About the Global Parallel System
The Global Modeling Branch of the NCEP Environmental Modeling Center normally runs several parallel global systems in order to test various upgrades and permutations of the operational global system. These parallel experiments are run in nearly real-time using the current observed data so that the experiments can be diagnosed and validated and compared to the operational run using current atmospheric conditions.
The operational global system consists of the Final Global Data Assimilation System (FNL), the Global Forecast System forecasts (GFS), and the Ensemble forecasts (ENS). The GFS is a consolidation of the forecasts formerly known as the Aviation (AVN) and the Medium Range Forecast (MRF).
The FNL provides the backbone of the global system. It collects observations for at least 6 hours past synoptic time and makes a global analysis and 3, 6, and 9 hour forecasts (the "first guesses" for the next analysis) 4 times per day. The FNL is run at the highest resolution, currently T254 L64. That is, the horizontal coordinate is triangular spectral truncated after wavenumber 254 and the vertical coordinate is sigma with 64 levels. The horizontal resolution is roughly equivalent to a global 55 km mesh. The vertical resolution extends from 997.3 mb to 0.266 mb for a surface pressure of 1000 mb and approximates a 680 meter mesh at 250 mb.
The GFS collects observations for 2 hours and 45 minutes past synoptic time and makes a 16 day forecast from 4 synoptic times per day, namely 0000 UTC, 0600 UTC, 1200 UTC, and 1800 UTC. The GFS starts at the same highest resolution (T254 L64) as the FNL but drops resolution during the forecasts. After hour 84, the GFS drops to T170 L42 resolution. This horizontal resolution is roughly equivalent to a global 80 km mesh. This vertical resolution extends from 996 mb to 2.0 mb for a surface pressure of 1000 mb and approximates a 850 meter mesh at 250 mb. Then after hour 180, the GFS drops to T126 L28 resolution. This horizontal resolution is roughly equivalent to a global 105 km mesh. This vertical resolution extends from 995 mb to 2.7 mb for a surface pressure of 1000 mb and approximates a 1300 meter mesh at 250 mb.
The ENS consists of 22 extra 16 day perturbed forecasts, 11 at 0000 UDT and 11 at 1200 UDT. The ensemble starts at the same T126 L28 resolution described above, but the ensemble also drops resolution during the forecasts. After hour 84, the GFS drops to T62 L28 resolution. This horizontal resolution is roughly equivalent to a global 210 km mesh. This vertical resolution is the same as for the T126 L28 resolution.
The parallels often contain modifications to the operational system that will soon be implemented. However, not every parallel experiment is eventually implemented. Sometimes the raw parallel data files are made available to forecast users. Users should be forewarned that the parallel forecasts may fail or run late. The parallels are not maintained as operational systems. They are only provided to users for evaluation purposes.
There are several designated slots for the parallel experiments to run, each denoted by a letter. The parallel X (PRX) is the primary parallel used to test changes that will soon be implemented into the operational global system. Its control is the high resolution operational FNL and MRF. The parallel Z is identical to the operational FNL and MRF but at low resolution and usually acts as the control for other low resolution parallels.
The Global Modeling Branch has been running parallel experiments for decades and documenting them in journal papers, technical procedures bulletins, and internal technical notes. The public HTML documentation of the parallel experiments, however, began in June 1999.