”Tropical Clouds and Cloud Feedback” The importance of radiative constraints

1. ”Tropical Clouds and Cloud Feedback” The importance of radiative constraints Dennis L. Hartmann Department of Atmospheric Sciences University of Washington Seattle, Washington USA Workshop on Large-Scale Circulations in Moist Convecting Atmospheres October 15-16, 2009 Papers online: Google Dennis L. Hartmann

2. Outline • Motivation from AR4 simulations • Radiation-Convection-Dynamics Interaction • Fixed Anvil Temperature Hypothesis (FAT) • Application of FAT to AR4 GCM Simulation Interpretation

3. LW feedbacks positive and comparable magnitude. SW feedbacks positive/negative, and dominate total feedback. SW and LW cloud feedback Net cloud feedback from 1%/ yr CMIP3/AR4 simulations Courtesy of B. Soden

4. Atmospheric Energy Balance is Radiative –Convective Radiative Cooling = Latent Heating + Advection of Energy Clear-Sky Radiative Cooling is a key parameter. Clouds, Convection and RadiationAtmospheric Energy Balance

5. Clear-sky Radiative Cooling and Relaxation: for tropical climatological conditions In the tropical atmosphere, and the in the global atmosphere, radiative cooling approximately balances heating by latent heat release in convection. The global mean precipitation rate is about 1 meter per year, Adiabatic Heating which equals an energy input of about 80 Watts/sq. meter, Requiring a compensating atmospheric radiative cooling of about 0.7 ˚K/day, averaged over atmosphere. -2.0 -1.0

6. Atmospheric Radiative CoolingAltitude vs Frequency Upper Troposphere Cooling from Water Rotation Lines 10 m 50 m 20 m 5 m 6.7 m Lower Troposphere Cooling from Water Continuum Harries, QJRMS, 1996

8. The FAT Hypothesis, The Fixed Anvil Temperature Hypothesis. Tropical anvil clouds appear at a fixed temperature given by fundamental considerations of: • Clausius-Clapeyron definition of saturation vapor pressure dependence on temperature. • Dependence of emissivity of rotational lines of water vapor on vapor pressure.

9. Testing the FAT Hypothesis with a CRM. ‘Cloud-Resolving’ Model 1km horizontal resolution Doubly periodic domain 64km x 64km box with uniform SST (28, 30, 32C) Bulk microphysics RRTM radiation model Basically a radiative-convective model in which the Clouds are explicitly resolved at 1km resolution. Run to equilibrium and average last 50 days. Zhiming Kuang’s work: Updated by Bryce Harrop

10. Recreating Kuang & Hartmann (2007) Results Using SAM with CAM 5˚C

11. Radiation • Change the level of clear-sky convergence • Two possibilities • Remove water vapor to lower convergence level • Add more water vapor to raise convergence level • SAM model: Two different water vapor variables • Bulk microphysics • Radiation

12. Altering Water Vapor in the Radiation Code Part I Water vapor change only applied to radiation calculation!! Temperature Base Case Removal Case Base Case Reduces emissivity = Less cooling = ? qv, stratospheric Water Vapor (radiation only)

13. Removal of Water Vapor Comparison Base Removal

14. Altering Water Vapor in the Radiation Code Part II Water vapor change only applied to radiation calculation!! Temperature Base Case Removal Case Addition Case Base Case Removal Case qv, stratospheric Water Vapor (radiation only)

15. Addition of Water Vapor Comparison Addition Base

16. Radiative Control In radiative-convective equilibrium in a CRM If you change SST, cloud temperature remains about the same - FAT If you change the emissivity of the upper troposphere in the Tropics, you can change the cloud temperature and associated circulation.

17. LW feedbacks positive and comparable magnitude. SW feedbacks positive/negative, and dominate total feedback. SW and LW cloud feedback Net cloud feedback from 1%/ yr CMIP3/AR4 simulations Courtesy of B. Soden

18. Motivation: Why is the Longwave Cloud Feedback Robustly Positive in the AR4 GCMs? We hypothesize that it is largely due to the fact that tropical high clouds remain at approximately the same temperature as the climate warms The clouds become higher as the surface warms, but do so in such a way as to remain at approximately the same temperature If high cloud emission temperature stays constant (or warms less than the surface), then this would lead to a positive cloud feedback, assuming no change in cloud fraction.

