摘要:

The ERS–I spacecraft scatterometer, C-band VV polarization, acquired radar cross-section measurements over the global oceans during 1992 and 1993. We investigate the cross-section dependence on mean wind speed U using collocated buoys within ±25km of the scatterometer cells. These collocated measurements result in over 75000 matches in two diITerent oceanic regions. The buoys measure hourly mean wind speeds from 0·2–10 mS1and 0·2–18ms-1in the equatorial Pacific Ocean and at mid-latitudes off the North American coasts, respectively. We present experimental evidence for a new and compact exponential model dependence on wind speed. The previously used power–law form inadequately characterizes the cross-section measurements based on a single index over a large wind speed range. The cross-sectional slope varies from about zero dB/ms-1at high wind speeds U=18ms-1and small incidence angles 0=20° to about 1·3dB/ms-1at low wind speeds U=3ms-1and large incidence angles, 0=55°. The CMOD4 model significantly underestimates the radar cross section measurements for U≤3ms-1whereas the exponential model exhibits less bias.

ISSN:0143-1161    

DOI:10.1080/01431169508954651

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Ten years (1982–1991) of upper air and ocean surface observations over the Indian Ocean from Sagar Kanya and Monex–79 data have been used to examine the relationship between the precipitable water (W) and surface level humidity (Q) on an instantaneous scale. Our analysis of Q and W over the Indian Ocean shows that Q is correlated with Won an instantaneous scale with r=O·44, which is a statistically significant correlation. A regression between Q and W has been fitted and it has been found that a fifth order polynomial yields a lowest root-mean-square (rms) error of I-4gkg- 1 when compared with observed Q using an independent observation. The validity of earlier derived global relation between Q and W has been examined over the Indian Ocean. It has been found that Liu's global Q-W relation gave a large rms error of 4·1 gkg-1when compared with the observed instantaneous Q values over the Indian Ocean. The usefulness of the above derived Q-W relation and an earlier derived relation between the monthly mean Q and W has been examined for the estimation of latent heat fluxes (LHF) over the Indian Ocean using an independent observation. The LHF estimated from the bulk aerodynamic method using all quantities available from ship observations, called the direct method (M1), has been compared with the LHF computed by using a derived Q-W relation (M2). The rms error between MI versus M2 is found to be 56 Wm-2. The LHF estimated by Liu's Q -W relation, when compared with MI gave an rms error of 155Wm-2, which is suggestive of its unsuitability for the estimation of LHF over the Indian Ocean on an instantaneous basis. The difference between the sea surface humidity (Q,) and surface level humidity (Q) has been parametrized in terms of sea surface temperature (SST) and W, both obtainable from satellite sensors. This relation has then been used to compute LHF (M3) and was compared with MI, where it was found that M1 versus M3 gave an rms error of 58Wm-2. Thus, this study indicates that methods M2 and M3 are found to be more consistent and accurate in nature, and also suggests that it can be further applied to the LHF estimation using satellite based microwave/IR measurements for Wand SST on an instantaneous basis.

ISSN:0143-1161    

DOI:10.1080/01431169508954652

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Polar-orbiting meteorological satellites collect imagery across a wide range of solar illumination and atmospheric conditions. Algorithms used to create automated cloud analyses from these data must compensate for variations in cloud signatures caused by changes in atmospheric attenuation and solar scattering geometry that occur as the satellite orbits the Earth. In this paper, a methodology is presented that describes the variations in cloud spectral signatures that result from changes in satellite observational conditions. Relationships are developed that describe the impact of solar illumination and scattering geometry on the spectral signature of optically-thick water clouds in the daytime Advanced Very High Resolution Radiometer (AVHRR) visible and near-infrared imageryxs. Additional relationships are presented that describe the impact of total integrated water vapour on the spectral signatures of optically-thin cirrus and stratus clouds in night-time AVHRR infrared. Threshold functions are then derived from these relationships and demonstrated in the automated analysis of high resolution AVHRR imagery. The accuracy of each automated analysis is measured against a ground truth (manual) cloud-no-cloud analysis created from the multi-spectral imagery. It is concluded that highly accurate automated cloud analyses are achievable using bi-spectral cloud detection techniques that employ the threshold function methodology to compensate for global variations in cloud spectral signatures.

ISSN:0143-1161    

DOI:10.1080/01431169508954653

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

An automated algorithm for thresholding hot pixels in AVHRR data is presented. The algorithm, applied to cloud-free sub-images of channel 3 minus channel 4 brightness temperature, compares each target pixel with its immediate background and then compares this difference with the natural variation in the surrounding region. Application to images of Mount Etna identified thermally anomalous pixels containing active lava or vents.

ISSN:0143-1161    

DOI:10.1080/01431169508954654

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

ISSN:0143-1161    

DOI:10.1080/01431169508954655

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A new approach for feature tracking on sequential satellite sensor images using neural networks has been developed. The method defines the correspondence problem between features as the minimization of a cost function using a Hopfield neural network. It has been tested on Meteosat radiometer images by tracking a cloud with rotational movement and compared to the maximum cross-correlation method. The Hopfield net was found to be more accurate and faster.

ISSN:0143-1161    

DOI:10.1080/01431169508954656

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor