Thermal Satellite Imagery Research Articles

Utility of thermal sharpening over Texas high plains irrigated agricultural fields

Irrigated crop production in the Texas high plains (THP) is dependent on water extracted from the Ogallala Aquifer, an area suffering from sever water shortage. Water management in this area is therefore highly important. Thermal satellite imagery at high temporal (∼daily) and high spatial (∼100 m) resolutions could provide important surface boundary conditions for vegetation stress and water use monitoring, mainly through energy balance models such as DisALEXI. At present, however, no satellite platform collects such high spatiotemporal resolution data. The objective of this study is to examine the utility of an image sharpening technique (TsHARP) for retrieving land surface temperature at high spatial resolution (down to 60 m) from moderate spatial resolution (1 km) imagery, which is typically available at higher (∼daily) temporal frequency. A simulated sharpening experiment was applied to Landsat 7 imagery collected over the THP in September 2002 to examine its utility over both agricultural and natural vegetation cover. The Landsat thermal image was aggregated to 960 m resolution and then sharpened to its native resolution of 60 m and to various intermediate resolutions. The algorithm did not provide any measurable improvement in estimating high‐resolution temperature distributions over natural land cover. In contrast, TsHARP was shown to retrieve high‐resolution temperature information with good accuracy over much of the agricultural area within the scene. However, in recently irrigated fields, TsHARP could not reproduce the temperature patterns. Therefore we conclude that TsHARP is not an adequate substitute for 100‐m‐scale observations afforded by the current Landsat platforms. Should the thermal imager be removed from follow‐on Landsat platforms, we will lose valuable capacity to monitor water use at the field scale, particularly in many agricultural regions where the typical field size is ∼100 × 100 m. In this scenario, sharpened thermal imagery from instruments like MODIS or VIIRS would be the suboptimal alternative.

The effect of urban leaf area on summertime urban surface kinetic temperatures: A Terre Haute case study

The urban heat island effect (UHIE) has been documented in many temperate region cities. One cause of the UHIE is the replacement of green spaces with impervious materials as urbanization commences and the city builds up and fills in. During the summer, elevated urban temperatures result in increased electricity usage, higher pollution levels, and greater resident discomfort. Through evapotranspiration and the interception of solar radiation, increasing urban tree canopy cover can help mitigate the UHIE. While this is universally accepted, the exact statistical relationship between urban leaf area (as measured by leaf area index, LAI) and urban temperatures has not been extensively studied. In a case study conducted in urban/suburban Terre Haute, Indiana, USA, simple linear regression was employed to quantify the relationship between in situ ceptometer LAI measurements and surface kinetic temperatures (SKTs) measured using thermal satellite imagery acquired at 1100 local time. For the 143 sample sites located in the study area, LAI accounted for 62% of the variation in surface temperature. For every unit increase in LAI, surface temperature decreased by 1.2 °C.

Spring phytoplankton bloom in the fronts of the East China Sea

Frontal areas between warm and saline waters of the Kuroshio currents and colder and diluted waters of the East China Sea (ECS) influenced by the Changjiang River were identified from the satellite thermal imagery and hydrological data obtained from the Coastal Ocean Process Experiment (COPEX) cruise during the period between March 1st and 10th, 1997. High chlorophyll concentrations appeared in the fronts of the East China Seas with the highest chlorophyll-a concentration in the southwestern area of Jeju Island (~2.9 mg/m3) and the eastern area of the Changjiang River Mouth (~2.8 mg/m3). Vertical structures of temperature, salinity and density were similar, showing the fronts between ECS and Kuroshio waters. The water column was well mixed in the shelf waters and was stratified around the fronts. It is inferred that the optimal condition for light utilization and nutrients induced both from the coastal and deep waters enhances the high phytoplankton productivity in the fronts of the ECS. In addition, the high chlorophyll-a in the fronts seems to have been associated with the water column stability as well.

