1. 19 February 2008 1 1 The Tripleclouds scheme Jon Shonk and Robin Hogan

2. 19 February 2008 2 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 water content / kg m –3

3. 19 February 2008 3 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. P 0 = 244.6 – 2.328 φ

4. 19 February 2008 4 Global model data calculations Short- wave CRF. Long- wave CRF. Net CRF. PP: plane-parallel TC: Tripleclouds max: maximum- random d(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

5. 19 February 2008 5 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.

6. 19 February 2008 6 Global model data calculations Short-wave CRF biases. Long-wave CRF biases. Net CRF 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%

7. 19 February 2008 7 Random animations... 0.8 × mean water content vertical separation overlap parameter