Tropospheric transit time distributions

Abstract:

The time scales for tropospheric transport from the northern hemisphere midlatitude surface are examined using three-dimensional chemical transport model simulations and trace gas measurements. Simulations of the transit time distributions (TTDs) show broad distributions with long tails and mean transit times (ages) significantly larger than the modal ages at all locations. However the TTD shape varies, e.g., the ratio of the width to mean of the TTD is larger than 2 in northern high latitudes but is less than 0.7 in the SH. Estimates of the mean age and width from trace gases show similar spatial variations, including large meridional gradients in the mean age in northern subtropics, and a decrease in the width/mean shape parameter from southern subtropics to the south pole. The above variations in the TTDs will be discussed in terms of the contributions of fast transport paths relative to slow eddy-diffusive recirculation, and related to tropospheric dynamics.