Thin Cirrus and Horizontal Transport Studies

 

Research Staff: Henry Selkirk, Leonhard Pfister, Marion Legg, Eric Jensen

This is a three-year program of diagnostic and modeling studies to elucidate two key issues that bear upon our understanding of the maintenance of water vapor and ozone distributions in the tropical tropopause layer (TTL) between 14 and 19 km. The primary issue is the origin and persistence of high-altitude, thin cirrus clouds and their role, either direct or indirect, in controlling the concentrations of water vapor and ozone in the TTL. A second issue is the spatial distribution of quasi-horizontal in-mixing from higher latitudes into the TTL that may impact the vertical structure of ozone in the TTL.

We are presently engaged in convective influence analyses of airborne differential absorption lidar (DIAL) observations of thin cirrus from NASA missions in the tropical Pacific and Atlantic oceans over the course of the past decade. This analysis will allow us to determine if these thin cirrus layers in the TTL are residuals left over from deep convective events or independent cloud features formed in situ from slow, radiatively-forced ascent.

At the completion of the convective influence analysis we will proceed to simple modeling calculations of the thin cirrus events using the explicit microphysical model developed at NASA-Ames. In these calculations, we will attempt to reproduce the gross microphysical characteristics of the observed thin cloud in the context of the thermal history of the air parcels derived from the convective influence analyses.

The investigations of horizontal transport employ trajectory analysis techniques to address the spatial variations of ozone in the TTL that may be due to the horizontal transport from higher latitudes, in particular the longitudinal gradient in ozone that has been observed in the equatorial Pacific.

Point of Contact: Henry B. Selkirk, 650/604-6489, hselkirk@mail.arc.nasa.gov