Upwelling along the western barrier reef of New Caledonia

Temperature decreases of up to 5°C at an automatic station at ~10 m depth at the outer reef of western New Caledonia signalled the occurrence of upwelling. The decreases are related to south-easterly wind events and they can take up to 5 days to develop. Successive wind events can depress the temperature for as much as 1 month. An upwelling shows in thermal satellite imagery as a band of cold water ~10 km wide and as long as several hundred kilometres just outside the reef. The cold water frequently spreads out to sea. The upwellings are more common in summer (October to March) than in winter. During lengthy periods of low wind speeds, the waters outside the reef appear to be overrun by warm water from farther north. The frequent occurrence of the warm water layer in the north may partly explain why upwellings are more common along the southern half of the reef. The entire lagoon cools in winter; most of the lagoon warms in summer, except at the southern end where the waters are cool, perhaps from tidal mixing.

Active mud volcanism observed with Landsat 7 ETM+

Mud volcanoes are relatively small spatter cones that erupt water-laden mud and gases, and occur throughout the world. For many mud volcanoes, the eruption of warm mud (10–40°C) can be detected with high-resolution thermal satellite imagery. We demonstrate the utility of Landsat 7 Enhanced Thematic Mapper Plus (ETM+) imagery for thermal monitoring of active mud volcanism. We constrain the temperature and area of active mud discharge and estimate surface heat flux for two isolated mud volcanoes in the Copper River Basin, Alaska using Band 6 (10.4–12.5 μm). The heat flux results span a wide range due to uncertainties in the environmental conditions at the time of image acquisition, but can be constrained to be less than 0.24 MW for each of the two mud volcanoes considering previously published field measurements. With this higher-resolution Band 6 on the ETM+ sensor, as well as the high-resolution thermal bands on the ASTER sensor, reliable monitoring of mud volcanism on this scale is possible for the first time.

SeaWiFS observations of the Arabian Sea southwest monsoon bloom for the year 2000

During the southwest monsoon (SWM) in the northern Arabian Sea, the phytoplankton bloom associated with upwelling has remained incompletely characterized by satellite ocean-color observations. This is due to the prevalence of dust storms in the region from May to September that prevents good satellite-derived chlorophyll retrievals. To ameliorate this problem, SeaWiFS data from May to September 2000 were processed with the spectral-matching algorithm (SMA) using a suite of Saharan dust models. The use of SMA significantly increased chlorophyll data recovery in areas otherwise completely masked by standard SeaWiFS processing, and permitted a description of the bloom over the SWM period. Maps of the frequency and spatial distribution of TOMS-absorbing aerosol index indicate that Africa and Arabia are the most probable dust sources, and that the nearshore area parallel to the Arabian coast was the most affected by dust events during the SWM, which is also where chlorophyll retrievals are also poorest. Weekly averages of the SeaWiFS images produced using the SMA show that at the onset of upwelling in June, the bloom along the Omani coast has limited offshore extent except for a few small thin filaments. From July to September, the high-chlorophyll features evolve into more complex structures that extend offshore into the Arabian Sea interior. In October, when the SWM has ended, the filaments appear to merge, forming a high-chlorophyll band off the Omani coast. Quikscat wind data suggest that the widespread filament formation occurs only when the SWM winds begin to weaken, which results in greater spatial variability in the zone of maximum wind stress (the Findlater Jet). Although the bloom is generally restricted to the cold-water upwelling area (as indicated by TMI sea-surface temperatures) that lies northwest of the Findlater Jet axis, the high-chlorophyll structures do not occur solely in association with coldest upwelling features. This suggests that thermal satellite imagery cannot be used to infer chlorophyll distributions for periods in which no ocean-color data are available, e.g., the earlier JGOFS cruises.

