Project Description

Interest is growing in the long range transport of dust on a global scale and the overall impact of dust on climate. The largest dust sources occur in the northern hemisphere with major contributors being the Sahara and Sahel regions in North Africa which emit between 60-200 millions of tons per year. Every year, large amounts of African dust (AD) are transported by the trade winds over vast areas of the North Atlantic, the Caribbean, and the Amazon basin. These dust particles influence the Earth’s radiative budget directly by scattering solar radiation in the atmosphere and indirectly by affecting cloud formation and, thus, cloud albedo. As yet, the underlying physico-chemical processes of dust-aerosol interactions are poorly understood; even less understood is how aging impacts cloud properties and climate as the particles travel from Africa to the Caribbean region. This is especially true for the highly under-studied Caribbean region, a region heavily influenced by trade winds and hurricane storm tracks, which has frequent input of long-range transported AD, and plays a crucial role in global climate by serving as an atmospheric conduit between tropical and extra-tropical regions. The proposed projects will combine field and modeling investigations to address how the physico-chemical properties of long range transported African dust (LRTAD) aerosols influence Caribbean cloud properties and precipitation levels in a unique Puerto Rican tropical montane cloud forest (TMCF). TMCFs occur globally in the Americas, Africa, and Asia and represent a primary source of drinking water. Two major hypotheses will be tested: (1) Cloud properties in the TMCF are different during intense LRTAD periods and (2) LRTAD has unique chemical and physical properties which influence cloud properties and processes. 

The PRADACS study will be performed in Puerto Rico (PR), at Pico del Este (PE) and Cape San Juan (CSJ) stations. PE is in a TMCF with an elevation of 1051m asl which is at/above the cloud condensation level. This location facilitates the study of clouds without the need for aircraft studies which reduces the overall complexity and costs while allowing longer term observations within a single cloud. CSJ is a near sea level regional station of the Global Atmosphere Watch, upwind from PE, and is the first contact point for the trade wind air masses in PR. This station allows the sampling of the properties of the aerosol that enter the cloud (below–cloud aerosol). We will collect data on the chemical and physical properties of aerosols, clouds, and rainwater samples via long term measurements and during an intensive field campaign in year 2. In the second phase of the project, using these results in conjunction with existing data sets of precipitation, cloud cover, temperature and humidity profiles and surface energy budgets, we will use the Weather Research and Forecasting model to simulate the sensitivity of precipitation distribution to LRTAD events, particularly focusing on the length and intensity of the summer dry season in the Caribbean. 

This project will contribute to “closing the gap” in understanding the potential impacts of AD transport events on the Caribbean region. It will demonstrate how AD inputs change cloud properties in a TMCF and the potential consequences for precipitation. It will also improve predictions of aerosol/climate interactions and the capability of climate models to simulate the influence of AD on the radiative balance of the Earth’s atmosphere. AD events in the Caribbean are seasonal, and their occurrence coincides with the summer dry-wet cycle. Increases in the frequency and intensity of AD events could in turn intensify the dry cycles; this could enhance summer drought, with potentially important consequences to endemic species in the TMCF, for the water supply, and to terrestrial and aquatic ecosystems in the Caribbean region. We envision that our findings will apply to other tropical regions that are highly vulnerable to climate change.

 El Yunque National Forest (Pico del Este) as seen from Cabezas San Juan with and without African dust