Verlag des Forschungszentrums Jülich

JUEL-4286
Gensch, Iulia
Water and nitric acid in cirrus clouds: microphysical kinetical modeling and a closure to field observations
IX, 105 S., 2009



Abstract

Upper tropospheric relative humidities over ice (RHice) of up to 200% have been reported frequently in recent times. This unexpectedly high supersaturation out- and inside of cold cirrus clouds may have significant impact on the Earth’s climate. In the first case, clear sky supersaturation could be justified when the critical supersaturation for ice cloud formation is higher than until now assumed. This would lead to a decrease in high cloud cover and thus impact on the radiation budget. In the second case, high supersaturation inside of cirrus clouds could suggest the existence of unknown microphysical and radiative properties with consequences for climate and the vertical redistribution of water and nitric acid. Peter et al. (2006) summarized possible reasons for the observed supersaturation in a ’supersaturation puzzle’, calling into question whether this puzzle can be solved by solely using the conventional ice cloud microphysics. Another important question raised in this study is whether the supersaturation may result from uncertainties or flaws in the water measurements. The aim of this PhD thesis is to puzzle out these questions. Therefore upper tropospheric field observations are simulated with an adequate conventional kinetic model in order to analyze the origin and persistence of high ice supersaturation, particularly inside cold cirrus clouds.
The proposed Model for Aerosol and Ice Dynamics (MAID) handles widely aerosol and ice microphysics, including: gas-diffusive particle growth and evaporation, homogeneous and heterogeneous ice nucleation, water vapor deposition and nitric acid uptake on growing ice crystal. Special emphasis of MAID is the exact balancing of chemical species among different physical states. MAID is validated here, based on observations during the field campaign POLSTAR–1 1997 (Polar Stratospheric Aerosol Experiment). Further, a detailed analysis of cirrus cloud observations during CR-AVE 2006, the tropical Costa Rica –Aura Validation Experiment, is performed with MAID. The model is initialized with different aerosol properties, water mixing ratios, accommodation factors of water on ice and amplitudes of mesoscale temperature fluctuations.
A notable feature here is to vary the freezing mechanism in the simulations. The model results indicate high sensitivity of the cloud microphysical evolution to the freezing pathway. The ice microphysics, as well as the partitioning of water and nitric acid inside the cloud derived from all sensitivity studies are compared at last with the the microphysical and chemical in-situ observations, to determine the most probable constellation of initial conditions and processes that led to the very cold, sub-visible tropical cirrus cloud observed during CR–AVE. The best agreement between model results and measurements is given when the cirrus cloud forms heterogeneously, with total accommodation of water on ice. By varying the freezing pathway, the accommodation factor of water on ice, or the amount of available water, clouds with completely different microphysical properties form.
As a summary, this work demonstrates that it is possible to simulate significant supersaturation inside cold cirrus (T<200 K) with conventional microphysics when assuming heterogeneous ice nucleation as the freezing mechanism. Thus, heterogeneous freezing appears to be an important pathway for cold cirrus cloud formation. More generally, a freezing mechanism producing low number densities of ice crystals could explain the frequent high supersaturation inside cirrus clouds observed in this temperature range.

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Letzte Änderung: 07.06.2022