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Investigating the use of a 2-channel IR Retrieval on Daytime Polar Optical Depth

Investigating the use of a 2-channel IR Retrieval on Daytime Polar Optical Depth. 2 case studies…. RGB. VISST Only τ. 4/5 retrieval used where: VISST τ < 0 or VISST τ =150. VISST + 2-channel IR(1) τ.

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Investigating the use of a 2-channel IR Retrieval on Daytime Polar Optical Depth

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  1. Investigating the use of a 2-channel IR Retrieval on Daytime Polar Optical Depth 2 case studies…

  2. RGB

  3. VISST Only τ

  4. 4/5 retrieval used where: • VISST τ < 0 or • VISST τ=150 VISST + 2-channel IR(1) τ Start by applying this retrieval where the VISST result is bad or pegged on the upper end. This may help along cloud edges, but not much (if any) benefit elsewhere Is this decrease in τ realistic??? High zenith angle and cloud shadows make this area difficult…

  5. Cloud Phase Discontinuities in τ exist where retrieved cloud phase is different…

  6. Some of these discontinuities in phase may be real, but some are certainly not accurate. VISST + 2-channel IR(1) τ Often, large τ = cold water cloud and small τ = ice cloud Since τ tends to be larger for supercooled water clouds, maybe it would help if we try using 2-channel IR for high τsupercooled water clouds…

  7. 4/5 retrieval used where: • VISST τ < 0 or • VISST τ=150 or • {VISST phase = water and • VISST Tcloud <273K and • VISST τ > 100} VISST + 2-channel IR(2) τ The result looks very much like when the final criteria is not included… Optical depth can be pegged on the lower end too, usually for ice clouds. Try running 4/5 for those cases too…

  8. 4/5 retrieval used where: • VISST τ < 0 or • VISST τ=150 or • {VISST phase = water and • VISST Tcloud <273K and • VISST τ > 100} or • VISST τ=0.05 VISST + 2-channel IR(3) τ Wow, this made almost no difference. Perhaps it will make more of a difference for a different scene??

  9. Looking at another case, over Greenland… RGB

  10. Cloud Phase

  11. Similar to the previous case, supercooled water clouds often exhibit extremely high τ, while ice clouds exhibit extremely low τ (in this case, < 1, but not pegged!) VISST Only τ

  12. 4/5 retrieval used where: • VISST τ < 0 or • VISST τ=150 or • {VISST phase = water and • VISST Tcloud <273K and • VISST τ > 100} or • VISST τ=0.05 VISST + 2-channel IR(3) τ Little change is seen in the low τ values across central Greenland. High τ clouds in this region are likely too thick for 4/5 retrieval to be of assistance.

  13. 4/5 retrieval used where: • VISST τ < 0 or • VISST τ=150 or • VISST τ=0.05 or • {VISST phase = water and • VISST Tcloud <273K and • VISST τ > 100} or • {VISST phase = ice and • VISST τ < 1} VISST + 2-channel IR(4) τ Applying 4/5 retrieval for all very low τ (< 1) ice clouds seems to increase the optical depth slightly in some areas. Are these changes realistic??

  14. RGB This one seems iffy Higher τ is better here This looks like clear-sky How would the original case we considered be affected by using 4/5 for low τ ice clouds?

  15. 4/5 retrieval used where: • VISST τ < 0 or • VISST τ=150 or • VISST τ=0.05 or • {VISST phase = water and • VISST Tcloud <273K and • VISST τ > 100} VISST + 2-channel IR(3) τ

  16. 4/5 retrieval used where: • VISST τ < 0 or • VISST τ=150 or • VISST τ=0.05 or • {VISST phase = water and • VISST Tcloud <273K and • VISST τ > 100} or • {VISST phase = ice and • VISST τ < 1} VISST + 2-channel IR(4) τ Running 4/5 retrieval for low τ ice clouds increases τ along cloud edges, but also in a few areas that could be false cloud.

  17. Hard to tell if increasing the τ in these areas offers an overall improvement. RGB Here, increasing τ is probably good Here, increasing τ is probably not so good Clear???

  18. Conclusions • 2-channel (4/5) IR retrieval may be applied over the polar regions in areas where the VISST retrieval is questionable • After examination of 2 scenes, large discontinuities in τ are observed to coordinate with discontinuities in cloud phase. Using the 4/5 retrieval for high τ water clouds and low τ ice cloud seems to provide some improvement • 4/5 retrieval is only used if τ < 6… this makes the assumption that this retrieval will yield τ > 6 for clouds that are actually thick (not necessarily valid) • Infrared retrieval is very sensitive to background T… need to make sure that clear-sky brightness temperatures are accurate and not cloud-contaminated • More cases need to be examined

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