Aerosol specification in single-column Community Atmosphere Model version 5

B Lebassi-Habtezion,P M Caldwell

doi:10.5194/gmd-8-817-2015

B Lebassi-Habtezion, P M Caldwell

Open Access

https://doi.org/10.5194/gmd-8-817-2015

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Abstract

Abstract. Single-column model (SCM) capability is an important tool for general circulation model development. In this study, the SCM mode of version 5 of the Community Atmosphere Model (CAM5) is shown to handle aerosol initialization and advection improperly, resulting in aerosol, cloud-droplet, and ice crystal concentrations which are typically much lower than observed or simulated by CAM5 in global mode. This deficiency has a major impact on stratiform cloud simulations but has little impact on convective case studies because aerosol is currently not used by CAM5 convective schemes and convective cases are typically longer in duration (so initialization is less important). By imposing fixed aerosol or cloud-droplet and crystal number concentrations, the aerosol issues described above can be avoided. Sensitivity studies using these idealizations suggest that the Meyers et al. (1992) ice nucleation scheme prevents mixed-phase cloud from existing by producing too many ice crystals. Microphysics is shown to strongly deplete cloud water in stratiform cases, indicating problems with sequential splitting in CAM5 and the need for careful interpretation of output from sequentially split climate models. Droplet concentration in the general circulation model (GCM) version of CAM5 is also shown to be far too low (~ 25 cm−3) at the southern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) site.

Highlights

The single-column model (SCM) version of the Community Atmosphere Model (CAM) is a very important tool for development of model parameterizations
Variables normally predicted by general circulation models (GCMs) are often hardcoded to observed values in SCM studies in order to separate errors in their prediction from errors in other parts of the model
In addition to Nd and surface precipitation (Pr), we include liquid water path (LWP) both before and after microphysics was called (LWPpre and LWPpost, respectively). These values are different because CAM5 sequentially updates the model state after each parameterization is applied

Summary

Introduction

The single-column model (SCM) version of the Community Atmosphere Model (CAM) is a very important tool for development of model parameterizations. One advantage of the SCM is that it is much more computationally affordable, which allows developers to test a wide variety of model changes Another advantage is that there exists a large number of standard SCM cases which can be used to evaluate model behavior over a wide variety of climate regimes. In the first GCSS intercomparison (Moeng et al, 1996), liquid water path (LWP) in nocturnal stratocumulus were found to vary by a factor of 5 between different large-eddy simulation (LES) models The source of this spread could not be identified because the LES models differed widely in all aspects of their design. By idealizing or specifying aspects of a simulation, the processes responsible for model bias can be illuminated, providing a pathway towards model improvement

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