A surface‐trapped intrusion of slope water onto the continental shelf in the Mid‐Atlantic Bight

A high‐resolution hydrographic transect across the shelfbreak south of Nantucket Shoals in August, 1995, revealed an unusual surface‐trapped intrusion of saline Slope water onto the continental shelf. The intrusion was confined to the upper 25 m of the water column and penetrated 15 km shoreward of the 100‐m isobath, the typical position of the shelfbreak front. The maximum salinity within the intrusion was 35.0. Several strong jets were present within the surface layer with maximum velocities between 0.30 and 0.44 m s−1 in the alongshelf direction. Satellite thermal imagery confirms the presence of a warm‐core ring at the offshore edge of the section and shows a warm feature penetrating shoreward of the 100‐m isobath.

Urban Mesoclimate Effects on Winter Survival and Damage of Ornamental Species in Fort Worth, Texas

Urban areas are typically 2 to 3°C warmer than surrounding rural areas throughout the year. Winter minimum temperatures are often 4 to 5 °C warmer in the city and, during extreme episodes may exhibit differences of 12 to 13°C. Because the USDA Hardiness Maps compile readings from individual stations in an area, temperature differences may not be apparent at the local scale. This study was conducted to compare ornamental plant damage during Winter 1995–96 in Fort Worth, Texas. AVHRR 1-km thermal satellite imagery was used to determine the warmest and coolest portions in Fort Worth, Texas. Each temperature area was divided into five 0.5-km blocks on the basis of similar landscape features and plant types. During Mar. 1996, these areas were evaluated on the basis of plant damage to several species. Asian jasmine (Trachelospermum asiaticum), indian hawthorn (Raphiolepis indica), St. augustine turf (Stenotaphrum secundatum), southern magnolia (Magnolia grandiflora), and Live Oak (Quercus virginiana) were the primary species damaged. Asian jasmine and St. Augustine turf were either completely killed or severely damaged in the coldest areas but suffered only moderate or light damage in the warmest areas. Indian hawthorn, live oak, and southern magnolia suffered leaf and stem damage in the coldest areas but little to no damage in the warmer areas.

Analysis of the urban thermal environment with LANDSAT data

Research was carried out with two dates of thermal satellite imagery and field climatic data in six high-rise housing estates in Singapore to investigate the relationship between satellite-derived values and the urban microclimate. GIS techniques were used to register the image data for two detailed study areas to digital plans of street and building outlines, including street canyons at different orientations to solar azimuth, and detailed comparisons between image data and urban morphology were undertaken. A method is described for enhancing the image spatial resolution to an appropriate level for evaluating the thermal character of individual streets, buildings and building complexes, and roadside trees. Field data obtained in two street canyons at the same time of year as the image data suggest a significant relationship between surface and air temperature and suggest that the satellite view of the high-rise urban environment in the study area is representative.

Gulf Stream warm surges: Thermal anomalies resulting from cold core ring interactions

Observations from thermal satellite imagery reveal a thermal anomaly pattern in the sea surface temperature (SST) field of the Gulf Stream downstream of Cape Hatteras which is a direct result of Gulf Stream‐eddy interactions upstream of Cape Hatteras. The general sequence of events observed in the images is as follows: (1) Detrainment of Gulf Stream surface water occurs as a result of an interaction with a cold core ring; (2) downstream of the interaction the Gulf Stream's SST decreases several degrees over a few days owing to the loss of the upstream surface flow; (3) a visible “jet” of warm surface water is advected down the major axis of the Gulf Stream after the stream‐ring interaction subsides and the detrainment diminishes. The jet maintains a 1–3°C temperature difference with the surrounding Gulf Stream water as it propagates downstream. This paired cooling/warming process is labeled a “warm surge event” because the sharp leading edge of the warm jet (i.e., the surge) is easily detectable in the satellite images, while the anomalous surface cooling preceding the passage of the warm jet is difficult to observe. Vertical profiles of temperature, downstream velocity, and cross‐stream velocity all indicate that the upper 100–150 m of the Gulf Stream are significantly altered during an event. Satellite and in situ data also indicate that just downstream of the interaction there is enhanced entrainment of Sargasso Sea surface waters into the Gulf Stream to replenish the detrained Gulf Stream water.

