Observed Southern Ocean Cloud Properties and Shortwave Reflection

Transcription

1 Observed Southern Ocean Cloud Properties and Shortwave Reflection Daniel T McCoy* 1, Dennis L Hartmann 1, and Daniel P Grosvenor 2 University of Washington 1 University of Leeds 2 *dtmccoy@atmosuwedu

2 Introduction The Southern Ocean ( S 6 S) Dominant cloud cover is low (Haynes et al, 211) Robust strong negative cloud optical depth feedback (Zelinka et al, 213) Feedback is thought to be contributed to by transition from icy to liquid clouds with warming

3 Outline We present observations of cloud properties that influence SW SW consistent with these observations is calculated Sensitivity of SW to seasonal variability in each cloud property is tested A simple calculation of the increased SW due to a ice to liquid transition is performed Sensitivity in this increased SW is found to both the mean-climate r e and the assumed future CCN behavior

4 Obs Cloud Fraction Summer time peak in EIS (Wood and Bretherton, 26) peak in low cloud fraction Enhanced winter synoptic activity upper level cloud peak Data shown is from MISR

5 Obs Effective Radius Near cloud-top effective radius from MODIS Minima in summer EffectiveRadius

6 Obs Effective Radius Effective radius from MODIS filtering described in Grosvenor and Wood (21) (SZA< 65 liquid CF> ) Persistent minimum in summer across retrieval bands+filtering µ m Collection µ m 21 µ m 37 µ m J A S O N D J F M A M J

7 Obs In-Cloud Water Path (P > 6hPa) IWP Low :2C-ICE Lat (a) Low level in cloud IWP 12 J A S O N D J F M A M J g/m 2 LWP Low :ISC Lat (a) Low level in cloud LWP 12 J A S O N D J F M A M J g/m 2 Efficient LW cooling in winter promotes buoyant production of turbulence and (b) Mid level in cloud IWP (b) Mid level in cloud LWP an increase in TWP fall and winter time peaks in liquid and ice (Morrison 5 1 et al, 211; Solomon et al, 211) Lat Lat 5 1

8 SISYPHUS Calculating SW 1732 JOURNAL OF CLIMATE The area-averaged cloud properties (example on the right) and the distribution of clouds in terms of τ and CTH (example on the left) MISR CTH-OD (Marchand et al, 21) Area-averaged data (LWP,IWP,r e) example from: (O Dell et al, 2) FIG 6 Mean values of LWP averaged over the years for (a) January, (b Black pixels denote land, while gray pixels denote missing data, either from the presen of land These are combined to create plane-parallel clouds consistent with observed cloud properties RRTMG is then used to calculate SW It is also instructive to compare our results with those those from ERA- from reanalysis products in order to identify potential NCAR reanalysis is model deficiencies or issues with the derived LWP observations does not contain th In particular, we compare our results with that the LWP produ

9 SISYPHUS Reflectivity (R 65) 55 Seasonal Cycle 27 2 SW (R 95) 2 Seasonal Cycle Reflectivity 5 Upwelling SW (W m 2 ) J A S O N D J F M A M J 2 J A S O N D J F M A M J Comparison to CERES EBAF 26r Dashed lines show the uncertainty due to input uncertainty

10 Methodology 12 1 Contribution to SW by seasonal cycles CF r e Phase(r e =const) Phase(Nd=const) 6 Wm J A S O N D J F M A M J Contributions calculated by setting each cloud property equal to annual mean and recalculating SW Phase contribution assumes TWP=constant and shifts LWP IWP Must also assume microphysics (eg CCN concentration) Strength highly dependent on assumed microphysics

11 Methodology Calculation of SW change due to transition from ice to liquid with warming The seasonal cycle of phase contributes to summertime brightening Let s calculate how much SW would increase from a ice liquid transition in warming climate and compare to the overall cloud optical depth feedback We assume: TWP is constant The seasonal cycle of LWP (T ) is analogous to the ice liquid transition IWP in a warmed climate Microphysics in a future climate (eg the future CCN concentrations in the Southern Ocean) We compare this to the optical depth feedback that it contributes to

12 Methodology Changes in thermodynamic phase (1K warming) Change in SW due to altering LWP/IWP of similar magnitude to τ feedback Increased reflectivity is shown as negative Change in SW sensitive to microphysics Increasingreflectivity ConstantNd Constan tr e OpticalDepthFeedback

13 W m 2 K Observations Methodology 6 Changes in thermodynamic phase (1K warming) 1 Constant r e Constant N d CFMIP1 CFMIP2 16 µ m 21 µ m 37 µ m r e at cloud top adiabatic profile of r e Change in SW due to altered LWP/IWP is sensitive to microphysics assumed (eg CCN) and mean state r e r e decreasing OpticalDepthFeedback

14 CF + r e: Seasonal variations in cloud fraction and cloud effective radius both significantly affect reflected solar radiation Ice Liquid (Seasonal): The seasonal variation of the ice to liquid fraction significantly affects reflected solar radiation Very dependent on assumed microphysics Ice Liquid (+1K all year): The observed dependence of the ice to liquid fraction on temperature implies a significant response of reflected shortwave to warming Very dependent on assumed microphysics and mean-climate r e Overall: These results imply that better understanding of cloud microphysical processes is needed to better constrain optical depth feedback

15 Grosvenor, D P and Wood, R (21) The effect of solar zenith angle on modis cloud optical and microphysical retrievals Atmos Chem Phys Discuss, 1: Haynes, J M, Jakob, C, Rossow, W B, Tselioudis, G, and Brown, J (211) Major characteristics of southern ocean cloud regimes and their effects on the energy budget Journal of Climate, 2:561 5 Korhonen, H, Carslaw, K S, Spracklen, D V, Mann, G W, and Woodhouse, M T (2) Influence of oceanic dimethyl sulfide emissions on cloud condensation nuclei concentrations and seasonality over the remote southern hemisphere oceans: A global model study Journal of Geophysical Research-Atmospheres, 113 Marchand, R, Ackerman, T, Smyth, M, and Rossow, W B (21) A review of cloud top height and optical depth histograms from misr, isccp, and modis Journal of Geophysical Research-Atmospheres, 115 Morrison, H, de Boer, G, Feingold, G, Harrington, J, Shupe, M D, and Sulia, K (211) Resilience of persistent arctic mixed-phase clouds Nature Geoscience, 5:11 17 O Dell, C W, Wentz, F J, and Bennartz, R (2) Cloud liquid water path from satellite-based passive microwave observations: A new climatology over the global oceans Journal of Climate, 21: Solomon, A, Shupe, M D, Persson, P O G, and Morrison, H (211) Moisture and dynamical interactions maintaining decoupled arctic mixed-phase stratocumulus in the presence of a humidity inversion Atmospheric Chemistry and Physics, 11: Wood, R and Bretherton, C S (26) On the relationship between stratiform low cloud cover and lower-tropospheric stability Journal of Climate, 19: Zelinka, M D, Klein, S A, Taylor, K E, Andrews, T, Webb, M J, Gregory, J M, and Forster, P M (213) Contributions of different cloud types to feedbacks and rapid adjustments in cmip5* Journal of Climate, 26:57 527

16 Wintertime brightening In-cloud LWP Low W m LWP Low 1 δ=21(3) 5 δ =56(59) J A S O N D J F M A M J % In-cloud IWP Low W m IWP Low δ= 92( 3) δ =2(17) J A S O N D J F M A M J %