Numerical Simulation and Analysis on the Phoenix Typhoon Rainstorm Using WRF
Rainstorm caused by the Typhoon Phoenix, between July 28th and 31st 2008, is studied by using the meso-scale numerical simulation model WRF and some NCEP reanalysis data of 1¡ã¡Á1¡ãwith 6h interval. Results show that, the distribution of the SLP, wind and rain belt was simulated successfully by the WRF model. The research shows that the strong upward motion at the 600 -700hPa and 100 -200hPa of 24¡ãN and two closure centers of near 21¡ãN-22¡ãN and 31¡ãN-33¡ãN at the high troposphere induced convective cloud cluster to develop.
2 NUMERICAL WEATHER PREDICTION 2 Numerical weather prediction
National meteorological services are required to provide short and medium range weather forecasts, warnings and alerts for their territory. Modern-day weather forecasting is highly relying on numerical weather prediction (NWP). The medium range forecasts are produced with global models using the whole of Earth as a domain having a horizontal resolution of about 20-40 km. For short range applications it is more efficient to run the NWP systems for only a small part of the globe using a Limited Area Model (LAM) operating on smaller scale grid and using boundary conditions from global models. [Nie06] In Europe there are a number of consortia that collaborate on Limited Area Modelling. The High Resolution Limited Area Modelling project HIRLAM was the first group to be established in the Nordic Countries in 1985. It has expanded to include other countries in Western and Southern Europe and it has been a successful collaboration programme. The developments of earlier projects are carried over and further developed in new projects. In January 2006 the programme advanced to a new phase called HIRLAM-A, which inherited all the knowledge from previous HIRLAM-6 project and its predecessors. The main objectives, targets and priorities of each project are listed in a memorandum of understanding, which is available at HIRLAM webpage www.hirlam.org. In addition to developing the HIRLAM model, the programme targets to provide and maintain NWP expertise among the HIRLAM members, enhance collaboration both between scientists in the HIRLAM institutes and with co-operating consortia and with other NWP groups. The HIRLAM forecast system is at the moment used in routine weather forecasting by several European NMS including Finnish FMI. [HIR06] A NWP-system comprises two main components, which are the atmospheric model and the data assimilation system. The atmospheric model is further divided into two main parts: the model dynamics and the physical parameterization [Nie06]. In this paper, the basics of NWP modelling are presented via the High Resolution Limited Area Modelling project HIRLAM.
On the behaviour of some physical parameterization methods in high-resolution numerical weather prediction models
Modern-day weather forecasting is highly dependent on Numerical Weather Prediction (NWP) models as the main data source. The evolving state of the atmosphere with time can be numerically predicted by solving a set of hydrodynamic equations, if the initial state is known. However, such a modelling approach always contains approximations that by and large depend on the purpose of use and resolution of the models. Present-day NWP systems operate with horizontal model resolutions in the range from about 40 km to 10 km. Recently, the aim has been to reach operationally to scales of 1 – 4 km. This requires less approximations in the model equations, more complex treatment of physical processes and, furthermore, more computing power. This thesis concentrates on the physical parameterization methods used in high-resolution NWP models. The main emphasis is on the validation of the grid-size-dependent convection parameterization in the High Resolution Limited Area Model (HIRLAM) and on a comprehensive intercomparison of radiative-flux parameterizations. In addition, the problems related to wind prediction near the coastline are addressed with high-resolution meso-scale models. The grid-size-dependent convection parameterization is clearly beneficial for NWP models operating with a dense grid. Results show that the current convection scheme in HIRLAM is still applicable down to a 5.6 km grid size. However, with further improved model resolution, the tendency of the model to overestimate strong precipitation intensities increases in all the experiment runs. For the clear-sky longwave radiation parameterization, schemes used in NWP-models provide much better results in comparison with simple empirical schemes. On the other hand, for the shortwave part of the spectrum, the empirical schemes are more competitive for producing fairly accurate surface fluxes. Overall, even the complex radiation parameterization schemes used in NWP-models seem to be slightly too transparent for both longand shortwave radiation in clear-sky conditions. For cloudy conditions, simple cloud correction functions are tested. In case of longwave radiation, the empirical cloud correction methods provide rather accurate results, whereas for shortwave radiation the benefit is only marginal. Idealised high-resolution two-dimensional meso-scale model experiments suggest that the reason for the observed formation of the afternoon low level jet (LLJ) over the Gulf of Finland is an inertial oscillation mechanism, when the largescale flow is from the south-east or west directions. The LLJ is further enhanced by the sea-breeze circulation. A threedimensional HIRLAM experiment, with a 7.7 km grid size, is able to generate a similar LLJ flow structure as suggested by the 2D-experiments and observations. It is also pointed out that improved model resolution does not necessary lead to better wind forecasts in the statistical sense. In nested systems, the quality of the large-scale host model is really important, especially if the inner meso-scale model domain is small. Publishing unit Finnish Meteorological Institute, Meteorological Research Unit Classification (UDC)