Near-real time operational applications of NOAA satellite enhanced thermal infrared imagery to snow monitoring for river flood forecasts, and a photographic overlay technique of imagery to enhance snowcover are presented. Ground truth comparisons show a thermal accuracy of approximately + or - 1 C for detection of surface radiative temperatures. The application of NOAA imagery to flood mapping is also presented

Carlson, R. F.

Kane, D. L.

Seifert, R. D.

English

NASA Technical Reports Server

Paper No. 11 N 7 6 - 1 6 5 7 2
OPERATIONAL APPLICATIONS OF NOAA-VHRR IMAGERY IN ALASKA
R. D. Seifert, R. F. Carlson, D. L. Kane, Institute of Water Resources, University of
Alaska, Fairbanks, Alaska
ABSTRACT
A discussion of near-real time operational applications
of NOAA satellite enhanced thermal infrared imagery to snow
monitoring for river flood forecasts, and a photographic over-
lay technique of imagery to enhance snowcover are presented.
Ground truth comparisons show a thermal accuracy of vtl°C for
detection of surface radiative temperatures. The application
of NOAA imagery to flood mapping is also presented.
INTRODUCTION
In Alaska, snow is a fact of life for most of the state for
at least 6 months of the year and, in the Arctic, it is present
for 8 to 9 months. It is surprising, therefore, that more is not
known about the snowcover at this stage of development. The
facts are, however, that observation stations and snowcourses are
sparse; the population is small; and what ground based data is
present tends to be clustered around the larger population cen-
ters: Anchorage, Fairbanks, and Juneau. For this reason, the
availability of daily synoptic NOAA - Very High Resolution
Radiometer (VHRR) imagery was a welcome addition to the sparse
data network of snowcover observations.
The NOAA series satellites are polar orbiting and sun-syn-
chronous. The VHRR is only one of many sensors which are carried
on board the satellite, but is the only one of interest for snow
observations. Imagery is produced in the visible portion of the
spectrum (0.6 - 0.7 urn) and in the thermal infrared (10.5 - 12.5
ym), hereinafter referred to as IR. Standard imagery is
^1:8.5x10 scale and a single orbital pass can scan an area 1700
km wide and about 6000 km long. Consequently the entire state
of Alaska can be monitored easily in two successive passes. No
other satellite provides this frequency of coverage at Alaska's
latitude. Resolution is 900 meters at nadir. The IR imagery
can also be calibrated to display radiative surface temperatures
to an accuracy of ±1°C (Barnes, et al., 1974), and has been
applied successfully to synoptic snowmelt monitoring.
143
https://ntrs.nasa.gov/search.jsp?R=19760009484 2020-03-22T18:08:11+00:00Z
Previous work on applications of NOAA-VHRR has centered
mainly on snowmapping for water resource information or for
scientific interest (Wiesnet, 1974; McGinnis, et al., 1975;
Rango, 1975; and Barnes, et dl., 1974; Barnes, et al., 1974).
Water contained in the snowpack is important as a source of
summer water supplies for communities and irrigation and for
aquifer recharge. Information obtained for these purposes can
be integrated into timely resource allocation plans. In Alaska,
however, applications of VHRR imagery are of a more expedient
nature. Since few communities in Alaska rely on snowmelt infor-
mation for water or irrigation, snowmapping is not usually nec-
essary. Instead, snowmelt itself is the important factor because
of the frequent flooding it causes. What is needed in Alaska is
synoptic, near real-time imagery which clearly delineates the
physical (thermal) state of the snowpack, allowing hydrologists
to analyze the progress of snowmelt and the possibilities of
flooding caused by rapid snowmelt and ice jams.
Since a data-link communication system exists between the
U.S.W.S. Offices in Fairbanks, Anchorage, Juneau, and the data
acquisition facility at Gilmore Creek (near Fairbanks), the
ability to transmit imagery in near-real time is limited only by
the necessity to calibrate an IR display. Calibration is neces-
sary because of variability in the VHRR sensor on board the
satellite. This problem was solved by programming the calibra-
tion functions on a small HP-65 calculator on site at Gilmore
tracking station so that the necessary calibration time was
only about twenty minutes. Consequently, calibrated IR displays
could be sent to the Regional Hydrologist's Office in Anchorage
and Fairbanks about two hours after they were acquired.
With snowmelt information as a first priority, an enhanced
thermal IR display was felt to have the most value as an opera-
tional tool. The enhanced IR display was specially developed for
hydrologic applications. Since the 0°C isotherm is the most
significant feature during the spring snowmelt, this temperature
was singled out for enhancement. After tests of a few display
schemes, the following scheme was put into daily operational use:
