Land surface hydrology: parametrization in GCMs and impacts on the interannual variability

Herve Douville

In the framework of the Global Soil Wetness Project (GSWP), the ISBA land-surface scheme of the ARPEGE atmospheric general circulation model has been forced with meteorological observations and analyses in order to produce a two-year (1987-1988) soil moisture climatology at a 1 by 1 degree horizontal resolution. This climatology is model dependent, but it is the climatology that the ARPEGE model would produce if its precipitation and radiative fluxes were perfectly simulated. In the present study, ensembles of seasonal simulations (March to September) have been performed for 1987 and 1988, in which the total soil water content simulated by ARPEGE is relaxed towards the GSWP climatology. The results indicate that the relaxation has a positive impact on both the model's climatology and the simulated interannual variability, thereby confirming the utility of the GSWP soil moisture data for prescribing initial or boundary conditions in comprehensive climate and numerical weather prediction models. They also demonstrate the relevance of soil moisture for achieving realistic simulations of the Northern Hemisphere summer climate. In order to get closer to the framework of seasonal predictions, additional experiments have been performed in which GSWP is only used for initialising soil moisture at the beginning of the summer season (the relaxation towards GSWP is removed on 1st June). The results show a limited improvement of the interannual variability, compared to the simulations initialised from the ARPEGE climatology. However, some regional patterns of the precipitation differences between 1987 and 1988 are better captured, suggesting that seasonal predictions can benefit from a better initialisation of soil moisture. Finally, other ensembles have been performed in which the soil water content is relaxed or constrained (below the wilting point or above the field capacity) only over south Asia or Sudan-Sahel. These additional experiments show a different sensitivity of the Asian and African monsoons to the land surface hydrology. Whereas African rainfall increases with increasing soil moisture, such a clear and homogeneous response is not found over the Indian continent. Precipitation does increase over northern India as a consequence of wetter surface conditions, but the increased evaporation is counterbalanced by a reduced moisture convergence when averaging the results over the whole Indian peninsula. This contrasted behavior is partly related to the more dynamical nature of the Asian monsoon, where moisture convergence is about twice that found over Sudan-Sahel so that water recycling has a weaker influence on seasonal rainfall. It is also due to a different response of the frequency distribution of daily precipitation, and particularly to an increased number of strong convective events with decreasing soil moisture over India.

References

Douville H. (1998): Validation and sensitivity of the global hydrologic budget in stand-alone simulations with the ISBA land-surface scheme, Climate Dyn, 14:151-171.

Douville H, Chauvin F (2000): Relevance of soil moisture for seasonal climate predictions: a preliminary study. Climate Dynamics, 16, 719-736.

Douville H, Chauvin F, Broqua H (2001): Influence of soil moisture on the Asian and African monsoons. Part I: Mean monsoon and daily precipitation. J Climate, 14:2381-2403.