The Atmospheric Boundary Layer (ABL) includes the portion of the atmosphere which is directly influenced by the presence of the Earth´s surface, and usually has much higher aerosol concentration than free troposphere above. Lidar system measures the intensity of backscattered light mainly from aerosol particles as a function of distance. Thus, the significant change in the backscatter across the top of the BL provides a means of determining ABL heights. Whereas an urban area situated in a complex mountain valley of Colombian tropical Andean region, we present the first results of estimating the height of the daytime ABL for Medellin city (Lat: 6°15' 38.37'', Long: -75° 34' 40.46'', Alt: 1483 m a.s.l.), using the wavelet covariant transform method in processing the elastic backscatter signal collected by a lidar built in Medellin city.