Characterization of the atmosphere to the passage of squall lines at dakar from 1968 to 2006

By performing a composite analysis of the vertical profile of humidity, temperature and wind on the basis of radiosounding data and rain gauge Dakar Yoff (Senegal) at 14 ° 44 N and 17 ° 30 W, in the period 1968-2006, we studied exchanges water, heat and momentum within the squall line (SL) and between it and its immediate environment. A similar analysis has been performed with the re-analysis of NCEP / NCAR for the period 1981-1995 for the specific humidity at a grid point near Dakar (15° N and 17 ° 30 W). A statistical study of clouds and rain, combined with the composite analysis of moisture profile showed that the precipitations generated by the squall line (SL) are mainly from the convective part. With less than ten hours before the passage of the squall line, there is an increase in relative humidity in the lower layers of the atmosphere. The passage of this, the humidity is maximum in the layer 700-600 hPa. After its passage, humidity decreases gradually in the lower and middle layers. Analysis of the results obtained on the basic parameters of the data with NCEP / NCAR shows that the vertical distribution of the atmosphere is disturbed at the passage of a SL; the composite analysis yielded 48 temporal classes. The state of the atmosphere during the 24 hours preceding the passage of SL is characterized by: - A net warming at 700hPa up to 6 hours before; - A significant crowding in the layer 850-700 hPa up to 13 hours before; - A west wind at 925 hPa between 10 and 5 hours before; - An increase in wind speed at 700hPa up to 15 hours before. During the passage of SL, the characteristics of the atmosphere are: - A strong cooling in the layer located below 600hPa; - A noticeable humidification in the layer 850-700hPa; - A strong east wind at 700 hPa between 4 hours and 9 hours later.