The effects of mineral dust acting as both cloud condensation and ice nuclei within organized convective cloud systems over the tropical Atlantic basin will be investigated through a combination of observational analysis and numerical simulations. Idealized simulations will be conducted using the Regional Atmospheric Modeling System (RAMS) to further examine dust aerosol impacts on tropical convective clouds and precipitation are to be guided by observations collected during NAMMA, NASA's African Monsoon Multidisciplinary Activities project, conducted during the 2006 Atlantic hurricane season. The lifecycle of an actual storm (Tropical Storm Debby), which was apparently influenced by ingestion of Saharan dust over the eastern Atlantic, will be simulated in detail. The dynamical and radiative impacts of the Saharan Air Layer (SAL, which carries dust westward from Africa) will be studied, and the relative importance of direct and indirect aerosol effects as well as environmental influences of the SAL itself--including humidity, temperature and vertical wind shear--will be assessed. Observations of aged dust reaching the far western Atlantic upon deep convective clouds in that region will be examined using additional observations from the NSF-supported PREDICT (Pre-Depression Investigation of Cloud-systems in the Tropics) experiment in 2010.
The intellectual merit of this effort centers on improved understanding of cloud processes (and in particular cloud and precipitation microphysics) in the presence of influences by dust and other aerosols originating over distant continental regions, and determination of the extent to which these influences may favor or disfavor tropical storm and hurricane formation.
Broader impacts will come through support of two principal investigators drawn from an underrepresented group (viz. women in the atmospheric sciences), through a combination of university training and field experience for a graduate student, and ultimately through improved ability to anticipate dust-induced changes in tropical storm formation and/or intensity key to accurate forecasts of Atlantic basin hurricanes.
Throughout the year, vast quantities of soil-dust particles embedded in the dry Saharan Air Layer travel from Africa westward over the tropical Atlantic Ocean. These particles can act to seed droplets and ice crystals in tropical storms that form over the Atlantic, and may develop into hurricanes. Through these microphysical changes, particles can influence how strong and deep storms are, how much they rain, how long they last, and how they absorb heat and scatter light. These effects are likely to differ depending on whether dust particles form liquid droplets, ice crystals, or both. The main goal of this collaborative work was to understand the impacts of dust particles on tropical storms by simulating their effects with a cloud resolving model that includes realistic microphysics. Observations were used to guide and test the simulation results. The team included researchers experienced in both numerical modeling and field experiments, as well as graduate students just beginning their research career. Improvements to the numerical model were made to better calculate how dust particles form ice, and to track where dust particles move throughout cloud systems. Simulations included both idealized simulations of individual convective clouds and larger tropical cyclones, as well as a case study of the 2006 Tropical Storm Debby. Results showed that the properties of storms were very different depending on whether dust particles were allowed to form ice crystals only, or to form both liquid droplets and ice crystals in the cloud (the latter case thought to be more realistic). Specifically, those clouds where dust formed liquid drops as well as ice particles had fewer, larger ice crystals at the top of the clouds, had more dust removed to the ocean surface in rain, and thus had less residual dust in the clouds overall. In simulations of idealized tropical convective storms, the addition of dust particles that can form both liquid and ice particles helped to invigorate the storm at low to mid-levels compared to a clean maritime environment. When dust aerosols were only allowed to form ice particles, the invigoration was concentrated higher in the clouds where heterogeneous ice nucleation was most prominent. In the idealized tropical cyclone simulations it was found that dust particles caused the storm intensity to increase (stronger tangential winds, lower surface pressures) and storm size to decrease when the dust penetrated to the core of the storm. Also, in the Tropical Storm Debby simulation, the track and characteristics of the storm were better reproduced when dust particles were included. The idealized and Tropical Storm Debby simulations conducted provided training for a graduate student, and will form part of his PhD dissertation. Clouds sampled by aircraft over the Atlantic near Africa were analyzed for dust concentrations. It was found that dust was present in liquid drops and was the most common particle type in high-level ice crystals. However, dust concentrations in ice at high altitudes were orders of magnitude smaller than dust amounts at lower levels in the Saharan Air Layer. This is consistent with the model results only when dust forms both liquid drops and ice crystals in simulations. Other particle types in the atmosphere that can efficiently initiate ice formation in clouds are biological particles, such as bacteria and fungi. These particles sometimes co-exist with dust particles, but not much is known about their abundance in the atmosphere above ground-level. New instrumentation was tested on an aircraft to sample these particle types at different altitudes and in clouds over the U.S. Central Plains. This provided important validation of the instrument, as well as hands-on training for a graduate student. Results showed that concentrations of biological particles were quite low, but non-negligible, above the atmospheric boundary layer in this region. Our results, which have been published in a number of peer-reviewed journal publications, have important implications for how dust and biological particles interact with clouds. First, dust seems to be important in influencing the development and ice formation in tropical storms that may become hurricanes. However, because dust particles can also form liquid droplets, many of these particles will be removed and deposited on the ocean surface within tropical precipitation, thus limiting their effects in the atmosphere and storm systems. Thus dust particles have the potential to provide essential nutrients for microscopic life in the oceans, potentially impacting the amount of carbon that the ocean removes from the atmosphere. Further studies will include satellite data in order to quantify how much Saharan dust is removed from storms over the Atlantic, and how much remains to influence clouds over the American continents.
