85th AMS Annual Meeting

Tuesday, 11 January 2005
The offline comparison of an alternative infrared radiation scheme for use in NCEP's global spectral model and regional spectral model
Johnny Seymore, Howard University, Washington, DC; and E. Joseph
In a preliminary assessment of the National Centers for Environmental Prediction (NCEP) Global Spectral Model (GSM), a bias in the zonal mean temperature structure reveals a warming in the northern and cooling in the southern hemispheres for the month of December 1989 (Seymore, 2002). Comparing the GSM model output of top-of-the-atmosphere (TOA) clear-sky longwave radiation to NASA Earth Radiation Budget Experiment observed data, a discrepancy in the results arise. Improvements in these biases were achieved after NASA’s Chou and Suarez (1994) infrared radiation (IR) scheme was implemented into the GSM. Specifically, the tropics showed considerable improvements in the free troposphere where the water vapor continuum is a dominant factor (Seymore, 2002).

The Chou and Suarez IR scheme differs from the default IR code in the GSM (Fels and Schwarzkopf, 1975) as follows: the calculation of cooling rates, treatment of temperature profile, and the gas treatment of major and minor constituents in the atmosphere. In the present study detailed calculations are conducted to further evaluate the Chou scheme, and to examine the differences in atmospheric heating and surface energy budget as calculated by the two schemes. An update version of the Chou scheme is also evaluated: Chou et al. (2002). This scheme includes multiple scattering and cloud overlap features not present in the earlier version. Also a stand-alone version of the Fels and Schwarzkopf IR scheme was developed for this study. The following calculations are conducted: heating profiles, TOA and surface fluxes based on standard atmosphere (McClatchey, 1971) for tropical, mid-latitude summer, and sub-artic winter conditions; and TOA and surface fluxes based on observed data from the Atmospheric Radiation Measurement (ARM) and Howard University Beltsville, MD sites. In the latter case flux measured by surface radiometers and satellites are used to evaluate the surface and TOA fluxes, respectively. Both clear and cloudy sky conditions are considered.

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