A 42 year inference of cloud base height trends in the Luquillo Mountains of northeastern Puerto Rico

ABSTRACT: The Luquillo Mountains of eastern Puerto Rico are home to the only tropical rainforest managed by the United States Forest Service, with cloud-immersed forests historically occupying the highest elevations. However, within the past 50 yr, studies of the Luquillo cloud forest have suggested an increase in cloud base heights (CBH), although the CBH in the area was not quantified until recently. The present work uses radiosonde observations from nearby San Juan, Puerto Rico, to contextualize the present-day CBH within a 42 yr (1975-2016) proxy record and determine evidence for rising cloud base. Two key questions are addressed: (1) Can theoretical CBH calculations from San Juan provide a reasonable proxy for CBHs in the Luquillo Mountains? (2) Does a significant trend accompany the CBH lifting inferred from recent work in the region? The mean-layer lifted condensation level (MLLCL), a thermodynamic parameter expressing the altitude at which a rising air parcel reaches 100% relative humidity, serves as the proxy. The 42 yr MLLCL time series corroborates both the low CBHs claimed in the 1980s and the higher CBHs documented by recent work. When considering all available radiosonde data, statistically significant increasing CBH trends are detected for all seasons. However, when the record is standardized to correct for progressive vertical resolution improvements to radiosonde observations, recent CBH increases are more modest than initially indicated, and statistically significant increases are only apparent in the late rainfall season.