Diurnal cycle over the South American Altiplano: comparison of NCEP/NCAR reanalysis with upper-air observations during the Visviri field experiment

As part of a research project aimed at improving the understanding of the climate dynamics during the wet season over the highland region in the central Andes (Altiplano), intensive surface and upper-air measurements (4 rawinsoundings a day) were carried out at Visviri (17.60 °S, 69.50 °W, 4070 m.a.s.l) during two 10-days periods in January 1994 (Visviri I) and January 1995 (Visviri II). Soundings were performed at 09, 13, 17 and 21 hours (local time) in Visviri I, and at 09, 13, 18 and 24 hour (local time) during Visviri II.

The average vertical profiles of temperature, humidity, wind and geopotential height, and the associated diurnal cycles during the Visviri I and Visviri II field experiments are compared with those obtained from the 03, 09, 15 and 21 hour (local time) NCEP re-analysis at the closest grid point (17.50 °S, 70.00 °W). The re-analisis are able to capture the major features in the vertical structure of these variables and also the interannual changes from the relatively wet period in Visviri I to the much dried conditions in Visviri II. Nevertheless, there are some significant discrepancies both in the strenght and timing of the diurnal cycle and particularly in the westward transport of water vapor across the Andes, that is significantly under estimated by the re-analysis.