A multispectral technique has been developed which independently tests for the presence of clouds before a registered window radiance measurement is accepted as coming from the sea surface and the intervening atmosphere. The spatial resolution of ocean temperature mapping can be the same as that of the radiometer. With the 55 km subsatellite track resolution of the Nimbus 2 MRIR, current boundaries and upwelling areas have been successfully identified. Knowledge of the position of these regions and temperatures within them are important to the detection of areas of high chlorophyll concentrations

Salomonson, V. V.

Shenk, W. E.

English

NASA Technical Reports Server

10-1
SECTION 10
A MULTISPECTRAL METHOD OF MEASURING
SEA SURFACE TEMPERATURES FROM SATELLITES
by
William E. Shenk
and
Vincent V. Salomonson
Laboratory for Meteorology
and Earth Sciences
Goddard Space Flight Center
Greenbelt, Md. 20771
INTRODUCTION
High chlorophyll concentrations are associated with areas of the
oceans where nutrients are plentiful. Nutrients are supplied from the
ocean bottom in regions of upwelling characterized by relatively low
temperatures. Algae growth is aided in coastal waters where relatively
high temperatures are found. Therefore, measurements of sea surface
temperatures are important in the detection of the areas of upwelling
and warm coastal waters. As a result of satellite measurements it has
been possible to study the temporal sea surface temperature fluctuations
in areas where, before the satellite observations, changes were
virtually unknown. A good example is the study of the strong upwelling
off the Somali Coast. Upwelling along the northeast African coast
begins in connection with the development of the southwest monsoon in
May. From May to July the temperature gradient increases and the rela-
tively cold temperatures, which first appear near the coast, move off-
shore to form an anticyclonic gyre that has a characteristic diameter
of about 600 km. Figure 1 shows a view of the gyre on July 3, 1966
where the equivalent blackbody temperature pattern (uncorrected for
atmospheric effects) was obtained from the 3.5-4.1 um measurements of
the Nimbus 2 High Resolution Infrared Radiometer (HRIR). The gyre is
fully developed with a temperature gradient of 50K and the width of the
cold temperature tongue is about 100 km. The lower portion of Figure 1
depicts chlorophyll concentrations measured during a similar event by
an Indian Ocean expedition vessel. Highest concentrations are seen in
the meridional cross section near the coast and in the offshore area of
the gyre whereas the minimum concentrations were found in the relatively
warm waters in between.
There are two problems associated with remotely sensing the sea
https://ntrs.nasa.gov/search.jsp?R=19720021661 2020-03-23T09:10:07+00:00Z
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surface temperature with infrared measurements that dominate the total
error budget. There are the effects of the atmosphere on the emission
in an atmospheric window and clouds. The greatest potential error is
obscuration of the sea surface by clouds since the emission from the
clouds can come from a wide temperature range that is associated with
the cloud top levels. Thus, it is imperative that a remote sensing
technique for ocean temperature measurement contain a means for deter-
mining that the emitted energy is coming from a cloud free source. A
multispectral technique has been developed (Shenk and Salomonson, 1972)
using the measurements from the Nimbus 2 Medium Resolution Infrared
Radiometer (MRIR) that tests for the presence of clouds before accepting
the concurrent window infrared emission from what was assumed to be sea
surface and the intervening cloud free atmosphere.
METHOD
The Nimbus 2 MRIR was a scanning radiometer operating in 5 spectral
regions. All observations in these five channels were geographically
registered at the same point and had an instantaneous field-of-view at
the subsatellite point of 55 km. Three of these spectral regions were
used in this study; namely, the 0.2-4.0 pm region where the observed
reflectivity of clouds is generally quite high relative to the ocean
surface, the 6.4-6.9 Um region that responds to upper tropospheric water
vapor and clouds thereby offering a descriminator for the presence of
cirrus, and the 10-11 pm window region permitting the observation of sea
surface temperature in cloud free areas. This approach consists of esta-
blishing thresholds for the 0.2-4.0 pm reflectance channel and the
6.4-6.9 pm water vapor channel which indicate when a particular con-
current observation in the 10-11 pm channel is cloud free. In this
study several threshold combinations in the reflectance and water vapor
channels were selected based on observed frequency distributions of the
MRIR observations taken from four relatively cloud free days during a
one-month period from mid-June to mid-July, 1966 over an area encompassed
