Simultaneous Geosynchronous Operational Environmental Satellite (GOES) 1 km resolution visible image pairs can provide quantitative three dimensional measurements of clouds. These data have great potential for severe storms research and as a basic parameter measurement source for other areas of meteorology (e.g. climate). These stereo cloud height measurements are not subject to the errors and ambiguities caused by unknown cloud emissivity and temperature profiles that are associated with infrared techniques. This effort describes the display and measurement of stereo data using digital processing techniques

Desjardins, M.

Hasler, A. F.

Shenk, W. E.

English

NASA Technical Reports Server

Paper No. 12
FOUR DIMENSIONAL OBSERVATIONS OF CLOUDS FROM GEOSYN-
CHRONOUS ORBIT USING STEREO DISPLAY AND MEASUREMENT
TECHNIQUES ON AN INTERACTIVE INFORMATION PROCESSING SYSTEM
A. F. Hasler, M. desJardins, W. E. Shenk, Goddard Space Flight Center,
Greenbelt, Maryland
ABSTRACT
Simultaneous Geosynchronous Operational Environmental Satellite
(GOES) 1 km resolution visible image pairs can provide quantitative
three dimensional measurements of clouds. These data have great
potential for severe storms research and as a basic parameter mea-
surement source for other areas of meteorology (e.g. climate). These
stereo cloud height measurements are not subject to the errors and
ambiguities caused by unknown cloud emissivity and temperature
profiles that are associated with infrared techniques. Previous work
by Minzner et al. (1978a) has demonstrated the validity of stereo
measurements from geostationary satellites using techniques based
on conventional analogue stereography. This effort describes the
display and measurement of stereo data using digital processing
techniques.
Computer remapping of digital GOES image pairs allows the inter-
active display (on the Atmospheric and Oceanographic Information
Processing System, AOIPS) of time sequences of stereo images : a
true four dimensional representation of cloud structures. A des-
cription of similar work at the University of Wisconsin is given by
Bryson (1978). Interactive manual and semi-automatic height mea-
suring techniques have also been developed on AOIPS. Capabilities
under development include: 1) measurement of multilevel wind
fields with accurate height assignment; 2) estimation of thunder-
storm intensity from horizontal and vertical cloud growth rates; and
3) measurement of four dimensional cloud structure for comparison
with numerical models and radar observations. Stereo height verifi-
cation and error analyses were conducted using computer crosscor-
relation on AOIPS. Accuracies of -+0.5 km appear to be possible
for geographical features and clouds with precisions (relative accura-
cies) approaching -+0.1 km.
INTRODUCTION
The quantitative three-dimensional geometric descriptions of cloud struc-
ture as a function of time is extremely important in severe storm research as well
as other areas of meteorology. A few of the many uses include: 1) height assign-
ment for satellite cloud winds; 2) cloud growth rate measurements for storm in-
tensity estimation; and 3) input and verification for radiation models. This paper
describes the stereo display and measurement of the four dimensional cloud
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structure using digital data on an interactive image processing system (AOIPS)
which has many inherent advantages over previously used techniques.
The only method routinely used over the U.S. for measuring cloud heights
at frequent intervals depends on infrared equivalent blackbody temperatures (TBB's)
from the Geosynchronous Operational Environmental Satellites (GOES). Stereo
cloud height measuring techniques have the following advantages over the infrared
techniques: 1) The GOES visible resolution is 8 times greater than the IR reso-
lution, making it possible to measure the .height of much smaller clouds and cloud
features. 2) The infrared techniques require a knowledge of cloud emissivity
which cannot be determined accurately from GOES images so that thin clouds
(especially cirrus) often have large height errors. 3) The infrared techniques re-
quire an atmospheric temperature profile which is generally not available and in
the case of isothermal layers cannot be accurately used for height assignment. 4)
The stereo representation allows the operator to scan large areas while observing
height differences, whereas with infrared techniques, height is generally shown
only as a TBB change which is not as easily interpreted. 5) Multilevel height
determination is possible using stereo including measurements beneath thin cirrus
which are impossible using the infrared.
Previous stereo analyses from geostationary satellites by Minzner et al.
(1978a, 1978b) involves the use of hard copy images and optical mechanical sys-
tems. The processing techniques used in this work operate directly on the digital
data in its most pure form. The measurements made directly from the digital
data do not differ from the degradation and inaccuracies inherent in all image dis-
