Development of a Highly Accurate and Flexible Radiative Transfer Package for Planetary Atmospheres, 15-R9561

Principal Investigators
Mark A. Bullock
Kandis Lea Jessup

Inclusive Dates:  07/01/05 – 01/01/07

Background - Detailed, high spectral resolution calculations of infrared fluxes emitted (or absorbed) by any mixture of gases at any temperature and pressure can in principle be obtained by solving the radiative transfer equation using optical data from the HITRAN database. Comparisons with remote infrared spectroscopic observations can be made by iterating the radiative transfer calculations until they yield the best-fit concentration of the gas species of interest. With the appropriate optical data and numerical techniques, the effects of a wide range of aerosols on infrared fluxes can be calculated. These techniques are extremely computationally expensive, suited to the retrieval of trace gas abundances in the atmosphere of a planet from a spacecraft in orbit, for example, once the data have been acquired. There are many applications where a high level of accuracy is required, but where the detection of a specific gas or gas mixture must be made in real time. Highly efficient, but lower-accuracy algorithms do exist that can calculate the average fluxes in predefined spectral windows, lowering the ability to discriminate unknown gases, but greatly speeding up the calculations. 

Approach - The purpose of the project has been to develop a fast, highly accurate, and versatile radiative transfer computer package. Utilizing recent databases on the spectral properties of gases, the performance of this package exceeds the capabilities of existing radiative transfer programs that are used for remote sensing of trace gas emissions, remote retrieval of atmospheric constituents of planetary atmospheres, the removal of atmospheric contamination of remotely sensed solid surfaces, and the determination of atmospheric temperatures in general circulation models. It incorporates full multiple scattering calculations so that the effects of hazes, clouds, and dust are self-consistently treated in the atmospheric radiation model.

Accomplishments - The radiative transfer package, SwPRT, has been built and tested. The primary inputs are the pressures, temperatures, and gaseous mixing ratios of the atmosphere or portion of the atmosphere for which the calculations are made. One of two paths may be chosen at the outset — a slow, computationally expensive, but highly accurate method that calculates line-by-line absorption coefficients. Speed versus accuracy is adjustable over a range of about 20 by adjusting the number of points used to describe each spectral line. A fast, less accurate, but more adjustable calculation is possible through the use of correlated-k coefficients. Cumulative absorption coefficients for each gas are calculated and tabulated on a coarse pressure-temperature grid for each gas. Adjustable speed versus accuracy knobs are the number of windows the spectrum is divided into and the number of Gauss points that are used for integration. A tradeoff of about a factor of 50 in speed versus accuracy is possible.

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