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

Abstract

[1] 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.

Topographical Effects on Internally Produced MJO-Like Disturbances in an Aqua-Planet Version of NICAM

“…The aqua-planet experiments were first designed by Hayashi and Sumi (1988) for studying super cloud clusters and are nowadays used for deeper understanding of tropical convective systems (Blackburn and Hoskins 2013). In fact, the aqua-planet experiments of NICAM were extensively performed and analyzed previously by Tomita et al (2005), Satoh et al (2005), Miura et al (2005), Nasuno et al (2007), Mapes et al (2008), Kubokawa et al (2010), Sherwood et al (2010), Yoshizaki et al (2012a, b), and .…”

Analysis of the tropical tropopause layer using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM): 2. An experiment under the atmospheric conditions of December 2006 to January 2007

“…In this paper we use experimental data from the NICAM simulation performed by Miura et al [2007]. In the current model experiment, as in the aqua planet experiment [ Tomita et al , 2005; Kubokawa et al , 2010], the horizontal spacing of the icosahedral grid system was set as 3.5 km and 7 km. In this study, we concentrate on the 7 km grid data because the 3.5 km grid experiment was made only for 7 days, while the 7 km grid experiment was made for one month.…”

“…As discussed in our previous paper [ Kubokawa et al , 2010], many studies have investigated the role of deep convective clouds in the temperature field around the tropopause. Both mesoscale, and cloud‐resolving models have been used to examine the role of deep convection in controlling dehydration within the TTL [ Potter and Holton , 1995; Chaboureau et al , 2007; Grosvenor et al , 2007], in the heat and mass budget of the TTL [ Kuang and Bretherton , 2004; Küpper et al , 2004], and in rapid transport of near‐surface air into the TTL [ Barth et al , 2007].…”

“…We will therefore investigate the roles of these cloud disturbances on temperature variability in the TTL. Compared with the aqua planet experiment analyzed by Kubokawa et al [2010], the MJO experiment include a realistic orography and sea surface temperature distribution that produced realistic diurnal variations with geographical features and cyclone activity, as well as large‐scale wave disturbances in the TTL. Furthermore, the regional and local‐time characteristics of penetrating high clouds are important for rapid vertical transport of near‐surface air into the TTL and the lower stratosphere.…”

The Non-hydrostatic Icosahedral Atmospheric Model: description and development

“…This was further investigated by Yasunaga et al (2013) by using the NICAM aqua planet experiment. The detailed structures of convective systems near the tropopause were analyzed by Kubokawa et al (2010).…”