Cloud frequency climatology at the Andes/Amazon transition: 1. Seasonal and diurnal cycles

Tropical montane regions present a complex local climate but one that may be very sensitive to local and global change. Therefore, it is important to assess their current climatological state, and to understand how the large-scale circulation may affect local-scale cloud patterns. We examine the cloud climatology of a tropical Andean montane region in the context of tropical South American climate in terms of seasonal/diurnal cycles using a corrected ISCCP (International Satellite Cloud Climatology Project) DX cloud product (1983-2008), MODIS (Moderate Resolution Imaging Spectroradiometer) MOD35 visible cloud flags (2000-2008) and ground-based cloud observations. Cloud climatologies were compared for three elevation zones: highlands (puna grassland), eastern slope (the montane forest) and lowlands. We found that in the dry season (JJA) the study area is part of a localized region of higher cloud frequency relative to other parts the eastern slope, and also relative to the adjacent highlands and lowlands. The highlands exhibited the greatest amplitude mean annual cycle of cloud frequency, with a minimum in June for all times of day. There were contrasts between the three zones with regard to the month in which the minimum cloud frequency occurs between different times of day. Higher lowland and eastern slope cloud frequencies compared with those on the puna in the early hours in the wet season suggest low-level convergence at lower elevations. Comparisons between satellite products show that ISCCP and MODIS produce very similar annual cycles although the absolute cloud frequencies are higher in ISCCP data.