The Radiative Impact of Water Ice Clouds from a Reanalysis of Mars Climate Sounder Data

Abstract:

We use a data assimilation scheme coupled to a global climate model (GCM) to investigate the radiative impact of water ice clouds in the atmosphere of Mars. Temperature profiles from Mars Climate Sounder (MCS) are assimilated directly into the GCM. The water ice opacity profiles are used to build a four-dimensional time-space map of water ice clouds, which is then used by the GCM to produce the radiative forcing associated with clouds. This procedure allows clouds to be inserted into the model at the correct time and location, producing the most realistic state from which to analyse cloud radiative effects. The resulting data set allows a detailed study of the atmospheric state that is not possible using observations or models alone. The results show that tropical clouds have a greater impact on the local atmosphere than polar hood clouds, increasing diurnally-averaged temperatures at the 10 Pa level by around 10–15 K. The small radiative impact of the polar hood clouds may be caused by limitations of the MCS retrieval algorithm, meaning optically thick near-surface clouds are not retrieved. Tropical clouds also strengthen the meridional overturning circulation, leading to increased temperatures in the polar warmings by around 6–8 K, and increased temperatures in the tropics by around 2 K due to increased dust loading. The positions and wind speeds of the tropical and high-latitude jets are also modified through changes to the meridional temperature gradients. Work is ongoing to couple the assimilation of ice opacities to a model including an active water cycle.