Conclusion

With the arrival of new and powerful observational datasets, particularly the new space-based active radar and lidar sensors in the "A-Train," we are entering a new era for the evaluation of clouds in large-scale models. Observations with active sensors have already demonstrated that NWP models have skill in representing clouds in the right place and the right time, and have also identified some shortcomings. It will soon be possible to assess key aspects of the simulation of clouds, such as the three-dimensional distribution of cloud layers, the cloud water phase, the cloud precipitation efficiency, and the physical and radiative properties of shallow-level clouds. As these aspects have the potential to affect the response of these crucial shallow tropical clouds to climate change, their evaluation in the current climate under a large variety of environmental conditions will allow us to assess better the realism of their change in the future. Moreover, as high-resolution models are increasingly used to assess and develop physical parameterizations, as well as to investigate cloud-in-climate issues, we recommend that new satellite data be used to evaluate the cloud distributions produced by high-resolution models, including operational NWP models.

We emphasize that the better our physical understanding is of the response of clouds to climate change, the more efficient the strategy for evaluating this response will be. Thus, developing a strategy of evaluation of climate change-cloud feedbacks requires efforts in analyzing and unraveling the physical mechanisms underlying these feedbacks. For this purpose, a promising approach consists of conducting idealized studies using a hierarchy of climate models of different complexities. However, to reduce the uncertainties in cloud-climate feedback processes and improve climate models, it is not suffi cient to point out deficiencies in a particular process; physical parameterizations must be improved if we want these deficiencies to be remedied. For this, it is important to keep developing collaborations between the large- and mesoscale cloud communities, as well as between the modeling and observational communities.

The radars and lidars now in space should enable us to observe the global vertical distribution of clouds and aerosols and aid in ascertaining the degree to which aerosols are modifying both warm and cold clouds, as well as the geographic extent of any modification. These measurements should allow us to quantify, for the first time, the effect aerosols are having on the present climate, and hence reduce the large uncertainties in the effects of aerosols on the future climate.

Turning to future satellite-observing systems, we have some concern that the long time series of global monitoring of TOA radiation, which is now being carried out with the CERES sensors, may not continue, although Mega-Tropique may fill the gap but only at low latitudes. We look forward to the launch of the ESA/JAXA EarthCARE mission (in 2013), which will embark a cloud radar and lidar on the same platform. The high spectral resolution li-dar should provide direct observations of the optical depths of thin cirrus and aerosols and characterize the aerosol and ice cloud particles. The radar will have improved sensitivity and so should detect more of the high thin ice high clouds as well as the lower-level water clouds, while the Doppler capability should help to characterize the vertical cloud motions and thus contribute to the evaluation of convective parameterization schemes, provide information on ice sedimentation velocities within extensive cirrus decks to inform the model ice schemes, and quantify the drizzling rates in low-level water clouds.

Many small-scale cloud processes remain which must be parameterized in the models; a better understanding of them is needed but cannot be provided from space. Examples include the entrainment and detrainment for both layer and convective clouds; the growth of ice particles from the vapor, their aggregation, riming, and subsequent evaporation; the warm rain coalescence process and the mechanisms that lead to the production and persistence of supercooled layer clouds. Progress can best be provided through detailed observation, whether in situ or remotely from the ground, of the evolving physical and dynamic variables. One particularly glaring gap remains: We still have no technique to observe the humidity structure of the atmosphere in three dimensions with a high enough resolution to characterize its PDF within the model grid box even in clear air. To achieve this, when clouds are present, is an even greater challenge.

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