On the wind-driven coastal upwelling in the Gulf of Lions

Satellite imagery has revealed the existence of discrete and relatively cool zones of surface upwell water along the Mediterranean coast of the Gulf of Lions off the South of France. The imagery relates to a period of time during which the character of the local surface wind field corresponds to the onset and subsequent persistence of a Mistral-Tramontane event, this consisting of strong northerly and northwesterly winds. Using a multi-level three-dimensional numerical model of wind-driven coastal upwelling dynamics in which turbulence closure is achieved at the level of the turbulence energy equation, experiments have been performed with a parameter setting appropriate to the onset of a Mistral-Tramontane event in the Gulf of Lions. Good agreement is obtained between the predicted zones of coastal upwelling (characterized by local reductions in sea-surface temperature) and the results of the satellite thermal imagery. In process-orientated evaluations, an account is given of the depth-structure of the momentum and thermal fields supporting the coastal upwelling.

Double structure of the coastal front in the Kii Channel, Japan, during winter

Analysis of satellite thermal imagery showed that the coastal front in the Kii Channel, Japan, has a double structure that consists of the well‐known S‐shaped front and a newly found mouth front. The S‐shaped front is in a state of equilibrium as has been shown by previous numerical experiments, whereas the mouth front is produced as a direct response to the intermittent arrival of warm water from the Kuroshio. As was shown by a previous study, the warm water comes along the eastern or western coast, and the western path (the Geitou Branch) occurs less frequently than the eastern (the Kinan Branch). We found, however, that each appearance of the western path has a greater effect on the heat budget of the channel and the shape of the front than the eastern path does. The western path tends to produce a large temperature difference and to form a sharp mouth front which is often accompanied by tongue and outflowing structures. Both structures are produced by water intruding across the mouth front; the tongue structure by warm water intruding toward the coast, and the outflowing structure by cold water intruding toward the offshore region. The tongue structure is a transient process toward equilibrium, the S‐shaped front. A hypothetical model that shows how the tongue and outflowing structures are formed also is presented.

Observations of offshore shelf-water transport induced by a warm-core ring

Gulf Stream warm-core rings often are observed to entrain water from the continental shelf and transport filaments of shelf water out over the continental slope. In June 1982 the R. V. Endeavor made a transect through one such shelf-water “streamer” that was seen from satellite thermal imagery to the east of warm-core ring 82B. Acoustic doppler current profiles together with repeated CTD stations were made through the streamer. The CTD system was also equipped with a dissolved oxygen sensor and transmissometer. We were able to make an unprecedented section of velocity and water mass structure of the shelf-water filament. The offshore velocity structure observed was complicated with velocities ranging from 10 to 50 cm s −1, temperatures, in a sub-surface temperature minimum, of less than 8°C, surface salinities less than 33‰, with high levels of suspended particulate matter and manganese, and with shelf-water properties as deep as 100 m in the center of the streamer. Transports calculated in different temperature and salinity classes showed a total volume transport of waters in the filament of 0.8–0.9 × 10 6 m 3 s −1 for water with a salinity <35‰. Thermal imagery shows that the streamer was not “wrapped around” the ring but was advected by the ring and deposited at the inshore edge of the Gulf Stream. Using adjacent stations not in the streamer, but in the Slope Water, as a reference for water that presumably replaced the above amount of shelf water, estimates have been made of the net exchange of heat, salt, particulates, dissolved oxygen and Mn due to the offshore transport of shelf water by 82B. These estimates indicate that warm-core rings are capable of inducing significant exchange across the Shelf/Slope Water front off the northeast coast of the U.S.A.

