Understanding Hydrometeorology Using Global Models

The land surface coupling, a crucial element of the climate system, is explored in the recent 40-yr European Centre for Medium–Range Forecasts (ECMWF) reanalysis (ERA-40) model. In seasonal forecasts for the Northern Hemisphere summer, initialized with idealized soil moisture fields, the ERA-40 model has a large evaporation–precipitation feedback over the continents, and the memory of initial soil moisture is longest at high northern latitudes. Thirty years of hourly data from the ERA-40 reanalysis are averaged over the Madeira, Red–Arkansas, and Athabasca River basins. Although the model fully resolves the diurnal cycle and has an interactive prognostic cloud field, the transitions in the boundary layer climate over land can be mapped with remarkable precision by the daily mean state and daily flux averages. The coupling to cloud processes plays an essential role in the surface and boundary layer equilibrium. Soil moisture, cloud base, cloud cover, radiation fields, and evaporative fraction are coupled quite tightly on daily time scales. The long wave flux control by cloud-base height and cloud cover is particularly strong across all basins. Evaporation can be regarded as being determined somewhat indirectly by the dependence of net radiation on cloud cover and cloud base, and sensible heat flux on subcloud-layer processes. Cloud and boundary layer processes and the land surface components of a model must be evaluated as a tightly coupled system, not as independent components. This analysis provides a new framework for comparing global models with each other, and for evaluating them against observations.