Ascent rates, dehydration and vertical diffusion in the tropical tropopause region and lower stratosphere

Abstract

The transport of water vapour and short lived tropospheric species into the stratospheric overworld is determined by processes in the tropical tropopause region. The interaction between horizontal and vertical transport through this region of the atmosphere determines the amount of water vapour that is transported into the stratosphere and the timescale for chemical processing of tropospheric species before they reach the stratosphere. We have developed approaches to quantify vertical transport rates from (a) meteorological fields from assimilation models throughout the upper TTL and lower tropical stratosphere, and (b) from ozone profile data in the lower stratosphere. We show that the quantification of ascent rates from assimilation data usually suffers from excessive noise in the vertical wind fields. This results in vertical diffusion rates that are several orders of magnitude larger than those derived from measurements. Also, net vertical transport rates are often significantly too large if based on vertical winds. We have used an approach to base the vertical motion in a lagrangian transport model on calculated diabatic heating rates, either directly on isentropic surfaces or by solving the thermodynamic equation in pressure coordinates. We show that both approaches agree well and result in values for vertical diffusion and net transport that are close to values derived from measurements. A multiannual time series of vertical transport rates and corresponding residence times in the upper TTL and lower tropical stratosphere will be presented. Geographical distribuions of the Langrangian Cold Points (CLPs), conditions in these and corresponding water vapour transport will be shown. The results are compared with two Chemical Climate Models. The interannual variability and the impact of the QBO will be discussed.