Influence of a future climate on the microphysical and optical properties of orographic cirrus clouds in ECHAM5

Abstract

The European Centre/Hamburg 5 (ECHAM5) general circulation model is used in order to investigate the influence of a warmer climate on the microphysical and optical properties of orographic cirrus clouds. The main goal of this study is to highlight the variety of processes influencing the formation of orographic cirrus and to emphasize the importance of coupling dynamics and cloud microphysics in order to provide realistic predictions of the influence of a future climate on cloud microphysical and radiative properties. Therefore, a coupling of gravity wave dynamics and cloud microphysics is implemented in the model. The influence of additional moisture on the propagation of gravity waves is investigated by using the dry and moist Brunt-Väisäla frequency in the calculation of the gravity wave-induced vertical velocity in two different simulations. In both simulations, the vertical velocities increase in the warmer climate. The additional moisture decreases the Brunt-Väisäla frequencies leading to less flow blocking and thus higher effective mountain heights. As this effect dominates over the decrease of the gravity wave amplitude due to more moisture, higher vertical velocities occur in a future climate. The opposite effect of a decreased vertical velocity in a future climate can be seen over the dry regions. From the present to the future climate, the ice crystal number concentration decreases despite the increased vertical velocities. Higher temperatures lead to a faster growth of the ice crystals, and the supersaturation is depleted faster such that no new crystals can be formed. The ice water content increases as more water vapor is available in a warmer climate. The net effect of a decreased ice crystal number concentration and an increased ice water content is an increased optical depth in a future climate. This result is in good agreement with recent cloud resolving studies. The effect of orographic cirrus clouds on the radiation is given by an increased short- and long-wave cloud forcing, whereas the latter dominates. However, from the present to the future climate, no changes in orographic cloud cover and cloud forcing over mountains can be seen. Copyright 2010 by the American Geophysical Union.