One of the complications in retreiving the mass of ash in a volcanic clouds
using RADIANNET is that the radiative transfer code assumes that the
background is homogeneous. Although, the background can sometimes be
approxiamated this way, there are other cases where this is clearly a
major problem. This case study illustrates this problem, how to
recognize it, and outlines a processing proceedure for improving the
AVHRR brightness temperature images
The image on the left is a band 4 image containing the
volcanic cloud from Mt. Spurr on September 19, 1992. In this image, the
boundary of the volcanic cloud (as deteremined by the band 4-5
proceedure) is indicated by the red line. In this image, the warmest
features (Great Lakes) are dark, and the coldest features (high clouds)
are bright white. Note that the cloud is underlain by several
backgrounds; water, land, low clouds, and high clouds.
The image on the right is the corresponding band 4-5 brightness
temperature image. Only values of -0.5 C and below are shown in color.
Although the temperature differnce image appears continuous across all
of the backgrounds, the problem that an inhomogeneous background causes
is apparent when retrieving the mass (below).
Radiannet Mass Retreivals
A cloud underlain by a mostly homogeneous background has a radius-
optical depth scatter plot like the one shown in the left plot.
This plot, shows the band 4-5 temperature difference and band 4
brightness temperature as a function of the effective radius (solid
lines) and optical depth (dashed lines). The AVHRR pixels are shown
overlain in red. Note that the pixels converge towards the right side
of the radius-optical depth envelop, which is defined by the temperature
of the underlying surface. If there are multiple background temperatures
the pixels will converge at several points, as shown in the middle
plot. Background 1 is the portion of the cloud underlain by land, and
background 2 is the portion underlain by the warmer water.
The image on the right is a map of the volcanic ash mass
which was retreived using the boundary conditions shown in the middle plot.
Note that the mass decreases over Lake Michigan, compared to values over
Wisconsin and Michigan. The of the surface temperature boundary
condition was set at 290K, which results in an overestimate of the optical
depth for pixels with Background 1, which should have a background temperature
closer to 280 K. As a result, the mass of of ash over the land is
overestimated. Also note that the portion of areas with the highest
"apparent" mass, are those portions underlain by low clouds, and high clouds.
In summary, if the background temperature is too high, the mass of the
portion of the cloud with a cooler background temperature will be overestimated.
The Cookie Cutter Approach
One method which can be used to reduce the problem of an inhomogeneous
background is the "cookie cutter" technique. In this method, portions
of the cloud are subset based on their background temperature. Each
cloud portion is then processed using its representative background
temperature. Due to their complex and changing shapes, it is difficult
proceedure to separate regions of the volcanic cloud underlain by
meterological clouds, however, it is possible to separate those regions
underlain by land and water.
The first step is to generate a land/water mask (left image), a
proceedure which can be automated using
Terascan. This mask is a 2-bit image file that has values of 1 for
the land, and 0 for the water. The data file is then divide by the
mask, and land pixels remain the same (middle image), and the
water pixels are undefined. The mask can then be inverted, so that
water pixels have a value of 1 and the land pixels are 0. Dividing the
data file by this mask will extract the water pixels (right
image) and eliminate the land pixels.
Resulting Scatter Plots
The resulting scatter plots from RADIANNET are shown above. The plot
on the left is the portion of the volcanic cloud underlain by land, and
the plot on the right is the portion of the volcanic cloud underlain
by water. Once these backgrounds are separated, the RADIANNET retrievals can
be done on each piece using an appropriate background temperature.
This page maintained by Dave Schneider (djschnei@mt