Fortmann, Martin
Influence of tropospheric aerosols on the Arctic climate
Ber. Polarforsch. Meeresforsch. 486 (2004)
ISSN 1618 - 3193
"Abstract
Tropospheric aerosols are particles of liquid and solid material suspended in the air and derived from natural and anthropogenic sources. They directiy influence the radiative balance of the earth-atmosphere system by scattering and absorbing solar radiation and, to less degree, by absorbing infrared radiation. Tropospheric aerosols also alter the cloud microphysical and optical properties by acting as condensation and ice nuclei and affect thereby radiation budget indirectiy. The magnitudes of both the direct and indirect aerosol effects are largely unknown and therefore the tropospheric aerosols are one of the key uncertainty parameters in climate change estimates.

During winter and spring seasons enhanced aerosol concentrations in the Arctic troposphere are observed. This phenomenon, known as 'Arctic Haze', results from the long range transport of anthropogenic pollution from industrial areas in mid-latitudes into the Arctic. Because estimates for future climate show largest warming signal over Arctic region it is important to investigate the climate forcing of these aerosols.
In the present work the direct climate effect of 'Arctic Haz' has been studied with a regional atmospheric climate model covering the whole Arctic north of 65ON. Two alternative methods were applied to incorporate the Arctic aerosol into the model. For the first method the microphysical and optical parameters of the Arctic aerosol components (sulfate, black carbon and sea salt particles) are taken from the Global Aerosol Data Set [GADS] (KOPKE et al., 1997), whereas the second method uses aerosol input Parameters derived from data transformation of Arctic airborne aerosol measurements.

At first, simulations with and without included 'Arctic Haze' were performed for a March ensemble of the years 1983 and'1989-95. The 'Arctic Haze' was described via a mixture of different components of the GADS (method 1) and was considered to be horizontally uniformly distibuted over the Arctic. At the top of the atmosphere the aerosols cause a positive change from 1 to 8 W/m2 in the net radiation fluxes over the ice- and snow-covered Arctic areas, whereas mostly a radiative energy loss up to 2 W/m2 results over the ice-free North Atlantic and Pacific. At the surface the aerosols lead to a shortwave energy loss by reducing the insolution. This effect is largest over the water (up to -7.5 W/m2). Averaging over the whole Arctic domain, the shortwave energy loss is about -2.4 W/m2. At the Same time the longwave energy loss decreases by 0.7 W/m2 at the surface, resulting in a total radiative forcing of -1.7 W/m2.

Although the aerosol acts only via the direct radiative effect by scattering and absorption of radiation, it affects not only the radiative fluxes but also the dynamical variables via the nonlinear interaction within the model. An important interaction is the aerosolradiation-cloud-circulation-feedback. The direct radiative forcing leads to diabatic heating rate changes in the vertical column of each model grid cells, which modify the circulation pattern, the humidity and cloud structures which in turn alter the aerosol optical depth and therefore the radiative forcing closing this feedback loop. The mean aerosol effect On the 2m air temperature shows a large spatial variability and ranges from -1 K (over the East Sibirian Sea and the Canadian Archipelgo) to +1 K (between Spitsbergen-Barents and Kara Sex-Taimyr peninsula). The monthly mean temperature anomaly is characterized by a large interannual variability up to 2 K. This can be explained by the strong synoptic variability which is connected with pronounced regional weather changes. Especially changes of air humidity, cloud Cover and surface albedo contribute to significant
changes of the magnitude and sign of the aerosol effect. In the zonal mean the simulations show a mean cooling in the near surface layers in contrast to a warming within the aerosol layer. 'Arctic Haze' therefore leads to a change of atmospheric stability. By amplifying the prevalent temperature Inversion in the lower Arctic troposphere the vertical exchange processes of turbulent fluxes of momentum, heat and water vapour between atmosphere and surface are reduced.

Several sensitivity studies were performed. The strength of the calculated aerosol effect also depends strongly on the effective absorption of the solar radiation by 'Arctic Haze'. It was also investigated how the applied specific cloud parameterizations affect the calculated direct aerosol effect. The regional pattern of the aerosol effect changed dramatically by using a new ice cloud parameterization which works better for Arctic conditions. In a further model simulation the influence of sulfate aerosols on the number of cloud condensation nuclei was considered. The calculation shows that the direct and indirect effect of 'Arctic Haze' seems to be of similar importance.
The last part of this work presents a model run for the ASTAR (Arctic Study of Tropospheric Aerosol and Radiation) capaign month March 2000 using aerosol parameters derived from airborne measurements in the vicinity of Spitsbergen (method 2). In this case the signal in the near surface temperature is characterized by substantial variability
at regional scale and varies between a cooling of -2 K in the Baffin Bay and Laptevsee and a warming of 3 K in the Beaufort Sea."
http://epic.awi.de/26665/1/10491.pdf