I. Radiative temperatures from -20°C to just below
freezing (-0.5°C) are displayed in an ascending gray scale.
This means that any temperatures which are detected to be-20°C
or colder are displayed as white, and temperatures intermediate
between -20°C and -0.5°C are displayed in a shade of gray. The
darker the grays, the warmer the temperatures, until at -0.5°C
the display is black. This is essentially the same type of dis-
play used in a standard IR image, but the display has been
adjusted to cover a much narrower band than usual (usually
185-315°K).
144
II. Next, the display was designed to enhance the areas at
the freezing temperature by showing them as white. The abrupt
change from black to white results in a clear depiction of the
areas of melting and ripe snow. A range from 0 to +1°C is dis-
played as white, because if only 0°C is displayed as white the
resolution of the satellite may not detect melting areas clearly,
since the zone of melting snow may be very narrow or patchy,
especially in mountainous areas. By displaying the 0°C to 1°C
range as white, the display clearly shows the melting regions
without adding much error. This is because surface temperatures
rise rapidly once an area becomes snowfree, and the actual area
which is snowfree and at 1°C is rather small. Essentially what
results is a compromised, but very useful and informative snow-
map. Melting snow can be seen in the imagery as a stippled white
and black area instead of the expected continuous white region
(Figure 1).
III. Finally, any areas with detected radiative tempera-
tures equal to or greater than 2°C were displayed as black. This
has two advantages; first it produces the same logical display as
a visible image, since snowfree areas are always the darker areas
in the visible band imagery. Secondly, it results in another
abrupt change, this time from white to black to delineate those
areas which are snowfree from those which are still at or just
above the freezing temperature.
An example of this display scheme on June 4, 1975 is shown
in Figure 1, along with the visible band image of the same gen-
eral area, the Brooks Range and North Slope regions of Alaska.
It should be pointed out that both images are always more useful
and have more information content when used together. In the
thermal IR, the sensor is essentially viewing a field of "glow
ing" objects and, since normal visual images are not analogous,
orientation is extremely difficult using just the IR imagery.
By comparing the visible with the IR, considerable additional
information is gained. In the example for June 4 (Figure 1) the
stippled white area (0 to +1°C) in the enhanced IR image clearly
shows the area of melting snow lying in a band across the entire
region with still frozen snow to the north and snowfree condi-
tions to the south. A unique situation exists here also, since
the snowfree areas are generally at a higher elevation than most
of the remaining snowcover. Snowmelt on Alaska's North Slope
initiates in the middle elevations (600-900 m) and proceeds in
both directions, with the highest peaks and the lowest coastal
areas the last to melt. When comparing this "band" of ripe and
melting snow with the visible image, it is evident that it corre-
sponds closely to the edge of the snowcover. A limitation of the
IR imagery is also shown because the presence of low stratus
cloud cover on the coastal plain obscures the snow cover in the
IR. The visible imagery shows that a large, open, sea-ice lead
is present off the northwest coast and the IR imagery shows that
145
.w^UbW o ^ ^^*6
b)
JUNE 4,1975, ORBIT No2519, NOAA-4-VHRR
a) Vis Enlargement
b) Enhanced I.R. Enlargement
Figure 1
146
ORIGINAL PAGE IS POOR
it is isothermal with the stratus cloud cover. Water can be seen
flowing out over the shorefast sea ice (ice frozen to the shore)
at the mouths of the Colville, Sagavanirktok, and Kuparuk Rivers.
This is also a unique feature of Arctic rivers. It is due to the
pattern of snowmelt and the presence of shorefast ice. Because
the coastal plain and shorefast ice are still frozen and snow-
covered when runoff begins to flow down the river channels, some
of the water simply continues out over the channel ice and shore-
fast ice at the mouths, flooding large areas of the sea ice.
Fortunately, there were some extreme weather conditions this
spring which permitted tests of the response of the enhanced
imagery. On May 10 and 11, 1975, record high temperatures of
27°C (80°F) and 26°C (79°F) respectively, were recorded at the
Fairbanks WSO and record high temperatures were experienced
generally throughout the Tanana and Yukon Valleys. This was
also the mid-point of the snowmelt period and consequently
snowmelt was rapidly accelerated. Figure 2 shows a series of
images, the visible and IR images from May 10, and the same
images for May 13. A comparison of the enhanced IR images of
both days shows some striking changes and discrepancies. In