play systems and optical mechanical devices. Computer remapping of the images
gives a three dimensional view of a large area with no apparent distortion. When
time sequences of remapped stereo image pairs are displayed on AOIPS the 3-
dimensional evolution of clouds can be observed: a true 4--dimensional represen-
tation. The unique image shift capabilities of AOIPS allow the interactive manual
measurements of height as well as automatic computer cross correlation height
measurement and interactive image enhancement.
Another paper in this volume by Minzner et al. (1978b) describes: 1) The
useful area of coverage for the two operational GOES satellites; 2) Stereo cloud
height errors caused by imprecise image pair scan synchronization; and 3) Theo-
retical error limits for satellites with various longitude separations.
TECHNIQUES
Basic Remapping, Display and Height Measurement
Image sequences from the two GOES satellites are precisely navigated. A
512 X 512 visible picture element (pixel) area of interest from one satellite is
chosen. A somewhat larger area from the second GOES satellite is then remapped
digitally* into the coordinate system of the first satellite such that sea level fea-
tures are superimposed. Features above or below sea level will be displaced to the
left or right. This parallax gives a three--dimensional view when the image pair is
projected through the red and green channels of a color television system and
viewed through red and green glasses. The magnitude of the parallax is a function
*See Moik (1977) for remapping programs used on the IBM 360-91.
68
of the altitude and is used to calculate the height. Time sequences of remapped
image pairs are then displayed on AOIPS. The heights are measured on AOIPS using
the following procedure: a cursor or box is moved under joystick control to the
cloud or geographical feature to be measured. One of the images is shifted also using
the joystick (a unique AOIPS function) until the parallax for that feature is elimin-
ated. Observed through the stereo glasses the cursor appears to move up or down un-
til it is at the height of the feature. This manual measurement of the parallax can
only be made to the nearest pixel, therefore a cross correlation algorithm must be used
when subpixel accuracy is required. The parallax is used to calculate the height
taking into consideration the positions and attitudes of the satellites.
Cloud Motion and Height Measuring Techniques
Software is under development at GSFC on AOIPS to allow the measure-
ment of cloud motion and height simultaneously from stereo image sequences.
Observation of three-dimensional cloud motion fields makes it much easier to
measure winds at many levels and to minimize confusion between cloud tracers,
because even small altitude differences between cloud layers are observed. This
software will also aid in the measurement of cloud height variations as a function
of time. Methods are also being developed to contour cloud height as a function
of time on AOIPS. These methods will be adapted for the cloud growth tech-
niques used to estimate vertical velocities and storm intensity which is now done
using infrared techniques by Adler & Fenn (1978a, 1978b).
STEREO HEIGHT VERIFICATION AND ERROR ANALYSIS
Measurements of the altitude of known geographical features, repeatibility
studies on cloud heights, and stereo vs. IR height comparisons have been made to
evaluate the stereo cloud height accuracy. The altitude of the coastline over a 5 °
latitude by 5 ° longitude area of the Gulf of Baja California was measured to de-
termine bias errors and to estimate altitude measurement variability. The average
bias error was Z = -0.67 km with a standard deviation oz = 0.29 km as shown in
Table 1. Over a small 5 x 5 pixel area the standard deviation az = 0.10 km. The
coastline altitude bias error was also measured in central California and near Lake
Titicaca in South America where Z = 0.11 km and 0.26 km respectively (also
shown in Table 1).
Also, the altitude of several lakes in and near California, and Lake Titicaca
were used for verification. The differences between the stereo heights and the
known (map) altitudes ranged from 0.01km to 0.29 km once the sea level bias
error was removed. Although we have no rigorous verification of cloud top height
we conclude from these terrestrial data comparisons that the absolute accuracy of
the cloud height measurements is about +0.5 km and if a feature of known alti-
tude is available in the image, measurements of precision approaching -+0.1 km are
possible (relative accuracy). The absolute accuracy is dependent upon having a
very accurate navigation solution. Repeatibility measurements of heights of various
cloud types indicate that cloud layer altitudes can be measured.within about 0.1
km even for very thin cirrus as shown by the standard deviation values ranging
from o z = 0.08 to o z = 0.19 in Table 2. Repeatibility over a few pixels (o z local)
is also excellent. All measurements were made using a crosscorrelation algorithm
