Blind tests of radar/lidar retrievals: Assessment of errors in terms of radiative flux profiles

1. Blind tests of radar/lidar retrievals: Assessment of errors in terms of radiative flux profiles Malcolm Brooks Robin Hogan and Anthony Illingworth David Donovan and Claire Tinel

2. Introduction • First blind test showed that • Both Donovan and Tinel algorithms could retrieve extinction coefficient very accurately • Effective radius and IWC depend on assumption of habit (i.e. “density” or the mass-size relationship) • Second blind test included multiple scattering, molecular scattering and instrument noise: • Reasonable extinction profiles were generally obtained if multiple scattering was included in the retrieval, otherwise extinction was underestimated • Retrieval only possible where lidar still has good signal • What are the radiative implications? • How do these retrievals compare to radar-only?

3. Blind test 1 (From aggregation study) • No instrument noise • No multiple scattering • No molecular scattering • High lidar sensitivity • Two versions of each profile provided, with variable or constant extinction/backscatter ratio “k”, which was not known by the algorithms

4. Blind test 1:Results 1 • Constant k: • Both Donovan and Tinel (after modification) algorithms produce highly accurate extinction • Variable k: • Error in extinction varies with k, but not unstable

5. Blind test 1:Results 2 • Effective radius: • Good, but difficult if re > 80 microns because of radar Mie scattering • Sensitive to particle habit • Ice water content: • Extinction ~ IWC/re • Hence if extinction is correct then the % error in effective radius is equal to the % error in IWC

6. Best case: radar/lidar retrieval • Excellent extinction, good re if same mass-size relationship is used (otherwise 40% too low) Extinction coefficient Effective radius Francis et al. relationship Radar only retrieval Mitchell relationship

7. Best case: radiation calculations • Used Edwards-Slingo radiation code • Excellent longwave, good shortwave but slight effect of habit and k; better than radar alone Longwave up Clear sky profile Error 20-40 W m-2 depending on habit and k Cloudy profile Shortwave up

8. Worst case: radar/lidar retrieval • Radar/lidar extinction excellent, re underestimated • Extinction poor from radar only Extinction coefficient Effective radius Poor radar-only retrieval, particularly at cloud top Effective radius underestimate

9. Worst case: radiation calculations • Excellent longwave, still good shortwave! • Effective radius not very important? Error 20-30 W m-2 Longwave up Shortwave up

10. Blind test 1: Heating rates • Radar/lidar: very accurate • Radar alone: OK but some biases Error due to higher Z here Best case Worst case

11. Blind test 2 (From EUCREX) • Instrument noise • Multiple scattering • Molecular scattering • True lidar sensitivity • Constant extinction to backscatter ratio • Note: radar-only relationships derived using this dataset so not independent!

12. Donovan retrieval: with multiple scattering

13. Tinel retrieval: no multiple scattering

14. Good case: radar/lidar retrieval • Extinction and effective radius reasonable when use same habit and include multiple scattering Extinction coefficient Effective radius Full profile retrieved Difference between Mitchell and Francis et al.

15. Good case: radiation calculations • OLR and albedo good for both radar/lidar and radar-only (but radar-only not independent) Longwave up Mass-size relationship has modest effect: Error<10 Wm-2 Underestimate radiative effect if multiple scattering neglected Shortwave up

16. Poor case: radar/lidar retrievals • No retrieval in lower part of cloud Extinction coefficient Effective radius Good retrieval at cloud top Wild retrieval where lidar runs out of signal

17. Poor case: radiation calculations • At top-of-atmosphere, lower part of cloud important for shortwave but not for longwave OLR excellent despite lower part not retrieved Albedo underestimated (90 W m-2): lower part of cloud is important Longwave up Shortwave up

18. Blind test 2: Heating rates • Heating profile reasonable if full profile retrieved Erroneous 80 K/day heating No cloud observed so no heating by cloud here Best case Worst case

19. Sensitivity of radiation to retrievals • Longwave: easy! • Sensitive to extinction coefficient • Insensitive to effective radius, habit or extinction/backscatter • OLR insensitive to lower half of cloud undetected by lidar • Shortwave: difficult to get to better than 20 W m-2 • Most sensitive to extinction coefficient • Need full cloud profile to get correct albedo • Some sensitivity to habit and therefore effective radius • Slight sensitivity to extinction/backscatter ratio • Note: not included habit dependence of asymmetry parameter or single-scattering albedo

20. Conclusions • Extinction much the most important parameter: • Good news: this can be retrieved accurately independent of assumption of crystal type • But need to include multiple scattering in retrieval • Need to retrieve something when no more lidar: • Switch to radar-only retrieval? • Assign error to both radar/lidar and radar-only retrievals and produce a consensus value, weighted accordingly? • Must avoid erroneous spikes where lidar loses signal! • Use imager (VIS & IR) synergy to give top-of-atmosphere radiances and provide a constraint for the retrieval: how would this be incorporated into the algorithms? • Do SW radiances provide multiple-scattering information?

21. Scaling the radar-only retrieval • Where radar/lidar retrieval fails, can we scale the radar-only retrieval to get a seamless join? • Dubious: the profiles are not real but simulated! Good fit Partial fit