Lower continental slope cyclonic eddies in the central Gulf of Mexico

Current meters, an inverted echo sounder, hydrography, drifters, and satellite imagery are used to characterize relatively small (100‐ to 150‐km diameter) cold cyclonic eddies in the central basin and on the lower Louisiana slope of the Gulf of Mexico. These cyclones are shown to be long‐lived (6 months or more), have limited movements when compared with Loop Current anticyclones, and be fairly vigorous, with upper layer currents of 30–50 cm s−1. They usually have only small temperature differences with surrounding water masses in the upper 50 to 100 m of the water column and are therefore not readily apparent in satellite thermal imagery. The largest isotherm displacements occur over depths of 200–800 m. In one case, a cyclone was traced from the deep basin near 92°W to the northern slope as a major anticyclone propagated southwestward through the gulf. The presence of cyclones and anticyclones on the Louisiana slope is consistent with observed current meter measurements in the upper half of the water column that have long (several months) periods and approximately equal variances in the cross‐slope and along‐slope directions.

Analysis of the Niagara River plume dynamics

The Niagara River discharges into Lake Ontario at a typical rate of 6500 m3/s, and is the lake's principal source of suspended and dissolved materials. The discharge plume is frequently warmer and thus more buoyant than the surrounding lake surface water. Using thermal satellite imagery collected over a 4‐year period, it was found that the plume executes an anticyclonic turn after exiting the river mouth in 53% of the images in which the plume is visible, while in 26% of the images the plume moves offshore. The cyclonic plume was the least common trajectory occurring in only 9% of the images. Temperature and velocity data are used to examine the buoyant anticyclonic plume. Estimates of the bulk shear Froude number suggest that the plume is unstable at the offshore edges of the turning region and stable in the interior. The stability of the interior is enhanced by the presence of a strong seasonal thermocline directly beneath the plume. The momentum balance within the plume is examined using a local coordinate system. Near the river mouth the plume is vertically mixed to the bottom and the large initial flow deceleration is balanced by bottom stress. Away from the river mouth the principal balance is between Coriolis acceleration, relative acceleration (inertial turning), and the cross‐stream baroclinic pressure gradient.

An analytical model for remote sensing determination of the mixed layer depth

An analytical mixed layer thermal inertia model based on remote sensing data is presented to estimate the diurnal mixed layer thickness and the thermal inertia of the mixed layer. The model depends on the concept that the diurnal sea surface temperature range (δSST) is primarily a function of the thermal inertia of the mixed layer, of local meteorological conditions, and physical characteristics of the sea surface. The value of δSST can be determined from satellite thermal infra-red imagery, albedo can be calculated from satellite broad-band visible and near infra-red imagery, wind speed can be estimated from satellite microwave radiometer imagery, and heat flux can be estimated using a standard method. Given these characteristics, the thermal inertia and diurnal mixed layer depth images are generated. The diurnal mixed layer depths predicted by the analytical remote sensing model agree well with observed data. Although it is limited by linearization of the upper boundary conditions and by some simplifying assumptions, the analytical method in the mixed layer thermal inertia model has a major advantage in its apparent simplicity in calculation. Special solutions are obtained in a closed analytical form, and the computation based on these solutions is made rapidly compared with the turbulence closure models. This advantage is most desirable for a remote sensing system, since the data base for image processing is orders of magnitude larger than the data base for any conventional observation. For further application of the model, the term scaled thermal inertia is defined and calibrated to generate surface wind images. We compare such images with microwave radiometer imagery and also discuss the limitations of the model.