which can measure to subpixel precision. For maximum precision it is necessary
to have a fairly large, high contrast feature oriented in a north south direction.
69
Table1. AOIPSdigital stereo error analysis based on geographical feature height
verification from GOES East (75°W) and GOES West (135°W)
BAJA
(1 km _ 1.5 pixels)
5° Lat. X
'5° Long. area
5 X 5 pixel area
CALIFORNIA
(Lake Tahoe Area/
1 km _ 2 pixels)
Coast Line
Walker Lake
Pyramid Lake
Lake Tahoe
Mono Lake
Altitude (km)
ZMA P -
ZMAP Z _CORRECTED oZ ZCORRECTED
0 -0.67 0.29
0 -0.46 0.10
0 0.11 0.25
1.21 1.03 0.92 0.17 0.29
1.16 1.20 1.09 0.07
1.90 2.0 1.89 0.01
1.95 2.0 1.89 0.06
SOUTH AMERICA
(Lake Titicaca area/
1 km _ 5 pixels)
Coast Line 0 0.26 -
Lake Titicaca 3.82 4.21 3.95
0.11
0.19 0.13
Table 2. AOIPS digital stereo error analysis based on cloud height measure-
ments from GOES East (75°W) and GOES West (135°W)
(km)
o Z Z-
Z o Z (LOCAL) ZIR ZIR
REPEATIBILITY
(Baja area/1 km _ 1.5 pixels)
Stratocumulus 1.2 0.19
Cirrus 8.2 0.12
Very Thin Cirrus 7.8 0.08
STEREO/IR COMPARISON
(Florida area/l km _ 3 pixels)
Stratocumulus
Middle Level Cumulus
Small Cumulonimbus Tower
Cumulonimbus Cirrus Anvil
0.01
0.06
0.7 0.1 0.6
4.9 4.9 0.0
5.8 6.1 -0.3
9.1 10.6 -1.5
11.1 12.4 -1.3
70
Manual measurements made on the 1 km resolution displays used for this study
and on features of about 1 pixel area can be made only to the nearest pixel. For
the four areas used in this study the ratio of horizontal parallax to vertical resolution
ranged from 1.5 to 5 pixels per km. Therefore measurements to 1 pixel would give
precision ranging from 0.67 to 0.2 km. For large features where the cross correla-
tion was used subpixel measurements with precision of 0.I 5 to 0.5 pixels would
be required to give the approximately 0.1 km vertical re_olution indicated by the
data. Comparison of stereo cloud height with IR cloud height on high emissivity
clouds show agreement to approximately 1 km as shown in Table 2. Since only
a climatological atmospheric temperature profile was used for determining the
infrared derived heights this gives only an order of magnitude verification.
RECOMMENDATIONS AND FUTURE WORK
Tests using 8 km resolution infrared images from GOES indicate that stereo
techniques work equally well in the infrared, but since the resolution is low it is
likely that little useful height information may be obtained over other methods.
It is recommended that future geosynchronous satellites have an infrared resolu-
tion of 1 km so that this extremely powerful stereo technique is available 24 hrs/
day.
It is also extremely important that 5 minute interval synchronized stereo
image pairs be obtained in the future, since this is required for cloud growth and
motion measurements in the complicated weather situations where stereo has its
greatest impact. These intervals are needed because many clouds and cloud fea-
tures have such short lifetimes that the continuity provided by 5 minute intervals
is absolutely necessary to measure their motion or growth.
Techniques are proposed to display mesoscale and submesoscale numerical
model results on AOIPS in a graphical four--dimensional manner. When combined
with the four-dimensional representation of cloud structures from the GOES sat-
tellites this should provide a powerful diagnostic tool for understanding the model
output and comparing it with observed weather.
REFERENCES
1. Adler, R. F., and D. D. Fenn, 1978a: 'Thunderstorm Vertical Velocities Esti-
mated from Satellite Data", Conference on Weather Forecastin_ and Analysis
and Aviation Meteorology, Oct. 16-19, 1978, Silver Spring, MD, AMS.
2. Adler, R. F., and D. D. Fenn, 1978b: "Satellite-Based Thunderstorms Intensity
Parameters", NASA Technical Memorandum 78094, May 1978, NASA/GSFC.
Submitted to Journal of Applied Meteorol.
3. Bracken, P. A., J. T. Dalton, J. B. Billingsley, J. J. Quonn, 1978: Atmospheric
and Oceanographic Information Processing System (AOIPS) System Description,
March 1977, NASA/GSFC X-933-77-148.
4. Bryson, W. 1978: Cloud Height Determination from Geosynchronous Satellite
Images. University of Wisconsin, Ph.D. Thesis.
5. Minzner, R. A., W. E. Shenk, and T. Steranka, 1978a: Stereographic Cloud Heights
Heights from Images of SMS/GOES Satellites, Geophysical Research Letters, Vol.
5, No. 1, pp. 21-24.
6. Minzner, R. A., R. D. Teagle, J. Steranka, and W. E. Shenk, 1978b: Stereographic
Ooud Height from the Imagery of Two Scan-Synchronized Geostationary
71
7.
Satellites, 4th NASA Weather and Climate Program Science Review, NASA Conf.
Pub. in printing.
Moik, J. G., 1977: Users Guide for Batch Operation of the SMIPS/VICAR Image
Processing System, NASA/GSFC X-933-76-114.
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Four dimensional observations of clouds from geosynchronous orbit using stereo display and measurement techniques on an interactive information processing system

Four dimensional observations of clouds from geosynchronous orbit using stereo display and measurement techniques on an interactive information processing system

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