Entrainment and Mixing in Cumulus Clouds

Transcription

1 Introduction NCAS, University of Leeds 25 March 2009

2 Outline Introduction Outline Cloud microphysics

3 Cumulus clouds

4 Start Movie

5 Liquid water content Introduction Outline Cloud microphysics From: Warner (1955) Raga et al (1990)

6 Introduction Outline Cloud microphysics From: Blyth and Latham 1990

7 Liquid water content Introduction Outline Cloud microphysics Trade-wind cumulus clouds 2005

8 LWC - COPS Introduction Outline Cloud microphysics COPS cumulus clouds, 15 July Justin Peter

9 Introduction Outline Cloud microphysics Concentration (N) and mean diameter (d) Trade-wind cumulus clouds 2005

10 Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds Two scales: Cloud scale - where does entrainment occur and what happens to entrained air? Small scale at boundaries - what is the mechanism for air exchange across boundaries?

11 Plumes, Thermals and Starting Plumes

12 in Plume Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds Expect entrained air from below

13 Thermals From: Woodward 1959 Expect entrained air from above and below

14 Introduction Thermodynamic arguments Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds Use thermodynamic tracers in cloud - conserved variables Use θ q and Q (Paluch 1979). Invariant in adiabatic altitude changes and mix linearly θ q = T and p d (Rd /C p)/[1+c w /C pd Q] ql exp C pd T / 1 + Cw Q C pd Q = q v + q l

15 From Paluch 1979

16 Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds From: Blyth et al. 1988

17

18 Taylor and Baker Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

19 Taylor and Baker Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds Distribution of points on Paluch diagram does not necessarily mean ascent of adiabatic parcel followed by mixing same distribution if mixing occurs continuously as parcel ascends as long as +vely buoyant. Different from Raymond and Blyth

20 Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds Wang and Geerts: cooling of reverse flow temperature probe

21 Wang and Geerts Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds The EC [evaporative cooling] correction proposed herein should be applied to all those studies. Since most of the measurements in the papers listed above occurred at temperatures above -12C, their analyses are affected and some of their conclusions may be flawed.

22 Downdrafts Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

23 Observations of Thermals Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

24 19 July 1981, CCOPE, Montana:

25 July, 1995, SCMS, Florida

26 Damiani et al

27 Damiani et al

28 Damiani et al Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

29 Damiani et al Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

30 Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds Schematic picture of thermals and entrainment

31 Erosion of the core Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

32 Reduced LWC in middle of updraft

33 Introduction Drop size distribution in the hole Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

34 Introduction Cloud models: Richard Carpenter Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds From Carpenter et al (1990)

35 Introduction Model of Cu cloud showing thermal Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

36 Introduction Model of Cu cloud showing thermal Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

37 Introduction Model of Cu cloud showing thermal Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

38 Introduction Model of Cu cloud showing thermal Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

39 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

40 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

41 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

42 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

43 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

44 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

45 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

46 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

47 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

48 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

49 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

50 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

51 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

52 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

53 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

54 How clouds entrain Courtesy of Dr. Sonia Lasher-Trapp

55 Introduction Heus et al: Modelling results Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds The obtained Paluch diagrams are found to be similar to many results in the literature, but the source of entrained air found by particle tracking deviates from the source inferred from the Paluch analysis. Whereas the classical Paluch analysis seems to provide some evidence for cloud-top mixing, particle tracking shows that virtually all mixing occurs laterally. Particle trajectories averaged over the entire cloud ensemble also clearly indicate the absence of significant cloud-top mixing in shallow cumulus clouds.

56 Heus et al Introduction Plumes and thermals Observations of source of entrained air Thermals Modelling of Clouds

57 Introduction What happens at the boundaries? Cloud Boundaries From Baker et al. (1984)

58 Introduction Cloud Boundaries From Jensen and Baker (1989)

59 Introduction Cloud Boundaries

60 Cloud edges Introduction Cloud Boundaries The sharpness of cloud edges must tell us something about the relative magnitudes of the processes strengthening and weakening the gradients there Gradients are sharpened due to buoyancy and smoothed out due to turbulent eddies

61 Introduction Cloud Boundaries Gradients in LWC and vertical wind; convergence

62 Introduction Cloud Boundaries Does this happen at the edges of Clouds? From Grabowski and Clark (1993)

63 Introduction Cloud Boundaries Heus and Jonkers: cloud-edge downdrafts

64 Introduction Cloud Boundaries Heus and Jonkers, 2003

65 Introduction Cloud Boundaries Schematic of cloud with descending shell Rodts et al., 2003

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68 Introduction Does entrainment occur at ascending cloud top? Not the edges of thermal At rear of thermal Some model results suggest lateral entrainment? Thermals important for circulating the entrained air down the edges and into centre Dilution of thermal about 3-5 mins? How long for molecular mixing vs bulk transport Need improved in-cloud temperature probes to make progress

69 Introduction Acknowledgements Many thanks to: William Cooper, Sonia Lasher-Trapp and Jorgen Jensen,