great lakes thermal studies using infrared imagery

Great Lakes Thermal Studies Using Infrared ImageryAuthor(s): Robert K. LaneSource: Limnology and Oceanography, Vol. 15, No. 2 (Mar., 1970), pp. 296-300Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2833874 .

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296 NOTES AND COMMENT

bus data of 7 September and 6 October may be the precursor of the temperature gradient structure defined by the thermal mapping flight of 18 October 1966.

DISCUSSION

The persistence of the thermal structure along 430 57.2' N lat for 48 hr between 18 and 20 October 1966 (Fig. 2) indicates sufficient equilibrium for computation of a geostrophic current by the dynamic height method (approx 35 hr pendulum day). Therefore, in spite of a lack of direct mea- surement, the existence of this current and its relation to the surface temperature structure can be inferred.

The evidence of a precursor thermal pat- term as derived from the radiation data of Nimbus II is somewhat weak because of the obvious limitations of the data, but the scale and persistence of the thermal feature would seem to imply a process more sig- nificant than simple, irregularly occurring, wind-generated upwelling.

Finally, in spite of the differences between the observations of 1966 and 1967, the scales of the thermal features and the magnitudes of the computed geostrophic currents seem to suggest a similarity of causal forces. We would like, therefore, to suggest that the possibility of the seasonal recurrence of this temperature and circulation feature in Lake Michigan is suffi- ciently strong to warrant reexamination of this phenomenon. If additional experiments should document the seasonal recurrence of

this feature, we suggest that it be named the Coyote Current (Noble 1967b) after El Coyote, the research aircraft that carried out the flights.

VINCENT E. NOBLE2

Great Lakes Research Division, University of Michigan, Ann Arbor.

JOHN C. WILKFRSON Airborne Remote Sensing

Oceanography Project, U.S. Naval Oceanographic Office, Washington, D.C. 20390.

REFEBENCES

AYERs, J. C. 1956. A dynamic height method for the determination of currents in deep lakes. Limnol. Oceanog. 1: 150-161.

, AND R. BACHMANN. 1957. Simplified computations for the dynamic height method of current determination in lakes. Limnol. Oceanog. 2: 155-157.

NOBLE, V. E. 1967a. Temperature structure of Lake Michigan. Mich. Univ., Great Lakes Res. Div. Spec. Rep. 30, p. 340-365.

1967b. Evidences of geostrophically defined circulation in Lake Michigan. Proc. Annu. Conf. Great Lakes Res. 1967: 289-298.

STRONG, A. E. 1967. The overwater develop- ment of a lake breeze in an offshore gradient flow. Proc. Annu. Conf. Great Lakes Res. 1967: 240-250.

WILuiRSON, J. C., AND J. F. ROPEK. 1967. An application of Naval oceanographic develop- ment to inland waters. Proc. Annu. Conf. Great Lakes Res. 1967: 456-460.

2 Present address: Airborne Remote Sensing Oceanography Project, U.S. Naval Oceanographic Office, Washington, D.C. 20390.

GREAT LAKES THERMAL STUDIES USING INFRARED IMAGERY

ABSTRACT

Examples of mosaics of infrared imagery of the western end of Lake Ontario are used to demonstrate their value in revealing de- tailed surface thermal patterns. Additional data from airborne thermometry and ship- board measurements confirm the interpretation of large-scale dynamic processes. Smaller scale phenomena, such as internal wave patterns and small eddies, are also interpretable.

During spring to autumn 1968, seven airbome surveys of western Lake Ontario were made with an infrared scanner. The main instrument was a modified Singer Reconofax IV scanner, flown and main- tained by the National Aeronautics Estab- lishment, National Research Council, Ot- tawa. Other instruments included a Barnes PRT-5 infrared thermometer, upward- and



298 NOTES AND COMMENTr

| *l =E .!Ae

FIG2Msi of teml.iagr fo wetr Lak Onaio15 Auus 196

water, associated with small cyclonic vor- tices, appear to extend out well into the central part of the lake; and d) directly above the "o" in Burlington, in the warm

10. S

FIG. 3. Radiation thermometer traces taken on 15 August 1968 along three lines shown in Fig. 2.

nearshore zone, the occurrence of parallel bands suggests the presence of internal waves.

The vast detail apparent in the imagery is useful in a variety of analyses; for example, the surface configurations of effluents from rivers are of interest in pollution studies. In Fig. 1, the water from the Ni- agara River is cooler than the nearshore lake water in that vicinity and so is not traceable except close to shore.

The second example (Fig. 2), taken 15 August at 1,829 m, shows the effectiveness of thermal imagery in delineating river effluents warmer than the ambient lake water. The large fanlike plume from the Niagara River and the two smaller effluents from sources westward of it are typ- ical examples.



NOTES AND COMMENT 297

Thermalz r .

T O

;ThermalI Bar WMI

BURLINGTO

FIG. 1. Mosaic of thermal imagery for western Lake Ontario, 23 May 1968. White bands mask classified data in all mosaics presented here.

downward-viewing solarimeters, and aerial cameras.

