In May 2007, forecast time of MSM is expanded to 33 hours 4 times a day. At the same time the new model (MSM0705), in which many physical processes such as radiation, turbulence, cloud physics and cumulus convection are improved, is installed instead of the current operational model (MSM0603). It has been confirmed that MSM0705 is superior to MSM0603 and the current operational regional spectral model of JMA (RSM) on accuracy of prediction of precipitation, vertical profiles of temperature and wind velocity, and diurnal change of surface temperature and wind.
In particular, the introduction of improved Mellor-Yamada level 3 scheme (MY3) and partial condensation scheme have remarkable impact on the performance of MSM, and they contribute to the considerable part of the improvement of MSM. The turbulence scheme of MSM0603 is based on Klemp and Wilhelmson(1978) with non-local like effect by Sun and Chang(1986). Turbulent kinematic energy (TKE) is diagnosed assuming the balance between local producing and dissipation of TKE. Because of diagnostic scheme to calculate TKE, the variation of TKE at each time step is considerably large and it possibly disturbs boundary layer in the model. It is also found that the maximum height at which TKE exists seems to be excessively restrained, and momentum, heat and vapor are not enough transported from upper of boundary layer to surface in MSM0603. That is one reason for insufficient diurnal changes of surface temperature and wind. Another reason is the shortage of shortwave radiation flux to surface due to overestimated cloud fraction diagnosed with relative humidity.
In the improved MY3 suggested by Nakanishi (2002), Nakanishi and Niino(2005, 2006), closure constants and mixing length in the original Mellor-Yamada model are revised based on the results of Large Eddy Simulation (LES) and stabilization on time integration of turbulent variables is taken. Furthermore, the partial condensation scheme with outputs by MY3 is applied to provide cloud fraction and cloud water content for the radiation scheme.
With the schemes, transportation of momentum, heat and vapor in boundary layer can be predicted more suitably and the negative bias of shortwave radiation flux can be much reduced. Consequently reduction of errors in vertical profiles of temperature and wind, and more diurnal changes of surface temperature and wind are realized in MSM0705. In some cases, rainband which caused severe disaster can be predicted more clearly.
J.B. Klemp and R.B. Wilhelmson. The simulation of three-dimensional convective storm dynamics. J.Atmos.Sci., 35, 1070-1096, 1978.
W.Y. Sun and C.Z. Chang. Diffusion model for a convective layer. part I: Numerical simulation of convective boundary layer. J. Climate Appl. Meteor., 25, 1445-1453, 1986.
M.Nakanishi. Improvement of the Mellor-Yamada turbulence closure model based on large-eddy simulation data. Bound.-Layer Meteor., 99, 349-378, 2001.
M.Nakanishi and H.Niino. An improved Mellor-Yamada level 3 model with condensation physics : Its design and verification. Bound.-Layer Meteor., 112, 1-31, 2004.
M.Nakanishi and H.Niino. An improved Mellor-Yamada level-3 model: Its numerical stability and application to a regional prediction of advection fog. Bound.-Layer Meteor., 119, 397-407, 2006.