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Abraham H. Oort

Abstract

Numerical output from a GFDL global climate model was used to determine whether the present distribution of rawinsonde stations is adequate to deduce the atmospheric structure and its variability in space and time over the globe. Spatial data gaps were found to cause typical rms wind errors averaged over a hemisphere of 2 to 3 m s−1, increasing for the zonal wind component to 5 or 6 m s−1 at jet stream levels. In temperature the spatial data gaps led to rms errors on the order of 0.5 to 1°C in the free atmosphere, in geopotential height between 20 and 30 gpm in, the upper troposphere, and in specific humidity between 1 and 2 g kg−1 near the surface and about 0.3 g kg−1 at 500 mb.

Errors due to instrumental deficiencies, unrepresentativeness of the local soudings, deficiencies in the analysis technique and gaps in the time series were found to he less important than those due to the, spatial gaps, even in the Northern Hemisphere.

In the Northern Hemisphere, the rawinsonde network was found to be generally adequate to measure large-scale circulation statistics. However, in the Southern Hemisphere the incorporation of additional data sources (rawinsonde, satellite or otherwise) is necessary, especially for defining the fluxes by the mean meridional and stationary eddy circulations.

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Abraham H. Oort

Abstract

The annual cycles in the atmospheric storage and in the meridional transport of energy are discussed. The calculations are based on a five-year sample from more than 500 radiosonde stations mainly located in the Northern Hemisphere. All statistics represent values integrated vertically between the earth's surface and 75 mb and horizontally along a latitude circle.

Several new and interesting features of the eddy and mean transports of potential energy, sensible heat, latent heat and kinetic energy become apparent by the breakdown according to calendar month.

In December through February more than half of the sensible heat is transported poleward by the standing eddies. The transient eddy heat flux does not peak in winter but in April and November.

The strong annual cycle in the tropical Hadley circulation does not contribute to the poleward transfer of energy for the year as a whole.

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Abraham H. Oort

Some of the contributions of Victor Paul Starr (1909–76) as a scholar and teacher at the Massachusetts Institute of Technology are described. His work on the atmospheric branch of the earth's angular momentum cycle is emphasized. Certain recent efforts to include the oceanic and solid earth branches of the cycle are discussed.

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ABRAHAM H. OORT

Abstract

Several estimates of the rate at which the mean and eddy forms of both kinetic energy and available potential energy are generated, converted, and dissipated in the atmosphere are compared in tabular form. From these tables a selection is made of those values which are, in the author's opinion, representative for the yearly energy cycle in the Northern Hemisphere.

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JoséP. Peixoto and Abraham H. Oort

Abstract

The present paper deals with the analysis of the time-average relative humidity fields in the atmosphere. Twice-daily estimates of relative humidity are used.

After some theoretical considerations on the relations between relative humidity, other moisture parameters, and temperature, a critical analysis of the various sources of data is made considering their possible limitations. Various methods of computing relative humidity are formulated and discussed.

The global distribution of relative humidity at various levels shows that it is not zonally uniform with centers of various intensities at all latitudes. The global maps show maxima in the equatorial zone and minima in the dry subtropical belts around 30°N and 30°S. The land–sea contrast and variations related to the orographic relief are also apparent. The general pattern of relative humidity is similar at all levels but its magnitude decreases with altitude. The seasonal analyses show a similar pattern as the annual analyses but are slightly shifted toward the summer pole.

The saturation deficit is also evaluated. Cross sections of the saturation deficit show that the maxima are found in the middle to lower troposphere at subtropical latitudes, being most intense in the Northern Hemisphere during the summer season.

The temporal variability of the relative humidity due to transient eddies exhibits a bimodal structure with maxima in the midlatitudes of each hemisphere around 700 mb. The stationary eddy distributions are less pronounced than the transient ones and do not change substantially from one season to another.

To validate our results, several comparisons are made using independent sources of humidity data as well as cloud distributions at various levels. Thus, COADS data are used to obtain independent analyses of the surface relative humidity over the oceans, and satellite observations by SAGE are used at the 300-mb level. The rawinsonde-SAGE differences are on the order of 10% in the Tropics and 20% in the high latitudes, due in part to a clear-sky (dry) bias in the SAGE data. Our results are further compared with those obtained from operational analyses by the ECMWF. The differences do not exceed 5% in the Tropics but tend to be larger in the tropical upper troposphere and at all levels in the extratropies of the Southern Hemisphere, where the radiosonde network is quite sparse. In view of the obvious connections between the moisture distribution in the atmosphere and cloudiness, a cloud climatology is used to cheek its consistency with the present results. The latitudinal and interseasonal variations of cloudiness and relative humidity are similar, with maxima in the equatorial belt and at high latitudes and minima in the subtropics that shift poleward during summer and equatorward during winter.

