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Donald P. Wylie
and
Barry B. Hinton

Abstract

Cloud motions over the Indian Ocean for May–July 1979 were used to obtain spatial auto correlations of the deviations of the wind components from local means. Best correlations were associated with u′, low altitude clouds and alongwind displacements. Worst correlations arose from v′, high clouds and crosswind displacements. The crosswind anisotropy was ∼15%. All correlations were 0.49 or greater at 5° separation or less.

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David W. Martin
and
Barry B. Hinton

Abstract

Rainfall is analyzed for the Indian and west Pacific Oceans. The analysis uses a multichannel scheme to retrieve open-ocean rain rate from brightness temperatures measured between 1979 and 1986 by the Nimbus-7 Scanning Multichannel Microwave Radiometer. Rain rates were averaged over calendar months for 1° boxes. These rain rates were checked against two published climatologies. They were analyzed in light of historical climatologies of rainfall over the Indian and west Pacific Oceans.

Except for the Somali jet, the scheme adequately represented ambient conditions over the Indian and west Pacific Oceans. Rain tended to fall in two bands paired across the equator. Over the Indian Ocean, the southern member consistently dominated the northern member. Over the west Pacific Ocean, at times through the course of the year, each member dominated the other. Close to the East Indies northern and southern members merged. Bands were modulated by a pair of wavelike conglomerates. Following the sun, each wave conglomerate strengthened on the poleward legs of its track and weakened on the equatorward legs. One wave conglomerate appeared to follow a clockwise loop connecting waters near Madagascar with the Arabian Sea, India, and the Bay of Bengal. The other appeared to follow a counterclockwise loop connecting Austral–Melanesian waters with the Philippine Sea, the South China Sea, and the Bay of Bengal. Converging in boreal spring on northbound legs, the wave conglomerates appeared to merge over South Asia.

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Barry B. Hinton
and
Donald P. Wylie

Abstract

The errors in ship wind reports of light winds tend to significantly bias their mean. This occurs because wind speed is a scalar quantity that is constrained to zero or positive values. Therefore, observations tend to overestimate the light winds because of the one-sided distribution of errors, but the bias disappears under stronger winds. A method for removing this bias from ship data is presented. In particular, the method is applied to interpreting the ratios of wind speeds observed by ships to those obtained from tracking low level clouds. Contracted ratios allow low cloud speeds to serve as proxy data for surface based observations.

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Donald P. Wylie
and
Barry B. Hinton

Abstract

A detailed analysis of the wind stress patterns over the Indian Ocean was made from 1 May to 31 July 1979. A combination of cloud motion and ship data obtained once per day was used to diagnose the surface-wind patterns to a degree of detail not possible in the past for an individual season. These data show the monsoon development and the fluctuations of the Somali Jet and the Southern Hemispheric tradewinds. Wind stress patterns produced by two traveling tropical storms are discussed. These combined to exert an unusually high westerly wind stress on the equator before the monsoon developed.

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Donald Wylie
,
Barry B. Hinton
, and
Kevin Kloesel

Abstract

We have studied the changes in marine stratocumulus cloud cover observed during the FIRF, Program and how cloud cover related to synoptic conditions. Statistical analyses of the 21 day FIRE period show that marine stratocumulus cloud cover over the eastern Pacific ocean was related to wind direction and temperature advection. Good coorelations were found between the cloud cover fraction observed on satellite imagery and the NMC Global Spectral Model analyses of surface winds and temperature advection. This comparison was made in even locations in the Eastern Pacific. Regional differences were found between the area of FIRE operations several hundred kilometers west of San Diego and the other oceanic areas studied.

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Roy W. Spencer
,
Barry B. Hinton
, and
William S. Olson

Abstract

In a comparison between 37 GHz brightness temperatures from the Nimbus 7 Scanning Multichannel Microwave Radiometer and rain rates derived from the WSR-57 radars at Galveston, Texas and Apalachicola, Florida, it was found that the brightness temperatures explained 72% of the variance of the rain rates. The functional form relating these two types of data was significantly different from that predicted by models of radiative transfer through plane-parallel clouds. Most of the difference can be explained in terms of the partial coverage of footprints by convective showers. Because residual polarization is always present, even for large obscuring storms over land and water, it is hypothesized that emission by nonspherical hydrometeors is at least partly responsible for the observed polarization.

