ISDAC

Indirect and Semi-Direct Aerosol Campaign (ISDAC)

1 April 2008 - 30 April 2008

Lead Scientist: Steven Ghan

Observatory: AAF , NSA

An intensive cloud and aerosol observing system was deployed to the ARM Climate Research Facility’s (ACRF) North Slope of Alaska (NSA) locale for three weeks in April 2008. This period was chosen because it was during the International Polar Year when many ancillary observing systems were collecting data that would be synergistic for interpreting the Indirect and Semi-Direct Aerosol Campaign (ISDAC) data. It also provided an important contrast with the October 2004 Mixed-Phase Arctic Cloud Experiment (M-PACE). Thirty to 45 hours of flight time were required with aircraft capable of measuring temperature, humidity, total particle number, aerosol size distribution, aerosol hygroscopicity, cloud condensation nuclei concentration, ice nuclei concentration, optical scattering and absorption, updraft velocity, cloud liquid water and ice contents, cloud droplet and crystal size distributions, cloud particle shape, and cloud extinction. In addition to these aircraft measurements, there was a surface deployment of a spectroradiometer for retrieving cloud optical depth and effective radius. These measurements will be used by members of the ARM Science Team to answer the following key questions: 1. How do properties of the arctic aerosol during April differ from those measured during the MPACE in October? 2. To what extent do the different properties of the arctic aerosol during April produce differences in the microphysical and macrophysical properties of clouds and the surface energy balance? 3. To what extent can cloud models and the cloud parameterizations used in climate models simulate the sensitivity of arctic clouds and the surface energy budget to the differences in aerosol between April and October? 4. How well can long-term surface-based measurements at the ACRF NSA locale provide retrievals of aerosol, cloud, precipitation, and radiative heating in the Arctic? By using many of the same instruments used during M-PACE, we were able to contrast the arctic aerosol and cloud properties during October and April. The aerosol measurements can be used in cloud models driven by objectively analyzed boundary conditions to test whether the cloud models can simulate the aerosol influence on the clouds. The influence of aerosol and boundary conditions on the simulated clouds can be separated by running the cloud models with all four combinations of M-PACE and ISDAC aerosol and boundary conditions: M-PACE aerosol and boundary conditions, M-PACE aerosol and ISDAC boundary conditions, ISDAC aerosol and M-PACE boundary conditions, and ISDAC aerosol and boundary conditions. ISDAC and M-PACE boundary conditions were likely to be very different because of the much more extensive ocean water during M-PACE. The uniformity of the surface conditions during ISDAC greatly simplifies the objective analysis (surface fluxes and precipitation are very weak), so that it can largely rely on the European Centre for Medium-Range Weather Forecasts analysis. The ISDAC cloud measurements can be used to evaluate the cloud simulations and to evaluate cloud retrievals. The aerosol measurements can also be used to evaluate the aerosol retrievals. By running the cloud models with and without solar absorption by the aerosols, we can determine the semi-direct effect of the aerosol on the clouds.