IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Lecture 4 Oceanographic Applications: Infrared Measurement of Sea Surface Temperature IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers 1. Infrared radiometry 2. Interpretation of sea surface temperature 3. Advanced Very High Resolution Radiometer (AVHRR) 4. Multi-Channel Sea Surface Temperature (MCSST) algorithm 5. Geostationary Operational Environmental Satellites (GOES) 6. CoastWatch sea surface temperature data source and software IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers All surfaces emit radiation, the strength of which depends on the surface temperature. The higher is the temperature, the greater is the radiant energy. In simple form, the total emitted energy is M = σ • T4, where σ is 5.699 x 10-8 W m-2 K-4 (Stefan’s constant). Maximum wavelength of the emitted energy can be estimated from the Wien’s displacement law: Λmax• T = C3, where C3 = 2897 μm K-1. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers The brightness temperature of the radiation is defined as the temperature of the black body which would emit the measured radiance. In practice, the measured brightness temperature differs from the actual temperature of the observed surface because of non-unit emissivity and the effect of the intervening atmosphere. For infrared (IR) radiation the emissivity (i.e., the ratio between real exitance and a perfect emitter at this temperature) of sea surface is between 0.98 and 0.99. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers The atmosphere is most transparent to infrared at 3.5-4.1 μm and 10.0-12.5 μm. At 10 μm, solar emittance is about 300 times the sea emittance. However, as a result of the distance between the sun and the earth, the solar irradiance reaching the top of the atmosphere is about 10-5 of its value near the solar surface, that is about 1/300 of the sea surface emittance. At 3.7 μm, the incoming solar irradiance is the same order as the surface emittance. As a result, this wavelength can be used during nighttime only. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers For IR sensor calibration, a target of known temperature is used. This temperature is measured and transmitted to ground receiving station along with the signal measured by the IR sensor. Atmospheric correction is based on multispectral approach, when the differences between brightness temperatures measured at different wavelengths are used to estimate the contribution of the atmosphere to the signal (more detail later, in AVHRR section). For cloud detection, the thermal and near-infrared waveband thresholds are used, as well as different spatial coherency tests. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Interpretation of Sea Surface Temperature • The actual thickens of the layer whose temperature is remotely sensed varies between 3 and 14 m. It is called skin SST and written Tskin or sometimes SSST. • At the same time, the measured in situ SST (called also bulk SST) corresponds to at least few centimeters or more, depending on waves. The SST measurements on buoys may be anything between 0.5 and 3 m deep. • Three physical effects may increase the difference between skin and bulk SSTs: • Diurnal thermocline; • Thermal skin layer effect; • 3) The presence of surface film. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Interpretation of Sea Surface Temperature Diurnal thermocline As a result of insolation, daytime temperature in the upper layer of up to 50 cm can differ from deeper layers as much as 4C. Since open skies are part of the requirement of diurnal warming, there is a higher probability of daytime satellite observations encountering diurnal warming events. To control this effect, we should analyze the differences between day and night satellite SST observations. The bulk SST (i.e., the parameter we are measuring) is invariant over the diurnal cycle. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Interpretation of Sea Surface Temperature Diurnal thermocline Wind plays an important role in the erosion of diurnal thermocline, transporting heat to the deep layers. After Yokoyama et al., 1995) IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Interpretation of Sea Surface Temperature The thermal skin layer of the ocean surface Heat flux from ocean surface to the atmosphere results in decrease of the skin temperature. This effect is observed during both day and night. The difference between the skin and the sub-skin temperature is typically –0.17ºC at wind speed >5 m s-1. At lower wind speeds, the picture is more complex, resulting from heat flux, which is different during day and night, humidity, swell, etc. The skin layer is very robust. Experiments show that if the surface is completely broken and the skin is destroyed, for example by breaking wave, the skin layer reforms again in a few seconds. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Interpretation of Sea Surface Temperature Effect of surface film Surface film may be a naturally produced organic material or oil from shipping. When the slick is thicker than a single layer of molecules, the emitted radiance and the resulting brightness temperature are lower. A surface slick affects the thermal structure of the near-surface, inhibiting wind mixing and increasing diurnal thermocline. The slick also reduces evaporation. Moreover, a thick oil slick absorbs solar radiation effectively and becomes warmer than the underlying sea water. With such a variety of opposing effects, it is not possible to predict whether the observed radiation temperature will be reduced or increased by a slick. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Advanced Very High Resolution Radiometers (AVHRR) The monitoring of sea surface temperature (SST) from earth-orbiting infrared radiometers had the widest impact on oceanographic science. First of all, this impact resulted from regular and continuous supply of information by AVHRR (Advanced Very High Resolution Radiometer) on NOAA satellites since 1978. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers VHRR (Very High Resolution Radiometer) used before had just one visible and one infrared channel. AVHRR (Advanced Very High Resolution Radiometer) was first mounted on TIROS-N (Television Infrared Observation Satellite) in 1978. NOAA-11 The satellites of NOAA series are near-polar sun-synchronous satellites. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Temporal coverage of NOAA satellites with AVHRR IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Sensor characteristics IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Sensor characteristics The scanner has an IFOV of approximately 1.3 mrads and a cross-track scan of ±55.4º. With a nominal height of 833 km the ground FOV in nadir is 1.1 km and the swath width about 2500 km. The orbit period is about 102 min and 14 orbits are completed per day. The swath of adjacent orbits overlap, ensuring that the whole Earth surface is viewed at least twice a day, once from the ascending (daylight) passes and once from the descending (night) overpasses. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Monitoring of the Earth’ surface by the NOAA satellites with AVHRR sensors enable control of fires over vast land areas. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers The ratio between near-infrared and infrared wavebands, called Normalized Digital Vegetation Index (NDVI) is a wide-used method of the analysis of land vegetation. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers AVHRR observations of sea surface temperature (SST) are very important for oceanographers, because they enable the analysis of spatial and temporal variations of ocean currents. At this image you see the Gulf Stream Current in North Atlantic. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers AVHRR data are acquired in three formats: High Resolution Picture Transmission (HRPT) HRPT data are full resolution image data transmitted to a ground station as they are collected. Local Area Coverage (LAC) LAC are also full resolution data, but recorded with an on-board tape recorder for subsequent transmission during a station overpass. Global Area Coverage (GAC) GAC data provide daily subsampled global coverage recorded on the tape recorders and then transmitted to a ground station. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers The data are collected in several scientific centers: EROS - Earth Resources Observation Systems data center; EDC - Earth Resources Observation Systems Data Center; NOAA/NESDIS - National Environmental Satellite, Data and Information Service of National Oceanic and Atmospheric Administration; And some others. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Processing steps: Georegistration: The position of the satellite is determined by an orbital model updated by ephemeris data (a table of predicted satellite orbital locations for specific time intervals) received daily from NAVY Space Surveillance. A refinement to the sensor model accounts for the displacement in longitude due to the rotation of the Earth under the satellite. The positional accuracy of a systematic georegistration is approximately 5000 m. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Processing steps - Georegistration : To avoid georegistration errors like shown above, more precise georegistration methods are applied, which can achieve a positional accuracy of 1000 m (I.e., 1 IFOV). The method includes correlation of image features with accurately registered cartographic or image-based maps, extracting easily identifiable features such as coastlines, water bodies, and rivers and correlating them with the matching raw image locations using various techniques. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Processing steps - Georegistration : IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Processing steps – Calculating SST on the example of MCSST algorithm The idea of the first step of atmospheric correction in Multi-Channel Sea Surface Temperature (MCSST) algorithm is that the contribution of the atmosphere water vapor to the signal is different at different channels. We assume that the temperature deficit in one channel, which results from atmospheric absorption by water vapor, is a linear function of the brightness temperature difference of the two different channels. SST = A + B * (T1 – T2) + T1. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Processing steps – Calculating SST on the example of MCSST algorithm During daytime observations the channels 11 and 12 µm are used: SST = 1.0346 * T11 + 2.5779 * (T11-T12) - 283.21; During nighttime we can also use the channel 3.7 µm, which during daytime is contaminated with sunlight: SST1 = 1.5018 * T3.7 - 0.4930 * T11 - 273.34; SST2 = 3.6139 * T11 - 2.5789 * T12 - 283.18; SST3 = 1.0170 * T11 + 0.9694 * (T3.7 - T12) - 276.58; (SST in degrees Celsius, T in degrees Kelvin). IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Processing steps – Calculating SST on the example of MCSST algorithm • Atmospheric correction: • Visible or IR reflectance test (during daytime only): • The reflectance of the cloud-free ocean as measured at a satellite is generally less than 10%, whereas the reflectance of the most clouds is greater than 50%. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Processing steps – Calculating SST on the example of MCSST algorithm Atmospheric correction: 2. Uniformity test Threshold of the variation of measurement values from adjacent cloud-free field of view is set to be slightly in excess of instrumental noise. With partially cloud-filled fields of view, the variations are generally larger. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Processing steps – Calculating SST on the example of MCSST algorithm Atmospheric correction: 3. Channel intercomparison test. At night three independent measures of SST can be obtained from different channels: SST1 = 1.5018 * T3.7 - 0.4930 * T11 - 273.34; SST2 = 3.6139 * T11 - 2.5789 * T12 - 283.18; SST3 = 1.0170 * T11 + 0.9694 * (T3.7 - T12) - 276.58; When the contribution of the atmosphere is too strong, the difference between SST1, SST2 and SST3 exceeds the assumed threshold and the resulting SST is marked as invalid. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Processing steps – Calculating SST on the example of MCSST algorithm Atmospheric correction: 4. The retrieved SSTs are compared with climatology and with SSTs retrieved using alternative algorithms. First, the SST is subject to “unreasonableness” test, i.e., SST must be within the range from 2ºC to +35ºC. Second, the retrieved SST must pass a climatology test, meaning that it must agree with monthly climatology at its location within 10ºC. As a result, 80–90% of AVHRR pixels are considered cloudy. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers The AVHRR data obtained during one week contain many areas where no data was collected due to cloud cover. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers The zones where the observations are absent can be filled with interpolated data, but the validity of these data is doubtful. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers During recent years AVHRR data are step-by-step reanalyzed within “Pathfinder” Project at NASA Jet Propulsion Laboratory (JPL) using sophisticated algorithm bases on numerous contact measurements of sea surface temperature. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Sea Surface Temperatures obtained during daytime and nighttime are essentially different and should not be compared at the series of images. This difference results from not only the daytime thermocline, but from different algorithms also. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Sea Surface Temperatures have been derived from the series of NOAA's Geostationary Operational Environmental Satellites (GOES). The data set includes data from two satellites: GOES East (GOES-10) and GOES West (GOES-12). Gridded Level 3 SSTs with a nominal spatial resolution of 6 km are available between 180W to 30W and 45S to 60N. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers Each satellite is equipped with GOES Imager radiometer which collects information on 5 channels (1 visible and 4 infrared). The scans are every hour, IFOV is 4 km. • Brightness temperatures from the 5-channel instrument are regressed against buoy data to derive a set of coefficients. These coefficients are then used to convert the brightness temperatures to an SST measurement. The theory itself is very similar to the non-linear algorithm used to process AVHRR-derived SSTs. IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

Sea surface temperature from infrared radiometers IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors

IoE 184 - The Basics of Satellite Oceanography. 4. Oceanographic Applications: Infrared Sensors