Two-Dimensional Dynamics of Ice Crystal Parcels in a Cirrus Uncinus

A simple 2D non-stationary parcel model has been elaborated by coupling kinematics and microphysics of ice crystals, for the purpose of retrieving realistic trajectories in a cirrus uncinus, with a hooked-shape head or cap, a trail or virga, and observed lifetimes. Prescribed kinematics and ambient physics includes vertical profiles of horizontal winds and updraft, and of air temperature and humidity. Modelled microphysics combines mass variation by deposition/sublimation of water vapour, radiative transfer and ventilation due to friction with ambient air. Upon changing the altitude of the updraft base, other parameters being unchanged, two distinct regimes of motions are found: setting the base at an altitude of 10 km yields a steady, non-oscillatory motion, with a hooked cap and a tail, typical of a cirrus uncinus, whereas fixing the base at a lower altitude (7–9 km), yields a self-sustained damped oscillatory motion with a long time period (7–10 hours) and a long wavelength (70–200 km), depending on supersaturation level and radiative conditions. This latter phenomenon may be related to the turrets occurring with “long-lasting” cirrus or to the Mesoscale Uncinus Complex (MUC) reported in published works. The sensitivity to the ice crystal size is analysed in order to estimate the spreading of trajectories, and the impact of radiative transfer is examined on the oscillatory regime at the light of three contrasting examples. A simple analytic model with kinematics and constant updraft is aimed at an outline of salient features and derivation of characteristic time- and spatial scales of the phenomena.