Wavenumber Dependent Investigation of the Terrestrial Infrared Radiation Budget with Two Versions of the LOWTRAN5 Band Model

Thomas P. Charlock National Center for Atmospheric Research, Boulder, CO 80307

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Abstract

Two versions of the LOWTRAN5 radiance code are used in a study of the earth's clear sky infrared radiation budget in the interval 30 cm−1 (333.3 μm) to 3530 cm−1 (2.8 μm). One version uses 5 cm−1 resolution and temperature dependent molecular absorption coefficients, and the second uses 20 cm−1 resolution and temperature independent molecular absorption coefficients. Both versions compare well with Nimbus 3 IRIS spectra, with some discrepancies at particular wavenumber intervals.

Up and downgoing fluxes, calculated as functions of latitude, are displayed for wavenumbers at which the principle absorbers are active. Most of the variation of the fluxes with latitude is found in the higher wavenumber intervals for both clear and cloudy skies. The main features of the wavenumber integrated cooling rates are explained with reference to calculations in more restricted wavenumber intervals. A tropical lower tropospheric cooling maximum is produced by water vapor continuum effects in the 760–1240 cm−1 window. A secondary upper tropospheric cooling maximum, with wide meridional extent, is produced by water vapor rotational lines between 30–430 cm−1. Water vapor lines throughout the terrestrial infrared spectrum prevent the upflux maximum from coinciding with the surface temperature maximum.

Abstract

Two versions of the LOWTRAN5 radiance code are used in a study of the earth's clear sky infrared radiation budget in the interval 30 cm−1 (333.3 μm) to 3530 cm−1 (2.8 μm). One version uses 5 cm−1 resolution and temperature dependent molecular absorption coefficients, and the second uses 20 cm−1 resolution and temperature independent molecular absorption coefficients. Both versions compare well with Nimbus 3 IRIS spectra, with some discrepancies at particular wavenumber intervals.

Up and downgoing fluxes, calculated as functions of latitude, are displayed for wavenumbers at which the principle absorbers are active. Most of the variation of the fluxes with latitude is found in the higher wavenumber intervals for both clear and cloudy skies. The main features of the wavenumber integrated cooling rates are explained with reference to calculations in more restricted wavenumber intervals. A tropical lower tropospheric cooling maximum is produced by water vapor continuum effects in the 760–1240 cm−1 window. A secondary upper tropospheric cooling maximum, with wide meridional extent, is produced by water vapor rotational lines between 30–430 cm−1. Water vapor lines throughout the terrestrial infrared spectrum prevent the upflux maximum from coinciding with the surface temperature maximum.

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