Short-term Prediction Research
and Transition Center

Retrieval of Geophysical Parameters from GOES

Land Surface Temperature and Precipitable Water - The retrieval of land surface temperature (LST) (also known as skin temperature) and total precipitable water (TPW) from GOES measurements is accomplished with an algorithm and the 11 and 12 micrometer channels of either the Imager or the Sounder. The algorithm, know as the Physical Split Window (PSW) technique, is derived from a perturbation form of the radiative transfer equation that is simplified through parameterization to retrieve bulk layer parameters rather than profile information. The physical approach requires a priori information, which includes estimates of temperature and mixing ratio profiles, TPW, and skin temperature. The guess information is used with forward radiative transfer code and GOES spectral response information to calculate channel transmittances and brightness temperatures, which are required for the solution equations. Variations in the retrieval methodology used for particular applications can be found in Haines et al (2004). Often GOES field of views (FOVs) are averaged together over a limited region before the retrieval process to reduce the affect of random noise on the resulting products. Proper screening of the individual FOVs for cloud contamination is crucial for successful retrievals. The quality of the TPW retrievals depends on the appropriateness of the first guess which is typically a short term forecast from the Eta model. LST retrievals are only weakly dependent on the guess profile information. The retrieval accuracy varies with application. The quality of both TPW and LST degrades under inversion conditions (either in the first guess or retrieval environment). Under optimal observing conditions, TPW retrieval errors will approach 2.0 mm, while LST errors are as small as 0.2 K. Variations in surface thermal emissivity unaccounted for in the retrieval process will increase the magnitude of the errors.

Cloud Mask - The GOES cloud mask algorithm (Jedlovec and Laws 2003) uses the 3.9 and 11 micrometer window channels in five spectral/spatial tests to determine the sky conditions at 4 km resolution for day and night scenes on an hourly basis. The unique aspect of the algorithm is that it uses hourly image composites to define dynamic threshold values used in the spectral tests. The GOES cloud mask has been validated with manually determined sky conditions.

Cloud Top Pressure (CTP) - Each cloudy pixel in the GOES 4 or 10km NSSTC/GHCC cloud mask is assigned a cloud top pressure based on an infrared method which matches the observed window channel brightness temperature with an adjacent thermodynamic profile as described in Haines et al (2004) or a CO2 approach as in McCarty et al. (2006). The infrared method is used where the CO2 approach fails (mainly with low clouds) and assumes an opaque cloud. The CO2 approach also retrieves an effective cloud fraction product which describes partial footprint coverage of the clouds and cloud emissivity. Both techniques use forecast model information as a first guess or reference profile.

Insolation and Albedo - The amount of solar energy reaching the Earth's surface (insolation) is estimated from the broadband visible channel on GOES. It is desirable to estimate both direct and diffuse radiation (scattering from the atmosphere and clouds). The albedo of the surface is required to accurately compute these components. The surface albedo (for each hour) is calculated using a short term history of GOES visible channel reflectance measurements from cloud-free images (minimum visible value over the history), the solar constant, and an estimate of the water vapor content of the atmosphere. In cloudy regions a historical estimate of the albedo (from cloud-free data) is used. The procedure includes three processes: attenuation of downward flux of solar radiation in cloud-free regions by molecular scattering and absorption by atmospheric water vapor, absorption and scattering of solar radiation by clouds, and the attenuation of solar radiation by the atmosphere below the clouds. Atmospheric absorption is calculated with a parameterized radiative transfer model appropriate for shortwave radiation and is dependent on water vapor (total precipitable water in our case), and satellite and solar viewing geometry. Cloud absorption is parameterized solely on visible reflectance and Rayleigh scattering with a molecular path length.

GOES Information
General Satellite Information GSFC-NASA or NOAA-NESDIS
Schedules and Scan Sectors
Current GOES images
GHCC GOES Imager and Sounder Products

More information regarding the Infrared Processes Group of the GHCC.