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Donald P. Wylie
,
Barry B. Hinton
, and
Kellie M. Millett

Abstract

The feasibility of using satellites for providing surface winds or wind stress data was explored. Three popular methods were compared using nearly colocated data to assess the accuracies of each and the coverage that each could provide. The three methods tested were 1) the use of the sun glitter reflection seen on visible images of the ocean surface; 2) the use of active microwave sensors (flown on SEASAT) which reflect microwaves off the ocean surface; and 3) the use of cloud motions as indicators of the surface winds.

Close agreement in wind speed estimates was found among the three methods. The biases were <0.6 m s−1 for comparisons between comparable methods of estimating surface winds (1 and 2). Cloud motion comparisons to the other methods exhibited biases of <3.0 m s−1. Individual point-by-point comparisons between wind measurements had an average scatter of 2.0 m s−1 (rms) or less after the mean biases were removed. Atmospheric variability caused as many of the differences as the instrumental errors indicating that meaningful wind information could be obtained from all three methods.

Very detailed spacial coverage was obtained with the sun-glitter method for wind speeds. However, the coverage was restricted to a narrow band 5° of latitude wide in the tropics. SEASAT also provided good coverage for two swaths (4° longitude wide) on each side of the satellite's orbit. Gaps between the swaths and orbits (polar non-synchronous orbits) were left unsampled. Both methods required external data on the wind directions which were obtained from cloud motions. The cloud motions provided coverage over larger areas than the other two methods because of the abundance of low-level cumuli.

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Donald P. Wylie
,
Barry B. Hinton
, and
Kellie M. Millett

Abstract

Statistics on boundary layer vertical wind shear were gathered from rawinsonde soundings taken from three small islands and one weather ship. These soundings show a high correlation between surface and 1829 m altitude wind directions. Wind speeds also were vertically correlated except at one tropical station where the winds were light. The directional results suggest that cloud motions could be used with instruments which measure only scalar surface wind speed (such as radar altimeters or microwave radiometers) to infer vector direction and sea surface stress.

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David W. Martin
,
Barry B. Hinton
, and
Brian A. Auvine

Monthly rain falling on the Indian Ocean is mapped for the period 1979 through 1981 by means of observations of the Nimbus-7 Scanning Multichannel Microwave Radiometer. Both stationary and mobile parts were found in the pattern of rain. The stationary part consisted of three zonal and two meridional bands. Only one, the band along and south of the equator, maintained a strong presence through all seasons. A north equatorial counterpart to this south equatorial band also was persistent, but weak. The mobile part of the pattern took the form of a wave. The locus of this wave was an eastward-tilted figure eight, which straddled the equator. The wave moved clockwise along the north loop of the figure eight, counterclockwise along the south loop. The crest of the wave crossed the equator from south to north in May or June and crossed the equator from north to south between August and October. Along its path the equatorial bands were alternately amplified and damped, and the transient bands were activated and suppressed. The effect of the bands and wave was to produce a strong “monsoon” (annual cycle, summer peak) signature in rain falling over both the northeastern and southwestern reaches of the Indian Ocean.

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Donald P. Wylie
,
Barry B. Hinton
,
Michael R. Howland
, and
Raymond J. Lord

Abstract

Autocorrelation and variance statistics were calculated for seven types of wind data in the western hemispheric tropics. Most of these data came from the Global Weather Experiment (GWE) in January 1979. They were: 1) cloud motion measurements from four different sources, 2) rawinsonde wind reports, 3) synoptic land station reports, 4) marine ship reports, 5) aircraft pilot reports, 6) automatic aircraft reports for the GWE, and 7) Seasat scatterometer winds from September 1978. Winds were analyzed within a target area from 30°N to 30°S latitude and 0° to 180°W longitude.

The Seasat scatterometer data had the highest autocorrelations and lowest standard deviations over short distances (<500 km). Cloud motions and rawinsondes had lower autocorrelations than Seasat, while synoptic land stations, ship reports, and aircraft pilot reports had the poorest autocorrelations. These correlations imply that synoptic land stations, ship reports, and aircraft reports were either more sensitive to small‐scale fluctuations than other sensors, or had higher intrinsic noise levels. Structure function plots of autocovariances against separation distance between observations indicated that Seasat was most sensitive to wind field structure by having low autovariance at short distances (100 km) that also grew with distance. The other structure function plots for low‐level wind observations indicated a lack of structure sensitivity to scalar wind speeds because of very small growth rates of the autocovariances with distance. However, all observations were sensitive to structure in the wind direction patterns.

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