Abstract

ABSTRACT 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 that isentropic surfaces stay nearly flat. Alternatively, 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, regardless of its optical density. 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, subgrid-scale radiative-dynamic atmospheric interactions might be efficiently parameterized within slower, coarse-grid climate models.