Do cloud properties in a Puerto Rican tropical montane cloud forest depend on occurrence of long-range transported African dust?

TitleDo cloud properties in a Puerto Rican tropical montane cloud forest depend on occurrence of long-range transported African dust?
Publication TypeJournal Article
Year of Publication2014
AuthorsSpiegel J.K, Buchmann N., Mayol-Bracero O.L, Cuadra-Rodriguez LA, Diaz C.JV, Prather KA, Mertes S., Eugster W.
JournalPure and Applied Geophysics
Date Published2014/09
Type of ArticleArticle
ISBN Number0033-4553
Accession NumberWOS:000341927200022
Keywordsaerosol; aerosol indirect; Caribbean; Caribbean trade wind cumulus; desert dust; droplet; effect; fog; fogwater fluxes; mineral dust; particles; precipitation; saharan dust; trade winds; tropical montane cloud forest

We investigated cloud properties of warm clouds in a tropical montane cloud forest at Pico del Este (1,051 m a.s.l.) in the northeastern part of Puerto Rico to address the question of whether cloud properties in the Caribbean could potentially be affected by African dust transported across the Atlantic Ocean. We analyzed data collected during 12 days in July 2011. Cloud droplet size spectra were measured using the FM-100 fog droplet spectrometer that measured droplet size distributions in the range from 2 to 49 A mu m, primarily during fog events. The droplet size spectra revealed a bimodal structure, with the first peak (D < 6 A mu m) being more pronounced in terms of droplet number concentrations, whereas the second peak (10 A mu m < D < 20 A mu m) was found to be the one relevant for total liquid water content (LWC) of the cloud. We identified three major clusters of characteristic droplet size spectra by means of hierarchical clustering. All clusters differed significantly from each other in droplet number concentration (), effective diameter (ED), and median volume diameter (MVD). For the cluster comprising the largest droplets and the lowest droplet number concentrations, we found evidence of inhomogeneous mixing in the cloud. Contrastingly, the other two clusters revealed microphysical behavior, which could be expected under homogeneous mixing conditions. For those conditions, an increase in cloud condensation nuclei-e.g., from processed African dust transported to the site-is supposed to lead to an increased droplet concentration. In fact, one of these two clusters showed a clear shift of cloud droplet size spectra towards smaller droplet diameters. Since this cluster occurred during periods with strong evidence for the presence of long-range transported African dust, we hypothesize a link between the observed dust episodes and cloud characteristics in the Caribbean at our site, which is similar to the anthropogenic aerosol indirect effect.

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