Theoretical Calculation of Water Vapor Continuum Absorption

The absorption coefficient as a function of frequency for H₂O-H₂O calculated at 10-K intervals
from T=220 K to T=330 K, from top to bottom, respectively.

Research Objectives

To calculate the water vapor continuum absorption from a first-principles theoretical formalism, valid for all frequencies from microwave through visible, and applicable over the full range of atmospheric pressures and temperatures.

Computational Approach

We have improved the computational efficiency and convergence rate of our previous far-wing formalism for computing continuum absorption due to H₂O-H₂O and other molecular systems. In our new formalism, the eigenfunctions of the orientation of the system, rather than the states themselves, are chosen as the complete set of basis functions in Hilbert space. With this new approach, the computational task is transformed from the previous procedure of diagonalization of large matrices to carrying out multidimensional integrations over the continuous orientational variables.

Accomplishments

The new formalism has been applied to linear molecular systems such as CO₂-CO₂ and CO₂-N₂, and converged results have been obtained. We have also obtained successful convergence for systems involving symmetric-top and asymmetric-top molecules such as H₂O-H₂O and H₂O-N₂. Detailed computations of self-broadened H₂O continuum absorption have been carried out from the microwave to visible wavelengths for a broad range of atmospheric temperatures. Given that experimental measurements of absorption coefficients for frequencies close to band centers and those in the window regions vary by many orders of magnitude, the agreement between theory and experiment is excellent, being best in the window regions where the quasi-static theory is valid, but underestimating the measured continuum absorption near band centers.

Significance

The new formalism more readily accommodates the utilization of more realistic interaction potentials involving water molecules (for which no detailed representation is currently available). The short-range anisotropic potential interactions that we have been able to include make the total potential more realistic, and thus lead to better agreement with observations. Accurate representation of the water vapor continuum absorption, and its dependence on temperature, is important for accurate radiative transfer modeling in climate energy balance applications, as well as in the remote sensing of surface and atmospheric temperatures from satellite platforms.

Publications

Q. Ma, R. H. Tipping, and C. Boulet, "A far-wing line shape theory which satisfies the detailed balance principle," J. Quant. Spectrosc. Radiat. Transfer 59, 245-257 (1998).

Q. Ma, R. H. Tipping, G. Birnbaum, and C. Boulet, "Sum rules and the symmetry of the memory function in spectral line shape theories," J. Quant. Spectrosc. Radiat. Transfer 59, 259-271 (1998).

Q. Ma and R. H. Tipping, "The distribution of density matrices over potential-energy surfaces: Application to the calculation of the far-wing line shapes for CO₂," J. Chem. Phys. 108, 3386-3399 (1998).