Surface radiative impacts of ash deposits from the 2009 eruption of Redoubt volcano

The solar broadband albedo change, surface radiative forcing, and snowmelt rate associated with ash deposits based on those from the 2009 eruption of Redoubt volcano were calculated using the field-corroborated loadings from the Fall3D and the SNow, ICe, and Aerosol Radiation models. The optical properties of ash were calculated from Mie theory, using size information from the Fall3D model. Two sizes of snow grains were used in order to simulate a young and old snowpack. The results show concentrations of aerosol-sized ash in snow range from similar to 6.9 x 10(4) to 1 x 10(8) ppb, for the distal edge of the deposits (located similar to 100-570 km from the vent) to the vent, and integrated solar albedo reductions of similar to 0-59% for new snow and similar to 0-85% for old snow. These albedo reductions are much larger than those typical for black carbon and are of the same order of magnitude as those reported for volcanic deposits in Antarctica. The daily mean surface shortwave forcings associated with ash deposits on snow were similar to 0-96 Wm(-2) from the most distal deposits to the near-vent deposits. We show that forcings caused by ash deposits can be greater than those caused by dust deposits. There were no accelerated snowmelts calculated for the edges of the deposits. However, for areas of higher ash concentrations, daily melting rates were conservatively estimated to be similar to 140-160% higher than those of pure snow. We find that ash deposits from midsized volcanic eruptions can be a major agent of deposit-induced snowmelt.