Land atmosphere coupling in the
RACMO West-Africa simulations
Bart van den Hurk
Erik van Meijgaard
Presented by Geert Lenderink
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Status runs
• Model used: RACMO cy31 (≈ EC-Earth
configuration,  European RT2b/RT3 simulations)
– old (shallower) soil scheme (TESSEL)
– different convection
• ERA40/Interim run
– 1986 – 1988: ERA40
– 1989 – 2005: ERA-Interim
– 2006 – 2007: ERA-Interim currently running
(until?)
• ECHAM5r3
– (almost) finished (at time of leave 2047)
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Science questions
• Does RACMO reproduce precipitation climate well?
• Which areas expose strong land influence on
precipitation variability on monthly timescales?
• Is this interaction explained by local atmospheric
processes?
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Seasonal patterns of
precipitation 1986 – 2005
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Reproduction of precipitation
• CRU TS3 monthly precipitation
• Mean land area 15S-25N, 15E-30E
ERA40 > ERA-Interim
discard 1986 – 1988
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Anomaly correlation
modelled-observed precipitation
1989 – 2005
Poor reconstruction
local processes in
soil/atmosphere
of importance
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Where is land effective in
changing precipitation?
•
Criteria (After Dirmeyer et al, 2008):
1. Surface perspective: Evaporation sensitive to
soil moisture
• significant +corr. E and soil moisture
2. Atmospheric perspective: moisture recycling
depends on local moisture supply
• significant +corr. recycling ratio and soil
moisture
* Recycling ratio: “Lagrangian” algorithm that
accumulates evaporation in upstream grid points,
and measures E/q within L km radius (100 < L <
1000 km)
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Recycling ratio L = 200km
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Recycling ratio L = 500km
Length scale
unimportant for
spatial structure
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Area of significant correlation
Evap – soil moisture anomaly
E controlled by soil
moisture except in
convective rainfall
areas (where E is
energy limited)
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Area of significant correlation
recycling – soil moisture
• + correlation: recycling senses local moisture supply
In the retreat area
of the wet season
(wet  dry transition)
In the landward
part of the
monsoon area
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Soil memory* in areas with both
positive correlations
Interesting areas
where land-atm
interaction may affect
precipitation on
indicated time scale
*autocorrelation time scale
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Science questions – Conclusions
• Does RACMO reproduce
precipitation climate well?
– mean climate yes, but free
interior leaves many degrees of
freedom for local processes
• Which areas expose strong land
influence on precipitation
variability on monthly timescales?
– at wet  dry transitions
• Is this interaction explained by
local atmospheric processes?
– work in progress
– [email protected]
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