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R. A. Antonia
and
A. J. Chambers

Abstract

Measurements of velocity and scalar fluctuations were made using a stable platform in Bass Strait at a height of ∼5 m above the mean surface of the ocean. These measurements were obtained over a period of time where the wind velocity increased steadily before reaching an approximately constant value that was maintained for a duration of ∼10 h. During the initial period of the experiment, induced wave fluctuations, centered about the dominant wave frequency, are observed an spectra of the longitudinal u and vertical w velocity fluctuations, and on the uw cross spectrum. The cross spectrum indicates a relatively important transfer of momentum in the direction sea to air, at the wave frequency. No measurable wave influence is detected on either temperature θ or humidity q spectra, but wθ and wq cross spectra are either negligible or show a change of sign at the wave frequency. Although the effect on w persists, wave-induced disturbances on the cross spectra are small enough to be neglected when the ratio of wave phase speed to friction velocity becomes smaller than ∼40.

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A. J. Chambers
and
R. A. Antonia

Abstract

The influence of surface waves on the Reynolds shear stress and the heat flux, measured in the atmospheric surface layer ∼5 m above the ocean, is discussed using the method of Lu and Willmarth (1973). During the observational period, the phase velocity C associated with the dominant wave frequency is 33–80 times larger than the friction velocity u *. When contributions to the momentum flux − uw are sorted out into the four quadrants of the (u,w) plane, the contribution from the interaction quadrants (u>0, w>0; u<0, w<0) increases as C/u * increases. The contributions to the heat flux are not appreciably affected by C/u *. While the probability that sweep and ejection quadrants associated with instantaneous shear-stress and heat-flux fluctuations occur at the same time is large and approximately independent of C/u *, the probability of simultaneous occurrence of interaction events increases as C/u * increases. The period between ejection events also increases with C/u *.

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R. A. Antonia
,
S. Rajagopalan
, and
A. J. Chambers

Abstract

Conditional sampling and averaging techniques are used to obtain statistics of convectively-driven quasi-ordered structures at a height of 4 m within the atmospheric surface layer. The fraction of time 'y occupiedby these structures, and their frequency of occurrence I can depend on detection criteria parameters, suchas the threshold and hold time. The effect of these parameters on 'y and f is investigated for two conditionalsampling techniques. Both techniques indicate that y decreases continuously with increasing threshold,whereas there is a region in which I is independent of both parameters. When the parameters are suitablyselected, reasonable agreement for both 'y and f can be obtained between the techniques. This agreementdoes not depend on whether the velocity or the temperature fluctuation is used as the basis of detection forone of the techniques.

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A. P. Ballantyne
,
P. A. Baker
,
J. Q. Chambers
,
R. Villalba
, and
J. Argollo

Abstract

The authors present results on the relationship between tree-ring proxies and regional precipitation for several sites in tropical South America. The responsiveness of oxygen isotopes (δ 18O) and seasonal growth as precipitation proxies was first validated by high-resolution sampling of a Tachigali myrmecophila from Manaus, Brazil (3.1°S, 60.0°W). Monthly growth of Tachigali spp. was significantly correlated with monthly precipitation. Intra-annual measurements of cellulose δ 18O in Tachigali spp. were also significantly correlated with monthly precipitation at a lag of approximately one month. The annual ring widths of two tropical tree taxa, Cedrela odorata growing in the Amazon (12.6°S, 69.2°W) and Polylepis tarapacana growing in the Altiplano (22.0°S, 66.0°W), were validated using bomb-derived radiocarbon 14C. Estimated dates were within two to three years of bomb-inferred 14C dates, indicating that these species exhibit annual rings but uncertainties in our chronologies remain. A multiproxy record spanning 180 years from Cedrela spp. showed a significant negative relationship between cellulose δ 18O and January precipitation. A 150-yr record obtained from Polylepis spp. also showed a significant negative relationship between δ 18O and March precipitation, whereas annual ring width showed a significant positive correlation with December precipitation. These proxies were combined in a multivariate framework to reconstruct past precipitation, revealing a significant increase in monsoon precipitation at the Amazon site since 1890 and a significant decrease in monsoon precipitation at the Altiplano since 1880. Proxy time series also showed spatial and temporal coherence with precipitation variability due to El Niño forcing, suggesting that oxygen isotopes and ring widths in tropical trees may be important diagnostics for identifying regional differences in the response of the tropical hydrologic cycle to anthropogenic warming.

