ISSN:0143-1161    

DOI:10.1080/01431169108929699

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Convective instability can be induced by unstable vertical temperature profiles and can be supported by the release of latent heat of water vapour provided by a large quantity of moisture in the air. It can also be released by the presence of gravity waves. Large amplitude gravity waves have been linked to the onset of convective storm systems. In this study, the vertical velocity of convection initiated by gravity waves was investigated. The study of Ardmore, Oklahoma, storms showed that weak convection was initiated by gravity waves having wave periods of 35 min and the convection was enhanced by gravity waves having wave periods of 20 min. Evidence for cloud formation, due to the condensation of water vapour through convection initiated by the gravity waves, was obtained from rapid-scan satellite imagery and radar summaries. In this particular case, the convective motion, initiated and supported by the gravity-wave-induced activity (excluding contributions made by other mechanisms), reached its maximum value about one hour before the production of the funnel clouds. In this study, we did not rule out the contributions made by other mechanisms, such as low-level convergence and others, toward the initiation of convection; however, our main purpose is to discuss the role of gravity waves in wave-induced convection contributing to the fractions of formation and development of severe convective storms.

ISSN:0143-1161    

DOI:10.1080/01431169108929700

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Both rawinsonde data and geosynchronous satellite imagery were used to study the life cycles of Ardmore, Oklahoma's severe convective storms. Cloud modelling with input sounding data from Oklahoma City, Oklahoma and rapid-scan imagery from GOES were used to investigate storm-cloud formation, development and dissipation in terms of growth and collapse of cloud tops, as well as, the life cycle of the penetration of overshooting turrets above the tropopause. The results based on these two approaches showed: (1) storm clouds developed lo a mature stage with overshooting cloud tops that penetrated above the tropopause; (2) they collapsed at approximately 9min before the touchdown of tornadoes; and (3) at the time of storm dissipation, cloud tops collapsed at a high rate, approximately 6min before the lift-off time of the tornadoes. Cloud modelling also showed that the local tropopause height decreased during the time when overshooting cloud tops penetrated above the tropopause due to the local subsidence around the overshooting top.

ISSN:0143-1161    

DOI:10.1080/01431169108929701

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The spatial and temporal stability of the distributions of satellite-measured visible and infrared radiances, caused by variations in clouds and surfaces, are investigated using bidimensional and monodimensional histograms and time-composite images. Similar analysis of the histograms of the original and time-composite images provides separation of the contributions of the space and time variations to the total variations. The variability of both the surfaces and clouds is found to be larger at scales much larger than the minimum resolved by satellite imagery. This study shows that the shapes of these histograms are distinctive characteristics of the different climate regimes and that particular attributes of these histograms can be related to several general, though not universal, properties of clouds and surface variations at regional and synoptic scales. There are also significant exceptions to these relationships in particular climate regimes. The characteristics of these radiance histograms provide a stable well defined descriptor of the cloud and surface properties.

ISSN:0143-1161    

DOI:10.1080/01431169108929702

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The correlated distributions of satellite-measured visible and infrared radiances, caused by spatial and temporal variations in clouds and surfaces, have been found to be characteristic of the major climate regimes and can be described by the attributes of bidimensional and monodimensional histograms and time-composite images. Most of the variability of both the surfaces and clouds is found to occur at scales larger than the minimum resolved by satellite imagery. Since satellite imaging data sets are difficult to analyse because of their large volumes, many studies reduce the volume by various sampling or averaging schemes. The effects of data resolution and sampling on the radiance histogram statistics and on the time-composite image characteristics are examined. In particular, the sampling strategy used by the International Satellite Cloud Climatology Project is tested. This sampling strategy is found to preserve the statistics of smaller cloud variations for most regions, with the exception of very rare events, if they are accumulated over large enough areas (at least 500 km in dimension) and long enough time periods (at least one month).

ISSN:0143-1161    

DOI:10.1080/01431169108929703

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The problem of classifying clouds seen on meteorological satellite images into different types is one which requires the use of textural as well as spectral information. Since multi-spectral features are of prime importance, textural features must be considered as augmenting, rather than replacing, spectral measures. Several textural features are studied to determine their discriminating power across a number of cloud classes including those which have previously been found difficult to separate. Although several features in the frequency domain are tested they are found to be less useful than those in the spatial domain with only one exception. The specific features recommended for use in classification depend on the type of classification to be undertaken. Specifically, different features should be used for a multi-dimensional feature space analysis than for a binary-tree rule-based classification.

