1. On the sensitivity of BATS to heterogeneity in precipitation distribution Anne De Rudder, Patrick Samuelsson, Lü Jian-Hua What is the issue? Adopted approach Experiment design Results and discussion Conclusion Workshop on Land-Atmosphere Interactions in Climate Models Trieste, May 28 - June 8, 2001

2. The issue Could a coarse resolution of the model (e.g. imposed by a coupled GCM) miss subgrid-scale features that would otherwise have a non-negligeable impact on its output? The approach Design experiments with increased heterogeneity (in terms of precipitation distribution) and compare the averaged response of the model to its standard output, in order to obtain some kind of measure of the model response to the degree of heterogeneity (to be defined). De Rudder, Samuelsson & LüWorkshop on Land-Atmosphere Interactions in Climate ModelsTrieste, May 28 - June 8, 2001

3. The experiment  Control case: Cabauw study case (one-year integration with prescribed observed precipitation)  Heterogeneous cases: redistribution of precipitation in summer (June-July-August) associated to convection:  the grid cell is divided into 4, 9, 16 subcells  all the rain received by the control case grid cell in one (or 2, or 3,...) day(s) is concentrated over the surface area of one subgrid cell  each subgrid cell is watered (if it rains!) in turn  no other change (no update of solar incoming flux when precipitation is cut off) De Rudder, Samuelsson & LüWorkshop on Land-Atmosphere Interactions in Climate ModelsTrieste, May 28 - June 8, 2001

4. Cabauw Shortgrass 4 tiles Going from control to distributed case: dryer surface => albedo increase => more reflection of SW solar radiation => net radiation decrease dryer surface + higher temperature => more H and less LE De Rudder, Samuelsson & LüWorkshop on Land-Atmosphere Interactions in Climate ModelsTrieste, May 28 - June 8, 2001

5. Cabauw Shortgrass 4 tiles De Rudder, Samuelsson & LüWorkshop on Land-Atmosphere Interactions in Climate ModelsTrieste, May 28 - June 8, 2001

6. Cabauw Forest Multi-tile experiments Top soil moisture De Rudder, Samuelsson & LüWorkshop on Land-Atmosphere Interactions in Climate ModelsTrieste, May 28 - June 8, 2001

7. Cabauw Forest Multi-tile experiments Runoff De Rudder, Samuelsson & LüWorkshop on Land-Atmosphere Interactions in Climate ModelsTrieste, May 28 - June 8, 2001

8. Cabauw Forest 9 tiles Energy fluxes De Rudder, Samuelsson & LüWorkshop on Land-Atmosphere Interactions in Climate ModelsTrieste, May 28 - June 8, 2001

9. Model sensitivity to heterogeneity De Rudder, Samuelsson & Lü Workshop on Land-Atmosphere Interactions in Climate ModelsTrieste, May 28 - June 8, 2001

10. Cabauw 4 tiles De Rudder, Samuelsson & LüWorkshop on Land-Atmosphere Interactions in Climate ModelsTrieste, May 28 - June 8, 2001

11. Conclusions With regard to the control case, taking into account precipitation distribution heterogeneities lead to changes in energy budget and water balance. The degree of heterogeneity that needs to be taken into account in coupled modelling will depend on the potential impact of these effects on atmospheric circulation.  In a situation where heterogeneity occurs, the model AT THE SCALE OF AN ACM (compared to cases including some heterogeneity representation, assumed to be closer to reality), would overestimate latent heat flux, net radiation and soil moisture and underestimate sensible heat and canopy/soil temperature.  With the same experiment, the model response could be further investigated  with respect to a case with homogeneous time distribution of precipitation (e.g. perpetual one-day Cabauw - with rain! - or other "test" case)  for different cycle lengths  for precipitation redistributions less drastic than totally wet/totally dry  for different vegetation types  with analysis of stomatal resistance behaviour De Rudder, Samuelsson & LüWorkshop on Land-Atmosphere Interactions in Climate ModelsTrieste, May 28 - June 8, 2001