Analytic model of upper tropospheric clouds in the tropical Hadley cell

We have developed a two-dimensional analytic model that describes the behavior of upper tropospheric clouds in the tropical Hadley cell. The behavior of the model is characterized by two dimensionless parameters: one is proportional to the temperature lapse rate, and the other is relevant to the initial altitude of the cloud particles. We derived analytic expressions for the motion and evaporation of the cloud particles and calculate various cloud properties, such as the cloudiness and the column water content. We found that the outflow from the convective regions spreads out and generates a cirrus layer whose coverage has a maximum at the tropopause: the cloudiness is as large as 0.5-1 in the vicinity of the tropopause for small cloud particles with radii of less than 5 mu m. We suggest that the thin cirrus clouds observed near the tropopause in the tropic region are formed by the advection of cloud particles supplied from the ITCZ. Because of its simpleness, the present model may play a role in diagnosing cloud properties in climate models that are used to study climate changes over a long time span.