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

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?

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

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

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

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

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 Shortwave up Clear sky profile Cloudy profile Error W m -2 depending on habit and k

Worst case: radar/lidar retrieval Radar/lidar extinction excellent, r e underestimated Extinction poor from radar only Extinction coefficientEffective radius Effective radius underestimate Poor radar- only retrieval, particularly at cloud top

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

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

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!

Donovan retrieval: with multiple scattering

Tinel retrieval: no multiple scattering

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

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

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

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

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

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

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?

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 fitPartial fit