Special observations of the low-level flow over eastern Bolivia during the 1999 Atmospheric Mesoscale Campaign

During the period January-April 1999 special upper-air observations were made in Bolivia by a network of 4 pilot balloon and one radiosonde stations. Three of the sites were at low elevations in eastern Bolivia and two were established on the altiplano, at approximately 4 km altitude. Observations were made twice-daily, to identify diurnal variability. These observations were a complement to other observations made in the Brazilian state of Rondonia, as part of the Atmospheric Mesoscale Campaign. The primary objective of the Bolivian observations was to describe the spatial and synoptic variability of the lower tropospheric flow along the eastern flanks of the Andes, and especially in the vicinity of Santa Cruz. A limited observational program during 1998 had suggested that a strong low-level jet was present in the vicinity of Santa Cruz.

The 1999 observations showed weaker wind conditions than during the 1998 observation program, but that frequent low-level jets were present at Santa Cruz and the other lowland sites. The winds were strongest at Santa Cruz, and the level of maximum wind during northwesterly jets was near 1.5 km above ground level (AGL). Cold frontal passages are evident in the data set; during these episodes the maximum southerly winds were at altitudes near or below 1 km AGL. Low-level vertical shear was also noticeably stronger during southerly flow regimes.

During this presentation the network observations will be described, some preliminary results of our analysis of the data collected during 1999 will be shown, and some comparisons will be drawn with the observations of 1998. In particular we hope to show results related to the diurnal variability of the lower tropospheric flow over eastern Bolivia and the mean spatial variations of the jet.