Cirrus, transport and mixing in the tropical tropopause layer

The transport and mixing induced by the radiative heating associated with cirrus clouds in the tropical upper troposphere is investigated. Numerical simulations show that the interaction between the large-scale circulation induced by the cloud ensemble and the mesoscale circulations induced by individual clouds in the ensemble causes substantial horizontal mixing. Consequently, the effective diabatic transport timescale of air and tracers is significantly longer than the timescale indicated by the time average of the radiative heating rate of the cloud ensemble.

Model calculations show that mixing is persistent regardless of the lifetime and spatial dimension of individual cirrus in the cloud ensemble. In fact, it is relatively insensitive to characteristics of individual clouds, as long as these characteristics are typical for cirrus clouds in the atmosphere. Conversely, the amplitude of the large-scale radiative heating asymmetry between cloudy and non-cloudy regions has a large effect on mixing and hence transport timescale. Consequently, the transport timescale does not decrease linearly with increasing time-mean radiative heating in the cloudy region, because the effect of stronger cloud radiative heating (which tends to shorten the transport timescale) is partially compensated by the increase in mixing (which tends to increase the transport timescale).