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

Abstract

The roles of deep convection and generated waves in the Tropical Tropopause Layer (TTL) are investigated using a global nonhydrostatic model, the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), which runs on the Earth Simulator with a horizontal spacing of 7 km. The model data, which successfully simulated a Madden‐Julian Oscillation (MJO) event for the period between 15 December 2006 and 15 January 2007, are analyzed. The frequency of deep convective clouds that reach the TTL is one of the key diagnostics for dehydration and transport. The present results revealed that the proportion of cumulus clouds that penetrate the lapse‐rate tropopause and the bottom boundary of the TTL (defined as the lapse rate minimum) is ∼0.5% and ∼20%, respectively, in the region between 5°S and 5°N. This result is reasonably consistent with atmospheric observations. Deep convective activity that reaches the TTL was observed over southern Africa, the Indian Ocean, the Indonesian maritime continent, the western Pacific, and southern America. Deep convection over the continents was most active during the local evening period. Over the oceans, high clouds reaching the tropopause were seen over the Indian Ocean and the seas around Java, where two tropical cyclones were generated. Prominent diurnal variations in tropopause temperature associated with deep convection occurred over the Indonesian maritime continent. These diurnal variations were superimposed on large, low‐frequency temperature variations associated with equatorial Kelvin waves generated by the MJO convection. Probably because of coarse vertical resolution, temperature variations simulated by the NICAM are larger than those in the real atmosphere. The two tropical cyclones caused relatively large tropopause temperature variations with a cyclone scale (∼500 km horizontally). The gravity waves generated by tropical cyclones cause small tropopause temperature variations that extend for 1000 km from the cyclone. We conclude that the Kelvin waves associated with the MJO convection cause the largest amplitude of temperature variations in the TTL and that tropical cyclones and diurnal variations of convective activity have large local impacts on temperature variations in the TTL.