The sensor of the infrared scanner re- peatedly scans a path normal to flight across a 1200 field of view, detecting elec- tromagnetic radiation in the 8-14-pu band (McColl, in prep.). Spatial resolution is 16 mradians and thermal resolution is esti- mated at 0.01C. The data are from north- south parallel flightpaths in the region from the west end of Lake Ontario to just east of Toronto. Warmer water is lighter in tone than cooler water. However, tones in one part of the imagery are not comparable with those in another; the average signal strength is a constant for each scan, and consequently, in the nearshore regions, where the scanner views both land and water, the high thermal contrast eliminates all detail in the water region.

This account deals with only three of the surveys to demonstrate applications of remote sensing in Great Lakes research.

The imagery from the first survey, made at 1,510 m on 23 May is shown in Fig. 1. The lake is undergoing spring overturn and exhibits the "thermal bar" feature charac- teristic of Lake Ontario (Rodgers 1965). This thermal gradient, which generally separates warm, stratified nearshore water from the cooler water (less than 4C) in the central part of the lake, can be seen along the north shore. The value of the imagery in characterization of this important surface feature is evident: a) the wavelike character near Toronto suggests horizontal shear effects; b) parallel thermal "fronts" inshore of the thermal bar are stronger in thermal gradient than the bar (confirmed by surface data); c) narrow bands of warm



NOTES AND COMMENT 299

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.-.. ~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ......

, ~~~~~~~~,

FURLIm

FIG. 4. Mosaic of thermal imagery for western Lake Ontario, 8 October 196f9.

On the north side of the lake, the bands of cool water parallel to shore are regions of upwelling that occurred following 6 days of westerly and northwesterly winds. (The winds on the day of the flight were light and variable.)

The type of scanner used in this study does not produce quantitative thermal data. However, simultaneous surface temperature data from the infrared thermometer are shown -in Fig. 3 for lines ABCD, EFGH, and IJKL in Fig. 2. These data are presented in uncorrected form. Although their absolute accuracy may only be ?1.OC, their relative accuracy is assumed to be +0.2C. The cool, upwelled water, the central water of relatively uniform temper-

ature, and the warm Niagara River water are clearly shown.

The third example, taken 8 October at 2,653 m, is shown in Fig. 4. During this season, the thermocline is still quite strong. However, on this occasion, a period of strong westerly winds stimulated an exten- sive upwelling throughout the northwest quarter of the lake. The imagery reveals that the warmer water along the south shore (about 16C) was separated in a rather complex fashion from the cooler, upwelled water (8C near the separation and 4C near the north shore). The warm water is characteristically featureless, while the cooler surface water in the relatively unstratified region contains many complex



300 NOTES AND COMMENT

15.0|

(LINE 2, 5 OCT) 14.5 NIAGARA RlVER

X ~~~ ~ ~~~~~~~~~~12.0 X 0 0 0~0 0 13.0

00 4 4 ~ ' .14-0 0 0 0

TOROTO 0(LINE 1,5 OCT) \12.0

10.0 (

9<Z .0 ..

8.5\

FIG. 5. Surface temperature (western Lake Ontario) taken by CSS Limnos, 5 and 9 October 1968 (dashed line represents upwelling boundary).

patterns, including cyclonic gyrelike features. Temperatures obtained by towing a thermistor at 1-m depth along lines 1, 2, and 3 from the research vessel CSS Limnos are given in Fig. 5.

The interpretation of scanner data re- quires recognition of several technological and environmental factors. Scanners are now being produced or developed that provide a more quantitative result and that are not as severely affected by the dynamic range limitations of the gray scale in the film imagery. The problem of dealing with the attenuation effects of the atmosphere is more difficult, particularly since the scanner views through a path length that changes by a factor of 2 as it sweeps from nadir to 600. The use of at- mospheric profile data, the air-water temperature difference, and a narrower band detector offers the possibility for more quantitative data.

Although airborne infrared-sensing tech-

niques produce data on only the very surface of water bodies, they can collect nearly synoptic information. Other considerations in assessing the value of infrared-scanning programs are:

1. Infrared thermometers, used simulta- neously, can provide complementary information, very important for evaluating relative gradient values.

2. Simultaneous aerial photography, particularly if multispectral techniques are applied, can be used to help determine the character of processes that are revealed in the thermal imagery (e.g., many of the important surface thermal features in Lake Ontario, such as river effluents and diver- gence/convergence patterns, are also revealed by turbidity patterns).

3. Although considerable work must be done in studies of the "noise" effects of diurnal heating and cooling, it is clear that many important large-scale dynamic and thermodynamic processes in lakes can be interpreted from infrared imagery.

4. Infrared imagery clearly provides a means for observing small-scale features in great detail, something that the usual surface techniques cannot do.

Previous work of this type on the Great Lakes by the University of Michigan has been reported by Beeton, Moffett, and Parker (1969).

ROBERT K. LANE Department of Energy,

Mines and Resources, Great Lakes Division, Canada Centre for Inland Waters, Burlington, Ontario.

REFERENCES

B-EETON, A. M., J. W. MOFFETT, AND D. C. PARKER. 1969. Comparison of thermal data from airborne and surface vessels of Lake Erie. Proc. Annu. Conf. Great Lakes Res. 1969: in press.

RoDGERs, G. K. 1965. The thermal bar in the Laurentian Great Lakes. Mich. Univ., Great Lakes Res. Div. Publ. 13, p. 358-363.



great lakes thermal studies using infrared imagery

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