Development of aerosol microphysics, radiative and cloud microphysics schemes for the GISS climate model
ABSTRACT
Co-PIs: Surabi Menon, Dorothy Koch
MAP Proposal Objective
Atmospheric aerosols have been increasingly implicated in several studies as causing large changes in surface and top of the atmosphere (TOA) radiation budgets, circulation, cloud cover, warm and cold-phase precipitation (initiation, enhancement and suppression), melting of glaciers, etc. These climate effects are dependent on the aerosol properties -- composition, size distribution, mixing state, vertical distribution; and their impact on cloud properties -- cloud droplet number and size, cloud liquid water paths and cloud cover as well as other feedback effects from changing cloud macrophysical properties and underlying surface properties. To evaluate future climate change due to aerosols and aerosol-cloud effects, we propose to enhance the climate change prediction capability of the newly developed version of the Goddard Institute for Space Studies (GISS) climate model (ModelE) and participate in the Cloud Modeling and Analysis Initiative (CMAI).
With relevance to CMAI goals (as described at the April 2006 workshop at NASA GISS), our proposed objectives pertain to improving the cloud droplet nucleation scheme and the treatment of hydrometeor spectra. To that end, we will collaborate with other investigators from the MAP team (Sonia Kriedenweis and Paul De Mott) to improve ice nucleation parameterizations. For the improved treatment of hydrometeor spectra, we will begin evaluating the two-moment bulk cloud microphysical scheme (based on CAM) that is now implemented in ModelE. As a performance assessment tool, the GISS SCM is being improved such that it can function with (a) standard bulk microphysics scheme, similar to the one in ModelE; (b) bin-resolved cloud microphysics scheme; (c) two-moment bulk cloud scheme, similar to the one now implemented in ModelE. Aerosols will not be a focus within the SCM simulations; but rather we will focus on cloud treatment. To diagnose improvements or short-comings obtained through these different cloud schemes; evaluation with observations is essential (GCSS-DIME). These evaluations will be performed via standard correlation matrices, principal component analysis, cluster analysis, etc. For the cluster analysis, we will work with another MAP funded investigator (George Tselioudis) to relate model fields with those from ISCCP. For these investigations, ModelE will be used to assess the changes to cloud fields from aerosol interactions; as well as changes arising from different cloud microphysics schemes at regional locations of interest.
A useful approach, not feasible under current plans but plausible within the larger CMAI community would be to identify scale-invariant cloud properties for different meteorological regimes from a variety of observations and representation of scale-variant features in the model. This requires better co-ordination between small and large-scale cloud modelers and observationalists. This part of the objective needs to be fulfilled, hopefully through subsequent interactions with the CMAI community after the first phase (analyzing model fields after testing results from the different cloud schemes, in the SCM and ModelE). It is also highly desirable that various satellite observations pertaining to aerosols, clouds and precipitation be easily available within the CMAI website and if possible a select suite of relevant observations be available as data files rather than links to satellite sites for select years. This will allow easy access and use of satellite data for model comparison to assess suitable representations of aerosol-cloud interactions.
