Fog Events at Maceio Airport on the Northern Coast of Brazil During 2002–2005 and 2007

Abstract

There were eight fog events in five years at Maceio international airport on the northern coast of Brazil, and all were analyzed. Fog duration was short and its intensity was weak or moderate. The principal objectives of the study were: (1) analysis of the physical processes of fog formation (synoptic and thermodynamic conditions and processes), (2) PAFOG model testing, and (3) estimation of the effect of vegetation on fog forecast. Cyclonic curvature and divergence of the air current over the ocean at low levels and anticyclonic curvature at high levels were associated with the fog. Weak lifting at low levels was identified by the NCEP/DOE II, ECMWF, and WRF models for all eight events. Sinking at high levels was dominant in the ECMWF and WRF models. Absence of thermal inversion and conditional instability at low levels was identified by the NCEP/DOE II and ECMWF models. According to the WRF model a typical temperature profile during fog comprises three layers: (1) a very thin layer (up to 166 m, 985 hPa) of temperature inversion with very high humidity; (2) a conditional layer of instability from 985–860 hPa; and (3) a dry and stable layer above 860 hPa. Moderate fog with visibility between 200 and 300 m was associated with ocean cooling whereas weak fog was associated with ocean warming. A warm oscillation on the sea surface near the Brazilian northeast was observed for all fog events. It was found there was colder air over the warmer water near the coast. Weak confluence in troughs at low levels contributes to weak lifting at low levels. This current creates conditions resulting in humidity increase. A warmer sea surface contributes to more evaporation and, as a consequence, increases the amount of water vapor in the surrounding air at low levels near the coast. The PAFOG model was used to forecast the fog for three events (i.e., for all cases possible), and was satisfactory for two cases. Satisfactory results for fog duration and intensity were obtained with 9 h of antecedence. No significant effect of fast-growing sugarcane on the low visibility forecast was detected.