Multi-Dimensional Broadband IR Radiative Forcing of Marine Stratocumulus in a Large Eddy Simulation Model
In order to address the interactive and evolutionary nature of the cloud-radiation interaction, we have coupled to a Large Eddy Simulation (LES) model the sophisticated multi-dimensional radiative transfer (MDRT) scheme of Evans (Spherical Harmonics Discrete Ordinate Method; 1998). Because of computational expense, we are at this time only able to run 2D experiments. Preliminary runs consider only the broadband longwave component, in large part because IR cloud top cooling is the significant forcing mechanism for marine stratocumulus. Little difference is noted in the evolution of unbroken stratocumulus between three-hour runs using MDRT and independent pixel approximation (IPA) for 2D domains of 50 km in the horizontal and 1.5 km in the vertical. Local heating rates differ slightly near undulating regions of cloud top, and a slight bias in mean heating rate from 1 to 3 h is present, yet the differences are never strong enough to result in a pronounced evolutionary bias in typical boundary layer metrics (e.g. inversion height, vertical velocity variance, TKE). Longer integration times may eventually produce a physical response to the bias in radiative cooling rates. A low-CCN case, designed to produce significant drizzle and induce cloud breakup does show subtle differences between MDRT and IPA. Over the course of the 6 hour simulations, entrainment is slightly less in the MDRT case, and the transition to the surface-based trade cumulus regime is delayed. Mean cooling rates appear systematically weaker in the MDRT case, indicative of a less energetic PBL and reflected in profiles of vertical velocity variance and TKE.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 15010131
- Report Number(s):
- PNNL-SA-37837; KP1201030; TRN: US200502%%131
- Resource Relation:
- Conference: AMS 11th Conference on Cloud Physics, Paper 7.1 (CD)
- Country of Publication:
- United States
- Language:
- English
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