The representation of gravity wave activity in models of cirrus ice formation

Abstract. Internal gravity waves have a profound impact on the nucleation of cirrus ice crystals in the stably stratified upper troposphere. Cloud models require the use of parameterization schemes that represent gravity wave-induced variability in vertical wind speeds with high fidelity. Accounting for such variability in microphysical models is a stringent test of their ability to accurately simulate ice nucleation events. However, such schemes are sparse and have not been characterized in detail. This study investigates the impact of several wave-cirrus parameterizations on the formation of tropical tropopause layer cirrus and compares distributions of nucleated ice crystal number concentrations with the statistic obtained directly from long-duration, quasi-Lagrangian superpressure balloon measurements. Fluctuation distribution, autocorrelation function, and power spectral density from both measurements and parameterizations are analyzed. Simulating homogeneous freezing of supercooled aerosols based on the balloon-borne data and on parameterizations with exponentially-distributed vertical wind speed fluctuations yields consistent results. The causes of small discrepancies in ice concentrations are traced back to filtering of the balloon data necessary to remove non-geophysical artifacts and differences in power spectra across the buoyancy frequency between the measurements and models. A brief outline of future research topics motivated by this study offers pointers of how to augment and further improve the representation of gravity wave forcing in both process-based and global models.