The Effect of Deep Convection on Temperatures in the Tropical Tropopause Layer and Its Implications to the Regulation of Tropical Lower Stratospheric Humidity

Abstract

This dissertation focuses on the impact of deep convection on the thermal structure in the Tropical Tropopause Layer (TTL). Temperatures in this region play an important role in the regulation of water vapor, which in turn affects radiation, chemistry, and dynamics in the lower stratosphere. This dissertation includes two important conclusions concerning the regulation of temperature in the TTL.

First, significant cooling near the tropical tropopause is observed during the time when active convection is occurring. A composite technique is used to relate the local temperature anomalies to the evolution of local convection. Temperature profiles are measured by the Atmospheric Infrared Sounder (AIRS) onboard the Aqua satellite, and the time evolution of local convections are determined by the National Centers for Environmental Protection / Aviation Weather Center (NCEP/AWS) half-hourly infrared global geostationary composite. The observations demonstrate that the TTL is cooled by convection, in agreement with previous observations and model simulations. By using a global data set, the variations in this convective cooling are investigated by season and region. The estimated cooling rate during active convection is - 7 K/day. This exceeds the likely contribution from cloud-top radiative cooling, suggesting turbulent mixing of deep convection plays a role in cooling the TTL.

Second, height and thermal structure of the overshooting deep convection in the TTL are investigated using visible and infrared observations from the Visible and Infrared Scanner (VIRS) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. The heights of overshooting clouds are estimated from the sizes of the visible shadows that these clouds cast. The temperature information is obtained from the mid-infrared channel. From these, the lapse rate in the cloud is estimated. The result shows that the measured lapse rate of these clouds is significantly below adiabatic. Mixing between these clouds and the near-tropopause environment is the most likely explanation. As a result, these clouds will likely settle at a final altitude above the convections' initial level of neutral buoyancy.