Ceilometer Observation of Seasonal and Diurnal Variation in Cloud Cover Fraction, Cloud Base Height, and Visual Range in the Eastern Amazon Region Matthew Czikowsky *(1), David Fitzjarrald (1), Ricardo Sakai (1), Osvaldo Moraes (2), Otavio Acevedo (2), Rodrigo da Silva (2) (1) Atmospheric Sciences Research Center, University at Albany, State University of New York, USA (2) Universidade Federal de Santa Maria, RS, Brazil *Corresponding Author: Cloud Cover Fraction Cloud Base Backscatter Profile At an old-growth forest site of LBA-ECO (Km67), located in the Tapajos National Forest off Kilometer 67 of BR-163 south of Santarem, a Vaisala CT-25K ceilometer was installed in April, 2001 and remained operational through June, The ceilometer provides 15-second measurements of cloud base (three levels up to 7500 m), echo intensity, and a 30-m resolution backscatter profile. The ceilometer reports vertical visibility during periods when the sky is obscured but a cloud base is not detectable. The ceilometer was operational for a sufficient amount of time to examine wet-to-dry season variations in cloud cover fraction and cloud base height. Site Location Map of the weather stations and flux-measurement sites operating in the Santarem region (STM) of LBA-ECO. Elevation (m) is shaded. Ceilometer site Photograph of the ceilometer in operation at km67. Km67 cloud base, LCL; Km77 LCL, wet season Km67 cloud base, LCL; Km77 LCL, dry season Median cloud cover fraction at km67 by hour of day for the wet season (February through May, plotted in blue) and the dry season (September through December, plotted in black) for 2001 to The quartiles are indicated by the bars. Note the presence of convective cloudiness during the day in the dry season and the absence of clouds at night in the dry season. Cloud cover fraction peaks during the morning in the wet season. Top panel: Median monthly cloud cover fraction (red dots; bars denote quartiles) for the daytime period ( GMT) from April 2001 to June Bottom panel: As in top panel but for the nighttime period (before 1000 and after 2100 GMT) plotted in blue. The wet-to-dry season transitions are clearly seen in both day and night periods. The dry season is characterized by nearly clear nighttime conditions. In 2003, the onset of the dry season appears to have occurred early, as seen by the decrease to near-zero nighttime cloud fraction and near 0.2 daytime cloud fraction in May-June Such decreases were not observed until July in both 2001 and Top panel: Cloud base at km67 (black), lifting condensation level (LCL) at km67 (blue), and LCL at km77 (pink) during a wet season period in 2001 (May 2-11, days ). Bottom panel: As in top panel but for a dry season period in 2001 (October 2-12, days ). Note that the LCL at km77 (a cleared agricultural site) corresponds to the km67 cloud base better than the LCL at km67 (an old-growth forest site). Medians by hour of daytime cloud base at km67 during the dry season (September through December; black) and wet season (February through May; blue) for Medians by hour of daytime LCL at km77 for the dry season (red) and the wet season (green) for The bars denote the quartiles. The cloud base during dry season afternoons is around 1300m, during wet season afternoons this decreases to about 750m. The km77 LCL corresponds well to the km67 cloud base during the dry season, but slightly underestimates the km67 cloud base in the wet season. Backscatter profile (units are in 10000*srad*km) -1 at km67 for May 4-8, 2001 (days ). The enhanced echoes detected within 100m of the ground are indicative of precipitation. Top panel: Integrated backscatter from 0 to 100m (units are in 10000*srad*km) -1 at km67 for May 4-8, 2001 (days ). Bottom panel: Precipitation (mm) measured at km67 during the same time period. Note that in many cases the precipitation detected in the backscatter profile is measured at the ground (see days 124, 125). However, there are other cases (such as during day 128) when little or no precipitation was measured during periods when precipitation was detected in the backscatter profile, possibly due to factors such as forest canopy interception and wind.