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

The 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. 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-Vaisala-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 as the Brunt-Vaisala-frequencies decrease. This leads to less flow blocking, higher effective mountain heights and vertical velocities. 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 less new crystals can be formed. The ice water content increases as more water vapour 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.