BMRC Unified Atmosphere Model BAM
Bureau of Meteorology Research Centre
BAM version 3.0d
The Bureau of Meteorology unified climate/NWP model (BAM) version 3.0d was used as the atmospheric component of the coupled model. The performance of a similar version of this model forced with observed AMIPI SST is described by Colman et al (2002). The model version has a horizontal resolution of T47 (72x144 grid points for the physics grid), with 17 vertical levels. These levels are: 0.991, 0.965, 0.926, 0.874, 0.811, 0.741, 0.664, 0.583, 0.5 , 0.417, 0.336, 0.259, 0.189, 0.126, 0.0740, 0.0343 , 0.0089. The short-wave radiation is parameterised following Lacis and Hansen (1974). The long wave radiation scheme used is a modified version of the Fels-Schwarzkopf scheme developed at the Geophysical Fluid Dynamics Laboratory (GFDL) (Schwarzkopf and Fels (1991)). Penetrative, mid-level and shallow convection is parameterized using the mass flux scheme of Tiedtke (1989). The model has surface boundary layer parameterizations based on the formulations of Louis (1983). Oceanic evaporation is enhanced over low wind regions according to the formulation of Miller et al. (1992). Vertical diffusion also follows the stability dependent form of Louis (1983) as modified by McAvaney and Hess (1996). Soil moisture is represented by a single layer "bucket" model with a field capacity of 150mm after Manabe and Holloway (1975). Gravity wave drag is determined using the formulation of Palmer et al. (1986).
The model includes the Rotstayn (1997) prognostic cloud scheme for stratiform clouds. Features of the scheme include two prognostic variables (cloud liquid water and ice) with physically based treatment of associated cloud microphysical processes. A triangular probability function is assumed for the subgrid scale distribution of moisture within a gridbox. Cloud formation is based on the statistical condensation scheme of Smith (1990). Clouds are permitted at all model levels, except the lowest. An additional diagnostic treatment of convective clouds is included, with the fraction of cloud based on the convective rainfall rate. There is currently only vertical advection of cloud water variables by the model dynamics.
For the operational version of POAMA (version 1.0) a modified convection closure was used based on CAPE closure rather than moisture convergence closure. This version was found to better simulate intra-seasonal variability, such as the Madden-Julian Oscillation.
The CAPE closure mass flux convection scheme in BAM
The model's convective parameterization scheme is based on Tiedke's (1989) scheme with modifications suggested by Nordeng (1994). It distinguishes between 3 types of convection; penetrative convection, shallow convection and mid-level convection.
Penetrative and shallow convection are both triggered by moisture convergence in the planetary boundary layer. Once triggered a preliminary ascent is performed assuming no downdrafts. A preliminary choice of shallow convection is made if the primary moisture source is surface evaporation although the scheme changes it to penetrative convective if the preliminary ascent indicates a cloud depth larger than preset limits. In the original scheme the mass flux at the base of the convection was determined by the amount of moisture convergence and this is still true for shallow convection. For penetrative convection the base mass flux is chosen by relating it to the degree of convective instability present (Nordeng 1994) and is evaluated on the assumption that the convection removes convective available potential energy (CAPE) over some characteristic timescale ç. The choice of ç is determined by the resolution on the basis of a suggestion by Nordeng (1994). Note that because the CAPE is not known until the convective ascent has been performed the base mass flux for the initial ascent calculation is set to an arbitrary value.
If the profile cannot support convection from the boundary layer the layers above are tested to see if they can support mid-level convection. The base flux for mid-level convection is related to the large scale vertical moisture transport at the initiating level.
For both mid-level and penetrative convection the ascent allows for organised entrainment due to large scale moisture convergence at levels below the mid-level of the cloud. Organised detrainment occurs only at the top of the convective column which is determined as the level of neutral bouyancy. Entrainment due to turbulent mixing at the cloud edge is allowed for by assuming cylindrical cloud masses with radii varying depending on the type of convection.
After the preliminary ascent calculation downdrafts are initiated and the CAPE is evaluated allowing the base mass flux for penetrative convective to be determined. The ascent is then recalculated using the corrected values and allowing for downdrafts.
For further information: email the POAMA group
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