On computing screen temperatures, humidities and anemometer-height winds in large-scale models

Many large-scale models use the Louis approximate equations to parametrise boundary-layer transfer processes. In this paper a simple, efficient technique is described to derive screen temperatures and humidities and anemometer-height winds from model variables at the surface and the lowest model level. The method uses Dyer-Businger profile relationships to predict the wind, potential temperature and specific humidity at screen or anemometer height. The bulk Richardson number is then calculated and corrected values of the wind, potential temperature and specific humidity are calculated using the Louis profile equations. This ensures consistency in the estimates, since the fluxes were obtained from the Louis scheme. The same methodology could be used directly for other boundary-layer schemes based on Richardson number. For very stable conditions the Dyer-Businger scheme may give poor estimates, but the method still works, provided an upper bound is placed on the Richardson number used in the Louis correction. Convergence to less than five per cent for wind, 0.25 per cent for temperature and five per cent for humidity occurs within two corrections.