The MM5 (short for Fifth-Generation Penn State/NCAR Mesoscale Model) is a regional mesoscale model used for creating weather forecasts and climate projections. It is a community model maintained by Penn State University and the National Center for Atmospheric Research. The MM5 is a limited-area, terrain-following sigma coordinate model that is used to replicate or forecast mesoscale and regional scale atmospheric circulation. It has been updated many times since the 1970s to fix bugs, adapt to new technologies, and work on different types of computers and software. It is used in many different ways: for research and for weather prediction. In research, it is used to compare it to other models, to see what works and what does not work. It is also used for air quality models.
Active development on the model ended with version 3.7.2 in 2005, and it has been largely superseded by the Weather Research and Forecasting (WRF) model.
Features
MM5 is globally relocatable, which helps support different latitudes, terrain types, elevations, soil types, etc. . The model can be either hydrostatic or non-hydrostatic, depending on the desired outcome. The fact that the model is regional implies that it requires initial conditions and lateral boundary conditions. This means that each boundary (there are four) has initialized wind speeds, temperatures, pressure and moisture fields. Thus, gridded data is needed for this program. This model takes and then analyzes its data based on pressure surfaces. However, these surfaces must first be interpolated by a specific vertical coordinate before it can be analyzed. When Σ is close to the ground, the program follows the actual terrain, but when Σ is higher up, the program looks at isobaric surfaces. Σ ranges from 0 to 1. Then, the model guesses the different pressures that exist in the atmosphere throughout this plot of land, which is done through REGRID. RAWINS is an older version of LITTLE_R and thus LITTLE_R is more commonly used. Then INTERPF takes that data from RAWINS/LITTLE_R and REGRID to interpolate that data to the sigma coordinate explained above.
thumb|Flow chart of the functions of the MM5
Memory and code basics
MM5 is written in FORTRAN. These FORTRAN programs must be compiled on a local computer, and some need to be recompiled each time a model configuration is changed. The program uses pointers to assign variables to values. These pointers go into parts of the memory to assign specific values to the desired variables. MM5 can also perform multiple tasks at once. Specifically, two different tasks can be executed on different processors simultaneously and MM5 uses this as much as possible.
Source code
The MM5 code itself has over 220 subroutines, with more than 55,000 lines of code. It uses Standard Fortran 77 with “Cray” pointers. A list of the subroutines of all of the functions within the MM5, with a short description and the routines that are called within it can be found here.
Use
The MM5 had been adapted for use in many different types of atmospheric simulations:
- Real-time regional weather forecasts
- Tropical cyclone prediction
- Climate change simulation and projection
Developments and improvements
TERRAIN's latest update uses USGS global 30 sec terrain elevation data, which looks at the entire world, where before was only used in the continental United States. This new update also allows for a better quality terrain image that can be used in MM5. REGRID's improvement makes it simpler for its users to input the data and it also is more portable.