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Algorithm Methodology

Below is a brief overview of the GEWEX Longwave and Shortwave Algorithms, and the Langley Parameterized Longwave and Shortwave Algorithms. A more detailed description of these algorithms is available in the Model Documentation.

GEWEX Longwave (LW) Algorithm uses the Fu et al., (1997, JAS, Vol. 54, 2799-2812) Thermal Infrared radiative transfer code with cloud and surface parameters requiring cloud, atmospheric profile information, and surface properties. Inputs to the algorithm were obtained from the following sources: The ISCCP DX cloud pixels were separated into categories of high, middle and low where middle and low clouds could be composed of ice or water. Cloud fractions and cloud optical depths were determined within these categories. Cloud particle sizes were assumed and cloud physical thicknesses were also designed based upon information from literature. Random overlap is used between the high, middle and low layers to better approximate undercast conditions.

Langley Parameterized Longwave algorithm (LPLA) Detailed descriptions of this algorithm can be found in:

  • Gupta et al. (1992) - J. Appl.Meteor., 31, 1361-1367.
  • Gupta (1989) - J. Climate, 2, 305-320.
  • Wilber et al. (1999) - NASA/TP-1999-209362. (available here )

GEWEX Shortwave (SW) Algorithm The shortwave parameters in this data file were generated using the Pinker/Laszlo shortwave algorithm (R.T. Pinker and I. Laszlo, 1992: Modeling Surface Solar Irradiance for Satellite Applications on a Global Scale, J. Appl. Met., 31, 194-211).

Langley Parameterized Shortwave Algorithm (LPSA) Detailed descriptions of the LPSA algorithm can be found in:

  • Gupta et al. (2001) - NASA/TP-2001-211272, Dec. 2001. (available here )
  • Darnell et al. (1992) - J. Geophys. Res., 97, 15741-15760
  • Darnell et al. (1988) - J. Climate, 1, 820-835

Each of the algorithms use cloud parameters derived from the DX data of the International Satellite Cloud Climatology Project (ISCCP; Rossow and Schiffer, 1999, BAMS, 80, 2261-2287) and temperature and moisture profiles taken from 4-D data assimilation products provided by the Data Assimilation Office at NASA GSFC and produced with the Goddard Earth Observing System model version 4 (GEOS-4). GEOS-4 is used in Rel. 3.0. Surface emissivities used in several algorithms are taken from a map developed at NASA LaRC (Wilber et al. 1999; see reference above). Column ozone values for the entire duration of this dataset were obtained primarily from the Total Ozone Mapping Spectrometer (TOMS) archive. For the early period (July 1983-November 1994), TOMS data came from NIMBUS-7 and Meteor-3 satellites. There was an interruption of about 20 months (December 1994-July 1996) after which TOMS data from EP-TOMS became available in August 1996 and continued until December 2004. From January 2005 to June 2006, Stratosphere Monitoring Ozone Blended Analysis (SMOBA) is used. All gaps in TOMS data, including those over the polar night areas every year, were filled with column ozone values from TIROS Operational Vertical Sounder (TOVS) data. Surface albedos are derived with a parameterization using monthly climatological clear-sky TOA albedos which are based on ERBE measurements during the 1985-1989 period.