19. Predicting level of abundant high cloudiness from clear-sky balance Mark Zelinka’s Work Input to Fu-Liou code: tropical-mean profiles of temperature and humidity averaged over decades  calculate net (LW+SW) radiative cooling profiles Assume that this radiative cooling is balanced by diabatic subsidence  take vertical derivative to get clear-sky UT convergence  assume from mass continuity that this is balanced by convective detrainment  should see clouds there

20. Dashed: Clouds, Solid: Convergence

21. SRES A2 Ensemble-Mean

22. Radiative cooling Static stability (T/θ)dθ/dp 2000-2010 2070-2080 2090-2100 2000-2010 2070-2080 2090-2100 Diabatic convergence Diabatic ω 2000-2010 2070-2080 2090-2100 2000-2010 2070-2080 2090-2100

23. SRES A2 Ensemble-Mean CTT warms ~1 K Upper Troposphere warms ~6 K Sfc Warms ~3 K

24. Attempting to Quantify Contribution of FAT to Longwave Cloud Feedback First calculate ΔLWCF, then use radiative kernel technique to estimate LW Cloud Feedback Very difficult because cloud properties are not saved and so cannot calculate radiative effect of clouds

25. Compare ΔLWCF for ‘FAT’ and ‘FAP’ FAT ΔLWCFtropics = Δfhi(OLRclr– OLRhicld) – fhiΔOLRhicld – floΔOLRlocld + fΔOLRclr FAP ΔLWCFtropics = Δfhi(OLRclr–OLRhicld) – fhiΔOLRhicld– floΔOLRlocld + fΔOLRclrassuming that OLRhi = σCTT4 in which the CTT increases as much as the temperature at a fixed pressure level (the initial cloud-weighted pressure) Finally, apply the cloud mask as explained in Soden et al. 2008 to convert ΔLWCF to LW cloud feedback

26. ENSEMBLE MEAN LW CLOUD FEEDBACK Actual Actual FAP FAT FAP minus Actual FAT minus Actual

28. Conclusion. • Radiative Convective Equilibrium, constrained by Clausius Clapeyron and basic radiation physics, seems to be a strong constraint on the depth of the convective layer in the Tropics. • One result of this is that the detrainment layer in the Tropics tends to have a nearly fixed temperature as the climate changes, or a nearly fixed anvil cloud temperature. • Another result of this is that climate models tend to give a relatively strong positive cloud longwave feedback. • Also, the Hadley Cell will deepen in pressure thickness with global warming.

30. High Cloud-weighted P Red: 1:1 line, with nonzero y-intercept Each x is a decadal mean UT Convergence-weighted P

31. High Cloud-weighted T Red: 1:1 line, with nonzero y-intercept Each x is a decadal mean UT Convergence-weighted T

32. Radiative cooling Static stability (T/θ)dθ/dp 2000-2010 2070-2080 2090-2100 2000-2010 2070-2080 2090-2100 Diabatic convergence Diabatic ω 2000-2010 2070-2080 2090-2100 2000-2010 2070-2080 2090-2100

33. Attempting to Quantify Contribution of FAT to Longwave Cloud Feedback First calculate ΔLWCF, then use radiative kernel technique to estimate LW Cloud Feedback Very difficult because cloud properties are not saved and so cannot calculate radiative effect of clouds

34. Decomposing the change in LWCF for cloud fraction (f) and cloud properties If OLR = f OLRcld + (1-f)OLRclr thenLWCF = OLRclr – OLR = f (OLRclr – OLRcld) ΔLWCF = Δf (OLRclr–OLRcld) + f ΔOLRclr– fΔOLRcld

35. Actual HAD CM3 ΔLWCF Sum Δf(OLRclr – OLRcld) – fΔOLRcld fΔOLRclr Sum minus actual

36. Decomposing the change in LWCF LWCF = OLRclr – OLR = f(OLRclr – OLRcld) ΔLWCF = Δf(OLRclr–OLRcld) + fΔOLRclr– fΔOLRcld This term dominates, but not because of warming or cooling high clouds, but apparently because of different abundances of high vs. low clouds (see next slide)

