Analyzing cloud base at local and regional scales to understand tropical montane cloud forest vulnerability to climate change

摘要

The degree to which cloud immersion provides water in addition torainfall, suppresses transpiration, and sustains tropical montane cloudforests (TMCFs) during rainless periods is not well understood. Climate andland use changes represent a threat to these forests if cloud base altituderises as a result of regional warming or deforestation. To establish abaseline for quantifying future changes in cloud base, we installed aceilometer at 100 m altitude in the forest upwind of the TMCF that occupiesan altitude range from ∼ 600 m to the peaks at 1100 m in the LuquilloMountains of eastern Puerto Rico. Airport Automated Surface Observing System (ASOS)ceilometer data, radiosondedata, and Cloud-Aerosol Lidar and Infrared Pathfinder SatelliteObservation (CALIPSO) satellite data were obtained to investigate seasonal cloudbase dynamics, altitude of the trade-wind inversion (TWI), and typical cloudthickness for the surrounding Caribbean region. Cloud base is rarelyquantified near mountains, so these results represent a first look atseasonal and diurnal cloud base dynamics for the TMCF. FromMay 2013 to August 2016, cloud base was lowest during the midsummer dryseason, and cloud bases were lower than the mountaintops as often in thewinter dry season as in the wet seasons. The lowest cloud bases mostfrequently occurred at higher elevation than 600 m, from 740 to 964 m. TheLuquillo forest low cloud base altitudes were higher than six other sites inthe Caribbean by ∼ 200–600 m, highlighting the importance of siteselection to measure topographic influence on cloud height. Proximity to theoceanic cloud system where shallow cumulus clouds are seasonally invariant inaltitude and cover, along with local trade-wind orographic lifting and cloudformation, may explain the dry season low clouds. The results indicatethat climate change threats to low-elevation TMCFs are not limited to the dryseason; changes in synoptic-scale weather patterns that increase frequency ofdrought periods during the wet seasons (periods of higher cloud base) mayalso impact ecosystem health.