Several interferometric synthetic aperture radar applications could benefit from the availability of a closed-form model for the second-order statistics of atmospheric delay. Due to the variability of the latter, it would also be desirable for the model to be tunable to some acquisition-specific information, describing the atmospheric state. In this letter, a closed-form expression for the zenith delay structure function of tropospheric propagation delay is derived from a two-regime power spectral density function reported in the literature. The power at a specific spatial frequency is used as a free model parameter, which may be tuned to available measurements or, in the absence of these, to atmospheric statistics. The latter approach is used to compare the derived model with previously published results.