Poster presented at American Geophysical Union, Fall 1993 Meeting
The AVHRR sensor on polar orbiting NOAA satellites can discriminate between volcanic clouds and meteorological ones using two-band data in the thermal infrared. This paper is aimed at developing a retrieval of the particle sizes, optical depth and total masses of particles from AVHRR two-band data of volcanic clouds. Radiative transfer calculations are used with a semi- transparent cloud model that is based on assumptions of spherical particle shape, a homogeneous underlying surface and a simple thin cloud parallel to the surface. The model is applied to observed AVHRR data from a 13 hour old drifting cloud from the 19 August 1992 eruption of Crater Peak/Spurr Volcano, Alaska. The AVHRR data fit in the range of results calculated by the model, which supports its credibility. According to the model results, the average of effective particle radius in the test frame of this cloud is in the range of 2 to 2.5 um, the effective emissivity averages 0.6 and the optical depth is about 0.60 - 0.65. The total estimated mass of ash in the air amounts to 0.24 - 0.31 x 10^6 tons, which is about 0.7- 0.9% of the mass measured in the ashfall blanket. Sensitivity tests show that the mass estimate is more sensitive to the assumed ash size distribution than is to the ash composition.
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A linear model for observed radiances:
(a)Radiative transfer equation
(c)Selected size distribution of particles:
(d)Estimation of the total masses:
Assumptions and conditions:
(a)Cloud-top temperature retrieval model
(b)Particle radius retrieval model
(c)Optical depth retrieval model
(a) (b) (c)
(a)AVHRR band4 image of Spurr volcanic clouds The AVHRR Band 4 image of the Crater Peak/Spurr eruption cloud, August 19, 1992, 1338 GMT. The center square is the study frame, with an area of about 165 km 110 km. In the central part of the cloud shown Ac=1 for all pixels but at the edges sometimes this is not true.
(b)Two-band temperature difference model Two-band temperature difference model at 10.8 m and 12 m. The near horizontal solid lines represent different effective radii, and the near vertical dashed lines represent the dependence of optical depth at 10.8 m with particle radius.
(c)Retrieval of optical depth
(d)Retrieval of effective radius
Refractive index of different samples
Band4 (Real,Imaginary), Band5 (Real, Imaginary)
(a)refractive index of basalt The two-band temperature difference model with uniform size distribution and the Basalt (sample 2) refractive index.
(b)rafractive index of basalt-glass The two-band temperature difference model with uniform size distribution and the Basalt glass (sample 3) refractive index.
(c)refractive index of obsidian The two-band temperature difference model with uniform size distribution and the Obsidian (sample 4) refractive index.
(d)refractive index of dust The two-band temperature difference model with uniform size distribution and the Volcanic dust (sample 6) refractive index.
Pixel-scale retrieval of masses for different samples:
Efficiency facters for differnt size distributions
Relationship of effective radius and efficiency factors at 10.8 m for different size distributions of particles. (1) (solid line ) is associated with uniform distribution, (2) (dotted line) with gamma, and (3) (dashed line) with lognormal distribution.
Frame scale retrieval of masses for different size distributions
Mass calculations for different refractive index and distributions. The relatively higher calculated masses is due to the assumption of lognormal size distribution of particles, and lower calculated masses is due to the assumption of the volcanic ashes only containing glassy basalt component.