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Abstract
One event of three consecutive days and another of four consecutive days of Stratus clouds were registered at the Maceio airport on the northeast coast of Brazil. This is a very rare phenomenon for equatorial regions and it was analyzed using the high resolution (10km) ETA model, NCEP reanalysis, satellite data and conventional data supplied by the Hydrometeorological Directory DHM-SERHI/AL and by the Department of Flight Protection of the Zumbi dos Palmares airport. Wave disturbance in the trade winds and weak ascendant movements at low levels (1000 and 925hPa) were identified. At the same time, anticyclonic curvature at high levels formed descendant movements and subsidence inversion. These vertical movements combined with a stable layer between 850 and 700hPa created a very humid layer at the low levels. The precipitation forecasted by the ETA model with 24h antecedence presented a satisfactory result for an average quantity of precipitation. Nevertheless, the spatial distributions of forecasted and observed precipitation were different. Precipitation during all days was not forecasted for the coastal region, and for the west region the forecasted precipitation was more intense than the observed ones.
Highlights
The Stratus (St) cloud formation and related low visibility create adverse conditions for all transport and especially for aviation
This wave disturbance in the trade winds (WDTW) was observed in NCEP reanalysis data and in high resolution ETA model (Figures 3 and 4)
Fog and stratus-clouds were associated with WDTW at low levels, which was localized at the southern periphery of the ITCZ and on the northwest periphery of the South Atlantic subtropical High
Summary
The Stratus (St) cloud formation and related low visibility create adverse conditions for all transport and especially for aviation. Very few investigations of St cloud formation and associated precipitation have been elaborated for the northern Brazilian coast. Frequencies of fog and stratus cloudiness formation were higher in the south than on the northern coast (Silveira, 2003, Fedorova et al, 2008). Stratus clouds were observed during 61 and 33 days per year for these regions, respectively. Fog/stratus formation was associated, in general, with wave disturbance in the trade winds (WDTW) field on the north coast. Classification of the temperature and dew-point vertical profiles for different fog/stratus physical processes was developed (Fedorova et al, 2008). Stable layers in the vertical profiles were not observed on the north coast and the humid layer was very narrow in all cases
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