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R. A. Madden and P. Speth

Abstract

Atmospheric angular momentum (M), friction (TF), and mountain torques (TM) are estimated from a 13-month period of European Centre for Medium-Range Weather Forecasts (ECMWF) data. Cross-spectrum analysis between M and total torques results in high coherence and one-quarter cycle phase angles (TF + TM leading M) for timescales between 5 and 66 days, suggesting that variations of the total torque are reasonably well estimated for these slower variations. However, cross spectra between M and TF, and TM separately reveal that the relatively high coherence is present between M and TF only at periods longer than 20 days. Also comparison with other published values and the considerable lack of balance between TF + TM and M over a full year implies that our estimates of TF, based on the parameterization of surface wind stress in short-term forecasts of the ECMWF, are negatively biased. For the 13-month period, the average bias is about −15.2 Hadleys (1018 kg m2 s−2).

During the period there are a few near 50-day oscillations in the M. Similar variations have been reported before and related to tropical intraseasonal oscillations of the same timescale. Two oscillations in M that are coincident with eastward-propagating cloud complexes of tropical intraseasonal oscillations are examined more closely. It is found that TF and TM work together to alter the M on the 50-day timescale, but that TM's contribution is three times larger than that of TF. During the two oscillations TF, reaches maxima when cloud complexes of tropical intraseasonal oscillations are in the vicinity of 90°E. It then declines but maintains positive anomalies at least until the cloud complexes reach the Central Pacific. The M reaches its maxima shortly thereafter. TM has sharp minima shortly before the cloud complexes are strongly developed in the Indian Ocean. Contributors to these minima are strong cast to west pressure gradients primarily across the Rocky Mountains.

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P. Speth, W. May, and R. A. Madden

Abstract

No abstract available.

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M. Christoph, U. Ulbrich, and P. Speth

Abstract

The seasonal cycle of the North Pacific and the North Atlantic storm track activity is investigated on the basis of daily National Meteorological Center (now known as NCEP) upper-air analyses (1946–89) and of data from the ECHAM3 T42 atmospheric general circulation model.

Emphasis is put on the midwinter suppression of the Pacific storm track. This feature of seasonal variability is not sensitive to a particular definition of midlatitude synoptic wave activity, as is shown by applying a common definition of area mean storm track intensity.

The suppression is reproduced by the atmospheric model with very similar characteristics. It is attributed to a negative correlation between the storm track intensity and the speed of the subtropical jet at 250 hPa for average zonal winds exceeding the threshold of approximately 45 m s−1, contrasting with a positive correlation below this value. The lack of an analogous behavior over the Atlantic may be assigned to the lower wind speeds there. In a 3·CO2 time-slice experiment with the ECHAM3 model, very intense jet streams occur more often in winter and the suppression becomes more pronounced. At the same time, the level of climatological storm track activity over the Pacific during winter is higher than in the control run. This is explained by the fact that the time-slice experiment produces statistically higher levels of activity for every given jet intensity.

The suppression is dominated by a decrease in synoptic-scale wave activity. Two possible reasons for this decrease were investigated but had to be rejected: there is neither a seasonal shift in the energy spectrum to frequencies that are outside the range sampled by the typical bandpass filter, nor evidence that the suppression is attributed to Pacific blocking activity occurring preferably during midwinter.

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A. H. Fink, D. G. Vincent, P. M. Reiner, and P. Speth

Abstract

Using ECMWF's second-generation reanalysis, ERA-40, the large-scale mean state and synoptic-scale features associated with African easterly wave disturbances (AEWs) are examined over West Africa and the adjacent eastern Atlantic Ocean during the three 21-day observing periods of the Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE) in 1974 (Phase I, 26 June–16 July; Phase II, 28 July–17 August; Phase III, 30 August–19 September). Results are partitioned into four geographical boxes, in order to highlight differences among the AEW vortices as they propagate westward along two tracks (northern and southern) over West Africa (land) and the adjacent eastern Atlantic Ocean (water). This marks the first time that a detailed diagnosis of the northerly track AEWs has been conducted. Results are also compared to previous GATE studies and a 30-yr climatology is extracted from ERA-40.

In general, the subjectively analyzed wind fields presented in earlier studies compare favorably with the ERA-40 horizontal wind fields. The vertical motion field is one of the parameters that shows the largest differences to previously published results. In the area of the GATE A–B-scale ship array in the eastern Atlantic Ocean, low-level ascent during GATE is twice as large as in the ERA-40 climatology, most likely due to the dense upper-air network that allowed for an exceptionally good analysis of the divergent wind field. The midtropospheric outflow layer found over the ship array is absent in the ERA-40 climatology. Detrimental to the ERA-40 analyses of the upper-level easterly jet over the central Gulf of Guinea and along parts of the Guinea coast, were the assimilation of erroneous aircraft data.

