On the feasibility of a fast forward model for Doppler interferometry in the infrared

Abstract

AbstractA fast forward radiative transfer model to compute the emitted limb radiances for use in recovering line‐of‐sight Doppler winds from thermal line emission has been developed and evaluated. This study was formulated around the Stratospheric Wind Interferometer for Transport Studies (SWIFT) instrument—a limb imaging, field widened, phase‐stepping Michelson interferometer. The fast forward model is required to simulate a four‐point interferogram from which line‐of‐sight winds may be computed. The model, RT‐SWIFT (Radiative Transfer for SWIFT), is part of an effort to develop the capability of providing simultaneous measurements of horizontal wind velocity vectors and ozone concentration in the stratosphere for improving our understanding of global stratospheric dynamics and for studies in ozone transport.Based in part on RT‐MIPAS (Radiative Transfer for the Michelson Interferometer for Passive Atmospheric Sounding), RT‐SWIFT uses a linear regression algorithm to parametrize the effective layer optical depths and can simulate the effect of variable ozone, nitrous oxide, water vapour and atmospheric winds. Trace gases are included as fixed climatological profiles.The model development involved the selection of two suitable databases consisting of appropriate atmospheric absorber and wind profiles, and the accurate line‐by‐line modelling of their transmittances; one for generating regression coefficients and one consisting of simulated measurements for independent evaluation. The development also required selecting suitable predictors for the absorbers, winds and viewing geometry.One‐dimensional inversion results using RT‐SWIFT with simulated measurements indicate that the model leads to wind and ozone error levels of about 3 to 5 m s−1 and 3 to 15% respectively for altitudes above ∼24 km. While further investment in model development and optimization is warranted, this demonstration study indicates that a sufficiently accurate fast forward model for near real‐time assimilation, and inversion, of horizontal Doppler wind measurements from thermal line emission is feasible. © 2011 Crown in the right of Canada. Published by John Wiley & Sons Ltd.