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R. A. Antonia
,
A. J. Chambers
,
C. A. Friehe
, and
C. W. Van Atta

Abstract

A review of the evidence for the organized temperature structure observed in both the atmospheric surface layer and the laboratory boundary layer reveals similar features between the two turbulent flows. This similarity suggests that the atmospheric temperature ramp may be interpreted as the signature of an organized large-scale motion rather than a necessary consequence of the presence of buoyant plumes. An experiment was conducted in which the translation velocity Ut of the sharp edge of the temperature ramp is determined from the transit time of the ramp between two thermistors placed at the same height in the marine surface layer but separated in a direction parallel to the wind. Ut was found to be in more nearly constant ratio to the local velocity than to the friction velocity. Velocities determined from the phase angle of the temperature cross spectrum and from the optimum temperature cross correlation obtained from the two thermistors are in reasonable agreement with Ut . Cross correlations of temperature signals from thermistors separated in either vertical or lateral directions are briefly discussed in the context of the spatial geometry of the organized temperature structure.

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R. A. Antonia
,
A. J. Chambers
,
S. Rajagopalan
,
K. R. Sreenivasan
, and
C. A. Friehe

Abstract

Measurements of turbulent momentum, heat and moisture fluxes have been made in Bass Strait from a stable platform, at a height of approximately 5 m above water. Direct measurements of these fluxes are compared with estimates obtained from spectra of velocity, temperature and humidity fluctuations with the use of the inertial dissipation technique. Directly measured momentum and moisture flux values are in reasonable agreement with inertial dissipation values. The sensible heal flux obtained by the inertial dissipation technique is about twice as large as the directly measured heat flux. The dependence on wind speed of bulk transfer coefficients of momentum, heat and moisture and of variances of velocity and scalar fluctuations is discussed and compared with available data.

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P. A. Mangarella
,
A. J. Chambers
,
R. L. Street
, and
E. Y. Hsu

Abstract

Interfacial evaporation and energy flux are presented for a series of experiments in a laboratory wind wave flume (neutral stratification) with varying free stream velocity and with and without mechanically generated waves. Integral conservation or budget techniques are the means of flux determination. Fluxes normalized to a 6-cm height are independent of fetch, increase with wind speed, and decrease if mechanically generated waves are introduced. Excellent correspondence between normalized energy and mass transfer exists except for the case involving spray which causes a reduction in the thermal field.

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Justin J. Hartnett
,
Jennifer M. Collins
,
Martin A. Baxter
, and
Don P. Chambers

Abstract

Central New York State, located at the intersection of the northeastern United States and the Great Lakes basin, is impacted by snowfall produced by lake-effect and non-lake-effect snowstorms. The purpose of this study is to determine the spatiotemporal patterns of snowfall in central New York and their possible underlying causes. Ninety-three Cooperative Observer Program stations are used in this study. Spatiotemporal patterns are analyzed using simple linear regressions, Pearson correlations, principal component analysis to identify regional clustering, and spatial snowfall distribution maps in the ArcGIS software. There are three key findings. First, when the long-term snowfall trend (1931/32–2011/12) is divided into two halves, a strong increase is present during the first half (1931/32–1971/72), followed by a lesser decrease in the second half (1971/72–2011/12). This result suggests that snowfall trends behave nonlinearly over the period of record. Second, central New York spatial snowfall patterns are similar to those for the whole Great Lakes basin. For example, for five distinct regions identified within central New York, regions closer to and leeward of Lake Ontario experience higher snowfall trends than regions farther away and not leeward of the lake. Third, as compared with precipitation totals (0.02), average air temperatures had the largest significant (ρ < 0.05) correlation (−0.56) with seasonal snowfall totals in central New York. Findings from this study are valuable because they provide a basis for understanding snowfall patterns in a region that is affected by both non-lake-effect and lake-effect snowstorms.

