GOES Sounder Details
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Land Surface Temperature
Skin temperature is the radiating temperature of the soil, vegetation, buildings, water etc. (often defined as land/sea surface temperature), and an image of it describes the spatial variability of surface thermal emission over the region. During periods of solar heating, the time tendency of skin temperature is related to the surface moisture availiability and is a useful parameter to assimilate into a numerical forecast model. Skin temperature is derived simultaneously with total precipitable water using data from at least two split window channels in a physical technique to solve the radiative transfer equation.
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Total Precipitable Water
Total precipitable water is the total amount of water vapor contained in a vertical column of unit cross-sectional area extending from the surface to the top of the atmosphere, and an image of it describes the spatial variability of the total water vapor in the atmosphere, and is useful for isolating regions of potential convective development. Total precipitable water is derived simultaneously with skin temperature using a physical split-window technique.
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20 Day Clear Sky Composite
The 20 day clear sky visible composite is the minimum visible channel radiance value for each pixel from the past 20 days, and is an intermediate product that provides an estimate of the minimum in surface reflectivity for each pixel, assuming no pixel is cloud covered for all 20 days. It is used to produce the surface albedo and insolation products.
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Cloud Mask
Jedlovec and Laws (2001) and Jedlovec and Laws (2003) describe the methodology for the GOES Sounder cloud mask. In summary, the cloud mask method applies two spatial tests and one threshold test on a 11 - 3.7 µm difference image. An additional test implemented since the writing of the paper is a temperature threshold test. This fourth test compares the temperature from the 11 µm channel to a 20 day clear sky composite of 11 µm temperatures, and labels the pixel as cloudy if the difference is greater than the threshold. This additional test is intended to produce improvements mainly during the nighttime hours.
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SPoRT Spectral Difference
Generated in the same way as the Fog Depth product, the SPoRT Spectral Difference product provides a different view of fog and low cloud locations. The product is at a slightly higher spatial resolution and can also be used to determine the location of low-level clouds.
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Cloud Top Pressure - CO2 Slicing
The CO2 slicing technique is being utilized to produce a cloud top pressure product from the GOES Sounder. The technique, presented by Smith and Platt 1978, is a multispectral approach utilizing five channels from the GOES Sounder to retrieve cloud top pressure. The multispectral nature of the technique allows for the effective cloud fraction, or the product of the physical cloud fraction and the cloud emissivity, to vary, allowing for a more accurate height assignment than the infrared window lookup technique in regions of non-uniform cloud coverage.
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Effective Cloud Fraction
The effective cloud fraction (ECF) product is a by-product of the CO2 slicing technique. After retrieving the cloud top pressure, the ECF, which is the product of the cloud emissivity and physical cloud fraction, can be calculated. Ranging from 0 to 100 percent, it is representative of the fractional contribution of the cloud to the measured radiances.
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Cloud Top Pressure - Combined
This cloud top pressure product combines the CO2 slicing technique, which is capable of more accurately determining the cloud height in regions where the cloud fraction and emissivity is non-unity, with the IR Window technique. Both algorithms are combined to first fill in cloudy scenes where the CO2 slicing algorithm fails to retrieve a cloud height. If both retrieval techniques were successful, this product selects the cloud height that was determined to be higher vertically in the atmosphere. This selection criterion is based on the fact that the IR Window technique will typically assign a cloud height lower than the CO2 slicing technique due to an assumption of unity in both the cloud fraction and emissivity, thus the cloud height is generally from the more advanced technique.
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Cloud Top Pressure - Infrared Window
Cloud top pressure images describe the spatial variation of height and type of clouds over a region, and are useful for nowcasting and forecast-model data assimilation. The presence or lack of clouds over a region is very important for modeling and diagnostic studies. When a cloud is present in a satellite image, its height (or cloud top pressure) is an important additional parameter. Cloud top pressure is derived using a method that correlates the cloud top temperature in an infrared window channel to a thermodynamic profile from the NAM forecast model (Fritz and Winston 1962). The cloud top pressure product incorporates the GHCC operational cloud mask, produced using a technique that relies upon a 11 - 3.7 µm difference image to enhance surface versus cloud contrast to delineate cloudy pixels from clear.
