MMM Research Highlight: May, 2010
Numerical simulation of turbulence
Large-Eddy Simulation (LES) has been widely used to examine turbulence in the PBL, however most LES applications have been limited to PBLs over idealized terrain and meteorological conditions. To extend the LES applications to real-world PBLs, we simulated two complex PBL regimes: (1) PBL under a deep convection system and (2) PBL over a ridge-valley surface.
(1) PBL under a deep convection:
As part of the research funded by the NSF Center for Multiscale Modeling for Atmospheric Processes (CMMAP), we performed a large-domain LES of a tropical deep convection system over a domain of about 205 km x 205 km x 27 km with a grid mesh of 100 m x 100 m in the horizontal and 50-150 m in the vertical (Khairoutdinov et al. 2009). This simulation thus resolves not only a deep convection system, but also energy-containing individual clouds and turbulent eddies, and can be used as benchmark to study the scale interaction between cloud-system and small convection (including turbulence) scales. In Moeng et al (2009) we applied a Gaussian filter with various filter scales to separate the LES flow into the filter-scale (FS, cloud-system scales) and the subfilter-scale (SFS, small convection and turbulence scales) and also to retrieve the SFS fluxes. These FS and SFS fields can be referred to as the resolvable and subgrid-scale (SGS) fields, respectively, in typical CRMs. We used this information to test a simple SGS K-model commonly used in CRMs. Figure 1 compares the spatial distributions of the retrieved SFS fluxes and the K-model predicted SGS fluxes. The differences between the right and left panels of Fig. 1 reveal the deficiencies of the K model. In the near future, we will analyze the flow fields to find variables that are responsible for the deficiencies and then to improve the existing K model.
(2) PBL over a ridge-valley surface:
Collaborating with Sapienza University (Rome, Italy), we investigated the thermally driven circulation and turbulence over a ridge-valley surface by performing a WRF-LES.The model was designed with a symmetrical geometry, i.e., a periodic ridge-slope-valley surface in x and uniform in y. The anabatic turbulent flow was generated by imposing a time varying surface temperature and geostrophic wind along the valley. The presence of the orography introduced numerous complexities both in the mean and turbulence features, which differ significantly from the idealized PBL. Catalano and Moeng (2010) reports the different structure of the PBL at various regions of the domain by presenting the first- and second-moment statistics and the TKE budgets.
Chin-Hoh Moeng, M A LeMone, M F Khairoutdinov, S K Krueger, P A Bogenschutz, D A Randall, 2009: The tropical marine boundary layer under a deep convection system: a large-eddy simulation study, J. Adv. Model. Earth Syst., Vol. 1, Art. #16, 13 pp., doi:10.3894/JAMES.2009.1.16 (Published 30 Dec. '09)
Marat F. Khairoutdinov, Steve K Krueger, Chin-Hoh Moeng, Peter A Bogenschutz, David A Randall, 2009: Large-eddy simulation of maritime deep tropical convection, J. Adv. Model. Earth Syst., Vol. 1, Art. #15, 13 pp., doi:10.3894/JAMES.2009.1.15 (Published 31 Dec. '09)
Franco Catalano and Chin-Hoh Moeng, 2010: Large eddy simulation of a complex turbulent anabatic flow over a valley using the WRF model. Submitted to Boundary Layer Meteorology.
Horizontal distributions of (left panels) the LES-retrieved SFS fluxes (with a filter width of 4 km as an example) and (right panels) the K-modeled fluxes of water vapor at (a) top: z = 600 m and (b) bottom: z = 200 m.