Influence of gravity wave temperature anomalies and their vertical gradients on cirrus clouds in the tropical tropopause layer – a satellite-based view

Abstract

Negative temperature perturbations (T′) from gravity waves are known to be favorable to tropical tropopause layer (TTL) clouds, and recent studies have further suggested a possible role of dT′/dz in facilitating TTL cloud formation and maintenance. With a focus on exploring the influence of dT′/dz on TTL clouds, this study utilizes radio occultation temperature retrievals and cloud layers from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) to understand how gravity wave perturbations modulate cloud occurrence in the tropics. Cloud populations were evaluated in four phases corresponding to positive or negative T′ and dT′/dz. We find that 55 % of TTL clouds are found where T′ and dT′/dz are both negative. Regions of frequent convection are associated with higher cloud populations in the warm phase T′>0. We show that the partitioning of cloud population among wave phases exhibits dependence on background relative humidity. In the phase where T′ and dT′/dz are both negative, the mean cloud effective radius is the smallest of all four phases, but the differences are small. It is shown that the strongest mean negative T′ anomaly is centered on the cloud top, resulting in positive dT′/dz above the cloud top and negative dT′/dz below. This negative T′ anomaly propagates downward with time, characteristic of upward propagating gravity waves. Negative (positive) T′ anomalies are associated with increased (decreased) probability of being occupied by clouds. The magnitude of T′ correlates with the increase or decrease in cloud occurrence, giving evidence that the wave amplitude influences the probability of cloud occurrence. While the decrease in cloud occurrence in the warm phase is centered on the altitude of T′ maxima, we show that the increase in cloud occurrence around T′ minima occurs below the minima in height, indicating that cloud formation or maintenance is facilitated mainly inside negative dT′/dz. Together with existing studies, our results suggest that the cold phase of gravity waves is favorable to TTL clouds mainly through the region where dT′/dz is negative.