Supersaturation, dehydration, and denitrification in Arctic cirrus

Abstract

Abstract.A polar cirrus case study is discussed with the help of a one-dimensional model with explicit aerosol and ice microphysics. It is demonstrated that continuous cooling of air in regions with small amounts of ice and slow ice deposition rates of water vapor drives significant in-cloud supersaturations over ice, with potentially important consequences for heterogeneous halogen activation. Radiatively important cloud properties such as ice crystal size distributions are investigated, showing the presence of high number concentrations of small crystals in the cloud top region at the tropopause, broad but highly variable size spectra in the cloud interior, and mostly large crystals at the cloud base. It is found that weakly forced Arctic cirrostratus are highly efficient at dehydrating upper tropospheric air. Estimating nitric acid uptake in cirrus with an unprecedented treatment of diffusion-limited trapping in growing ice crystals suggests that such clouds could also denitrify upper tropospheric air masses efficiently, but a closer comparison to suitable observations is needed to draw a definite conclusion on this point. It is also shown that low temperatures, high ice supersaturations, and the absence of ice above but close to the cloud top region cause efficient uptake of nitric acid in background aerosol particles.

Modelling of cirrus clouds – Part 1a: Model description and validation

“…Additionally, some sensitivity studies are carried out. Second, we simulate a well-documented case of arctic cirrostratus and compare our results with 1-D simulations (Lin et al, 2005;Kärcher, 2005). Kärcher and Lohmann (2002a) used a detailed box model (particle tracking, highly resolved aerosol size distribution) for testing an analytically derived relationship between the maximum possible ice crystal number density, formed by homogeneous nucleation, and vertical velocity.…”

“…In this section we compare the performance of our bulk microphysics scheme with the spectrally resolving schemes of Lin et al (2005) and Kärcher (2005) for the case of an arctic cirrostratus triggered by a constant vertical updraught.…”

“…For detailed process studies, cloud resolving models (Starr and Cox, 1985;Jensen et al, 1994;Lin et al, 2005;Kärcher, 2005) and box models (Sassen and Dodd, 1989;Lin et al, 2002;Gierens, 2003;Haag and Kärcher, 2004;Hoyle et al, 2005;Bunz et al, 2008) were used. Many box models and cloud resolving models have very detailed microphysics schemes which require high spatial and temporal resolution.…”

In-situ observations and modeling of small nitric acid-containing ice crystals

“…In cirrus with sufficiently high IWC (>0.1-1 mg m −3 ) in the absence of very high ice supersaturations, HNO 3 is redistributed between aerosol and ice particles and uptake in ice dominates dissolution in liquid particles (Kärcher, 2005; . Thermodynamic equilibrium properties of STS aerosol particles containing 0.1 nmol mol −1 H 2 SO 4 as a function of the ice saturation ratio S i for various combinations of the HNO 3 partial pressure p n and T prevailing during the TROCCINOX observations.…”

“…In what follows we make use of a one-dimensional, cirrus cloud-resolving model with detailed STS aerosol and ice microphysics (Kärcher, 2005).…”