fact , the first impression is that the snow cover has increased,
even though no precipitation fell during the four-day period.
The weather on May 13 was generally cooler (10 - 15°C) and
moderately strong winds (16 - 20 km/hr) were recorded at most of
the interior Alaska weather stations. Interpretation is further
confused in that comparisons of both days' visible images with
the enhanced IR images shows discrepancies both days between
indicated snow cover in each type of imagery.
The worst 1975 case of discrepancy of the imagery is pre-
sented here in order to show the dependence of interpretation on
surface weather conditions. Under conditions of very high sur-
face temperatures the imagery will indicate greater snowmelt than
is actually present. This is probably the result of advection
of warm air over the melting snow areas and the integrated ther-
mal effect of warm air and emergent vegetation warmed by the air
and sun over the remaining snow cover, causing it to appear
"thermally snowfree." The opposite effect occurs on windy,
lightly cloudy days, as air movement obliterates any surface
heating and low light cloud cover lowers the radiative tempera-
ture. There may be other, subtler effects as well, but a
theoretical analysis will not be attempted in this paper. The
important point is that interpretation of the enhanced IR imagery
must be tempered with the synoptic context of surface weather
conditions and use of visible, standard IR, and enhanced IR
imagery. This will enable an operational river forecaster to
give his forecast with more confidence in lieu of the limitations
inherent in the enhanced IR imagery.
147
2l:07M6 220€ ' izri 10MHY75 M4 13S DIE !; 21:07:10 !8 10TWY75 N4 13S DIE
0) 10 MAY VIS 10 MAY I R
i3: 20: 0 3 : 0 2 4 7 V2F MnYTS H4 12S
b) 13 MAY VIS
A COMPARISON OF THERMAL I R ENHANCEMENTS UNDER DIFFERENT SURFACE CONDITIONS :
a) May 10,1975, Record High Temperatures Which Induced Flood Warnings
b) May 13,1975, Strong Winds
Figure 2 - These examples show the possibility for errors in
interpreting enhanced IR imagery if it is not used with the
visible and standard IR images. Measurements of the snowfree
areas centered around the Yukon River on the two days shown
indicated 27% less snowfree area on 13 May than had been mea-
sured three days earlier.
148
REPRODUCIBILITY OF THE
ORIGINAL PAGE IS POOR
GROUND TRUTH TEST
A test of the imagery was made with ground truth water tem-
perature measurements made in the Yukon River at various villages
in the interior of Alaska. These measurements are taken by
village residents with U.S.W.S. equipment. The comparisons are
tabulated in Table 1.
TABLE 1
COMPARISONS OF SATELLITE VHRR-IR MEASUREMENTS
WITH YUKON RIVER WATER TEMPERATURE MEASUREMENTS
Location of River
Temp. Meas.
Beaver, Alaska
Eagle, Alaska
Circle, Alaska
Beaver, Alaska
Beaver, Alaska
Date
14 May
16 May
16 May
16 May
19 May
Thermal IR "Temp."
Measured From
Enhanced VHRR Imagery
0-1°C
>2°C
0-1 °C
0-1°C
0-2°C
(Stippled & Black)
*Measured
River Temp.
1.7°C
2.2°C
1.7°C
1.1°C
1.7°C
*Courtesy of U.S.W.S. Regional Hydrologists Office.
The temperatures compare closely with a maximum difference
of 0.7°C. This confirms the stated accuracy of NOAA thermal IR
imagery of ±1°C (Barnes, et al., 1974b). When temperatures rose
above 2°C (36°F), the calibrated IR could no longer differentiate
temperatures because the display scheme was designed to show all
temperatures above 2°C as black.
ADDITIONAL APPLICATIONS
Another opportunity to provide snowcover information in a
synoptic form is available through the technique of photographic
combination of the visible and enhanced IR images. By over-
laying one image negative on top of the other and aligning them,
a print can be made of an entire frame of the imagery which
enhances snow cover (Figures 3, 4, and 5). Since an IR enhance*-
ment is used, areas which are "thermally snowfree" will be dis-
played as black, similar to the visible imagery. Terrain shad-
owing effects can be somewhat eliminated by this technique because
the IR temperatures will cause them to be displayed as white, i.e.
snowcovered, when they might be displayed dark in the visible
imagery. Because this process uses negatives, snow cover is
always retained and enhanced in the image, because its image
density in the enhanced IR will be greater than, for instance,
the density of a corresponding shadowed area. This also means
149
I
•I
GIL 134:20:57:12 2256 V2F2276 14MAY75 N4 13S 04E
MAY 14, 1975
Figure 3 - An exceptionally clear day during the snow melt
period, and nearly the entire state is visible. The negative
of this standard visible band image was overlain on the nega-
tive of the enhanced IR image (Figure A) to produce the print
shown in Figure 5.
150
GIL 134:20:57:06 2256 I2F2238 14MAY75 N4 13S 04E
MAY 14, 1975
Figure 4 - This enhanced IR image was used in producing the
overlay print shown in Figure 5. Note the stippled white areas
indicating melting or ripe snow. Nearly the entire Brooks
