On the sensitivity of BATS to heterogeneity in precipitation distribution Anne De Rudder, Patrick...

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

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 & Lü Workshop on Land-Atmosphere Interactions in Climate Models Trieste, May 28 - June 8, 2001

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)


CabauwShortgrass4 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


CabauwShortgrass4 tiles


CabauwForestMulti-tileexperiments

Top soilmoisture


CabauwForestMulti-tileexperiments

Runoff


CabauwForest9 tiles

Energyfluxes


Modelsensitivity toheterogeneity


Cabauw4 tiles


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


On the sensitivity of BATS to heterogeneity in precipitation distribution Anne De Rudder, Patrick...

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