by the latitudes from 30-500 N and longitudes stretching from 30'W to a
line that was parallel to and no closer than 50 nautical miles (more
than 1 MRIR resolution element diameter) from the United States coast-
line. The four relatively cloud free days were selected over this region
by looking at the pictures taken by the Advanced Vidicon Camera System
(AVCS) on the Nimbus 2 satellite which had a nominal spatial resolution
of 0.9 km.
Figure 2 shows the frequency distribution of the observations taken
for the four days in the reflectance region. Normalization of the 0.2-
4.0 pm reflected energy for solar zenith angle was accomplished with the
expression:
10-3
wN
r
H*cos 6
where Y is the normalized reflectance, N is the effective radiance,
T* is the effective solar constant and 6 is the solar zenith angle.
The strong peak at low reflectances corresponds to the normalized
reflectances occurring over the ocean surface. Larger reflectances
are associated with clouds and cloud/clear sky combinations. The cloud-
no cloud threshold was positioned at a point displaced a distance Ar
on the high reflectance side of the frequency peak resulting from
observations from what was most likely the cloud-free ocean surface.
The magnitude of Ar was set equal to the difference between the lowest
observed reflectance and the reflectance corresponding to the above-
mentioned frequency peak. In Figure 1 there were no i values less than
3. Thus, with the frequency peak at r = 6, Ar = 3 and the reflectance
cutoff was placed at r = 9. The Ar on the low reflectance side of the
peak was assumed to be produced by random instrument noise and other
minor factors such as sea surface glitter.
The layer emitting most of the radiation from which the sensed
radiance emanated in the 6.4-6.9 pm spectral band is from 7-11 km for a
U.S. Standard Atmosphere with 2.0 cm of precipitable water. This is
within or slightly below the level where cirrus clouds are generally
located. When this layer was relatively dry the radiation was emitted
from a slightly lower layer and thus, in general, from a region of
warmer temperatures. With a dry upper troposphere the likelihood of
the presence of cirrus clouds was considered to be small.
A frequency distribution approach was also followed in setting the
thresholds for the water vapor channel equivalent blackbody temperature
observations. Two separate criteria were used to establish these
thresholds. One criterion consisted of setting the threshold at the
la location on the warm side of the mean of the water vapor channel
observations that were taken over specified latitude zones (Figure 3).
The second criterion was less restrictive and the threshold was
established at the 70% point of the cumulative frequency distribution
of equivalent blackbody temperatures (where the cumulative frequency
curve started at the lowest temperatures).
RESULTS AND DISCUSSION
Using the threshold selection techniques described above, Nimbus 2
window channel observations, which were objectively selected as being
cloud free, were collected over the month period for the area in the
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Atlantic already described. Using different threshold combinations and
ranges in radiometer observation nadir angle (n), 4 different sets of
these measurements were assembled and composited on 1:5 million Mercator
grid print maps (Nimbus Project, 19662) with a grid point spacing of
0.6250 of longitude.
For comparison purposes with the window radiation measurements all
available ship observations of sea surface temperature were obtained for
the time period already specified and from 31-43°N and 580 W westward to
the U.S. coast. These ship observations (about 4500) were composited
on the same map projection and scale as that employed for the radio-
metric observations. The differences between the ship observed tempera-
tures and the window channel temperatures at a grid point were computed
wherever 4 or more radiometric and ship observations occurred. The
results were tabulated in histograms for each of four equally-spaced
latitude bands from 31-430 N. Figure 4 shows the results for the set
with the most relaxed thresholds where the 70% cumulative frequency
threshold for the water vapor channel was used and the nadir angle
limits were 0-50° . The differences between the ship observations and
the observed window equivalent blackbody temperatures (AT) are caused
by atmospheric effects. These effects were greatest in the 31-34°N
region where the frequency peak occurs at AT = 8K whereas AT = 4-5K
for the frequency peak at 400 N-430 N. The dispersion in the results
within a latitude band occur because of variability in atmospheric
attenuation effects, n, some nonconcurrence in time of the ship and
radiometric observations, possible clouds, and random errors in the
ship reports. The best comparisons were achieved between 31-340 N where
a total dispersion of 2.OK about the mean difference between the ship
