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D. N. Sikdar and V. E. Suomi

Abstract

In this paper we have used time-lapse ATS-I satellite cloud photographs as our data source for the evaluation of the convective transport of latent heat from the lower troposphere to the tropical upper troposphere.

The analysis reveals that the meso- to subsynoptic-scale convection systems over the tropical mid-Pacific are well organized on a time scale of a few days and are controlled by the large-scale motion field. The time variation of this beat transport, in the sector 12OW–18OW, 15N–15S, indicates an approximate periodicity of five days. Furthermore, this pulsating feature seems to be tied to a wave-like disturbance field of wavelength nearly 75° of longitude and moving westward with an approximate speed of 15° of longitude per day.

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D. N. Sikdar and V. E. Suomi

Abstract

This paper develops an objective technique for estimating the mass and energy exchange in convection systems corresponding to altocumulus cumulogenitus and cumulonimbus intensities using measurements of the area change of the cirrus outflow on a sequence of satellite cloud photographs obtained at geostationary altitude. The data clearly show that: 1) the technique is able to isolate vigorous and moderate convection regimes on the geostationary satellite cloud photos; and 2) the model-estimated mass and energy are consistent with ground-based measurements such as those of Braham and Brown.

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John A. Young and D. N. Sikdar

Abstract

Patterns of fluctuating cloud amount in the tropical Pacific are decomposed into individual frequency bands and presented in longitude-time and latitude-time sections. The natural variability of propagating “clusters” is identified by considering zonal deviation fields, While patterns closer to planetary scale are studied in the zonal means for this limited domain. Periods of both regular and irregular transient activity are made apparent as a function of location. Regimes of “regular” activity are found to differ in direction of cloud propagation, horizontal scale, and waveform tilts.

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Soon-Ung Park, D. N. Sikdar, and V. E. Suomi

Abstract

Nimbus 4 data (11.5-μm) channel and ATS 3 digitized data were analyzed for three days during April and May, 1970. Cloud-top temperature, as measured by the 11.5-μm channel of the satellite radiometer, shows a close negative correlation with cloud brightness measured by the ATS 3 satellite. This is especially true in well-developed cumulonimbus clouds.

An estimate of cloud heights corresponding to cloud-top temperature was obtained by using radiosonde data and radar images, and the relationships between the cloud thickness and brightness counts were derived for the cumulonimbus clouds.

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D. N. Sikdar, J. A. Young, and V. E. Suomi

Abstract

Time series of areal cloud coverage over the central Pacific are studied to determine the large-scale variations of tropical disturbance activity for a four-month period in 1967. The persistent and fluctuating components are found to be more prevalent in regions of high and low mean cloudiness, respectively, with major longitudinal variations in activity most evident in the Southern Hemisphere. Normalized power spectra show that the transient activity occurring in the equatorial zone is dominated by shorter periods of about four days, while longer periods are dominant away from the equator. Coherency magnitudes and phases between different locations indicate that propagating cloud systems are most identifiable at lower frequencies in the Southern Hemisphere, and at somewhat higher frequencies in the Northern Hemisphere. Activity with periods in excess of five days consists largely of both westward and eastward propagation on the planetary wave scale. The shorter periods appear to consist of both synoptic-scale and planetary-scale modes, with westward propagation dominant near the equator.

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