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A. P. Sturman
and
M. R. Anderson

Abstract

A comparison is made of seven Antarctic sea ice data sets developed since 1980, on the basis of techniques of analysis and inferred temporal variations. Navy-NOAA Joint Ice Center sea ice charts are the basic data for all seven studies, but techniques used to derive ice areas vary significantly between studies. Sources of variation include the choice of a single week to represent a month, the characteristic measured (i.e., latitude of ice edge or actual ice area—with or without polynya), and the sea ice concentration used to determine the ice edge. The resulting data sets tend to indicate similar long term trends between 1973 and 1982. However, the estimates of mean annual and mean monthly ice areas vary distinctly between studies. This variability is often explainable in terms of the different techniques of analysis, but in some cases is not (e.g., Chiu's apparent overestimation of ice areas). The differences identified between these analyses suggest that caution should be taken in applying or extending these data sets.

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R. A. Weller
and
S. P. Anderson

Abstract

A major goal of the Coupled Ocean-Atmosphere Response Experiment (COARE) was to achieve significantly more accurate and complete descriptions of the surface meteorology and air-sea fluxes in the western equatorial warm pool region. Time series of near-surface meteorology from a buoy moored near the center of the COARE Intensive Flux Array (IFA) are described here. The accuracies of the measurements and the derived fluxes are quantified; agreement between average net heat fluxes at the buoy and two nearby research ships is better than 10 W m−2 during three intercomparisons. Variability in the surface meteorology and fluxes associated with westerly wind bursts, periods of low winds, and short-lived, deep convective events characteristic of the region was large compared to the 4-month means. The ECMWF (European Centre for Medium-Range Weather Forecasts) analysis and prediction fields differed most from the buoy data during periods of short-lived, deep convective events, when several day averages of the net heat flux differed by more than 70 W m−2 and had the opposite sign. A one-dimensional ocean model run to examine the sensitivity of the upper-ocean response to differences between the observed and the ECMWF fluxes illustrates the importance of the short-lived events as well as of the wind bursts in maintaining the temperature of the warm pool.

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Jason A. Otkin
,
Martha C. Anderson
,
John R. Mecikalski
, and
George R. Diak

Abstract

Reliable procedures that accurately map surface insolation over large domains at high spatial and temporal resolution are a great benefit for making the predictions of potential and actual evapotranspiration that are required by a variety of hydrological and agricultural applications. Here, estimates of hourly and daily integrated insolation at 20-km resolution, based on Geostationary Operational Environmental Satellite (GOES) visible imagery are compared to pyranometer measurements made at 11 sites in the U.S. Climate Reference Network (USCRN) over a continuous 15-month period. Such a comprehensive survey is necessary in order to examine the accuracy of the satellite insolation estimates over a diverse range of seasons and land surface types. The relatively simple physical model of insolation that is tested here yields good results, with seasonally averaged model errors of 62 (19%) and 15 (10%) W m−2 for hourly and daily-averaged insolation, respectively, including both clear- and cloudy-sky conditions. This level of accuracy is comparable, or superior, to results that have been obtained with more complex models of atmospheric radiative transfer. Model performance can be improved in the future by addressing a small elevation-related bias in the physical model, which is likely the result of inaccurate model precipitable water inputs or cloud-height assessments.

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R. C. Anderson
,
J. G. Pipes
,
A. L. Broadfoot
, and
L. Wallace

Abstract

The results of two Aerobee rocket flights are reported. One obtained spectra of Venus from 3200 to 1900 Å at 16.5 Å resolution and the other spectra of Jupiter from 32M to 1800 Å at 28 Å resolution. The spectra of both planets are of much higher statistical accuracy than those that have been obtained previously. The peculiarities indicated by the previous observations are not confirmed. In particular, there does not appear to be an absorption feature in the Jupiter spectrum at 2600 Å or an ozone absorption in the Venus spectrum.