Surface patterns in temperature, flow, phytoplankton biomass, and species composition in the coastal transition zone off Northern California

Satellite thermal imagery and in situ biological and physical data are presented that describe the spatial variability of phytoplankton biomass and species composition in relation to the physical structure at the sea surface during persistent upwelling off northern California. Surface patterns in temperature, geostrophic velocity, chlorophyll, and particle size structure show that the coastal zone was dominated by two water masses separated by a well‐defined physical boundary. This boundary is apparent in satellite thermal imagery as a cold front and in dynamic height as a meandering jet in the California Current. On the landward side of the front (jet) we observed cold, coastal water that was relatively enriched in phytoplankton biomass, due to the presence of large diatoms. On the seaward side of the front (jet) we observed relatively warm water where the phytoplankton biomass was low and the diatoms were largely absent. Broad tongues and narrow filaments of cold, chlorophyll‐rich water that extended over 100 km offshore were present in our study area. These features were bounded by the meandering coastal jet. The coldest, most chlorophyll‐rich water in these tongues and filaments was located between the onshore and offshore flows in relatively slow moving water, but entraimnent of coastal water into the jet was apparent.

On the seasonal response of the Lower St Lawrence Estuary to buoyancy forcing by regulated river runoff

The seasonal current fluctuations recorded from May to September 1979 in the Lower St Lawrence Estuary (LSLE) were re-examined using complex empirical orthogonal functions analysis. The first mode explained 88% of the seasonal variability, and revealed the presence of an estuary-wide anticyclonic eddy near the mouth, which lasted for 40 days in June and July. Careful inspection of the (regulated) 1979 freshwater runoff and salinity time series indicated that light surface water pulses from the St Lawrence River and the Saguenay fjord arrived in the LSLE during that time. Their duration was about 40 days. The contention is that the anticyclonic eddy results from buoyancy forcing by these light water pulses, isolated in the LSLE by denser waters upwelled upstream and by the buoyancy front at the mouth. A reduced gravity model is used to show that when the width of the LSLE becomes greater than two internal Rossby radii, an initial dynamic height elevation will adjust through geostrophy to an anticyclonic eddy. This seems to occur downstream of Rimouski. The eddy will form within a time scale 0 (f −1), and in the absence of instabilities in the current field, it will conserve potential energy for extended periods of time. During August, the advected river runoff decreased, unstable wave activity developed, and denser Gulf waters entered the LSLE from the north shore producing a cyclonic eddy near the mouth. Concurrent satellite thermal imagery tends to support these findings.

The morphology of shelfbreak eddies

We used a combination of buoy tracking, intensive hydrography, satellite thermal imagery, and moored current meters to resolve the structure of eddies at the shelfbreak front in the Middle Atlantic Bight south of New England. Eddylike features were always present at the front in our study area throughout the 15‐day period of observations in June 1984. We found that hydrographic features in our across‐shelf hydrographic transects that appeared to represent the detached parcels of shelf water often reported in the literature were, in fact, part of the three‐dimensional structure of shelfbreak eddies. Adequate alongshelf resolution, in particular, enabled us to determine that no detached parcels were present. The two prominent features of the eddy groups we found were plumes of lighter shelf water that protruded into slope water, curling “backward” opposite the direction of mean shelf flow, and neighboring cyclones with warmer, saltier slope water in their cores, partly or wholly encircled by the plumes. The plumes have the potential especially for producing vigorous across‐front exchange of heat, salt, and nutrients and may play roles analogous to the “squirts” found on the California shelf.

Instability of a buoyancy‐driven coastal jet: The Gaspé Current and its St. Lawrence precursor

The discharge of the St. Lawrence River drives a buoyant jet which extends along the south shore of the lower St. Lawrence estuary and, combining with the northwest Gulf of St. Lawrence cyclonic gyre, becomes the Gaspé Current. In both the estuarine jet and the Gaspé Current, strong lateral and vertical shears are present. Satellite thermal imagery has shown that the Gaspé Current is unstable. Its instabilities are manifested as prominent wave disturbances and eddy features. We use satellite thermal images from 3 different years to show that unstable disturbances are also characteristic of the estuarine jet. We update previous work to show that Gaspé Current is both baroclinically and barotropically unstable. Our model studies also show that this conclusion is valid for the estuarine jet as well. Finally, we discuss a sequence of satellite images showing the development of an unstable disturbance apparently triggered by an eastward wind pulse.