عنوان مقاله [English]
Representing the boundary layer processes is crucial in simulating atmospheric phenomena in operational hydrostatic weather forecast models. Moreover, evaluating the performance of different physical processes in a variety of numerical models is an essential subject of its own. This paper presents an objective assessment of a planetary boundary layer scheme based on turbulent kinetic energy in a single-column version of the innovative atmospheric general circulation model developed based on potential vorticity at the University of Tehran, which is called UTGAM. Single-column models are a complementary tool to the atmospheric general circulation models that provide a simple framework to investigate the fidelity of the simulated physical processes.
The reliable parameterization of the boundary layer processes has got significant impacts on weather forecasts. Most of the hydrostatic models have got deficiencies in the representation of these unresolved processes, especially in stably stratified conditions, and it seems that this problem is continuing in the forthcoming future. Here we have utilized the first GABLS intercomparison experiment set up as a simple tool to evaluate the performance of the diffusion scheme in the UTGAM. Two different sigma-theta and sigma-pressure single-column grid staggering combined with, respectively, 33 and 14 vertical levels below 3 km height have been used for the low- and high-resolution simulations. The GABLS1 LES results have been used as a benchmark for comparison. The boundary layer scheme that has been explored here is the same as the one in the ECHAM model, but some simplifications have been made. For instance, in this simulation, the effects of tracers have been ignored to circumvent the complexity of the problem.
Results depict subtle nuances between the sigma-theta and sigma-pressure coordinates in intercomparison between the low and high vertical resolutions separately, which are more apparent in the lower vertical resolution. Nevertheless, it seems that the diffusion processes have been simulated rather more accurately in the high-resolution sigma-pressure vertical coordinate. The boundary layer scheme analogous with most of the operational models in the GABLS1 intercomparison experiment overestimate the momentum and the heat diffusion coefficients. The wind profile with height, depicts maxima that are higher than the corresponding LES profile. It is inferred that the scheme mixes momentum over a deeper layer than the LES, but the simulated wind profile is better in comparison with the other operational models in GABLS1. Considering the vertical profiles of potential temperature revealed that the amount of heat mixing is not suitable in this experiment, and it causes a negative bias in the lower part of the simulated boundary layer. The simulated amounts of surface friction velocity have proved significant differences with the LES results in all separate experiments. However, the latter large amounts seem unlikely to have a detrimental effect on forecast scores in the operational model. Moreover, the sensitivity of the scheme to the lowest full level has been partially explored. Decreasing the lowest full-level height concurrent with increasing the vertical resolution leads to a modest influence on the simulation of the boundary layer processes. All the results confirm notable improvements by increasing the vertical resolution in both sigma-theta and sigma-pressure coordinates.