Influence of variations in gravity wave properties on cirrus ice number concentrations produced by homogeneous freezing in the tropical tropopause layer

Abstract

In this study, we examine the impact of variations in gravity wave properties on the ice crystal number concentrations in the tropical tropopause layer (TTL). Analysis of quasi‐Lagrangian superpressure balloon measurements from the Stratéole‐2 campaign shows that high‐frequency waves dominate the occurrence of relatively large vertical wind speeds. Both synthetic and observed time series of wave‐driven vertical wind perturbations are used to drive ensembles of homogeneous freezing ice nucleation simulations. Reducing high‐frequency variability from the time series reduces the occurrence of ice concentrations exceeding 1 cm− 3. In contrast, reducing the wave amplitudes at all frequencies simply shifts the entire frequency distribution of ice concentrations to lower values. We also derive analytic expressions for the ice concentration distribution and the mean ice concentration. The ice concentration statistics produced by these analytic expressions approximately reproduce the numerical results. Increasing high‐frequency variability increases the likelihood of vertical wind speed changing from cooling to warming during a nucleation event. If wave frequencies approaching the Brunt‐Väisälä frequency are included, this nucleation quenching effect has a significant influence on the mean ice concentration produced by homogeneous freezing. However, the increase in vertical wind speed amplitude associated with high‐frequency waves dominates over the increased occurrence of quenching events, such that the overall impact of the high‐frequency variability is to increase in ice concentrations. The observed natural variability in TTL gravity wave properties results in mean ice concentrations produced from homogeneous freezing ranging from about 0.8 to 4.9 cm− 3.