How well does MODIS +adiabaticity characterize the south-eastern Pacific stratocumulus deck?

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Vernon Armstrong
8 years ago
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1 How well does MODIS +adiabaticity characterize the south-eastern Pacific stratocumulus deck? -integral to VOCALS objectives David Painemal & Paquita Zuidema, RSMAS/U of Miami -we want to do the best we can: Chris * marine Fairall, stratus NOAA/ESRL over ocean is the idealization many retrievals of warm cloud properties are based on * VOCALS an opportunity to both nail down retrievals & a work in progress. use retrieval-breakdowns to better understand cloud/aerosol/precip behavior * unique characteristic/opportunity for VOCALS IUGG, July 4, 2007

2 MOTIVATION Global assessments of boundary layer cloud properties (& aerosol indirect effects) best achieved with satellite data Satellite cloud property retrieval assumptions optimized for stratocumulus over oceans Satellite cloud droplet concentration retrievals invoked in, e.g., Bennartz 2007; Schuller et al. 2005; Boers et al, 2006 Almost-annual buoy-tending cruises provide opportunity for closer examination of satellite/ship-based datasets

3 5 cruises so far cruise-coincident TMI SSTs show seasonal, interannual changes Oct (EPIC) Nov Dec Oct (EPIC) Oct Oct. 2006

4 Intriguing suggestion of aerosol indirect effects: MODIS monthly-mean AOD Nov. 03 & cloud effective radius Nov. 03

5 Intriguing suggestion of aerosol indirect effects: MODIS monthly-mean AOD Nov N aer high even far away from land (03 mean= 180/cc) ocean biology? & cloud effective radius Nov. 03

6 Intriguing suggestion of aerosol indirect effects: MODIS monthly-mean AOD Nov N aer high even far away from land (03 mean= 180/cc) ocean biology? & cloud effective radius Copper smelter effluents? Huneeus et al 06 Nov. 03 Wood et al., VOCALS program summary

7 Or is it cloud thinning near coast? MODIS monthly-mean aerosol optical depth Oct, 01 Oct, 05 & cloud effective radius Oct, 01 Oct, 05

8 Or is it cloud thinning near coast? MODIS monthly-mean aerosol optical depth Oct, 01 Oct, 05 Oct, 06 & cloud effective radius Oct, 01 Oct, 05 Oct, 06

9 How do we make sense of this? Ship-board instrumentation always include vertically-pointing ceilometer (cloud base height) rawindsondes flux tower measurements (lifting condensation level calc.) 5cm depth & since 2001 include surface-based aerosol particle counts (Lasair-II and/or Texas A&M DMA;Tomlinson et al., 2007) derive & compare MODIS&ship-derived cloud depths assuming adiabaticity derive MODIS cloud droplet numbers (CDN) assuming adiabaticity compare to surface-based N aer Should find CDN <~ N aer

10 We think adiabaticity should hold: retrieved LWP adiabatic LWP retrieved LWP adiabatic LWP Local time EPIC, 6 days at buoy Zuidema et al deviations from it either point to retrieval problems and/or provide physical insights (e.g. dependence of drizzle on LWP,N aer )

11 Satellite-derived cloud Depth: relies on TMI SST, MODIS CTT, lapse rate assumption TMI SST all cruises Ship SST TMI SST = Ship SST - 0.3K Ship SST- T inversion (sonde) cloud base height (sonde)

12 MODIS CCT = sonde CCT K MODIS cloud top temperature R=0.7 Red: Cloud Fraction < 0.9 Inversion base temperature (sonde) Good agreement, if offset & constrained to overcast conditions (MODIS, collection 5, level 2)

13 sonde CT height MODIS CT height Linear fit leads to reasonable derivation of cloud top height from satellite => useful for model validation of top of boundary layer

14 Derive a satellite cloud depth assuming adiabaticity: LWP modis = Γ ad (z top - z bottom ) 2 /2 = 5/9ρ water r eff τ In-situ cloud depth MODIS cloud depth Reasonable correlation but MODIS derived cloud depth tends to be too high

15 Γ ad (z top - z bottom ) 2 /2 = 5/9ρ water r eff τ (overcast conditions) WHY (does this overestimate cloud depth) MODIS optical depth too large? (can test this) MODIS mean droplet size too large? (can t test) modify 5/9 and/or Γ ad? Does it depend on coupling? (stratify by CB-LCL amount) (EPIC cruise best-coupled of the 5 cruises) ceilometer LCL can similarly derive an adiabatic cloud droplet number: N CDN = sqrt(10)/(4πρ 0.5 w)τ 0.5 /r 2.5 eff (Wood, 2006) Is this less than the surface-based N aerosol? (work in progress)

16 can similarly derive an adiabatic cloud droplet number: N CDN = sqrt(10)/(4πρ 0.5 w )τ0.5 /r 2.5 eff (Wood, 2006) Is this less than the surface-based N aerosol? (work in progress)

17 Comparison of monthly-mean CDN & AOD inconsistent Sampling? Retrieval issues? Meteorology? Oct 01 Oct 05 Oct 06

18 IN CONCLUSION.OUR MOTIVATION Global assessments of boundary layer cloud properties (& aerosol indirect effects) best achieved with satellite data Satellite cloud property retrieval assumptions optimized for stratocumulus over oceans Satellite cloud droplet concentration retrievals invoked in, e.g., Bennartz 2007; Schuller et al. 2005; Boers et al, 2006 Almost-annual buoy-tending cruises provide opportunity for closer examination of satellite/ship-based datasets towards elucidating best use of satellite data

Cloud Remote Sensing. retrieval-breakdown to better understand cloud/aerosol/precip. cloud/aerosol/precipbehavior. characteristic/opportunityfor

Cloud Remote Sensing. retrieval-breakdown to better understand cloud/aerosol/precip. cloud/aerosol/precipbehavior. characteristic/opportunityfor Cloud Remote Sensing -integral -integraltotovocals VOCALSobjectives objectives -we -wewant wanttotodo dothe thebest bestwe wecan: can: * *marine marinestratus stratusover overocean oceanisisthe theidealization