Fog and stratus formation on the coast of Brazil

The physical and synoptic processes of fog and stratus cloudiness formation at the northern and southern coasts of Brazil were investigated. The frequencies of both phenomena were higher at the southern coast. The synoptic situation patterns for fog/stratus formation were identified using the following products of NCEP reanalysis: streamlines, pressure, omega and relative humidity at different levels. Fog/stratus formation is associated, in general, with a wave disturbance in the trade winds (WDTW) field at the north coast. Moreover, stratus clouds were observed on the cold front periphery, Easterly wave and under the Upper Tropospheric Cyclonic Vortex. The principal synoptic processes of fog formation at the southern coast are a High dislocation along the east coast of South America and a warm core barotropic Low occurrence north of Argentina. Fog formation is initiated between these two synoptic systems. There were 1 or 2 h of fog duration at the north coast and on average 12 h at the south coast. A variety of meteorological elements and phenomena were studied before and during the fog/stratus days, and results of an atmosphere instability analysis are presented. A classification of vertical profiles (from NCEP reanalysis data) of temperature and dew-point for different fog/stratus physical processes was also developed. At the north coast no stable layers in the vertical profiles were observed and the humid layer was very narrow in all cases. An intense inversion or stable layer with a high humidity up to 950–670 hPa is responsible for fog development at the south coast.