Two extreme models are used to interpret the data: the reflecting layer model and the cloud model. We find that the CO2 abundances for Venus and the H2 abundances for Jupiter, deduced with the reflecting layer model from observations in the red and IR parts of the spectrum, are much ton large to be compatible with the UV albedos. In terms of the cloud model, the albedos of both planets down to 2200 Å are the result of the decreasing single scattering albedo of the cloud particles and the increasing Rayleigh scattering. These two effects produce an almost constant geometric albedo from 2800 to 2200 Å. Below 2200 Å, the Venus albedo drops sharply due to CO2, absorption; the Jupiter albedo drops off by a factor of 4 due to NH3 absorption, an unidentified absorber, a decrease in the cloud particle albedo, or Borne combination of these effects.

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Yun Fan
,
M. R. Allen
,
D. L. T. Anderson
, and
M. A. Balmaseda

Abstract

The predictability of any complex, inhomogeneous system depends critically on the definition of analysis and forecast errors. A simple and efficient singular vector analysis is used to study the predictability of a coupled model of El Niño–Southern Oscillation (ENSO). Error growth is found to depend critically on the desired properties of the forecast errors (“where and what one wants to predict”), as well as on the properties of the analysis error (“what information is available for that prediction”) and choice of optimization time. The time evolution of singular values and singular vectors shows that the predictability of the coupled model is clearly related to the seasonal cycle and to the phase of ENSO. It is found that the use of an approximation to the analysis error covariance to define the relative importance of errors in different variables gives very different results to the more frequently used “energy norm,” and indicates a much larger role for sea surface temperature information in seasonal (3–6-month timescale) predictability. Seasonal variations in the predictability of the coupled model are also investigated, addressing in particular the question of whether seasonal variations in the dominant singular values (the “spring predictability barrier”) may be largely due to the seasonality in the variance of SST anomalies.

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Bruce T. Anderson
,
Jeff R. Knight
,
Mark A. Ringer
,
Jin-Ho Yoon
, and
Annalisa Cherchi

Abstract

Global-scale variations in the climate system over the last half of the twentieth century, including long-term increases in global-mean near-surface temperatures, are consistent with concurrent human-induced emissions of radiatively active gases and aerosols. However, such consistency does not preclude the possible influence of other forcing agents, including internal modes of climate variability or unaccounted for aerosol effects. To test whether other unknown forcing agents may have contributed to multidecadal increases in global-mean near-surface temperatures from 1950 to 2000, data pertaining to observed changes in global-scale sea surface temperatures and observed changes in radiatively active atmospheric constituents are incorporated into numerical global climate models. Results indicate that the radiative forcing needed to produce the observed long-term trends in sea surface temperatures—and global-mean near-surface temperatures—is provided predominantly by known changes in greenhouse gases and aerosols. Further, results indicate that less than 10% of the long-term historical increase in global-mean near-surface temperatures over the last half of the twentieth century could have been the result of internal climate variability. In addition, they indicate that less than 25% of the total radiative forcing needed to produce the observed long-term trend in global-mean near-surface temperatures could have been provided by changes in net radiative forcing from unknown sources (either positive or negative). These results, which are derived from simple energy balance requirements, emphasize the important role humans have played in modifying the global climate over the last half of the twentieth century.

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A. Scotti
,
B. Butman
,
R. C. Beardsley
,
P. Soupy Alexander
, and
S. Anderson

Abstract

The algorithm used to transform velocity signals from beam coordinates to earth coordinates in an acoustic Doppler current profiler (ADCP) relies on the assumption that the currents are uniform over the horizontal distance separating the beams. This condition may be violated by (nonlinear) internal waves, which can have wavelengths as small as 100–200 m. In this case, the standard algorithm combines velocities measured at different phases of a wave and produces horizontal velocities that increasingly differ from true velocities with distance from the ADCP. Observations made in Massachusetts Bay show that currents measured with a bottom-mounted upward-looking ADCP during periods when short-wavelength internal waves are present differ significantly from currents measured by point current meters, except very close to the instrument. These periods are flagged with high error velocities by the standard ADCP algorithm. In this paper measurements from the four spatially diverging beams and the backscatter intensity signal are used to calculate the propagation direction and celerity of the internal waves. Once this information is known, a modified beam-to-earth transformation that combines appropriately lagged beam measurements can be used to obtain current estimates in earth coordinates that compare well with pointwise measurements.