and radiometer equivalent blackbody temperatures (AT) occurred. Larger
dispersions further north were believed to be mostly attributable to
greater ocean temperature fluctuations occurring during the month
period. Despite the relaxed water vapor channel threshold and the
wide nadir angle limits the results were similar to those for the other
sets with the more stringent thresholds.
Another important result was an experiment to examine the possibili-
ty of using just one of the two cloud filtering channels. The conclu-
sion was that the two channel cloud filter was substantially superior
to the use of either of the channels alone.
The magnitude of the atmospheric correction to the sensed window
radiance depends on the spectral interval, the vertical temperature
profile, particulate matter, n, and the vertical distribution of the
absorbing constituents. For the Nimbus 2 MRIR window channel the
principal absorbing constituents are ozone and water vapor. Therefore,
the smaller differences between observed ship temperatures and the
window equivalent blackbody temperatures at the higher latitudes was
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probably caused by smaller atmospheric water vapor content and colder
vertical temperature profiles. Any difference in the particulate
concentration as a function of latitude was uncertain.
In order to correct the radiometric observations for the atmospheric
effects, a stepwise multiple regression approach was employed. This
permitted the evaluation of a simple linear correlation which fitted
each grid print window channel radiometric observation average to each
grid print ship observation average. In addition, the multiple correla-
tion approach was used to assess the utility of including not only the
two parameters just mentioned, but also the observations in the reflec-
tance and water vapor channels, and the nadir angle at which the meas-
urements were made. The multiple regression approach permitted the
specification of the ship sea surface temperatures to an r.m.s. error
of approximately 1.5K for all latitudes. When the results were strati-
fied into two equal latitude bands, the temperatures in the lower lati-
tudes could be determined to approximately 1K.
The two regression equations were then used to process the radio-
metric information and arrive at a map of sea surface temperature. This
map is shown in Figure 5. The corrected sea surface temperature map
shows the basic ocean features that would be expected in the June-July
period over the western North Atlantic. A short distance east of the
United States coast is the warm axis of the Gulf Stream with sea surface
temperatures of about 300K. The strong thermal gradient of about 10K
occurs in connection with the north wall of the Gulf Stream just north
of 400 N. This gradient is not as steep as it normally is on a given day
due to the one month data collection period and the spatial resolution
of the MRIR.
Other investigations have successfully employed this technique over
different oceanic regions. Figure 6 shows the temperature gradients for
a consecutive 3-week period in June-July, 1966 over the southern Indian
Ocean, from 100 to 45°S (Shenk and Szekielda3). The 10-11 um equivalent
blackbody temperatures have not been corrected for atmospheric effects.
The radiation measurements detected the Agulhas Current flowing south-
westward along the African Coast to 38°S. An area of upwelling was
located just off the south coast of Africa and north of the warm axis
of the Agulhas Current.
The oceanic polar front is pronounced between 40-45°S which is asso-
ciated with a convergence of the surface currents. Thus, the front
separates the cold polar surface water from the warmer water masses of
the lower latitudes. The center of this front lies in the Indian Ocean
in the region where the water temperatures are, according to the
uncorrected satellite data, in the range of 281-283°K. This cold
water is found in the tropics at a depth of 300-400 m and in the
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subtropics between 500-1000 m which originates at the surface at about
40°S.
The analysis of remotely sensed radiation data from the satellite
gave a temperature gradient at 520 E and 430S of 0.055C km -1. This is a
sharper horizontal gradient than that associated with the upwelling
region along the Somali Coast and more pronounced than in the nautical
charts for this area. Considering the ground resolution of the radio-
meter and the time period of approximately three weeks during which the
radiation data were sampled, it is concluded that the temperature
gradient should be still greater if synoptic measurements over shorter
periods are made and that the polar front had a very stable position.
This gradient is comparable with similar temperature gradients in the
Gulf Stream. The Gulf Stream temperature boundary oscillates so much
that a temperature analysis over a few weeks substantially smoothes the
original sharp temperature gradients.