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M. Rodell
,
H. K. Beaudoing
,
T. S. L’Ecuyer
,
W. S. Olson
,
J. S. Famiglietti
,
P. R. Houser
,
R. Adler
,
M. G. Bosilovich
,
C. A. Clayson
,
D. Chambers
,
E. Clark
,
E. J. Fetzer
,
X. Gao
,
G. Gu
,
K. Hilburn
,
G. J. Huffman
,
D. P. Lettenmaier
,
W. T. Liu
,
F. R. Robertson
,
C. A. Schlosser
,
J. Sheffield
, and
E. F. Wood

Abstract

This study quantifies mean annual and monthly fluxes of Earth’s water cycle over continents and ocean basins during the first decade of the millennium. To the extent possible, the flux estimates are based on satellite measurements first and data-integrating models second. A careful accounting of uncertainty in the estimates is included. It is applied within a routine that enforces multiple water and energy budget constraints simultaneously in a variational framework in order to produce objectively determined optimized flux estimates. In the majority of cases, the observed annual surface and atmospheric water budgets over the continents and oceans close with much less than 10% residual. Observed residuals and optimized uncertainty estimates are considerably larger for monthly surface and atmospheric water budget closure, often nearing or exceeding 20% in North America, Eurasia, Australia and neighboring islands, and the Arctic and South Atlantic Oceans. The residuals in South America and Africa tend to be smaller, possibly because cold land processes are negligible. Fluxes were poorly observed over the Arctic Ocean, certain seas, Antarctica, and the Australasian and Indonesian islands, leading to reliance on atmospheric analysis estimates. Many of the satellite systems that contributed data have been or will soon be lost or replaced. Models that integrate ground-based and remote observations will be critical for ameliorating gaps and discontinuities in the data records caused by these transitions. Continued development of such models is essential for maximizing the value of the observations. Next-generation observing systems are the best hope for significantly improving global water budget accounting.

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Tristan S. L’Ecuyer
,
H. K. Beaudoing
,
M. Rodell
,
W. Olson
,
B. Lin
,
S. Kato
,
C. A. Clayson
,
E. Wood
,
J. Sheffield
,
R. Adler
,
G. Huffman
,
M. Bosilovich
,
G. Gu
,
F. Robertson
,
P. R. Houser
,
D. Chambers
,
J. S. Famiglietti
,
E. Fetzer
,
W. T. Liu
,
X. Gao
,
C. A. Schlosser
,
E. Clark
,
D. P. Lettenmaier
, and
K. Hilburn

Abstract

New objectively balanced observation-based reconstructions of global and continental energy budgets and their seasonal variability are presented that span the golden decade of Earth-observing satellites at the start of the twenty-first century. In the absence of balance constraints, various combinations of modern flux datasets reveal that current estimates of net radiation into Earth’s surface exceed corresponding turbulent heat fluxes by 13–24 W m−2. The largest imbalances occur over oceanic regions where the component algorithms operate independent of closure constraints. Recent uncertainty assessments suggest that these imbalances fall within anticipated error bounds for each dataset, but the systematic nature of required adjustments across different regions confirm the existence of biases in the component fluxes. To reintroduce energy and water cycle closure information lost in the development of independent flux datasets, a variational method is introduced that explicitly accounts for the relative accuracies in all component fluxes. Applying the technique to a 10-yr record of satellite observations yields new energy budget estimates that simultaneously satisfy all energy and water cycle balance constraints. Globally, 180 W m−2 of atmospheric longwave cooling is balanced by 74 W m−2 of shortwave absorption and 106 W m−2 of latent and sensible heat release. At the surface, 106 W m−2 of downwelling radiation is balanced by turbulent heat transfer to within a residual heat flux into the oceans of 0.45 W m−2, consistent with recent observations of changes in ocean heat content. Annual mean energy budgets and their seasonal cycles for each of seven continents and nine ocean basins are also presented.

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