The Mesoscale Dynamics of Thin Tropical Tropopause Cirrus

Thin cirrus clouds in the tropical tropopause layer (TTL) are warmed through the absorption of infrared radiation. The response of the cloud and the surrounding atmosphere to this thermal forcing is investigated through linear theory and nonlinear numerical simulation. Linear solutions for the circulations forced by a fixed heat source representative of TTL cirrus clouds show ascent in the region of the heating, accompanied by horizontal flow toward the heat source at the base of the heated layer and horizontal outflow at the top of the layer. Gravity waves propagate positive temperature perturbations well beyond the lateral edges of the heated region. Cool layers that also spread horizontally are produced immediately above and below the heated region. Numerical simulations with a cloud-resolving model allow the radiative heating to change in response to the redistribution of the cloud by the evolving velocity field. The basic atmospheric response in the numerical simulations is nevertheless similar to that generated by the fixed heat source. In the numerical simulations, the advection of ice crystals by the radiatively forced velocity field also lofts the cloud, while horizontally spreading its top and narrowing its base. Ice crystal sedimentation is neglected in these calculations, but it appears that the radiatively induced upward vertical velocities are likely strong enough to maintain clouds consisting of very small crystals (radii less than 4 μm) against sedimentation for many hours.