1. © Crown copyright Met Office Radiation developments Latest work on the radiation code in the Unified Model James Manners, Reading collaboration meeting 17/11/09

2. © Crown copyright Met Office Contents This presentation covers the following areas Clouds: Temporal sampling Representing sub-grid inhomogeneity Optical properties of ice Gaseous absorption Representing 3D effects: Solar zenith angle correction Effect of resolved orography

3. © Crown copyright Met Office Clouds

4. © Crown copyright Met Office Temporal sampling HadGEM3 does a full radiation calculation every 3 hours. A simple (3 k-term) calculation is done on every 20 minute time-step to calculate increments. This captures changes in flux in the window regions of the spectrum where clouds have the greatest effect. Net surface SW error (Wm -2 ) for 3 hour time-step (left) and with 1 hour increments (right). Can allow time for a more accurate ‘full’ radiation calculation that can be used across all models. Manners et al. (2009) Q.J.R. Meteorol. Soc. 135: 457-468

5. © Crown copyright Met Office Sub-grid cloud Aim to have a consistent representation of sub-grid cloud distribution for all model physics. ‘Raisanen 2004’ cloud generator now included in UM which assumes global values for: Decorrelation scale for vertical overlap of cloud and condensate Horizontal distribution of condensate Sampled using McICA for the full radiation calculation – Peter Hill has worked on methods to reduce noise. (Possibly ‘TripleClouds’ for the increment calculation?) Future Work: How to better characterise the sub-grid cloud from model variables? …climatologies? How to use the sub-grid representation within the cloud scheme / microphysics?

6. © Crown copyright Met Office Optical properties of ice Future work: Need better verification of cloud radiative properties vs. IWC and T. Current scheme based on relationship between effective dimension (D e ) and temperature. Optical properties calculated from D e, an assumed PSD, and an ice-aggregate model. But… D e ’s used are much too small (ice shattering in detectors). New scheme by Anthony Baran based on observations to relate IWC and T directly to optical properties. Final scheme will use an ensemble model to provide a more consistent estimate of optical properties over all wavelengths.

7. © Crown copyright Met Office Gaseous absorption

8. © Crown copyright Met Office Clear-sky RT updates New spectral files from Zhian Sun (BoM Australia): Look-up tables for T & P broadening k-terms for gas mixtures Solar flux sorted for each k-term New solar spectrum based on Lean (2000) New SW ozone k-terms from Wenyi Zhong (improvements in the stratosphere and response to solar cycle) Future Work: Improve response to CO 2 increases (+ other trace gases) for climate Accurate spectral files to be used across all models (with the incremental time-stepping scheme)

9. © Crown copyright Met Office Representing 3D effects

10. © Crown copyright Met Office Change in solar zenith angle over radiation time-step Transmission based solar zenith angle correction Reduces large errors seen at the beginning and end of a 3 hour radiation time-step Error in surface SW (Wm -2 ) for first 15 minutes of 3-hour radiation time-step: Original correction just using change in cos(zenith angle) New correction including change in atmospheric transmission due to zenith angle

11. © Crown copyright Met Office Effects of resolved orography SW correction for slopes – implemented SW shadows LW net emission depends on surface area + sky-view factor… both are needed to maintain energy conservation Future work: 3D cloud effects… high resolution models?

12. © Crown copyright Met Office Questions and answers

13. © Crown copyright Met Office McICA noise in the UM Timestep Cumulative frequency Boundary layer type in the SCM Well mixed BL Mid-level convection Noise causes a bias in some convection and BL type indicators Results seen in SCM and climate runs Cumulus + well-mixed BL types Cumulus BL only Single sampling (high noise) Full sampling (no noise) Optimal sampling (low noise)