Kinematics and dynamics of the stratocumulus average diurnal dissipation process at Atacama Desert

Abstract

Stratocumulus-type clouds constitute a stable system over the Southeast Pacific Ocean. Their phenomenology is well known particularly over the ocean, but a quantitative analysis of their diurnal cycle over the Pacific continental coastal desert strip of South America remained unaddressed. In this work, the stratocumulus average morning dissipation process at two sites in the Atacama Desert is documented through simple empirical mathematical equations. Based on the change rate analysis of the visible cloud optical depth (COD) inferred from solar irradiance measurements, the kinematics and the dynamics of the stratocumulus dissipation process is established, showing that it develops at an average constant “acceleration of dissipation” (d2COD/dt2) of about −3.3 h−2, equivalent to a cloud geometrical vertical thickness change at an average constant “acceleration of thinning” of about −40 m/h2. Towards the end of the dissipation process when cloudiness vanishes, the average “velocity of dissipation” (dCOD/dt) and “velocity of thinning” round −16 h−1 and −175 m/h, respectively. A simple plane-parallel optical and geometrical model explains the average behavior of the phenomenon in the region on a wholly radiative base, with an absorbed fraction of solar irradiance acting as exclusive driver of the average dissipation process. About 13 kJ/m2 of net irradiation is needed to change the COD of the stratocumulus deck by one unit. The progressive morning cloud dissipation from the land on the east towards the sea on the west is evidenced. The analysis of the cloud effective transmittance and a full set of checks, including test of robustness, support these results and their interpretation.