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Aylmer H. Thompson

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Aylmer H. Thompson

Abstract

A micrometeorological investigation was made of ground layer inversion formation, maintenance, and dissipation, and of the related thermal circulations as they were observed in and near the mouth of Red Butte Canyon in the Wasatch Range of northern Utah. Measurements of temperature and other parameters at various locations and levels were made near the mouth of the canyon by means of a semi-automatic instrument and recording network. Analysis of data obtained during the summer of 1957 showed that one basic pattern occurred on 90 per cent of the days. Detailed study of this basic pattern revealed the following significant features:

1) The formation of a thin film of cold air within the canyon an hour or two before (astronomical) sunset.

2) A definite wind shift from up-canyon to down-canyon within about 10 min, near the time of sunset.

3) Extremely rapid local cooling and increase in wind speed near the canyon mouth for about half an hour after the wind shift.

4) Quasi-steady wind and little cooling in the canyon within the first hour after the wind shift, although outside the canyon moderate cooling lasted several hours.

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Charles R. Holliday
and
Aylmer H. Thompson

Abstract

The occurrence of rapid deepening of tropical cyclones(≥42 mb in 24 h) in the western North Pacific is examined to determine the statistics of these events and to identify features peculiar to their onset. Seventy-nine cases of rapid growth during the period 1956-76 were selected to study climatological characteristics. These data show that the majority (75%) of the deep central pressures(≤920 mb) in the region are attained through the rapid deepening process. The bulk (67%) of these pressure reductions occur over a time interval of 18 h or less with the first 6 h most likely to account for the steepest fall.

The statistics reveal that development of a tropical cyclone to typhoon intensity over warm waters (temperature ≥28°C to a depth of 30 m) is a necessary (but not sufficient) prerequisite for rapid deepening. An eye dimension near 40 km also is a frequently observed feature at the onset of rapid deepening. The time of onset occurs most frequently at night. Investigation of typhoon track direction and speed (or changes of these two variables) in relation to abrupt intensificaion revealed little associaion.

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Charles R. Holliday
and
Aylmer H. Thompson

Abstract

Typhoon Kate (1970) formed, developed and reached major (50 m s−1) typhoon intensity while remaining at or equatorward of 5°N, normally considered as the extreme latitude of minimal tropical cyclone formation. The climatology of tropical storms occurring at low latitudes is presented. Details of the behavior of Kate are described. Kate is compared with other western North Pacific storms of record that developed near latitude 5°N.

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Aylmer H. Thompson
and
Morris Neiburger

Abstract

The net gain or loss of energy at 700 millibars over the eastern North Pacific Ocean due to the absorption and emission of radiation by water vapor was computed for nine days during a period when the subtropical anticyclone split in two and re-amalgamated. The maps of the distribution of the equivalent temperature change show organized patterns of maximum and minimum radiative cooling which changed from day to day with the changes in flow pattern. These patterns are shown to move regularly in response to the changes in the atmospheric structure, but not in such a simple fashion that they move with the winds or remain in the same relationship to the 700-mb contour pattern from day to day. The computed absorption of solar radiation corresponded to temperature rises of from 0.3 to l.0C; the long-wave radiational exchange corresponded to cooling from a few tenths to more than 2.5C. The net effect ranged from a slight warming to a cooling of more than 2.5C.

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AYLMER H. THOMPSON
and
PHILIP W. WEST

Abstract

This paper describes a scheme for classifying the cloudiness, as determined from satellite TV pictures, of small areas in the vicinity of radiosonde stations. The classification number, c, was correlated with the observed average relative humidity below 500 mb., H̄, at radiosonde stations within a few hundred miles of the Gulf of Mexico and the western Caribbean Sea. For over 100 dates distributed throughout the year the relationship was found to be H̄ = 24.6 + 7.30 c, with a correlation coefficient of 0.78. There is a slight suggestion of seasonal and latitudinal trends. Examples show suggested average relative humidity distributions based on observations made from meteorological satellites over the waters adjacent to the southeastern United States.

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James P. McGuirk
,
Aylmer H. Thompson
, and
Lloyd L. Anderson Jr.

