Engineering topics

Christopher, Sundar A.

Publications and source records attributed to Christopher, Sundar A..

22 records · Page 2

NASA NTRS1994

Radiative Effect of Sub-Sonic Jet Contrails

The study of jet contrails is of major importance to a wide range of disciplines from military planners to climate researchers. Contrails, which form at the wake of jet aircraft, act as tracers, which serve as potential intelligence to military planners. In terms of atmospheric effects, climate researchers are interested in the potential radiative effects of jet aircraft emissions. Since jet contrails are sunset of thin cirrus clouds, they also have regional impacts on climate. The major goal of this proposed effort was to use Satellite Remote Sensing to detect jet contrails in an automated fashion. The tasks accomplished for this project are: 1. Using AVHRR data, a new technique was developed to detect jet contrails. The technique and results were published in a paper recently. 2. A global survey of jet contrails was also conducted using AVHRR data. This work will also be submitted to a peer-reviewed journal. 3. Work is in progress to estimate the optical thickness of these jet contrails and the size of ice particles in these clouds. 4. Another key issue to be addressed is the differences between thin cirrus and jet contrails.

Welch, Ronald M.↗

NASA NTRS1992

Spatial resolution and cloud optical thickness retrievals

The impact of sensor spatial resolution and accurate cloud pixel identification on cloud property retrievals was investigated. Twelve fair weather cumulus (FWC) scenes of high spectral and spatial resolution Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) data are analyzed. A variation of the 3-band ratio technique of Gao and Goetz is used to discriminate clouds from the background, and then a discrete ordinate radiative transfer model is used to obtain optical thickness of cloudy regions for each scene. To study the effect of spatial resolution upon retrieved optical thickness, the 20 m AVIRIS data was spatially degraded to spatial resolutions ranging from 40 to 960 m. Cloud area, scene average optical thickness, and distribution of retrieved optical thickness are determined at each spatial resolution. Finally, a comparison between the 3-band ratio technique and monospectral reflectance thresholding, using 20 m spatial resolution data, is presented.

Feind, Rand E.↗

NASA NTRS1992

A comparison of cirrus clouds determined by ISCCP and SAGE-II and their relation to convection in the tropics

Results of tropical thin cirrus cloud retrievals using International Satellite Cloud Climatology Project (ISCCP) and Stratospheric Aerosol and Gaseous Experiment (SAGE-II) data from January 1985 are presented. A preliminary analysis of the results shows that thin cirrus increases with increasing height in both data sets, and SAGE-II exhibits a high frequency of occurrence. The thin cirrus extinction coefficient shows maxima around the convective regions of South America and the western Pacific Ocean.

Christopher, Sundar A.↗

NASA NTRS1992

The effect of spatial resolution upon cloud optical property retrievals. I - Optical thickness

High spectral and spatial resolution Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) imagery is used to study the effects of spatial resolution upon fair weather cumulus cloud optical thickness retrievals. As a preprocessing step, a variation of the Gao and Goetz three-band ratio technique is used to discriminate clouds from the background. The combination of the elimination of cloud shadow pixels and using the first derivative of the histogram allows for accurate cloud edge discrimination. The data are progressively degraded from 20 m to 960 m spatial resolution. The results show that retrieved cloud area increases with decreasing spatial resolution. The results also show that there is a monotonic decrease in retrieved cloud optical thickness with decreasing spatial resolution. It is also demonstrated that the use of a single, monospectral reflectance threshold is inadequate for identifying cloud pixels in fair weather cumulus scenes and presumably in any inhomogeneous cloud field. Cloud edges have a distribution of reflectance thresholds. The incorrect identification of cloud edges significantly impacts the accurate retrieval of cloud optical thickness values.

Feind, Rand E.↗