ISSN:0143-1161    

DOI:10.1080/01431169108929704

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

For a feasibility study setting up the maritime aerosol model, measurements of the radiance and degree of polarization of the skylight radiation were carried out over the Pacific ocean during the cruise of the Hakuho Maru KH-88-2. The wavelengths 0.4,0.6 and 0.9 μm were considered using the portable spectrora-diometer (resolution about 30nm). The derived visibility is in good agreement with that reported from ships. Furthermore the optical thickness and the mixing ratio of aerosol constituents are shown to be derived from the measurements of maximum degree of polarization for moderate solar elevations, provided that the maritime aerosol is a mixture of oceanic and water-soluble aerosols.

ISSN:0143-1161    

DOI:10.1080/01431169108929705

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The modulation of small-scale centimetric water waves induced by large-scale wave groups has been studied by using the dual-frequency radar technique. This paper contains results from a field experiment conducted onboard the research platform ‘Forschungsplattform Nordsee’ situated in the German Bight, plus a theoretical formulation of the relationship between the wave field and the radar measurements. Representing the gravity wave field as a superposition of linear Gaussian wave packets, the long wave modulation of the ripple waves manifests itself as a narrow linearly Doppler-shifted peak in the cross-product spectrum of the backscattered returns. This line is located at a frequency which is a function of the spectral peak frequency, the group velocity and the beat- wavenumber KΔ. The bandwidth of the peaks turns out to be dependent upon the radar footprint width in the line-of-sight direction, the group envelope length and the average period between subsequent wave packets. Monitoring the fluctuations in the magnitude of the resonance peaks, the variations are highly correlated for KΔin the range 0.08-0.9m−1. These results are shown to be interpretable by composite surface scattering theory.

ISSN:0143-1161    

DOI:10.1080/01431169108929706

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Coarse resolution satellite data provide a unique means by which to examine the spatial characteristics of surface phenomena at a wide range of spatial scales. In this study images of the Normalized Difference Vegetation Index (NDVI) with a resolution of 8 km derived from the National Oceanographic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) are studied at a variety of spatial scales ranging from 8 to 512 km. Scale variance analysis is used to examine the spatial characteristics of the NDVI up to the scale comparable with that used for global modelling. This technique identifies the scales at which spatial variation is taking place and the relative magnitude of the variation. Both annual and seasonal images of Africa from 1987 are examined. The analysis revealed substantial differences within the continent in the scale at which spatial variation takes place. Commonly, for the annual image there is an increase in spatial variation with coarsening spatial resolution, although certain areas of complex surface conditions show markedly different patterns. There are substantial changes in the spatial characteristics of the NDVI with time. Analysis of monthly maximum value composites for September and February revealed different responses in scale variance as a function of spatial resolution. These spatial differences were most marked for areas where vegetation possesses strong seasonality. Interpretation of these results leads us to believe that different factors appear to be controlling the spatial variation of the NDVI at different scales. Averaging at coarse grid cell sizes of 512 km as a means of representing surface conditions results in varied success in representing the NDVI. Averaging areas of transition and surface heterogeneity may result in a substantial over-simplification of surface conditions. Consideration needs to be given to the spatial characteristics of areas and their temporal variability if satellite-derived data are going to be validly applied to global models.

ISSN:0143-1161    

DOI:10.1080/01431169108929707

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

An analysis was undertaken of range conditions in the Botswana Kalahari during the 1980-1987 drought. Information from Landsat MSS print and tape data was field-checked both aerially and on the ground through two wet seasons. Comparison of 327 field and digitized aerial photograph sites with spectral data on MSS4, 5 and 7 in relation to hues on Landsat colour composite print data revealed three main spectral-vegetation types. Type 1 darkened areas comprise green tree and shrub vegetation, shown as varying tones of blue-green hues. The rate of darkening varied with the soil substrate but regression analysis showed that higher green vegetation densities occurred at lower MSS4 and 5 reflectance values. Type 2 areas combine darkened areas with areas of high near-infrared reflectance. Vegetation indices do not predict green leaf cover in type 1 or type 2 areas. This is best achieved through density slicing on MSS5. Type 3 areas show high near-infrared and low red reflectance. Green leaf cover can be predicted both by density slicing on MSS5 and through vegetation-index analysis. Since 70 per cent of the Kalahari exhibited darkening, an attempt was made to resolve darkening components using a hand-held radiometer. Results indicated that the within-plant shadow components on thinly leaved trees and shrubs gave rise to darkening. An assessment made of the remaining plant material (browse) showed that large areas of the Kalahari were suffering from poor to intermediate range conditions during the mid to late drought period.

ISSN:0143-1161    

DOI:10.1080/01431169108929708

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor