MODELING LOCAL-SCALE METEOROLOGY
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The Definition of the Mesoscale.
(Note: NOAA ARL is no longer running the RAMS mesoscale model.)
Mesoscale models are used to simulate sub-grid scale phenomena not resolved by regional-scale models. The
mesoscale ranges as defined by Orlanski (1975) :
| Scale | Range | Examples | Applicable Model |
| Micro-scale | < 2 km | Turbulent eddies | LES model |
| Meso-gamma | 2-20 km | Thunderstorm convection, complex terrain flows | Non-hydrostatic (ARPS) |
| Meso-beta | 20-200 km | land-water, sea breezes | Non-hydrostatic, hydrostatic (MM5, RAMS) |
| Meso-alpha | 200-2000 km | Fronts, low-pressure systems | Hydrostatic (Eta,AVN) |
Non-hydrostatic processes are important when the vertical motions in the atmosphere are
changing rapidly. The non-hydrostatic criteria is defined when the ratio of the height (H) to the length (L)
scale of an atmospheric circulation is much less than 1 (H/L << 1). For example, deep
convection non-hydrostatic effects must be parameterized since the length scale is order 1-2 km, but the height of the circulation can reach the top of the troposphere (10 km).
Mesoscale models normally include non-hydrostatic effects when the model grid spacing is
set finer than 10 km.
Popular mesoscale models in use today:
| Model | Institution | Grid-spacing | Air Quality Applications |
| MM5 | NCAR/PSU | 1-100 km | multi-events to seasonal air quality assessments. Key meteorological driver of EPA/MODELS-3 |
| RAMS: The Regional Atmospheric Modeling System | CSU/Mission Research Corp. | 100 m -100 km | LES, ozone studies along Lake Michigan and OTAG area, deposition over Chesapeake Bay |
| ARPS : Advanced Regional Prediction System | U. Oklahoma | 1-10 km | Primarily thunderstorm simulations |
| Eta | NWS/NCEP | 10-32 km | Regional modeling, seasonal assessments with Hy-Split |
| LAPS: Local Analysis and Prediction System | NOAA/FSL | 2-10 km | High resolution analysis of non-standard met.data used for emergency assessment |
| MAPS/RUC : Mesoscale Analysis and Prediction System | NOAA/FSL | 25 km | Regional analysis and prediction, Visiblity assessments |
| OMEGA : Operational Multi-scale Environmental model with Grid Adaptivity | SAIC | 1-30 km | Local analyses and prediction |