1. © Crown copyright Met Office Cloud parametrization: current issues relating to microphysics Adrian Lock

2. © Crown copyright Met Office Outline Summary of cloud parametrization in GCMs (General Circulation Models) Use of observations and LES (Large-Eddy Simulation) Current issues (related to microphysics) Impact of microphysics on boundary layer turbulence and entrainment Organisation on the mesoscale (10~100km) Fog and radiative transfer

3. © Crown copyright Met Office Clouds in GCMs Extremely important due to their role in the radiation budget and hydrological cycle Two aspects to accurate cloud modelling in GCMs: Accurate transports of heat and moisture (to give accurate grid-box mean quantities + potentially eg. variances) Accurate determination of cloud amount (cover and water content)

4. © Crown copyright Met Office Clouds in GCMs A Very basic overview Need to know how much cloud in a grid box ie. what fraction is cloudy and how much condensed water is there? given grid-box mean quantities (eg. temperature and humidity) and perhaps tendencies ?

5. © Crown copyright Met Office Cloud parametrization Calculate (or assume) some distribution of humidity about the grid-box mean ( ) Eg. Smith scheme assumes triangular distribution of specified width (=1-RH c ) Need to know width (and shape) of RH distribution (role of cloud scheme), as well as grid-box means (role of transport parametrizations) RH=(q T – q sat )/q sat 100% PDF Cloudy Clear 1-RH c

6. © Crown copyright Met Office Clouds in the marine sub-tropics Near Hawaii: cumulus Somewhere in between! Something in between! Near California: stratocumulus Mean wind direction Photos courtesy of Mark Webb

7. © Crown copyright Met Office Inversion Cloud base Tropopause Inversion Subsidence F q,T Entrainment Physics of clouds Figure courtesy of Pier Siebesma, KNMI Shallow cu: good progress with parametrization from improving functional dependencies (massflux closure, entrainment rates, etc) but microphysical interactions relatively subtle Stratocumulus: again good progress; interactions with microphysics are potentially important LW

8. © Crown copyright Met Office Parametrization of turbulent mixing in stratocumulus in the Met Office UM Simple 1 st order closure within the mixed layer Specified profiles of turbulent diffusivities, driven from cloud-top or the surface (extension of Holtslag and Boville, 1993) Empirically/physically based, largely on LES Explicit boundary layer top entrainment parametrization Related to cloud-top radiative and evaporative cooling, surface heating, wind shear Additional non-gradient fluxes for surface-driven heat-fluxes and wind stresses Explicit entrainment flux Explicit link to clouds and cloud properties

9. © Crown copyright Met Office Diagnosis of decoupling...or fine tuning the K-profile depths Mixed-layers could stay well-mixed forever without an additional contraint on the K-profiles We limit the buoyancy consumption of TKE: with D~0.1 from LES (Stevens, 2000; Lewellen and Lewellen, 2002) Similar to Turton and Nicholls (1987) Potential for interaction with microphysics: Profile stabilisation by drizzle can promote decoupling

10. © Crown copyright Met Office Use of LES for parametrization development

11. © Crown copyright Met Office Use of LES to improve the representation of BL clouds Cloud evolution strongly dependent on representation of turbulence fluxes Use LES to validate large-scale model (proxy for observations) understand key processes develop and quantify empirical relationships Important to know how far to trust the LES Use model intercomparisons and comparison with observations…

12. © Crown copyright Met Office GCSS Working Group on boundary layer clouds International group performing intercomparisons of LES, with observations (and single-column models) Aim to improve parametrizations of clouds and boundary layer turbulence used in GCMs Quantified LES sensitivities across a wide range of cloud regimes Inspired many productive lines of research

13. © Crown copyright Met Office Non-precipitating Stratocumulus Stevens et al (2005) Stratocu hard to simulate accurately due to sharp “capping inversion” Even using high resolution (eg. 30m x 30m x 5m) Intercomparison case based on DYCOMS II observations, aiming to constrain LES: Nocturnal, non-precipitating (so simple) Observations roughly steady for 6 hours 5 measures of the entrainment rate

14. © Crown copyright Met Office RF01 location and flight track Stevens et al 2003

