Analysis of the tropical tropopause layer using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM): Aqua planet experiments

Abstract

The dynamical characteristics of the tropical tropopause layer (TTL) are investigated using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) run on the Earth Simulator under an aqua planet condition. Two experiments are performed: one with a 3.5 km horizontal spacing and a three‐dimensional snapshot output and another with a 7 km horizontal spacing and 3‐hourly averages for 1 month. The number of vertical levels is 54 and the model top is at 40 km; the vertical spacing in and around the TTL is ∼700 m. Large‐scale organized convection associated with convectively coupled equatorial Kelvin waves prevails around the equator. The zonal mean vertical distribution of cloud top height near the equator shows a realistic trimodal structure. The simulation results reveal that cumulus clouds penetrate the lapse‐rate tropopause and the bottom boundary of the TTL (defined as the lapse rate minimum) for ∼0.1% and ∼25%, respectively, in the equatorial area. The frequency distribution of vertical wind may provide a good indicator of the TTL bottom boundary. A significant reduction in the speed of strong vertical winds is observed at ∼16 km. High variability in temperature and the water vapor mixing ratio observed around the tropopause is mainly caused by equatorial Kelvin waves generated by the organized convection in these experiments. Horizontal variability in tropopause height over a large‐scale convective system is much smaller than that in the area of Kelvin waves. The gravity waves generated by this large‐scale convective system locally control the temperature around the tropopause. In addition, large‐amplitude gravity waves with a scale of 600 km are superimposed on the cold phase of Kelvin waves, producing one of the coldest regions around the tropopause. It is suggested that the combination of Kelvin waves and gravity waves may be one of the most effective dehydration processes in the TTL.