Orographic cirrus in a future climate Institute for Atmospheric and Climate Science, ETH Zurich, Universitaetsstrasse 16, 8092 Zurich, Switzerland
Received: 11 March 2009 – Published in Atmos. Chem. Phys. Discuss.: 03 April 2009 Abstract. A cloud resolving model (CRM) is used to investigate the formation of orographic
cirrus clouds in the current and future climate. The formation of cirrus
clouds depends on a variety of dynamical and thermodynamical processes,
which act on different scales. First, the capability of the CRM in
realistically simulating orographic cirrus clouds has been tested by
comparing the simulated results to aircraft measurements of an orographic
cirrus cloud. The influence of a warmer climate on the microphysical and
optical properties of cirrus clouds has been investigated by initializing
the CRM with vertical profiles of horizontal wind, potential temperature
and equivalent potential temperature, respectively. The vertical profiles
are extracted from IPCC A1B simulations for the current climate and for the
period 2090–2099 for two regions representative for North and South America.
The influence of additional moisture in a future climate on the propagation
of gravity waves and the formation of orographic cirrus could be estimated.
In a future climate, the increase in moisture dampens the vertical propagation
of gravity waves and the occurring vertical velocities in the moist simulations.
Together with higher temperatures fewer ice crystals nucleate homogeneously.
Assuming that the relative humidity does not change in a warmer climate the
specific humidity in the model is increased. This increase in specific humidity
in a warmer climate results in a higher ice water content. The net effect of a
reduced ice crystal number concentration and a higher ice water content is an
increased optical depth. However, in some moist simulations dynamical changes
contribute to changes in the ice water content, ice crystal number concentration
and optical depth. For the corresponding dry simulations dynamical changes are
more pronounced leading to a decreased optical depth in a future climate in some cases.
Revised: 04 September 2009 – Accepted: 22 September 2009 – Published: 20 October 2009
Citation: Joos, H., Spichtinger, P., and Lohmann, U.: Orographic cirrus in a future climate, Atmos. Chem. Phys., 9, 7825-7845, doi:10.5194/acp-9-7825-2009, 2009.