A 42-Yr Assessment of Cloud-Base-Height Trends in the Luquillo Mountains of Eastern Puerto Using Radiosonde Observations from San Juan

The Luquillo Mountains of eastern Puerto Rico are home to the only tropical rainforest in the United States, with cloud-immersed forests historically occupying elevations greater than 600 m. These forests have the highest rate of rainfall in the country (nearly 5 m each year) and are home to dozens of endemic plants and animals. However, within the past 25 years, studies focusing on the Luquillo cloud forest have indicated progressively higher cloud base heights (CBH), a troubling trend for this ecosystem. The purpose of this research is to contextualize the present-day CBH within a 42-yr (1975–2016) CBH proxy record using radiosonde observations from nearby San Juan, PR. Three 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 changes inferred from recent work in the region? (3) Do CBHs during the last decade have a historical precedent within the period of record?

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 mean monthly MLLCLs closely align with in situ CBH observations made with a Vaisala CL31 laser ceilometer sited in the rainforest beginning in 2013. When the surface wind direction placed the ceilometer on the windward face of the mountain, the mean absolute error of the MLLCL calculation is 37.5 m.

The 42-yr MLLCL time series corroborates both the low CBHs claimed by papers in the 1980s and the higher CBHs reported by more recent work. However, the higher, present-day MLLCLs are shown to have a historical precedent in the late 1970s. Consequently, there is no statistically significant change in CBH when considering the entire period of record. However, a decrease (p < 0.05) in the seasonal CBH changes is detected, yielding a lower amplitude annual cycle of CBH.