Relaxation of soil variables in a regional climate model

Abstract

A regional climate model, HIRHAM4, has been used with perfect boundary conditions fromthe ECMWF reanalysis project with an integration area covering the Mediterranean area. Themodel has been applied iteratively to one particular year, 1982, in the sense that boundaryconditions corresponding to that year were applied for several “years” (iterations); soil temperatureand moisture have been allowed to evolve through the simulation, whereas all atmosphericvariables have been reinitialized for each iteration. This way the soil variables, in particulartemperature, approach a stationary state with a vanishing net energy and water exchange withthe atmosphere. This approach over most of the integration area follows an exponential functionafter a transient period of not more than 1 year. This has the consequence that the asymptoticsoil temperature can be calculated after just 4 iterations of the model, resulting in an estimatemuch closer to equilibrium than the value after 4 years. Soil water is much more noisy andreaches approximate equilibrium after a few iterations; the extrapolation procedure for thisquantity is only recommended for arid regions. In general, the present experiment constitutesa method to distinguish between the transient behavior of slowly varying fields and the interannualvariability. The asymptotic state is independent of the initial soil state before the firstiteration and is therefore a property only of the model and of the forcing through the integrationperiod. This way of obtaining a “canonical” soil state may, for instance, have uses for modelinter-comparisons, as the arbitrary influence from initial transients has been removed.