New Cloud System Metrics to Assess Bulk Ice Cloud Schemes in a GCM

• Published in: Journal of advances in modeling earth systems 2019-10, Vol.11 (10), p.3212-3234
• Main Authors: Stubenrauch, Claudia J., Bonazzola, Marine, Protopapadaki, Sofia E., Musat, Ionela
• Format: Article
• Language: English
• Subjects: Atmospheric sounding; bulk ice cloud scheme for climate models; Climate change; Cloud System Metrics for process‐oriented diagnostics; Cloud systems; Clouds; Earth Sciences; Evaluation; Feedback; General circulation models; Ice; Ice clouds; Ice crystal size; Ice crystals; Influence; Interferometers; IR Sounder cloud observation simulator; Moisture content; Probability distribution; Properties; Satellite observation; Sciences of the Universe; Simulators; Size distribution; Troposphere; Tropospheric clouds; Water content; Water distribution; Water temperature
• ISSN: 1942-2466; 1942-2466
• DOI: 10.1029/2019MS001642

Bulk microphysical properties of ice clouds, such as fall speed and ice crystal size distribution, strongly impact the life time and the radiative effects of these clouds. Three coherent bulk ice schemes, with fall speed and effective ice crystal diameter depending on both ice water content and temperature, have been constructed from published parameterizations. We present a novel upper tropospheric cloud system concept to study the impact of these schemes on the LMDZ climate simulations. For this evaluation, cloud data from hyperspectral infrared sounders Atmospheric InfraRed Sounder and IR Atmospheric Sounding Interferometer are used, because they include cirrus with visible optical depths as low as 0.2. The analogous satellite observation simulator, developed for this purpose, is also presented. The cloud system concept, applied to the data and to the simulator outputs, allows a process‐oriented evaluation. In general, the new bulk ice schemes lead to a better agreement with the cloud data, in particular concerning the cloud system property distributions and the relation between cloud system properties and proxies mimicking the life stage and the convective depth. Sensitivity studies have demonstrated that both the introduction of the new schemes as well as the necessary adjustment of the relative width of the upper tropospheric subgrid water distribution lead to these improvements. Our studies also suggest to revise the formulation of the latter. Plain Language Summary Bulk microphysical properties of ice clouds, such as fall speed and ice crystal size distribution, strongly impact the life time and the radiative effects of these clouds. Both have to be predicted from other variables in a climate model. Three bulk ice schemes, depending on both ice water content and temperature, have been constructed from existing parameterizations, based on a large statistics of observations. The impact of these new schemes, when integrated into the LMDZ climate model, is studied. For the evaluation with cloud data from infrared sounders, a specific satellite observation simulator was developed which makes the simulation results look like the observed cloud layers. A novel cloud system concept, applied on the data and on the simulator results, allows a process‐oriented evaluation and demonstrates improvements linked to the introduction of the new schemes themselves, but also to the necessary adjustment of another parameter which is linked to the subgrid variability of