  This paper presents a simple analytical framework for the dynamic response of
cirrus to a local radiative flux convergence, expressible in terms of three
independent modes of cloud evolution. Horizontally narrow and tenuous clouds
within a stable environment adjust to radiative heating by ascending gradually
across isentropes while spreading sufficiently fast so as to keep isentropic
surfaces nearly flat. More optically dense clouds experience very concentrated
heating, and if they are also very broad, they develop a convecting mixed
layer. Along isentropic spreading still occurs, but in the form of turbulent
density currents rather than laminar flows. A third adjustment mode relates to
evaporation, which erodes cloudy air as it lofts. The dominant mode is
determined from two dimensionless numbers, whose predictive power is shown in
comparisons with high resolution numerical cloud simulations. The power and
simplicity of the approach hints that fast, sub-grid scale radiative-dynamic
atmospheric interactions might be efficiently parameterized within slower,
coarse-grid climate models.
