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Spiros G. Geotis

Abstract

Average reflectivities of the rain falling on a ship, measured with a shipborne radar by observing at close range and high elevation angle, are compared with values computed from drop size measurements on the ship. Agreement is good and the technique appears to hold promise as a routine calibration check in locations where sidelobe echoes from nearby ground targets are not too predominant.

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Spiros G. Geotis

Abstract

The radar reflectivity of thunderstorms at 10 cm is shown to be a good indicator of hail and a rough measure of its size. The physical characteristics of the hailstorm, as deduced from 3- and 10-cm echoes of a large number of New England hailstorms of 1961, are described. It is shown that the hailstorm possesses no great singularity beyond that of its significantly high reflectivity. It is concluded that the larger hail-stones contribute little to the total liquid water content of the thunderstorm as the highest reflectivities measured are easily accounted for by low concentrations of large hail, wet or dry.

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Spiros G. Geotis

Abstract

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Spiros G. Geotis

Abstract

Total instantaneous water contents of several intense thunderstorms were calculated from smoothed representations drawn from quantitative radar observations; values typically ranged from 108–109 kg. Rain rates and accumulations were obtained from reflectivity measurements near the surface. The rainfall rates of the storms were about 105–106 kg sec−1 with total accumulations averaging 109–1010 kg. Dimensions and durations of the storms are also given.

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Spiros G. Geotis and Robert A. Houze Jr.

Abstract

Radar and rain gauge data are used to determine the precipitation pattern in the vicinity of the north coast of Borneo during Winter MONEX. The results show that separate maxima of rain occurred offshore and inland. Satellite data for other years suggest that this rainfall pattern typifies the winter monsoon in this area.

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Pauline M. Austin and Spiros G. Geotis

Abstract

Several sets of drop size measurements were made on ships and aircraft during GATE. The data were taken primarily as support for the radar measurements and have been analyzed to provide relations between radar reflectivity and desired meteorological quantities.

Uncertainties in the results because of instrumental difficulties and differences within and between the various data sets are examined and discussed. The overall Z-R relation based on the combined data sets is Z = 180R 1.35. Also included are relations of the reflectivity factor to rainwater content and attenuation of 5 cm radiation.

Comparison of the drop size distributions with measurements from other places suggests that tropical oceanic showers typically contain an abundance of medium-sized drops and relatively few large ones.

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Spiros G. Geotis, Earle R. Williams, and Chester Liu

Abstract

No abstract available.

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Earle R. Williams, Spiros G. Geotis, and A. B. Bhattacharya

Abstract

Radar measurements and model studies are combined to investigate the plasma condition and the physical structure of lightning in thunderclouds. The lightning radar target is inferred to be an arclike plasma whose temperature exceeds 5000 K, thereby implying overdense plasma at all meteorological wavelengths. Lightning echoes are treated as volume targets and are modeled as treelike assemblages of conductive channels which are each long and thin compared to the radar wavelength. The channel lengths per unit volume deduced from more than one thousand reflectivity measurements at 11 cm wavelength range from 10−3 to 102 km km−3. Comparisons with more than 200 measurements at 5 cm wavelength show that the wavelength dependence is highly variable. On the average, the apparent dependence is λ−2 but this is unreliable because of the masking effects of precipitation. The infrequent detection of lightning at short wavelengths (λ ≥ 5 cm) is also attributed to masking rather than to an underdense plasma condition.

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Robert A. Houze Jr., Spiros G. Geotis, Frank D. Marks Jr., and Arthur K. West

Abstract

Radar and satellite observations in the vicinity of northern Borneo obtained during the International Winter Monsoon Experiment (WMONEX) showed that the convection in that region underwent an extremely regular diurnal cycle. Over the sea to the north of Borneo, the general level of convective activity was increased during monsoon surges and during the passages of westward-propagating near-equatorial disturbances. Convective activity was decreased during monsoon lulls. The diurnal cycle was well-defined, regardless of whether, the general level of convective activity was enhanced or suppressed by synoptic-scale events.

The cycle of convection over the sea was especially well documented. It was typically initiated at about midnight when an offshore low-level wind began. Where this wind met the monsoonal northeasterly flow, usually just off the coast, convective cells formed. After midnight, the convection continued to develop and by 0800 LST it had evolved into an organized mesoscale system with a precipitation area often continuous over a horizontal distance of 200 km. The structure of this system resembled that of squall lines and other organized mesoscale systems observed in the tropics. The precipitation was composed partially of convective cells, but a considerable portion was also stratiform with a well-defined melting layer extending across much of the system. This precipitation fell from a large mid-to-upper level cloud shield. The mesoscale systems typically began dissipating at midday, when the offshore wind reverted to an onshore wind and low-level convergence became concentrated over land.

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