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David Suchman and David W. Martin

Abstract

In this study we explore the accuracy, representativeness and reproducibility of tracer winds in the area of the 1974 GARP Atlantic Tropical Experiment (GATE). These winds were generated by tracking clouds in Synchronous Meteorological Satellite (SMS) images displayed on the University of Wisconsin's Man-Computer Interactive Data Access System (McIDAS). Two questions are addressed: 1) How accurately can the cloud displacements be measured? and 2) To what extent do the cloud displacements represent the wind field?

Accuracy is evaluated in terms of data characteristics, McIDAS precision and consistency. We find that for full-resolution visible data neither navigation nor resolution errors significantly affect the tracking of clouds. An examination of consistency, defined as similarity of wind sets independently produced by several scientists tracking clouds from the same set of images, yields an rms reproducibility of 2 m s−1 for cirrus level and 1.3 m s−1 for cumulus level winds. This is smaller than the “random” error generally attributed to cloud winds. In addition, the vorticity and divergence fields are qualitatively reproducible.

The discussion of representativeness centers about cloud height determination, and relating cloud motion to winds. Representativeness is examined through 1) the internal consistency of consecutive sets; 2) the consistency of the cloud wind field, including divergence and vorticity with such features as clusters, vortices, and clear areas; and 3) the difference between proximate satellite and ship winds. These differences were all under 3 m s−1, which is close to the noise level of ship winds and better than radiosonde-radiosonde comparison. We conclude that the representativeness of cloud tracers to cumulus and cirrus level flow is good to within the accuracy of currently available ground truth data.

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Werner Schwerdtfeger and David W. Martin

Abstract

Data from all available sources have been used to construct vertical cross sections of the seasonal mean flow of the atmosphere between 900 and 30 mb, over the western and southern part of South America. Monthly zonal wind profiles are presented for the subpolar region where the stratospheric circumpolar westerlies in late winter are strongest. For these latitudes, an extrapolation of the zonal wind up to the 10-mb level is attempted.

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David W. Martin, Brian Auvine, and David Suchman

Abstract

An evolutionary view is sought Of a single cloud cluster. This cluster was chosen less for intensity than for comprehensive observations. The aim is to describe the principal outside controls on the cluster, including its relationship with nearby clusters. This is accomplished by combining observations from satellites with those from ships and aircraft.

The cluster represented the deepest of four overlapping layers of moist convection present on this day. It—and its neighbors—tended to occur along rings of cumulus clouds, somewhat larger in size, which were formed by the collapse of older clusters. There was no evidence of a migratory cyclonic synoptic disturbance in the lower troposphere. On the contrary, the cluster occurred entirely within southwest monsoon flow. Abruptly, early in the afternoon, as its cumulonimbus towers became aligned across the front face, the cluster accelerated and intensified. It is argued that this change toward squall line structure and behavior was due to strengthening vertical shear in the upper troposphere, which, together with a layer of dry northeasterlies near 600 mb, increased the strength of evaporationally forced downdrafts under the cirrus shield. The change, a kind of metamorphosis, points to more variability in tropical cloud clusters than has commonly been recognized.

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David W. Martin and Dhirendra N. Sikdar

Abstract

Ground based data from the Barbados Oceanographic and Meteorological Experiment (BOMEX) have been combined with satellite data in a study of the time changing cloud and thermodynamic structure of cloud clusters and their environment. Digital images from the third Applications Technology Satellite (ATS-3) were analyzed as “movie loop” sequences on a computer controlled image storage, display, and processing device called McIDAS. Three-to six-hourly soundings from the five BOMEX ships were corrected for radiation and lag errors in moisture, then analyzed in time sections of temperature anomaly, relative humidity, and equivalent potential temperature. The integration of satellite and surface sounding data was accomplished through plexiglas models in which time section strips were treated as space sections. Extended meridional and quasi-zonal space sections of temperature anomaly, relative humidity, and equivalent potential temperature then were constructed from the models.

The time sections reveal a complex stratification of the tropical troposphere, with multiple interwoven layers superimposed on a basic pattern of surface mixed and cloud layers, a trade inversion layer, and an upper tropospheric layer. These layers weakened and sometimes vanished in the vicinity of cloud clusters and the Intertropical Cloud Band (ITCB). Shallower convective systems and patterns were also reflected in this laminar structure, but mainly at lower levels. ATS picture-sequences covered three clusters: two along the ITCB and one to the north of the ITCB. The first of the ITCB clusters contained moderate convection with little organization as it traversed the ship array; the second had distinct squall characteristics. The northern cluster formed in two stages just east of the array: an arc of congestus cloud gradually increasing in area, then explosive development of cumulonimbi. This cluster and the squall cluster formed along the leading edge of a middle tropospheric layer of Saharan origin. Cluster formation occurred close to but not within the areas of greatest parcel instability, where deep convection apparently was inhibited by a strong trade inversion and a very dry mid-troposphere.

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David W. Martin and Anthony J. Schreiner

Abstract

This article describes the size, intensity, trajectory, lifetime and distribution of the GATE cloud clusters of West Africa and the eastern Atlantic Ocean and relates their distribution to the summer climate of the region. SMS-1 infrared and visible 3 h pictures for 85 days of GATE, starting 27 June 1974, were used. It was found that over 500 clusters occurred. Size averaged 2 × 105 km2; lifetime, one day. Although both were highly variable, in general, lifetime increased with maximum size. The clusters occurred in a band oriented west-southwest to east-northeast over the ocean and eastward over land. Nodes were observed at intervals of 5–7° along the axis of maximum frequency of occurrence. Clusters at all latitudes moved generally westward, having straighter tracks and faster speeds over land. From July to September the axis of the cluster band shifted northward 100–300 km, and tended to split over the ocean. Clusters on the northern flank of the band were associated with African easterly waves, especially during Phase III; however, most of the clusters of GATE occurred to the south of the surface pressure trough and surface confluence, apparently in association with convergence within the mean low-level monsoon circulation.