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L. A. Sromovsky
,
J. R. Anderson
,
F. A. Best
,
J. P. Boyle
,
C. A. Sisko
, and
V. E. Suomi

Abstract

An untended instrument to measure ocean surface heat flux has been developed for use in support of field experiments and the investigation of heat flux parameterization techniques. The sensing component of the Skin-Layer Ocean Heat Flux Instrument (SOHFI) consists of two simple thermopile heat flux sensors suspended by a fiberglass mesh mounted inside a ring-shaped surface float. These sensors make direct measurements within the conduction layer, where they are held in place by a balance between surface tension and float buoyancy. The two sensors are designed with differing solar absorption properties so that surface heat flux can be distinguished from direct solar irradiance. Under laboratory conditions, the SOHFI measurements agree well with calorimetric measurements (generally to within 10%). Performance in freshwater and ocean environments is discussed in a companion paper.

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L. A. Sromovsky
,
J. R. Anderson
,
F. A. Best
,
J. P. Boyle
,
C. A. Sisko
, and
V. E. Suomi

Abstract

The Skin-Layer Ocean Heat Flux Instrument (SOHFI) described by Sromovsky et al. (Part I, this issue) was field-tested in a combination of freshwater and ocean deployments. Solar irradiance monitoring and field calibration techniques were demonstrated by comparison with independent measurements. Tracking of solar irradiance diurnal variations appears to be accurate to within about 5% of full scale. Preliminary field tests of the SOHFI have shown reasonably close agreement with bulk aerodynamic heat flux estimates in freshwater and ocean environments (generally within about 20%) under low to moderate wind conditions. Performance under heavy weather suggests a need to develop better methods of submergence filtering. Ocean deployments and recoveries of drifting SOHFI-equipped buoys were made during May and June 1995, during the Combined Sensor Program of 1996 in the western tropical Pacific region, and in the Greenland Sea in May 1997. The Gulf Stream and Greenland Sea deployments pointed out the need for design modifications to improve resistance to seabird attacks. Better estimates of performance and limitations of this device require extended intercomparison tests under field conditions.

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T.L. Anderson
,
D.S. Covert
,
S.F. Marshall
,
M.L. Laucks
,
R.J. Charlson
,
A.P. Waggoner
,
J.A. Ogren
,
R. Caldow
,
R.L. Holm
,
F.R. Quant
,
G.J. Sem
,
A. Wiedensohler
,
N.A. Ahlquist
, and
T.S. Bates

Abstract

As designed in the 1940s by Beuttell and Brewer, the integrating nephelometer offers a direct method of measuring light scattering by airborne particles without assumptions about particle composition, shape, or physical state. A large number of such instruments have been deployed; however, only a limited number of validation experiments have been attempted. This paper reports a set of closure experiments in which a gas-calibrated nephelometer is used to measure the scattering coefficient of laboratory-generated particles of known size and refractive index.

Specifically, it evaluates the performance of a high-sensitivity, three-wavelength, total scatter/backscatter integrating nephelometer (TSI, Inc., model 3563). Sources of uncertainty associated with the gas-calibration procedure, with photon-counting statistics, and with nonidealities in wavelength and angular sensitivity are investigated. Tests with particle-free gases indicate that noise levels are well predicted by photon-counting statistics and that the nephelometer response is linear over a wide range of scattering coefficients. Tests with particles show average discrepancies between measured and predicted scattering of 4%–7%. Error analysis indicates that these discrepancies are within experimental uncertainty, which was dominated by particle generation uncertainty. The simulation of nephelometer response, which is validated by these tests, is used to show that errors arising from nephelometer nonidealities are less than 10% for accumulation-mode or smaller particles (i.e., size distributions for which the volume mean diameter is 0.4µm or less) and that significant differences exist between the total scatter and backscatter uncertainties. Based on these findings, appropriate applications of the model 3563 nephelometer am discussed.

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