Ubiquitous Influence of Waves on Cirrus Cloud Occurrence in the Tropical Tropopause Layer

Cirrus clouds in the tropical tropopause layer (TTL) and stratospheric water vapor, together with their influence on stratospheric ozone chemistry, have significant impacts on the global radiation budget. However, the TTL processes controlling these have been a great uncertainty in climate simulations. One of the key regulators of the TTL dehydration process accompanied by cirrus formation is wave-induced temperature fluctuation. While a number of modeling studies have shown that waves affect cirrus clouds and dehydration in the TTL, only a handful of observations have suggested wave impacts on cloud occurrence.

Here we report ubiquitous wave influence on cirrus cloud occurrence observed in the extensive survey of the TTL over the western to eastern Pacific by NASA's unmanned aircraft Global Hawk. During the Airborne Tropical TRopopause EXperiment (ATTREX), TTL clouds dominantly observed where time anomalies of local temperatures are negative over the central and eastern Pacific. The western Pacific clouds at high altitudes (> ~16.5 km) are also highly associated with cold anomalies, while lower TTL clouds seem to be under combined effects by wave-induced formation, ice particle sedimentation and sublimation, and convective injection. In addition to the dominant cloud occurrence at cold temperature anomalies, clouds preferentially exist when a warm wave phase changes into a cold phase, suggesting the importance of cooling rate on cirrus formation. The onboard cloud physics lidar further confirms the relationships of thin cirrus layers with waves. Our findings suggest that proper representation of waves in models is critical to simulate TTL cirrus clouds, and thus to represent feedbacks related to the TTL dehydration process in climate models.