15. © Crown copyright Met Office LES Ensemble mean Central half Full range LES intercomparison results Stevens et al (2005) LES entrainment rate varies between models (4-6 mm/s) and typically seems high compared to observations (4 +/- 1 mm/s) As a result of excessive entrainment: Cloud top rises slightly compared to that observed Cloud base rises significantly (entrained air is very dry and causes evaporation) Cloud

16. © Crown copyright Met Office DYCOMS II Non-precipitating nocturnal stratocumulus Inversions capping typical stratocumulus clouds are very strong and sharp, making accurate modelling by LES very hard Bretherton et al (1999) estimate of required resolution ~ 3m for RF01 So, LES-derived entrainment rates in stratocumulus should still be treated with caution

17. © Crown copyright Met Office Microphysics-turbulence interactions

18. © Crown copyright Met Office Potential microphysics-turbulence interactions in stratocumulus Suppression of turbulence by drizzle Suppression of cloud–top mixing (entrainment) by droplet sedimentation Why interested? Implications for cloud evolution Crucial for “2 nd indirect aerosol effect”: what impacts do aerosol changes have on cloudiness?

19. © Crown copyright Met Office Potential aerosol feedbacks Increased aerosol Smaller cloud drops Less rainMore cloud More vigorous turbulence More water near inversion More cloud-top entrainment Less cloud Stevens et al (1998) Ackerman et al (2009) Bretherton et al (2007) “2 nd indirect aerosol effect”

20. © Crown copyright Met Office Precipitating nocturnal stratocumulus intercomparison: DYCOMS II, RF02 Ackerman et al (2009) Drizzling nocturnal stratocumulus case, flight RF02 Thicker cloud layer, moister free atmosphere cf RF01 Particular emphasis on role of cloud droplet sedimentation Previously ignored in most LES studies Most LES do quite well at maintaining the cloud layer Some LES can get realistically high drizzle rates LES Ensemble mean Central half Full range No cloud sediment n Observation range

21. © Crown copyright Met Office Large difference in drizzle rates between different LES Apparently independent of microphysics parametrization – dominated by differences in the bulk turbulence dynamics (eg. resolved numerics, subgrid) giving differences in cloud water content Motivated a more idealised intercomparison (1D) of microphysics schemes (see Ben Shipway!) Precipitating nocturnal stratocumulus intercomparison: DYCOMS II, RF02 Ackerman et al (2009)

22. © Crown copyright Met Office What goes wrong with LES of RF02? LES can match observed LWP and precipitation But only through spurious decoupling of the cloud layer + weak BL turbulence Observations

23. © Crown copyright Met Office What goes wrong with LES of RF02? Why can’t we get correct cloud, rain and turbulence? Is it a model deficiency (numerical, subgrid, microphysical)? Is it the heterogeneity encountered in RF02? VOCALS campaign may shed some light here Van Zanten and Stevens (2005)

24. © Crown copyright Met Office DYCOMS II, RF02 Excessive LES sensitivity to drizzle? LES turbulence sensitive to presence of drizzle Reality apparently less sensitive (POC vs non-POC) Non POC POC

25. © Crown copyright Met Office DYCOMS II, RF02 LES sensitivity to droplet settling Gravitational settling of cloud droplets reduces entrainment for all LES (by reducing evaporative cooling of entrained air near cloud top) giving increased LWP and thence increased drizzle With drizzle only With drizzle and cloud sedimentation

26. © Crown copyright Met Office DYCOMS II, RF02 LES drizzle evaporation Tendency for LES to evaporate too much drizzle below cloud base Observational range

27. © Crown copyright Met Office DYCOMS II intercomparisons LES summary We know from RF01 intercomparison that LES can entrain too fast across strong inversions Including droplet settling makes LES perform better Just because it reduces entrainment so conveniently hiding an error? Correctly including a missing process Why do LES of precipitating stratocumulus underestimate turbulence? Too much evaporation perhaps, leading to excessive stabilisation of mean profiles? Too small domain to represent heterogeneity?