Abstract

Moisture observing capability is surveyed over the tropical northeast Pacific Ocean. Data are taken from late January 1979 during FGGE. Emphasis is on diagnosis of synoptic scale systems in data sparse areas. The capabilities and limitations of five observing systems are examined: surface observations, satellite cloud imagery, radio- and dropsondes, satellite individual channel brightness temperatures, and model analysis from the European Center for Medium Range Weather Forecasts. Both qualitative and quantitative intercomparisons are made.

Surface observations carry almost insignificant moisture information over the tropical oceans. The capability of GOES imagery is well known; however, clouds mask important moisture structure and do not always define moisture patterns well, even at cloud level. Soundings were adequate for synoptic diagnosis, ifthere were enough of them; however, FGGE dropsondes were limited in detail. Satellite channel data provide thorough coverage and show some detail even in nearly overcast regions; ambiguity of interpretation remains a problem. Given the lack of moisture observations for initialization, the ECMWF analysis provides surprisingly realistic moisture patterns.

Quantitative intercomparison of data is generally discouraging. Field comparisons of model analysis and satellite observations are poor, with only marginal statistical significance. Both systems, however, clearly define the synoptically active regions in their variability statistics; they both perform better in moist regions, where quantitative estimates of moisture are most important. Comparisons with radiosondes are poor as well. Correspondence of analysis, satellite and radiosondes is good in moist regions, but all three have serious observational problems when radiosonde-observed relative humidity falls below 50%.

Each of the five systems describes detail not contained in the other systems. Most importantly, quantitative satellite moisture channel data can be used synoptically, and model analysis provides useful synoptic moisture information, even without initial moisture observations.

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James P. Mcguirk
,
Aylmer H. Thompson
, and
Neil R. Smith

Abstract

“Moisture bursts” are bands of high clouds or middle and high clouds extending poleward and eastward from deep tropical locations into subtropical and middle latitudes. These events, synoptic in both temporal and spatial scales, are extremely common, particularly over the North Pacific Ocean.

We define moisture bursts objectively, in primarily geometric terms, to emphasize both their synoptic scale and their tropical-extratropical interaction. We apply this definition to infrared satellite imagery for four 6-month cool seasons (November–April) in the eastern North Pacific (160°E to the west coast of the Americas). The frequency of thew events is about ten bursts per month during normal cool seasons, distributed uniformly across the Pacific to the wen of 110°W; east of this longitude, few moisture bursts occur. Half of the bursts last 2 to 4 days, and no bunt lasted longer than 10 days.

Only 36 moisture bursts occurred during the 6-month El Niño cool season of 1982–83, with the location of occurrence shifted eastward. Few bursts occurred in the region of active tropical convection associated with the El Niño event.

Because moisture burst frequency decreases at times when the ITCZ strengthens, we hypothesize two modes in Hadley cell behavior: a strong zonally symmetric mode, and a weaker mode comprised of the statistical ensemble of a large number of transient moisture bursts. Through analysis of wind fields, zonal averages across moisture bursts are shown to resemble transient intensification of the mean meridional circulation in regions where the Hadley cell is typically weak.

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James P. McGuirk
,
Aylmer H. Thompson
, and
James R. Schaefer

Abstract

Synoptic-scale cloud systems, called tropical plumes, develop from disturbances in the eastern Pacific ITCZ and in conjunction with amplifying troughs to the north; a common spatial pattern and temporal evolution accompany most events. A blend of disparate data and analyses yields a three-dimensional description of a case study during the FGGE January 1979 special observing period. That this tropical plume is typical is corroborated with a climatology of 41 systems and a less detailed climatology encompassing over 200 plumes.

Tropical plumes are accompanied by intense drying and subsidence within the trough to the northwest and a strong subtropical jet within the plume. Wind observations for the case study show that the jet originates near the equator. A disturbance in the low-level easterly trades exists independently from the upper trough when the plume initiates; such anticyclonic curving wind patterns are common at initiation. The plume developed simultaneously out of several disturbances along its axis. A frontal pattern of intensifying moisture gradient, developing thermal gradient and inversion, strengthening jet level winds, solenoidal overturning, and deepening of the tradewind inversion appears along the northwest flank and downstream of the plume. Plumes cease normally when their tropical and nontropical aspects become separated.

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