37. HAD CM3 HAD CM3 ΔLWCF<<0 due to ΔOLRcld>>0 ΔLWCF>>0 due to ΔOLRcld<<0 Dashed: 2000-2010 Solid: 2090-2100

38. Another ΔLWCF decomposition Let’s assume we can break OLRcld and f into contributions from high and low clouds. We do this separation only in the Tropics Rather than trying to pretend like we know the effective high and low cloud fractions, lets assume that the high cloud-weighted temperature is a reasonable estimate of the high cloud emission temperature and that the low cloud emission is the same as clear-sky emission. Then we can determine what fhi and flo must be such that fOLRcld = fhiOLRhicld + floOLRlocld

39. [1] LWCF = OLRclr - OLR= f(OLRclr – OLRcld) [2] ΔLWCF = Δf(OLRclr – OLRcld) + fΔOLRclr – fΔOLRcld If we assume that f and OLRcld can be broken into a component from high and from low clouds: [3] fOLRcld = fhiOLRhicld + floOLRlocld, where flo is the fraction of area covered by low clouds that are not covered by high clouds Using a cloud-weighted temperature for clouds that are between the freezing level and the tropopause as CTT, we write [4] OLRhicld = σCTT4 Using f = fhi + flo, we can solve [3] for fhi: [5]where OLRcld is given by [1], OLRhicld is given by [4], and we assume OLRlocld = OLRclr [6] ΔLWCF = Δfhi(OLRclr– OLRhicld) – fhiΔOLRhicld – floΔOLRlocld + f ΔOLRclr

40. So the formulas are…. ΔLWCFtropics = Δfhi(OLRclr– OLRhicld) – fhiΔOLRhicld – floΔOLRlocld + f ΔOLRclr ΔLWCFextra-tropics = Δf(OLRclr–OLRcld) – fΔOLRcld + fΔOLRclr

41. In the Tropics we should see two or three levels of cloud. Boundary layer cloud - from strong radiative cooling of moist, warm low level air - H2O continuum High cloud from strong cooling under tropopause by rotation bands of H2O Middle cloud from 6.7 micron V/R band Predictions from Clear-SkyRadiative Cooling

42. MODIS Temperature-Optical Depth Histogram Eastern Equatorial Pacific Ocean Three Levels of Cloud Tropopause High High-Anvil and Cirrus Clouds Middle Middle- Congestus Low Low - Cumulus+ Stratocumulus Optical Depth Kubar et al. 2007

43. Fundamental energy balance in atmosphere is: Convective heating = Radiative Cooling Question is, Which places a more fundamental Constraint on the climate system in the tropics? Answer: In the deep tropics radiative cooling, particularly in clear skies, may provide a more fundamental prediction of the depth of the convective layer.

44. First Law of Thermodynamics In Tropics ~ Using continuity in pressure coordinates

45. Fact: The radiatively-driven divergence in the clear regions is related to the decrease of water vapor with temperature following the Clausius-Clapeyron relation and the consequent low emissivity of water vapor at those low temperatures. Fact: 200 hPa Convective outflow and associated large-scale divergence near 200 hPa are both associated with radiatively-driven divergence in clear skies. Hypothesis: The temperature at which the radiatively-driven divergence occurs will always remain the same, and so will the temperature of the cloud anvil tops.

46. t > 1 9% 22% 10%

47. Use Cloudsat to detect cloud tops and AMSR to estimate precipitation rate Heavy rain is 90th Percentile, 10% of frequency, but ~50% of total rainfall. West Pacific East Pacific Kubar & Hartmann 2008

48. Use Cloudsat to detect cloud tops and AMSR to estimate precipitation rate Heavy rain is 90th Percentile, 10% of frequency, but ~50% of total rainfall. Kubar & Hartmann 2008

49. Why should convection stop/detrain at a fixed temperature? Vapor pressure depends only on temperature, and decreases exponentially as T decreases with altitude. Emissivity (radiative relaxation time) depends most importantly on vapor pressure. Temperature where water vapor emissivity becomes small is only weakly dependent on relative humidity and pressure. Heating of air by condensation also becomes small at this temperature

50. Testing the FAT Hypothesis in a model. Larson and Hartmann (2002a,b) Model Study: MM5 in doubly periodic domain a) 16x16 box with uniform SST (297, 299, 301, 303K) b) 16x160 box with sinusoidal SST c) 16x16 box with uniform SST and rotation. Clouds and circulation are predicted Clouds interact with radiation Basically a radiative-convective model with parameterized convection, in which the large-scale circulation is allowed to play a role by dividing the domain into cloudy (rising) and clear (sinking) regions.