Downscaling MODIS-derived maps using GIS and boosted regression trees: The case of frost occurrence over the arid Andean highlands of Bolivia

Frost risk assessment is of critical importance in tropical highlands like the Andes where human activities thrives at altitudes up to 4200 m, and night frost may occur all the year round. In these semi-arid and cold regions with sparse meteorological networks, remote sensing and topographic modeling are of potential interest for understanding how physiography influences the local climate regime. After integrating night land surface temperature from the MODIS satellite, and physiographic descriptors derived from a digital elevation model, we explored how regional and landscape-scale features influence frost occurrence in the southern altiplano of Bolivia. Based on the high correlation between night land surface temperature and minimum air temperature, frost occurrence in early-, middle- and late-summer periods were calculated from satellite observations and mapped at a 1-km resolution over a 45,000 km² area. Physiographic modeling of frost occurrence was then conducted comparing multiple regression (MR) and boosted regression trees (BRT). Physiographic predictors were latitude, elevation, distance from salt lakes, slope steepness, potential insolation, and topographic convergence. Insolation influence on night frost was tested assuming that ground surface warming in the daytime reduces frost occurrence in the next night. Depending on the time period and the calibration domain, BRT models explained 74% to 90% of frost occurrence variation, outperforming the MR method. Inverted BRT models allowed the downscaling of frost occurrence maps at 100-m resolution, illustrating local processes like cold air drainage. Minimum temperature lapse rates showed seasonal variation and mean values higher than those reported for temperate mountains. When applied at regional and subregional scales successively, BRT models revealed prominent effects of elevation, latitude and distance to salt lakes at large scales, whereas slope, topographic convergence and insolation gained influence at local scales. Our results highlight the role of daytime insolation on night frost occurrence at local scale, particularly in the early- and mid-summer periods when solar astronomic forcing is maximum. Seasonal variations and interactions in physiographic effects are also shown. Nested effects of physiographic factors across scales are discussed, as well as potential applications of physiographic modeling to downscale ecological processes in complex terrains. (resume d'auteur)