Using a recently developed tracking method of midtropospheric African easterly waves, a complete tracking history of northerly and southerly AEW vortices is presented and discussed for all three phases of GATE. One important result is that the activity of the northern waves at about 20°N was, in contrast to the southern waves at about 9°N, already quite strong during Phase I. At the same time, the low-level monsoonal flow, the heat low, and the upward motion in the northern desert zone were strongest. In contrast, the midtropospheric African easterly jet (AEJ) and the related horizontal shear instabilities were strongest during Phase III. The AEJ is also found at the lowest altitude over land during Phase III and it extends out to the Atlantic Ocean without changing its height and strength. These factors are associated with the well-known peak in the activity of AEWs in the southern wet zone during Phase III. In contrast to earlier findings, no reduction of AEW energy, by lifting of anomalously cool low-level air along the southern moist AEW track, could be observed over land.

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C. M. Roithmayr, C. Lukashin, P. W. Speth, D. F. Young, B. A. Wielicki, K. J. Thome, and G. Kopp
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C. M. Roithmayr, C. Lukashin, P. W. Speth, D. F. Young, B. A. Wielicki, K. J. Thome, and G. Kopp

Abstract

Highly accurate measurements of Earth’s thermal infrared and reflected solar radiation are required for detecting and predicting long-term climate change. Consideration is given to the concept of using the International Space Station to test instruments and techniques that would eventually be used on a dedicated mission, such as the Climate Absolute Radiance and Refractivity Observatory (CLARREO). In particular, a quantitative investigation is performed to determine whether it is possible to use measurements obtained with a highly accurate (0.3%, with 95% confidence) reflected solar radiation spectrometer to calibrate similar, less accurate instruments in other low Earth orbits. Estimates of numbers of samples useful for intercalibration are made with the aid of yearlong simulations of orbital motion. Results of this study support the conclusion that the International Space Station orbit is ideally suited for the purpose of intercalibration between spaceborne sensors.

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Bruce A. Wielicki, D. F. Young, M. G. Mlynczak, K. J. Thome, S. Leroy, J. Corliss, J. G. Anderson, C. O. Ao, R. Bantges, F. Best, K. Bowman, H. Brindley, J. J. Butler, W. Collins, J. A. Dykema, D. R. Doelling, D. R. Feldman, N. Fox, X. Huang, R. Holz, Y. Huang, Z. Jin, D. Jennings, D. G. Johnson, K. Jucks, S. Kato, D. B. Kirk-Davidoff, R. Knuteson, G. Kopp, D. P. Kratz, X. Liu, C. Lukashin, A. J. Mannucci, N. Phojanamongkolkij, P. Pilewskie, V. Ramaswamy, H. Revercomb, J. Rice, Y. Roberts, C. M. Roithmayr, F. Rose, S. Sandford, E. L. Shirley, Sr. W. L. Smith, B. Soden, P. W. Speth, W. Sun, P. C. Taylor, D. Tobin, and X. Xiong

The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission will provide a calibration laboratory in orbit for the purpose of accurately measuring and attributing climate change. CLARREO measurements establish new climate change benchmarks with high absolute radiometric accuracy and high statistical confidence across a wide range of essential climate variables. CLARREO's inherently high absolute accuracy will be verified and traceable on orbit to Système Internationale (SI) units. The benchmarks established by CLARREO will be critical for assessing changes in the Earth system and climate model predictive capabilities for decades into the future as society works to meet the challenge of optimizing strategies for mitigating and adapting to climate change. The CLARREO benchmarks are derived from measurements of the Earth's thermal infrared spectrum (5–50 μm), the spectrum of solar radiation reflected by the Earth and its atmosphere (320–2300 nm), and radio occultation refractivity from which accurate temperature profiles are derived. The mission has the ability to provide new spectral fingerprints of climate change, as well as to provide the first orbiting radiometer with accuracy sufficient to serve as the reference transfer standard for other space sensors, in essence serving as a “NIST [National Institute of Standards and Technology] in orbit.” CLARREO will greatly improve the accuracy and relevance of a wide range of space-borne instruments for decadal climate change. Finally, CLARREO has developed new metrics and methods for determining the accuracy requirements of climate observations for a wide range of climate variables and uncertainty sources. These methods should be useful for improving our understanding of observing requirements for most climate change observations.

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