1. © Crown copyright Met Office A New Radiative Time- stepping Scheme A short description of the time-stepping scheme test Peter Hill, 19 Feb 2008

2. © Crown copyright Met Office Motivation Transmissivity = Regions with low transmissivity Absorption predominantly due to a strong gaseous absorber Absorption by clouds, aerosols and weak absorbers has little effect. Regions with high transmissivity Absorption predominantly due to clouds Gaseous absorbers are all weak. Treatment of main gaseous absorber is computationally more expensive. Clouds change on much shorter time-scales than the concentration of gases.

3. © Crown copyright Met Office Idea Replace existing spectral file with two new files A slow file for treating strong gaseous absorption, called less frequently. A fast file for treating clouds and minor gases, called more frequently.

4. © Crown copyright Met Office NAE Diagnostics SW Heating Rates,1 August 09.00 In experiment, fast is called every 3 time-steps, while slow is called every 18, the operational calls radiation every 12 time- steps.

5. © Crown copyright Met Office NAE Diagnostics LW Heating Rates,1 August 09.00

6. © Crown copyright Met Office Questions and answers