Low Visibility Formation and Forecasting on the Northern Coast of Brazil

Abstract

Visibility analysis and forecast at the Maceio International Airport in the Brazilian Northeast (NEB) was the principal goal of this investigation. Surface meteorological data of the Maceio International Airport were used for low visibility frequency study. Low visibility in NEB was provoked more frequently by light fog (LF) formation (1,098 or 92 h month−1 on average). Haze and fog were very rare (81 h and one event per year, respectively on average). Light fog with a visibility less than 2 km usually was detected together with rain or drizzle. Low visibility was observed more frequently at night and during the rainy season. Applications of the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model for light fog forecast were tested. Thermodynamic processes were studied by vertical profile, elaborated by: (1) National Centers for Environmental Prediction (NCEP) reanalysis data for Maceio (because of some radiosonde absence) and (2) forecast vertical temperature and humidity profiles were produced, using Air Parcels Trajectories of the HYSPLIT model at the pattern levels. The synoptic situations before and during low visibility phenomena were analyzed using different products of NCEP reanalysis, the high resolution (10 km) ETA model and infrared satellite images. Wave disturbance in the trade winds field, localized on the northwest periphery of the South Atlantic subtropical High, usually accompanied the phenomena. A humidity advection, weak ascendant movement and thermal inversion absence at the low levels were created by these waves. The middle level’s descendent movement provoked the humidity accumulation at levels below. Satisfactory results of the HYSPLIT model applications for light fog forecast were obtained with 12 h antecedence. In particular, stable level forecast by the ETA model was forecast satisfactorily with 12 h antecedence; vertical movements were predicted better with up to 48 h antecedence. The PSU/NCAR mesoscale model (MM5) and PAFOG models were tested for analysis and forecast of an intensive fog event. Intensive fog provoked a fatal accident of a small airplane near the Maceio Airport in 2007. These fog formation processes were studied by NCEP reanalysis data, the high resolution regional model MM5, and satellite and radar data. Fog formation was simulated by PAFOG model and satisfactory results were obtained with 10 h antecedence.