Finally, some comments are made on the radiosonde-observing systems in the light of recent satellite studies of humidity. Mainly at the upper levels systematic localized differences are found between electrical hygristor and organic sensors, but the differences almost disappear in the middle and lower troposphere.

In spite of the shortcomings, limitations, and errors of the radiosonde network, the present analyses describe for the first time the large-scale, three-dimensional characteristics of the relative humidity in the global atmosphere.

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Abraham H. Oort and Huanzhu Liu

Abstract

New time series of the hemispheric and global mean temperature anomalies in the troposphere and lower stratosphere are presented for the period May 1958 through December 1989. The statistics are based on objective monthly analyses of all available daily soundings from the global rawinsonde network (∼700–800 stations). The results are compared with Angell's earlier statistics based on a subset of 63 stations. Excellent agreement is found with these earlier results as well as with an 11-year set of satellite-derived microwave sounding unit data. These detailed comparisons support the conclusion that the rawinsonde network can provide reliable estimates of the actual interseasonal hemispheric-scale temperature changes that have occurred between the earth's surface and about 20 km (50 mb) height since the 1950s.

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Sydney Levitus and Abraham H. Oort

A project to objectively analyze a large quantity of oceanographic data for the world ocean is described. Preliminary results are encouraging within the limits of data available. Results are being used in a variety of ways but at present primarily for studies of the ocean's role in the global heat balance. A brief discussion of the data used, the method of analysis, and some preliminary results is presented.

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ABRAHAM H. OORT and ALBION TAYLOR

Abstract

For six stations in the northeastern United States, the spectrum of horizontal wind speed was analyzed using 10 yr of 1-min averaged, hourly surface reports. The fast Fourier transform technique was employed to estimate the spectrum between 1 cycle/2 hr and 1 cycle/2 yr.

The kinetic energy spectra show two major spikes at periods of 24 hr and 1 yr. However, most of the energy is contained in the traveling cyclones and anticyclones with periods between 2 and 7 days. The apparent discrepancy between Van der Hoven's results and our results concerning the existence of an important diurnal cycle in the kinetic energy can be explained by Blackadar's theory of the diurnal wind variation with height. Van der Hoven's spectrum represents conditions near the top of the surface layer, while our data were taken well within the surface layer. A line-by-line investigation of the diurnal peak reveals a very sharp line at 2400 hr with two side lobes 3.9 min away from the main line. These side lobes are probably caused by an annual modulation of the diurnal cycle.

The spectra tentatively corrected for aliasing give some indication of the existence of a spectral gap between small-scale turbulence and mesoscale phenomena.

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Abraham H. Oort and Thomas H. Vonder Haar

Abstract

Based on the best presently available satellite radiation, atmospheric and oceanic data sets the long-term mean heat balance of the earth and its normal seasonal variation are investigated over the Northern Hemisphere. Quantitative estimates for the various flux and storage terms in the atmospheric and terrestrial branches of the heat balance are given for 10° wide latitude belts and for each calendar month. The results are presented in both graphical and tabular form. As was known before, the storage of heat in the oceans is found to dominate the energy storage in the combined atmosphere-ocean-land-cryosphere system. In the tropics, large changes in oceanic heat storage are found in the 10°N–20°N belt with a maximum in spring and a minimum in late summer. The main new finding of this study is that the inferred oceanic heat transports appear to undergo very large seasonal variations especially in the tropics. Between 10°N and 20°N, maximum northward oceanic transports of 4 to 5 × 1015 W were competed in spring and late fall, which are as large as or larger than the corresponding mid-latitude atmospheric transports. Near the equator the oceanic fluxes were found to reverse seasonally and be directed generally toward the winter hemisphere with an absolute maximum of −8 × 1015 W in August.

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Thomas H. Vonder Haar and Abraham H. Oort

Abstract

Recent measurements of the earth's radiation budget from satellites, together with extensive atmospheric energy transport summaries based on rawinsonde data, allow a new estimate of the required poleward energy transport by Northern Hemisphere oceans for the mean annual case. In the region of maximum net northward energy transport (30–35N), the oceans transport 47% of the required energy (1.7×1022 cal year−1). At 20N, the peak ocean transport accounts for 74% at that latitude; for the region 0–70N the ocean contribution averages 40%.

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