Range is in this condition. The Yukon River is also visible
because it is at or near the 0-1°C temperature range. A water
temperature measurement at the village of Beaver was made thjs
same day, and the measured temperature was 1.7°C.
151
TO
)6 2
MAY 14,1975, ORBIT No. 2256
AN OVERLAY PRINT USING BOTH THE VISIBLE
IMAGE NEGATIVE & THE ENHANCED I.R. NEGATIVE
Figure 5 - This overlay technique enhances marginally snow
covered areas, and may eliminate some problems of shadowing
that often effects visible imagery.
Vtf
152
that marginally snowcovered (patchy) areas are enhanced, because
they are more likely to be at the freezing temperature and thus
displayed as white. Some shadowed areas may be displayed as dark
in the enhanced IR as well because they are just below the
freezing temperature. This problem could be remedied by
expanding the enhanced IR display scheme to display a wider
temperature range in white. This was not attempted in Alaska,
however, because it would eliminate detection of "ripe" snow,
which, in Alaska, is a more important factor. In snow surveys
where detection of melting snow is not critical, this technique
could prove very useful.
The overlay technique is not operational in real time be-
cause the images can only be produced photographically at present.
This is not a limitation for water resource inventories however,
and the possibility exists for daily or weekly snow cover over-
lay images which could be made available a day or two after
initial acquisition.
Snowmelt runoff is responsible for many floods in Alaska
and, during the river break-up, ice jams can cause rapid and
unpredictable rises in water levels both when they form and again
when they break. Two floods caused by ice jams were observed in
the standard IR imagery and in the enhanced IR during the river
breakup. Both were on the Yukon River and both flooded areas
were greater than 2500 km . The first flood occurred near Holy
Cross, Alaska, a Yukon River village, on May 24, 1975. It is
depicted best on the regular IR imagery, and an enlargement
showing the flooded area is shown in Figure 6. The flooded area
was measured with a polar compensating planimeter and found to be
2590 km2 ±5%. Just as a comparison, this is more than 5 times
the surface area of Lake Tahoe. NOAA imagery provided the only
areal measurement available for this flood.
A second flood caused by ice jams occurred May 29, 1975, in
the Yukon Delta. The flooded area was also measured with a
planimeter and found to be 2710 km^ ±5%. A comparison of the
mapping accuracy is pending in lieu of receipt of a LANDSAT image
made of this area on the same date.
CONCLUDING REMARKS
NOAA VHRR imagery has demonstrated its utility for daily
synoptic, near-real time snowmelt information. By using all the
available VHRR imagery (visible, IR, and enhanced IR), reliable
and timely supplemental river forecast information is available
by 3 p.m. each day. Ground truth comparisons are very encouraging
and more will be sought. A NOAA satellite field service station
is to become operational in November of this year and tests and
developments will continue as new data needs and uses evolve.
153
Figure 6 - An enlargement of a standard IR image demonstra-
ting the ability of the VHRR to detect flooded areas. This flood
was measured using a polar compensating planimeter and found to
be 2590 km2 in area. This is more than five times the surface
area of Lake Tahoe (499 km2).
ACKNOWLEDGEMENT
This work was accomplished under the Alaska Pilot Project
grant from the National Oceanic and Atmospheric Administration
(NOAA), National Environmental Satellite Service (NESS).
REFERENCES
Barnes, J. C.; Bowley, C. L.; and Simmes, D. A. (1974). The
application of ERTS Imagery to Mapping Snow Cover in the
Western United States. NASA ERT Document 9407-F, NASA GSFC,
Greenbelt, Md., January.
Barnes, J. C.; Bowley, C. J. ; Cogan, J. L. (1974). Snow Mapping
Applications of Thermal Infrared Data. NOAA VHRR, NASA-ERT
Doc. No. 0438-F, NASA-GSFC, Greenbelt, Md. , November.
Holmgren, B.; Benson, C.; and Weller, G. (1973). A Study of the
Breakup on the Arctic Slope of Alaska by Ground, Air, and
Satellite Observations. IN: Climate of the Arctic,
Proceedings of the 24th Alaska Science Conference,
Fairbanks, Alaska, Aug. 15-17, pp. 358-366.
McGinnis, D. F.; Pritchard, J. A.; andWiesnet, D. R. (1975).
Snow Depth and Snow Extent Using VHRR Data from the NOAA-2
Satellite. NOAA Tech. Memo NESS 63, Dept. of Commerce,
NOAA-NESS, Wash., D.C. , February.
Range, A. (1975). Applications of Remote Sensing to Watershed
Management. NASA-GSFC, Technical Info. Div. Code 250 GSFC,
Greenbelt, Md. 20771.
Wiesnet, D. R. (1974). The Role of Satellites in Snow and Ice
Measurements. NOAA Tech. Memo 58, Dept. of Commerce,
NOAA-NESS, Wash., B.C., August.
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Operational applications of NOAA-VHRR imagery in Alaska

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