28. © Crown copyright Met Office DYCOMS II, RF02 SCM results example All participants capable of maintaining a well-mixed boundary layer capped by stratocumulus cloud Significant variation in “details” (eg. LWP, precip) Obs LES

29. © Crown copyright Met Office DYCOMS II: RF02 SCM results Variation much reduced at consistent, high vertical resolution

30. © Crown copyright Met Office Missing links in GCMs between microphysics and turbulence If we trust the LES result that droplet sedimentation reduces entrainment, how should we represent this process in a GCM on a 200m vertical grid? Based on LES, Bretherton et al (2007) suggest a modification to Nicholls and Turton (1986)’s closure where J is a measure of evaporative enhancement NT found a 2 =60 from aircraft obs, Bretherton et al use a 2 =15 based on more recent observations The sedimentation dependent factor, a sed ~0.7 for typical marine Sc Parametrization of entrainment, down to the level of what it should depend on, remains uncertain

31. © Crown copyright Met Office Missing links in GCMs between microphysics and turbulence (continued) Does drizzle have a preference to fall in updraughts? Could then damp mixed layer turbulence directly, rather than through stabilisation of the mean profile? How moist is the air drizzle falls in? implications for evaporation rates and hence BL stabilisation Are we getting these right in LES?

32. © Crown copyright Met Office Potential aerosol feedbacks Increased aerosol Smaller cloud drops Less rainMore cloud More vigorous turbulence More water near inversion More entrainment Less cloud Stevens et al (1998) Ackerman et al (2009) Bretherton et al (2007) Important implications…

33. © Crown copyright Met Office Mesoscale variability of clouds

34. © Crown copyright Met Office VOCALS VAMOS Ocean-Cloud-Atmosphere-Land Study International campaign to study stratocumulus in the SE Pacific; organisation of drizzle, aerosol interactions, etc Met Office provided forecasts from the global UM (~40km) and a VOCALS regional UM at 17km UM found to give good general guidance of main low cloud areas Impressive, given there is no cloud assimilation Little skill at forecasting “detail”, for example POCs…

35. © Crown copyright Met Office VOCALS forecasts A “POC”

36. © Crown copyright Met Office Validation against VOCALS profiles Comparison of operational forecast model profiles with aircraft profiles off the coast of South America, October 2008 XXXX

37. © Crown copyright Met Office Validation against VOCALS profiles Model captures inversion height well in both air masses Largely dictated by balance between entrainment and subsidence, suggesting the former is reasonably well parametrizated

38. © Crown copyright Met Office How can we represent subgrid POCs in a GCM? Ie. how to generate an intermediate cloud fraction in an otherwise homogeneous cloud sheet? This won’t happen “naturally” (without high resolution) Typically (in cloud schemes) drizzle is expected to reduce moisture variance (rains more where wetter) so cloud fraction tends to 0 or 1 Organisation by drizzle, as hypotheised for POCs, means precip needs to increase/maintain variance Van Zanten and Stevens (2005)

39. © Crown copyright Met Office Microphysics and fog

40. © Crown copyright Met Office Droplet settling and fog Winter 07-08: systematic problems with fog being excessively thick and persistent Including cloud droplet settling reduces fogginess in test cases

41. © Crown copyright Met Office Impact of cloud droplet settling on fog Despite slow fall speeds (typically ~0.01m/s) droplet settling delays and reduces optical thickening of fog

42. © Crown copyright Met Office Microphysics of fog Observations from MRU Cardington (Jeremy Price) show very low droplet numbers (low aerosol activation rate) LES show strong sensitivity to imposed drop number (via impact on optical thickness and thence radiative cooling profile)

43. © Crown copyright Met Office Summary Cloud are important to get right for NWP LES is a key tool in developing subgrid models for NWP and climate models LES of stratocumulus must be treated with caution (strong gradients at inversion make it sensitive to resolution, resolved numerics, subgrid model, …) Drizzle can act to stabilise the BL and reduce turbulent mixing LES may over do this effect Microphysical effects may impact cloud near inversion which may affect entrainment, which affects turbulence, etc Complex interactions between processes Turbulent mixing in fog is particularly sensitive to stability and thence microphysical properties of the cloud

44. © Crown copyright Met Office Questions?

45. © Crown copyright Met Office Parametrization development strategy After Randall et al (2003) Improvements LES

46. © Crown copyright Met Office ‘BOMEX’ intercomparison Siebesma et al (2003) Non-precipitating shallow cumulus case Very good agreement between LES LES stay close to initial, observed profiles Numerically easy? Weak gradients Well-resolved (100m x 100m x 40m)