The daily cycle of the atmospheric boundary layer over pasture site in Amazonia

In the last decades a great effort has been made by the scientists and/or operational meteorologists from micrometeorology and boundary layer for a better understanding of the dynamics and the description of the properties of the atmospheric boundary layer (ABL), especially by its height. Thus, the main goal of this work is to integrate and compare data from the heights of ABL measured or estimated by several different instruments as radiosonde, tethered balloon and SODAR. This data set tends to be representative for the daily cycle evolution of daytime (Convective Boundary Layer) and nighttime (Nocturnal Boundary Layer) over a pasture area at southwest Amazonia during the transition period from the end of dry to the onset of wet season. The measurements were made at Nossa Senhora Aparecida farm (10° 45' S, 62° 22' W; 293 m), a cleared/pasture area, which has been used in LBAxs micrometeorological field campaigns since the 90s. The data-set was obtained during the RaCCI/LBA (Radiation, Cloud and Climate Interactions in the Amazonia/Large Scale Biosphere-Atmosphere Experiment in Amazonia) field experiment, conducted from September 6 up to November 3, 2002. In order to estimate the height of the CBLxs heights, 3 different procedures were used (a profile method, a parcel method and the Richardson number) using basically profiles of thermodynamic and dynamic paremeters. For the nighttime (NBL), the heights were computed by the profile method (using data from radiosondes and tetherballoon) and the SODARxs estimation using the height of maximum wind. During the daytime, the CBL calculated by the method of the profiles showed an initial height of 434 ± 44 m at 08 LT and a subsequent growth rate about 210.8 m h-1 up to 11 LT due to surface heating. At late afternoon due to the lower solar radiation flux, this rate reduced to approximately 9.2 m h-1 however reaching a maximum height of 1504 ± 257 m at 17 LT. The profile method has traditionally been used as an indicator of the height of the CBL. The parcel method has tendencies to overestimate CBL especially in areas with high incidence of solar radiation and superadiabatic gradients, which is the case of the pasture areas in the Amazon, and it showed a growth rate of 216.8 mm h-1 between 08 and 14 LT. The final height of the CBL by this method is 1584 ± 16 m at 17 LT after reaching a maximum about 1880 ± 125 m at 14 LT. The values obtained from the method of Richardson number are very similar to the profile method at 17 LT, only 22 m of difference, with an underestimation of 35 m for the entire daytime, representing a growth rate of 117.0 m h-1 (from 08 up to 17 LT ). The values obtained by SODARxs software for the CBLxs heights virtually do not present a daily cycle, with a constant value around 780 m. These figures do not represent the classical knowledge on the evolution of the CBL height and are quite different from those observed by other methods and previous work. Although the radiosondes are not the ideal instruments for determining the height of the NBL, their estimated values can provide a broader view of the NBL and showed a maximum development of 342 ± 92 m at 08 LT, associated with an average rate growth of 14.6 m h-1 from 20 LT to 08 LT. The SODARxs estimates, through the identification of the thermal inversion by an internal calculation presents an average height 757 ± 308 m and overestimated the heights measured by the radiosondes (509 ± 219 m). Previous work had noticed that this heights is not representative of the NBL top, but of a residual layer resulted from the previous day. Assuming the fact that Vmax can represent the height of the NBL and it is coincident with the level of occurrence of maximum wind are, these heights were re-computed and has an average of 262 ± 120 m. The heights of the NBL between 02 and 08 LT obtained by the Vmax method and by the estimates from radiosondes are very close, with an average difference of only 8 m. At the evening (20 LT) the height of the NBL was underestimated by -104 m. Performing a general analysis it was possible to observe that there is an abrupt growth of the CBL between 08 and 11 LT (late morning) with a stationary pattern between 14 and 17 LT (late afternoon). The maximum heights at late afternoon were typically around 1500 m. The onset of the NBL occurs before the sunset (18 LT) and its height is very stable during the whole night (typical values around 180-250 m). The erosion of the NBL lasts for 2 hours and it is fully eroded at 08 LT.