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J. S. Theon and W. S. Smith

Abstract

Twelve acoustic grenade experiments were conducted during the period September 1968–February 1969 from Barrow, Alaska (71N). These measurements were intended to monitor the transition in the thermal structure of the mesosphere from the persistent summertime case to the dynamic and highly variable winter-time case. The disturbed features typical of winter appeared in the high mesosphere in September, and at successively lower altitudes until December, at which time the full winter structure had been established. In early January, a warming at the stratopause began a chain of events which eventually would restore the summertime structure and thus complete the cycle.

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M. S. V. Rao and J. S. Theon

Analysis of satellite-derived oceanic rainfall maps reveals certain distinctive characteristics of global patterns for the years 1973–74. The main ones are: 1) the forking of the Intertropical Convergence Zone in the Pacific, 2) a previously unrecognized rain area in the South Atlantic, 3) the bimodal behavior of rain belts in the Indian Ocean, and 4) the large interannual variability in oceanic rainfall. These interesting features are discussed.

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W. Nordberg, L. Katchen, J. Theon, and W. S. Smith

Abstract

Pressure, density, temperature and wind measurements in the upper stratosphere and in the mesosphere resulted from a total of 53 rocket-grenade soundings conducted during the period 1960–1965. Most of the soundings were performed over North America (Wallops Island, 38N and Churchill, 59N) but some results were also obtained over the tropical Atlantic (Ascension Island, 8S) and over Northern Europe (Kronogard, 66N). Soundings were carried out simultaneously at these sites and were coordinated with soundings measing similar parameters over other areas of the globe.

Seasonal and latitudinal variations in the structure and circulation of this region of the atmosphere were derived from the results. Stratosphere temperatures vary with season and latitude in accordance with solar heating rates and with established circulation models. Temperatures above 65 km are substantially warmer in winter than in summer. Average seasonal temperature differences are about 40K at 80 km. They are very pronounced at midlatitudes (Wallops Island) and become even more extreme at high latitudes where in summer mesopause temperatures as low as 140K were observed. Maximum stratopause temperatures were observed during late winter-early summer. At Wallops Island these maxima of about 280K coincided with the period of transition from winter to summer circulation. Temperature profiles for all seasons at all sites intersect between 60 and 65 km at a temperature range of 230 to 240K.

The strong westerly flow in winter shows two pronounced cores, one persistent throughout the winter just above the stratopause, the other somewhat weaker and less persistent near 75 km. Deviations from the zonal flow indicate the existence of meteorological circulation cells on a synoptic scale with the average meridional flaw at Churchill strongly from the north during both summer and winter and at Wallops Island somewhat weaker from the south during the winter.

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J. S. Theon, W. Nordberg, L. B. Katchen, and J. J. Horvath

Abstract

Temperature and wind soundings of the stratosphere and mesosphere using the acoustic grenade technique were made over Barrow, Alaska (71N), during the arctic winter night and during the summer. The winter mesosphere temperatures were generally warmer than at any other location and season and were variable from day to day. Temperatures oscillated with height in wavelike fashion at magnitudes up to 30–40C between 70 and 90 km. These wavelike structures were found to exist also in the winter mesosphere at Churchill (59N) and Wallops Island (38N). However, the magnitude of the oscillations decreased with latitude to a value of 20–25C at Wallops. Conversely, variability In summer armed to decrease with increasing latitude and the wavelike structure was suppressed entirely at Barrow. Temperature variations in summer at Barrow were less than 10C from sounding to sounding at any given altitude from 40 to 85 km. Extremely low mesopause temperatures ranging from 130–140K near 85 km were observed in all of these soundings. One of the Barrow soundings, conducted during a noctilucent cloud display, when compared with a sounding conducted in the confirmed absence of noctilucent clouds, failed to show any significant temperature difference at the cloud level.

Simultaneous grenade soundings conducted from Wallops, Churchill and Barrow during 1965 indicate that a stratospheric warming which occurred over Barrow in late January-early February can be explained in terms of the circulation features at 50 km. Ridging of the Aleutian anticyclone over Alaska combined with the splitting and/or migration of the polar vortex center to a location southwest of Churchill, resulted in a 20–25C increase in the Barrow temperatures from 40–50 km during the 8-day period between soundings (27 January to 4 February). The third set of soundings made on 8 February shows that the original circulation pattern was restored with the polar vortex again centered north of the Arctic circle and the Aleutian anticyclone centered south of Alaska. The resulting circulation returned the Barrow temperature profile to its original value of 27 January and was accompanied by a weakening of the pressure gradient over North America which changed the strong northwesterly winds above Barrow to light and variable up to 70 km.

One set of grenade soundings at Wallops in November 1964, and four sets of pitot-tube soundings at Ascension Island in April 1964 and May 1965, and aboard ship in March and April 1965, were conducted each within one diurnal cycle. Wallops temperature variations observed at 45 km indicate a tendency toward a higher temperature near midnight than near local noon. This variation, a total of 15C at 45 km, increases with altitude to above 20C at 75 km. The phase of the cycle changes considerably also, and at 70 km the maximum occurs at local sunset and the minimum occurs near local sunrise. Results from the pitot-tube soundings indicate higher temperatures at night than in daytime at the stratopause, a rapidly changing phase of maximum temperature with height, and an increasing amplitude of the variations with height.

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T. T. Wilheit, J. S. Theon, W. E. Shenk, L. J. Allison, and E. B. Rodgers

Abstract

The Electrically Scanned Microwave Radiometer (ESMR) on the Nimbus 5 satellite measures the microwave radiation emitted by the earth and the atmosphere in a wavelength band centered at 1.55 cm. The ESMR scans perpendicularly to the spacecraft suborbital track from 50° left to 50° right in 78 steps every 4 s, producing an image which has a spatial resolution of 25 km at nadir.

At these wavelengths, the emissivity of the earth and atmosphere varies considerably more than at infrared wavelengths. Thus the contrast in radiance between land surfaces, which have high emissivities, and ocean surfaces, which have low emissivities, makes continents and islands readily distinguishable. There is a minimum of interference from clouds since most non-raining clouds are virtually transparent at these wavelengths. However, atmospheric moisture does modify the radiation emitted by the surface and when cloud droplets reach precipitable size, they enhance the radiation considerably over surfaces of low emissivity (e.g., over oceans), making it possible to map areas of rainfall as well as regions of heavy cloudiness.

In this application the ESMR images are meteorologically useful in determining the extent, structure and, qualitatively, the intensity of rainfall. It is then possible, over oceans, to determine the location of frontal rain, rain/snow boundaries, and the structure of tropical storms. Because of the generally high emissivities of land surfaces and the wide range of values they assume, interpretation of atmospheric parameters over land is not possible at present.

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T. T. Wilheit, A. T. C. Chang, M. S. V. Rao, E. B. Rodgers, and J. S. Theon

Abstract

A theoretical model for calculating microwave radiative transfer in raining atmospheres is developed. These calculations are compared with microwave brightness temperatures at a wavelength of 1.55 cm measured by the Electrically Scanning Microwave Radiometer (ESMR) on the Nimbus 5 satellite and rain rates derived from WSR-57 meteorological radar measurements. A specially designed ground-based verification experiment was also performed, wherein upward viewing microwave brightness temperature measurements at wavelengths of 1.55 and 0.81 cm were compared with directly measured rain rates. It is shown that over ocean areas, brightness temperature measurements from ESMR may be interpreted in terms of rain rate with about an accuracy of a factor of 2 over the range 1–25 mm h−1 rain rate.

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