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19 February 2008
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The Tripleclouds schemeThe Tripleclouds scheme
Jon Shonk and Robin Hogan
19 February 2008
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Plane-parallel approximation uses one cloudy region in each vertical layer.
Tripleclouds uses two regions: one containing optically thin cloud, the other containing optically thicker cloud.
Studies have shown that fractional standard deviation of cloud water content in a layer is typically around 0.8.
Horizontal structure
height / km log of ice watercontent / kg m–3
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Vertical structure
The Tripleclouds scheme uses decorrelation pressure overlap.
• Overlap parameter is allowed to decrease exponentially with layer separation.
• The decorrelation pressure is a function of latitude.
P0 = 244.6 – 2.328 φ
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Global model data calculations
Short-wave CRF.
Long-wave CRF.
Net CRF.
PP: plane-parallelTC: Triplecloudsmax: maximum- randomd(var): decorrelation pressure
Global mean biases in net CRF with respect to PPmax scheme...
TCmax: 6.53 W m–2
PPd(var): –1.86 W m–2
TCd(var): 3.78 W m–
2
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Summary
Tripleclouds reduces the plane-parallel biases.
• The largest reductions are in areas of marine stratocumulus, where the magnitude of net CRF is highest.• Implementing the horizontal component of Tripleclouds alone causes a reduction of the biases that is too large.• The vertical component of Tripleclouds acts to reduce the size of this bias.
The next steps for Tripleclouds...
• There is a nine-month project to implement the Tripleclouds scheme interactively in the Met Office Unified Model.
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Global model data calculations
Short-wave CRF biases.
Long-wave CRF biases.
NetCRF biases.
Left column: absolute biases.Right column: percentage biases.
Biases are TC minus PP.
4.6 W m–2 7.2%
–0.8 W m–2 –5.3%
3.8 W m–2 7.8%
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Random animations...
0.8 × meanwater content
vertical separation
overlap parameter
