Using stochastically-generated subcolumns to represent cloud structure in a large-scale model

Journal Article · Thu Dec 08 04:00:00 EST 2005 · Monthly Weather Review, vol. 134, no. 12, December 20, 2006, pp. 3644-3656

OSTI ID:898035

Pincus, R; Hemler, R; Klein, S A

A new method for representing subgrid-scale cloud structure, in which each model column is decomposed into a set of subcolumns, has been introduced into the Geophysical Fluid Dynamics Laboratory's global climate model AM2. Each subcolumn in the decomposition is homogeneous but the ensemble reproduces the initial profiles of cloud properties including cloud fraction, internal variability (if any) in cloud condensate, and arbitrary overlap assumptions that describe vertical correlations. These subcolumns are used in radiation and diagnostic calculations, and have allowed the introduction of more realistic overlap assumptions. This paper describes the impact of these new methods for representing cloud structure in instantaneous calculations and long-term integrations. Shortwave radiation computed using subcolumns and the random overlap assumption differs in the global annual average by more than 4 W/m{sup 2} from the operational radiation scheme in instantaneous calculations; much of this difference is counteracted by a change in the overlap assumption to one in which overlap varies continuously with the separation distance between layers. Internal variability in cloud condensate, diagnosed from the mean condensate amount and cloud fraction, has about the same effect on radiative fluxes as does the ad hoc tuning accounting for this effect in the operational radiation scheme. Long simulations with the new model configuration show little difference from the operational model configuration, while statistical tests indicate that the model does not respond systematically to the sampling noise introduced by the approximate radiative transfer techniques introduced to work with the subcolumns.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA
• Sponsoring Organization: USDOE
• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 898035
• Report Number(s): UCRL-JRNL-217661
• Journal Information: Monthly Weather Review, vol. 134, no. 12, December 20, 2006, pp. 3644-3656, Journal Name: Monthly Weather Review, vol. 134, no. 12, December 20, 2006, pp. 3644-3656
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
58 GEOSCIENCES
CLIMATE MODELS
CLOUDS
CONDENSATES
CONFIGURATION
RADIANT HEAT TRANSFER
RADIATIONS
SAMPLING
TUNING