The waves are allowed to propagate vertically, reducing static stability by ascent and increasing wind-shear, until the Richardson Number falls below a critical value. Where it does so 'breaking' of the wave is diagnosed and a drag is exerted, leading to a brake on atmospheric flow. Breaking has been shown to occur most commonly in the boundary layer and in the lower stratosphere, where isentrophic surfaces are tilted and overturned. The effect is largest over mountains in winter, where it reduces the strength of jet streams and causes surface depressions to be less deep and quicker to fill than would otherwise be the case. Long-wave fluxes depend upon the amount and temperature of the emitting medium and its emissivity.
Clouds interact to a significant degree with both long and short-wave radiation.
Unfortunately the water substance as a whole is poorly observed in the atmosphere and therefore the least well-forecast of all primary model variables. Because the parametrization scheme is so complex, a short cut is taken in that the fluxes are only calculated once every three hours of model time. The Unified Model employs a continuous assimilation technique whereby observations are repeatedly inserted into an evolving model around their validity time. The Global Model operates on a six-hour cycle, assimilating data in forecast mode whilst integrating the fields forward in time to provide the background for next run. For the 12Z and 00Z runs, this is the Global Model run which starts from a data time 12 hours earlier, while for the intermediate runs it is six hours earlier.