Presentation is loading. Please wait.

Presentation is loading. Please wait.

Blind tests of radar/lidar retrievals in ice clouds: Assessment in terms of radiative fluxes Robin Hogan, Malcolm Brooks, Anthony Illingworth University.

Similar presentations


Presentation on theme: "Blind tests of radar/lidar retrievals in ice clouds: Assessment in terms of radiative fluxes Robin Hogan, Malcolm Brooks, Anthony Illingworth University."— Presentation transcript:

1 Blind tests of radar/lidar retrievals in ice clouds: Assessment in terms of radiative fluxes Robin Hogan, Malcolm Brooks, Anthony Illingworth University of Reading, UK David Donovan Claire Tinel KNMI, The NetherlandsCETP, France

2 Ice cloud retrievals: the problem Radar only –Not well related to radiative properties Lidar only –Difficult to correct for attenuation

3 Combined radar and lidar Current plan for CloudSat and Calipso… –Combine Calipso Level 2 extinction profile with CloudSat reflectivity product to get IWC and effective radius (size is related to ratio of Z and extinction coefficient) –But the large errors in the extinction will feed through to very inaccurate retrievals, particularly at cloud base Alternative: use radar to constrain lidar inversion –Donovan et al. (2000): find lidar ratio that minimizes the variation in implied effective radius in furthest few gates –Tinel et al. (2000): find lidar ratio that minimizes the implied variation in number concentration in the profile –CloudSat must ingest Calipso Level 1 backscatter product

4 Blind Tests At last CloudSat science meeting a Blind Test of these algorithms was reported: –I used aircraft spectra to simulate radar/lidar profiles –Dave Donovan and Claire Tinel ran their algorithms to derive profiles of extinction, IWC and effective radius What we found: –Both algorithms retrieved extinction very accurately, even for 1-way optical depths of up to 7 (lidar reduced by ) –Stable to uncertain lidar extinction-to-backscatter ratio –However, r e and IWC retrievals sensitive to mass-size relation

5 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

6 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

7 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

8 In this talk… Ultimate test: are these retrievals accurate enough to constrain the radiation budget? –Perform radiation calculations on true & measured profiles –Assess errors in terms of fluxes and heating rates –What are the most sensitive of the retrieved parameters? But in the Blind Test, the measured profiles were noise-free and almost infinitely sensitive! Second Blind Test simulates instrumental limitations that will be faced by EarthCARE for: –10-km dwell (1.4 seconds), 400-km altitude, 355-nm lidar –Conclusions similar for CloudSat/Calipso Radar/lidar combination better than radar alone? –Also test Z-IWC, Z-, Z-r e relationships (from EUCREX)

9

10

11 Case from First Blind Test Excellent extinction (both Donovan and Tinel) Good r e if same mass-size relationship used (otherwise 40% too low) Mitchell relationship Francis et al. relationship Radar only retrieval Extinction coefficientEffective radius

12 What about radiative fluxes? Edwards-Slingo 1-D plane-parallel calculations –Excellent longwave, good shortwave, slight effect of habit and extinction-backscatter ratio, better than radar alone –Effective radius not very important? Longwave up Shortwave up Clear sky profile Cloudy profile Error W m -2 depending on habit and k

13 Heating rates Radar/lidar: very accurate Radar alone: almost as good

14 Case from First Blind Test Excellent extinction (both Donovan and Tinel) –r e depends on mass-size relationship, but can still be too low –Extinction poor from radar only Extinction coefficientEffective radius Effective radius underestimate Poor radar- only retrieval, particularly at cloud top Mitchell relationship Francis et al. relationship

15 Radiation (Edwards-Slingo code) Excellent longwave, good shortwave –Slight effect of habit and k; better than radar alone –Effective radius not very important? Longwave up Shortwave up Error Wm -2 depending on habit and k Clear sky profile Cloudy profile

16 Heating rates Radar/lidar: reasonably accurate Radar alone: OK but some biases Error due to higher Z here

17 Second Blind Test: more realistic Ice size distributions from EUCREX aircraft data –Correction for 2D-C undercounting of small ice crystals Radar –Simulate 100-m oversampling of 400-m Gaussian pulse –Noise added based on signal-to-noise and number of pulses Lidar –Add molecular scattering appropriate for 355 nm –Instrument noise: photon counting - Poisson statistics –True lidar sensitivity: incomplete penetration of cloud –Multiple scattering: Eloranta model with 20-m footprint –Extinction-to-backscatter unknown but constant with height –Night-time operation, negligible dark current

18 Accounting for small ice crystals –2D-C probe undercounts small crystals –Assume gamma distribution for crystals < 100 m diameter –Mode at 6 m –Same conc. at 100 m –Conc. 2.5 times higher at 25 m

19 Instrument noise Five new profiles from EUCREX dataset This is what they would look like without instrument noise or multiple scattering –Note strong lidar attenuation Radar Lidar

20 Instrument noise Five new profiles… With instrument noise & multiple scattering –Radar virtually unchanged except finite sensitivity –Lidar noise noticeable –Lidar multiple scattering increases return Note: radar-only relationships derived using same EUCREX dataset so not independent! Radar Lidar

21 Donovan retrieval: with multiple scattering

22 Tinel retrieval: no multiple scattering

23 Good case: lidar sees full profile Extinction and effective radius reasonable when use same habit and include multiple scattering Extinction coefficientEffective radius Donovan: includes multiple scattering Difference between Mitchell and Francis et al. mass-size Tinel: no multiple scattering

24 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: Error~10 Wm -2 Underestimate radiative effect if multiple scattering neglected

25 Typical case: radar/lidar retrievals No retrieval in lower part of cloud –Important to include multiple-scattering in retrieval Extinction coefficientEffective radius Wild retrieval where lidar runs out of signal Donovan: good retrieval at cloud top Difference between Mitchell and Francis et al. mass-size relation Tinel: no multiple scattering

26 Typical case: radiative fluxes At top-of-atmosphere, lower part of cloud important for shortwave but not for longwave Longwave up Shortwave up OLR excellent: lower part not important Albedo too low (80 W m -2 ): lower part of cloud is important but mass-size less so (10 W m -2 ) Underestimate radiative effect if multiple scattering neglected

27 Typical profile: Heating rates Heating profile would be reasonable if full profile was retrieved What do we do when the lidar runs out of signal? Erroneous 80 K/day heating No cloud observed so no heating by cloud here

28 Simple solution: blend profiles Where lidar runs out of steam, scale radar-only retrieval for seamless join –Much better result than pure radar/lidar or radar only Extinction coefficientShortwave up Lidar becomes unreliable here Scale radar-only retrieval to match Blended Radar only Radar/lidar only

29 Possible solution: blend profiles Where lidar runs out of steam, scale radar-only retrieval for seamless join –Better result than pure radar/lidar or radar only Extinction coefficientShortwave up Lidar becomes unreliable here Blended Radar only Radar/lidar only Scale radar- only retrieval to match here

30 Sensitivity of radiation to retrievals Longwave: Easy! (for plane-parallel clouds…) –Sensitive to extinction coefficient –Insensitive to effective radius, habit or extinction/backscatter –OLR insensitive to lower half of cloud undetected by lidar –Blending usually gets in-cloud fluxes to better than 5 W m -2 Shortwave: More difficult –Most sensitive to extinction coefficient –Need full cloud profile: blending enables TOA shortwave to be retrieved to ~10 W m-2, in-cloud fluxes less accurate –Some sensitivity to habit and therefore effective radius –Slight sensitivity to extinction/backscatter ratio

31 Conclusions Extinction much the most important parameter: –Good news: this can be retrieved accurately independent of assumption of crystal type –So lidar can provide the extra accuracy at cloud top necessary if retrievals are to be consistent with measured TOA fluxes –But need to include multiple scattering in retrieval –Must avoid erroneous spikes where lidar loses signal! What is the best way to blend profiles? –Scale radar-only retrieval? Or switch straight to radar-only? –Need to analyze aircraft spiral descents through ice cloud How could radiances from passive instruments be used to refine the retrievals? –E.g. do SW radiances provide multiple-scattering information?

32 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

33 An invitation! Try your algorithm on profiles of radar reflectivity and attenuated lidar backscatter from the first blind test (variable lidar ratio, no instrument noise): –http://www.met.rdg.ac.uk/radar/esa/blind_test.html If it passes the test, try profiles from the second blind test (multiple scattering, instrument noise): –http://www.met.rdg.ac.uk/radar/esa/blind_test2.html


Download ppt "Blind tests of radar/lidar retrievals in ice clouds: Assessment in terms of radiative fluxes Robin Hogan, Malcolm Brooks, Anthony Illingworth University."

Similar presentations


Ads by Google