什么是 Numerical weather prediction; data assimilation; ensemble forecast; predictability limit？

Numerical weather prediction is forecasting by computer: the equations that govern the motion, the energy and the water of the atmosphere are integrated forward in time from its observed state. Data assimilation combines the latest observations with a short-range forecast from earlier observations to obtain the best estimate of the present state, the analysis, from which the forecast starts. An ensemble forecast is a set of forecasts run from slightly different initial states, and with slightly different versions of the model, to show the range of possible weather and its likelihood. The predictability limit is the time beyond which a forecast of the state of the weather is no better than a guess from climate, because the errors of the initial state have grown as large as the differences between any two states of the atmosphere.