Abstract
Abstract. The degree to which cloud immersion provides water in addition to rainfall, suppresses transpiration, and sustains tropical montane cloud forests (TMCFs) during rainless periods is not well understood. Climate and land use changes represent a threat to these forests if cloud base altitude rises as a result of regional warming or deforestation. To establish a baseline for quantifying future changes in cloud base, we installed a ceilometer at 100 m altitude in the forest upwind of the TMCF that occupies an altitude range from ∼ 600 m to the peaks at 1100 m in the Luquillo Mountains of eastern Puerto Rico. Airport Automated Surface Observing System (ASOS) ceilometer data, radiosonde data, and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite data were obtained to investigate seasonal cloud base dynamics, altitude of the trade-wind inversion (TWI), and typical cloud thickness for the surrounding Caribbean region. Cloud base is rarely quantified near mountains, so these results represent a first look at seasonal and diurnal cloud base dynamics for the TMCF. From May 2013 to August 2016, cloud base was lowest during the midsummer dry season, and cloud bases were lower than the mountaintops as often in the winter dry season as in the wet seasons. The lowest cloud bases most frequently occurred at higher elevation than 600 m, from 740 to 964 m. The Luquillo forest low cloud base altitudes were higher than six other sites in the Caribbean by ∼ 200–600 m, highlighting the importance of site selection to measure topographic influence on cloud height. Proximity to the oceanic cloud system where shallow cumulus clouds are seasonally invariant in altitude and cover, along with local trade-wind orographic lifting and cloud formation, may explain the dry season low clouds. The results indicate that climate change threats to low-elevation TMCFs are not limited to the dry season; changes in synoptic-scale weather patterns that increase frequency of drought periods during the wet seasons (periods of higher cloud base) may also impact ecosystem health.
Highlights
Mountains play a key role in collecting atmospheric moisture in tropical regions (Wohl et al, 2012)
The data suggest that the tropical montane cloud forests (TMCFs) has a greater probability of cloud interaction during the midsummer dry season throughout the elevation range of the forest between 600 and 1000 m and during the winter dry season at the upper elevational reaches (< 1000 m) than it does in the wet seasons (Figs. 2, 3a)
No metrics of daily clouds correlated with Mean sea level pressure (MSLP) except at the Luquillo Experimental Forest (LEF) site, where the low and maximum cloud bases had good and very good correlation with MSLP, respectively (Table 3)
Summary
Mountains play a key role in collecting atmospheric moisture in tropical regions (Wohl et al, 2012). Smaller mountains are likely to have clouds at lower elevations due to slightly higher adiabatic lapse rates (more temperature loss with the same elevation gain) than larger mountains, which undergo greater heating of the land mass (the mass-elevation effect: Foster, 2001; Jarvis and Mulligan, 2011). This effect and the higher humidity near the ocean support TMCFs on small coastal mountains.
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