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Robert W. Pratt

Abstract

Spectra computed for the midlatitude troposphere indicate that both the NCAR and GFDL models have less variance than the atmosphere in most space and time scales. The deficiency is most serious for low-frequency planetary waves near 50°N. The low-frequency planetary-scale fluctuations also show less tendency for westward movement than is observed. In both models, especially NCAR's, the 4–5 day wavenumber 1 variance is relatively too large.

The GFDL spectra are generally closer to observed values than the NCAR spectra, probably because of the GFDL model's higher horizontal and vertical resolution. The NCAR spectra show an abnormal level of variance at a period of 5 days in zonal wavenumbers 8–12.

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Robert W. Pratt

Abstract

Several features of the one-sided space-time spectrum of the zonal and meridional wind components are discussed. The spectra show bands of energy corresponding to eastward moving disturbances, a transition from a red to a nearly white frequency spectrum with increasing wavenumber, and abrupt transitions to a power law dependence on zonal wavenumber for individual frequency components.

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Robert W. Pratt

Abstract

The frequency averaging implicit in the calculation of space-time spectra complicates the interpretation of eastward and westward variance and covariance components. One-sided (in frequency) space-time spectral definitions are proposed in an effort to avoid some of the ambiguities of such two-sided spectra, and better define the role of standing and zonally propagating fluctuations. Problems are discussed which affect both one- and two-sided spectra, especially for the mid-latitude planetary scales.

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Robert W. Pratt

Abstract

An attempt is made to provide a brief but comprehensive summary of sources of error in National Weather Service upper air data, and a guide to the relevant literature. Error analysis must be tailored for particular applications. Temperature errors are generally 1°C or less, but reports of relative humidity can be low by 20–30% under unfavorable conditions, especially at very low temperatures.

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Phillip D. Falconer
and
Robert W. Pratt

Abstract

No abstract available.

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Robert W. Pratt
and
John M. Wallace

Abstract

The general character of zonally propagating fluctuations in the mid-latitude troposphere is illustrated using the quadrature spectrum of the zonal sine and cosing Fourier coefficients. It is shown that for low frequencies (periods around 20 days) and low wavenumbers, fluctuations in the geopotential field tend to be westward, while those in the temperature field are eastward. Evidence is provided, by means of empirical orthogonal function analysis, that this situation is explained by the existence of two distinct modes of fluctuation, which together account for most of the large-scale, low-frequency variance. One mode has substantial surface amplitude, is nearly barotropic in structure, and propagates westward with respect to the earth. The other mode is more baroclinic and propagates eastward.

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Peigen Lin
,
Robert S. Pickart
,
Kerstin Jochumsen
,
G. W. K. Moore
,
Héðinn Valdimarsson
,
Tim Fristedt
, and
Lawrence J. Pratt

Abstract

The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic meridional overturning circulation, yet a description of the full velocity field across the strait remains incomplete. Here we analyze a set of 22 shipboard hydrographic–velocity sections occupied along the Látrabjarg transect at the Denmark Strait sill, obtained over the time period 1993–2018. The sections provide the first complete view of the kinematic components at the sill: the shelfbreak East Greenland Current (EGC), the combined flow of the separated EGC, and the North Icelandic Jet (NIJ), and the northward-flowing North Icelandic Irminger Current (NIIC). The total mean transport of overflow water is 3.54 ± 0.29 Sv (1 Sv ≡ 106 m3 s−1), comparable to previous estimates. The dense overflow is partitioned in terms of water mass constituents and flow components. The mean transports of the two types of overflow water—Atlantic-origin Overflow Water and Arctic-origin Overflow Water—are comparable in Denmark Strait, while the merged NIJ–separated EGC transports 55% more water than the shelfbreak EGC. A significant degree of water mass exchange takes place between the branches as they converge in Denmark Strait. There are two dominant time-varying configurations of the flow that are characterized as a cyclonic state and a noncyclonic state. These appear to be wind-driven. A potential vorticity analysis indicates that the flow through Denmark Strait is subject to symmetric instability. This occurs at the top of the overflow layer, implying that the mixing/entrainment process that modifies the overflow water begins at the sill.

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D. Baumgardner
,
L. Avallone
,
A. Bansemer
,
S. Borrmann
,
P. Brown
,
U. Bundke
,
P. Y. Chuang
,
D. Cziczo
,
P. Field
,
M. Gallagher
,
J.-F. Gayet
,
A. Heymsfield
,
A. Korolev
,
M. Krämer
,
G. McFarquhar
,
S. Mertes
,
O. Möhler
,
S. Lance
,
P. Lawson
,
M. D. Petters
,
K. Pratt
,
G. Roberts
,
D. Rogers
,
O. Stetzer
,
J. Stith
,
W. Strapp
,
C. Twohy
, and
M. Wendisch

No abstract available.

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