin operations. It can also be applied to any pollutant for which a lidar system is configured, such as particulate matter, carbon dioxide, and ammonia.

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Paul E. Ciesielski
,
Hungjui Yu
,
Richard H. Johnson
,
Kunio Yoneyama
,
Masaki Katsumata
,
Charles N. Long
,
Junhong Wang
,
Scot M. Loehrer
,
Kathryn Young
,
Steven F. Williams
,
William Brown
,
John Braun
, and
Teresa Van Hove

Abstract

The upper-air sounding network for Dynamics of the Madden–Julian Oscillation (DYNAMO) has provided an unprecedented set of observations for studying the MJO over the Indian Ocean, where coupling of this oscillation with deep convection first occurs. With 72 rawinsonde sites and dropsonde data from 13 aircraft missions, the sounding network covers the tropics from eastern Africa to the western Pacific. In total nearly 26 000 soundings were collected from this network during the experiment’s 6-month extended observing period (from October 2011 to March 2012). Slightly more than half of the soundings, collected from 33 sites, are at high vertical resolution. Rigorous post–field phase processing of the sonde data included several levels of quality checks and a variety of corrections that address a number of issues (e.g., daytime dry bias, baseline surface data errors, ship deck heating effects, and artificial dry spikes in slow-ascent soundings).

Because of the importance of an accurate description of the moisture field in meeting the scientific goals of the experiment, particular attention is given to humidity correction and its validation. The humidity corrections, though small relative to some previous field campaigns, produced high-fidelity moisture analyses in which sonde precipitable water compared well with independent estimates. An assessment of operational model moisture analyses using corrected sonde data shows an overall good agreement with the exception at upper levels, where model moisture and clouds are more abundant than the sonde data would indicate.

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