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David W. Martin and Michael R. Howland

Abstract

A new technique is described for estimating daily rainfall by means of visible and infrared geostationary satellite imagery. It is designed for the tropics and warm-season midlatitudes. Because it operates on a grid of points and measures time changes at these points, the technique has been named “grid history.”

It is assumed that at any grid point in some image belonging to a sequence, by means of spectral, textural and evolutionary information, it is possible to classify instantaneous rain rate as nil, light, moderate or heavy. Then the total rainfall over a day is the sum over three classes of the product of frequency and class average rate.

The class average rates have been determined by least-squares multivariate linear regression of frequencies on observed rainfalls. The areas treated are South China Sea, India, Arabian Sea, tropical North Atlantic Ocean and Amazonia. Inland India had the lowest (driest) class average rates (coefficients), coastal India the largest (wettest) coefficients. Differences in coefficients were least for the Arabian Sea and Atlantic Ocean. There the class average rates were roughly zero (by definition), 1.5, 6 and 15 mm h−1. For the strongly convective rain regimes treated here, it was found to be important to “look” at the area at least once per hour. A loss of accuracy in estimates over land apparently was due to unexpectedly large terrain and synoptic effects. Best-circumstance estimates of daily rainfall for an area 100 km on a side should be within a factor of 2 of true rainfall.

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David W. Martin and Dhirendra N. Sikdar

Abstract

We describe the behavior, dynamics and setting of three cloud clusters. The clusters occurred in the western Atlantic Ocean between 21 and 24 July 1969. Wind and cloud analyses made from intensive ship and satellite observations of the Barbados Oceanographic and Meteorological Experiment (BOMEX) were supplemented by analyses of thermodynamic structure (temperature, moisture and stability) from Martin and Sikdar (1975).

Each of the clusters moved west to northwest. In each cluster high-energy near-surface air was ingested on the front or right flank and vented in the upper troposphere toward the right flank or rear. The clusters were distinguished by a range of speeds (6–15 m s−1), maximum areas (30 000–200 000 km2) and lifetimes (<1 to ∼3 days). Downdrafts in the first cluster were weak. The second cluster was without significant downdrafts as it formed, but gradually assumed the appearance of tropical squall lines. Massive squall-line downdrafts were observed in the third cluster. Conditions favoring deep convection were high absolute moisture content of the subcloud layer, relatively high moisture content in the middle troposphere, a weak trade inversion, large-scale 950 mb convergence, and cyclonic or weakly anticyclonic 950 mb relative vorticity. Deep convection tended to parallel centers of cyclonic vorticity at 950 mb. Downdrafts were stronger where there was a distinct wind speed maximum in the middle troposphere.

The clusters occurred with a persistent westward moving cloud wave: one toward the apex, and two at the base close to the Intertropical Cloud Band (ITCB). Surface θe was high within the cloud wave, and 950 mb relative vorticity was mostly cyclonic. The trailing edge of the cloud wave marked a surge in the northeast trades. The cloud wave was linked with a layer of warm, dry Saharan air between 650 and 850 mb. Baroclinicity across the front of the Saharan air supported a 20 m s−1 east-southeasterly jet at 650 mb. There was a ridge over the trailing edge of the cloud wave, and a trough over the cloud wave, 200–500 km downstream. Air advanced relative to the wave, sinking as it approached the ridge and rising in passing from ridge to trough. In this case the strongest controls on deep convection were exercised from the middle troposphere.

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Elen Cutrim, David W. Martin, and Robert Rabin

A survey of shallow (fair weather) cumulus clouds over part of Amazonia yields evidence of enhanced frequency where the forest had been cleared. The survey covers one dry-season month from 1988. It employs a threshold algorithm to construct an image of cumulus cloud cover from sets of geostationary satellite visible–infrared image pairs. Cumulus images were constructed for two times. The morning image shows no association of the cumulus index with cultural features. However, in the afternoon image a patch of high index values coincides with deforestation along highway BR-364 in the state of Rondonia.

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David W. Martin and Verner E. Suomi

Signal enhancement is advanced as a method for isolating selected features of ATS images. The discussion covers enhancement theory, limitations, and one important application, the isolation of deep convection within tropical cloud clusters. Comparisons of brightness contoured ATS images with radar images from BOMEX test the validity of associating deep convection with very bright clouds. The enhancement technique is then applied in a census of Atlantic cloud clusters, and in case studies of individual clusters.

It is shown that in spite of difficulties involving control of the ATS signal, enhancement is an effective, precise tool for isolating selected features of ATS images. Comparisons of ATS and radar images establish a high correlation of bright areas on ATS with large radar echoes; therefore, enhanced ATS pictures emphasizing the upper levels of the brightness range effectively isolate deep convection. The brightness structure of convective clouds is such that they can be studied over a three-to four-hour period around local noon on pictures uncorrected for changes of incident and reflected radiation. A simple cosine law correction for incident radiation can appreciably extend this period.

The census and case studies show that the eastern Atlantic was at least as convectively active as the western Atlantic during June and July, 1969, and had a significantly greater total area of cloud clusters in 1969 and 1970. Convective cores have a great range of size, spacing, and lifetime: nevertheless, an ordering invariably can be perceived. This most often is in the form of lines or bands; waves, spirals, or solitary cores are also observed. Lifetimes are a few minutes to several hours or more; large cores last longer. Displacement of cloud clusters is accomplished by a complex combination of band and cell movement and propagation. Structure, as evidenced by core behavior, is varied and complex.

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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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