Global assimilation of air temperature, humidity, wind and pressure from surface stations

Originally the only surface data assimilated in the Met Office global forecasting system were pressure and marine winds but now most temperatures, humidities and land winds are also used. Adjustments for differences between station and model height are essential for pressure and temperature; new height adjustments for humidity and wind were introduced. These changes brought the global and regional forecasting systems much closer in their use of surface data and forecast performance for surface variables. Winds from islands and headlands not resolved in the forecast model and tropical land winds are excluded. Extra reports (notably Metars) have been introduced into the system. The assimilation of land station temperature and humidity reports gave a clear improvement to short-range forecasts of 'screen' temperature and humidity and small improvements to pressure forecasts. The assimilation of winds over land areas had little impact - wind speed biases, especially at night, are part of the problem. The surface pressure assimilation improves pressure and upper atmosphere forecasts but has little effect on other surface variables. The observation innovations reveal aspects of observation and model errors and other factors such as the proximity to the coast and the importance of the diurnal cycle.