Experiments are carried out for spectroscopic studies using hypersonic jet of carbon monoxide seeded in argon as a carrier gas. Probing of this jet using cavity ring-down spectroscopy revealed a double peak structure for various absorption lines. Flow field simulation using computational fluid dynamics is used to understand the shape of such lines integrated over line of sight. Absorption contribution from warmer non-isentropic part of the jet, owing to its transverse velocity variation, is found responsible for those line shapes. (C) 2016 Elsevier B.V. All rights reserved.