Szekielda4 has discussed in more detail upwelling along the south
African west and south coasts. Figure 7 shows his analysis from mid-
May to mid-June, 1966. The prominent warm axis of the Agulhas Current
is seen to extend south of 40°S with a small area of upwelling (285°K
uncorrected isotherm) appears just off the south African Coast. A
stronger region of upwelling produced by southerly winds is found asso-
ciated with the Benguela Current along the west coast between 300 S and
the Cape of Good Hope.
CONCLUSIONS
A multispectral technique has been developed which independently
tests for the presence of clouds before a registered window radiance
measurement is accepted as coming from the sea surface and the inter-
vening atmosphere. The spatial resolution of ocean temperature mapping
can be the same as that of the radiometer. With the 55 km subsatellite
track resolution of the Nimbus 2 MRIR, current boundaries and upwelling
areas have been successfully identified. Knowledge of the position of
these regions and temperatures within them are important to the detec-
tion of areas of high chlorophyll concentrations.
These three registered channels will hopefully be flown on radio-
meters that are proposed for space flight on the TIROS N, ATS-G, and
EOS missions. The spatial resolutions will vary from 0.9 km (TIROS-N)
to 11 km (ATS-G) for the 11 pm channel. In the interim it may be
possible to implement this technique where the appropriate channels are
flown on the same satellite but are not part of the same instrument
(Nimbus E).
10-7
REFERENCES
1. Shenk, W. E. and V. V. Salomonson, 1972: "A Multispectral
Technique to Determine Sea Surface Temperature Using Nimbus 2 Data",
J. of Phys. Ocean. (to be published in April, 1972).
2. Nimbus Project, 1966: "Nimbus 2 Usersl Guide", Goddard Space
Flight Center, Greenbelt, Maryland, 229p.
3. Shenk, W. E. and K-H. Szekielda, 1971: "Satellite Ocean
Temperature Analysis of the Indian Ocean", Paper presented at the
Symposinum on Indian Ocean and Adjacent Seas, Cochin, India, Jan.
12 - 18, 1971.
4. Szekielda, K-H., 1971: "Upwelling Studies with Satellites", NASA
X-651-71-298, 17p.
10-8
W4E 50E 52 5E 50°E °E °E
3.0
- 2.0 
E 52E 5E 
50E 521 54 ' W-
Figure 1 - Top - Nimbus 2 High Resolution Infrared Radiometer Equiva-
lent Blackbody Temperatures (uncorrected for atmospheric effects) of
the fully developed upwelling gyre off the Somali Coast on July 3,
1966 (after Szekielda). Isotherms are in °C and equivalent black-
body temperatures <25'C within the gyre are shaded.
Bottom - An east-west cross-section of chlorophyll concentrations
recorded by an Indian Ocean Expedition vessel during a similar event.
10-9
FREQUENCY
z
0
9-
N
m
0
M
Rr"m
A
-4
z
rlAK,
Figure 2 - Frequency distribution of normalized reflectance measure-
ments from the 0.2-4.0 um channel of the Nimbus 2 Medium Resolution
Infrared Radiometer for 4 relatively clear days over the western
North Atlantic between mid-June and mid-July, 1966.
10-10
200
180 _ 35- 40°N
160 -
140 -
120-
01
60
40 -
20i ~MiEAN
0
210 215 220 225 230 235 240 245 250 255
6.4 - 6.9km EQUIVALENT BLACKBODY TEMPERATURE
Figure 3 - Frequency distribution of 6.7 pm equivalent blackbody
temperatures (TBB's) for the same 4 days as in Figure 2. The mean
and 1 a to the warm side of the mean are indicated where the TBBvS
on the warm side of the 1 a point were assumed to be associated with
a cloud free upper troposphere.
10-11
40' - 43'N 37' - 40'N
40 40
35 -
- 35
30 - - 30
25 25
f 20 f 20
15 15
10 I - 10
0 . 1 0 -l -2 -3 -4 -5 -6 -7 -8 -9 -10-11 +1 0 -1 -2 -3 -4 -5-6 -7 -8 -9-10-11
AT AT
34F - 37'N 31' - 34°N
40 40
35... 35
30 -
- 30
25. . . 25
f 20.- . f 20
15- .- 15
10 - I 10 i
0 0
+1 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 10-11 1 0 -I -2 -3 -4 -5 -6 -7 -8 -9-10-11
AT AT
Figure 4 - Frequency distributions for 4 equally spaced latitude bands
over the western North Atlantic of the difference between ship sea
surface temperatures and 10-11 Um window equivalent blackbody
temperatures associated with assumed cloud free conditions.
10-12
Figure 5 - Satellite generated map of sea surface temperature (mid-June to mid-July, 1966) over the western North Atlantic where theassumed cloud free 10-11 pm equivalent blackbody temperatures havebeen corrected for the effects of the atmosphere. Isotherms are in
°K with the warmest areas (>298°K) stippled.
10-13
I 10 O0 Z" o. InC I I
Figure 6 - June, 1966, southern Indian Ocean map of assumed
10-11 pm equivalent blackbody temperatures (°K) that are
for atmospheric effects.
cloud free
uncorrected
_
I
- U
I 
- 0
In
I
To
en W W
0 0 o
oA W W N)
O n O 0 
Figure 7 - Mid-May to mid-June, 1966 chart of assumed cloud free
10-11 um equivalent blackbody temperatures (°K), uncorrected for the
atmosphere, over portions of the southwestern Indian Ocean and south-
eastern Atlantic Ocean.
10-14


A multispectral method of measuring sea surface temperatures from satellites

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