Best et al. (2004) propose a generic interface between land surface and atmospheric models, suitable for many existing GCMs and present modeling configurations, in order to facilitate modeling studies of the land–atmosphere exchange. As the authors state, a general interface would make model intercomparison (and therefore validation) easier and would allow differences between the simulated climates of different land– atmosphere coupled systems to be identified with specific modeling attributes or parameters. We propose a small modification to the generic interface that makes it more general in terms of the handling of the interaction between the atmosphere and a heterogeneous land surface. Best et al. (2004) state in their introduction that the tiles in a mosaic scheme can be coupled to a single, homogenized boundary layer because “[t]he idea is that the internal boundary layers from subsurfaces have merged at or below the bottom model level.” The authors themselves acknowledge that the assumption that the internal boundary layers have merged below the lowest model level may not be valid. The modification to the generic interface that we propose here would accommodate a model that does not make any specific assumption about the level of the blending height relative to the lowest model level. The importance of the suggested change in the generic interface is related to recent modeling and observational studies, summarized below, which suggest both that the blending height may be elevated in some regions (at 500 m or higher) and that model simulations are improved when the lowest model level is at 250 m (or less) above the ground. In addition, the choice of blending height in a model has been shown by Molod et al. (2004) to affect the simulated climate. Authors of modeling studies have considered the issue of an elevated blending height since the early discussion of mosaic techniques to model the interaction between a heterogeneous land surface and the atmosphere. Koster and Suarez (1992) first raised the issue that their assumption of a blended lowest layer may not be adequate, based on the scaling arguments that link the blending height and the horizontal scale of heterogeneity. Bringfelt (1999) also state that a mosaic assumes that the blending height is at the surface layer and that this might not be an adequate representation of the impact of surface heterogeneity. In addition, Arola (1999) introduced a technique designed to raise the level of the blending height in the model as an attempt to improve this drawback of the standard mosaic approach. The existence of organized mesoscale circulations, suggested by the modeling study of Avissar and Schmidt (1998), also raises the issue of whether the internal boundary layers have blended below some near-surface layer. The observational studies that suggest that the blending height may be elevated include Segal et al. (1989) in which patch-to-patch differences in temperature and moisture over an irrigated and a dry crop area were reported throughout much of the planetary boundary layer. An elevated blending height is also suggested by the lidar study reported in Angevine et al. (2003) and Banta and White (2003), where subCorresponding author address: Dr. Andrea Molod, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139. E-mail: amolod@mit.edu AUGUST 2006 N O T E S A N D C O R R E S P O N D E N C E 833
Read full abstract
Jun 7, 2023
Open Access