Stratus fog formation and dissipation: a six-day case study

 

Authors

Martial P. Haeffelin — Institut Pierre Simon Laplace

Alain Protat — Australian Bureau of Meterology
Jean-Charles Dupont — Laboratoire de Météorologie Dynamique (LMD), IPSL
Dominique Bouniol — GAME/CNRM, CNRS/Météo-France
Neda Boyouk — LMD/IPSL/Ecole Polytechnique

Category

Aerosol-Cloud-Radiation Interactions

Description

A suite of active and passive remote sensing instruments and in situ sensors deployed at the SIRTA Observatory (Instrumented Site for Atmospheric Remote Sensing Research) near Paris, France, for six months (October 2006 to March 2007) document simultaneously radiative, microphysical, and dynamic processes driving the continental fog life cycle. The study focuses on a six-day period between December 23–29, 2006, characterized by several stratus cloud lowering and lifting events and almost 18 hours of visibility lower than 1 km. The formation, the development, and the dissipation phases of four major stratus fogs are here analyzed to quantify the impact of each physical process. The coupling between the surface and the stratus cloud altitude shows a very good agreement for the six-day period except for some specific periods studied here. A first dense but short in duration fog event is driven by the in-cloud dynamics. The turbulent mixing rate and the sedimentation process (vertical velocity near -1 ms-1) are the dominant processes involved in the dissipation of the fog. In two other cases, the significant near-surface turbulence with a turbulent kinetic energy in excess of 1 m2s-2 induced by wind shear modulates the intensity of the stratus cloud lowering periods. The study also shows the impact of a high-altitude cloud on the infrared radiative cooling at the stratus cloud top. The cloud-top infrared flux budget ranges from -160 to -130 Wm-2, and it corresponds to strong changes in radar reflectivity (-10 to -22 dBZ) and in-cloud velocity (-0.3 to 0 ms-1), producing a strong variability in visibility.