1 00/XXXX © Crown copyright Carol Roadnight, Peter Clark Met Office, JCMM Halliwell Representing convection in convective scale NWP models : An idealised study

2 00/XXXX © Crown copyright Introduction Severe weather conditions occur on small spatial scales. It would be beneficial to be able to represent these phenomena in NWP models. Nature, location and organisation of convection not predicted very well by the current mesoscale model –the convection scheme only represents average effects of an ensemble of clouds. Difficulty in representing convection in models that aim to resolve scales between 1km and 10km is well known.

3 00/XXXX © Crown copyright Introduction cont... In the absence of an appropriate scheme decrease the grid length so that convection is resolved? Introduction of non-hydrostatic dynamics into the Met Office UM possibility of using high resolution grids. Current computer power restricts us to grid lengths of 1km - 10km subgrid representation of physical processes still required. JCMM: research into problems associated with modelling and Data Assimilation with partially resolved convection.

4 00/XXXX © Crown copyright Do we need a convection scheme? Target grid length 4km - 1km –convection partially resolved. Current convection scheme (Gregory Rowntree mass flux scheme) not designed for these resolutions. We have option of using –vertical mixing: convection scheme –horizontal mixing: diffusion scheme. –other schemes.

5 00/XXXX © Crown copyright Does the addition of the convection scheme and diffusion have an effect on the scales in the system? Do the choices of diffusion coefficient and CAPE closure timescale determine these scales?

6 00/XXXX © Crown copyright Idealised UM Uses same dynamics as full UM but allows physics schemes to be switched off Allows us to study processes of interest in controlled conditions. Radiative-convective equilibrium set up. –Bi-periodic domain: 4km grid length, 64x64 points in horizontal. 50 vertical levels. –Constant cooling profile –Run until equilibrium –Compare with LEM –Higher resolution reference run

7 00/XXXX © Crown copyright Does the scheme behave in a physically sensible way? Grid length CCT

8 00/XXXX © Crown copyright With and without convection scheme

9 00/XXXX © Crown copyright With and without convection scheme

10 00/XXXX © Crown copyright With and without convection scheme

11 00/XXXX © Crown copyright With and without convection scheme

12 00/XXXX © Crown copyright With and without convection scheme

13 00/XXXX © Crown copyright With and without convection scheme

14 00/XXXX © Crown copyright With and without convection scheme

15 00/XXXX © Crown copyright 4km, no diffusion

16 00/XXXX © Crown copyright 4km, no diffusion

17 00/XXXX © Crown copyright 4km, no diffusion

18 00/XXXX © Crown copyright 4km, no diffusion

19 00/XXXX © Crown copyright 4km, no diffusion

20 00/XXXX © Crown copyright 4km, no diffusion

21 00/XXXX © Crown copyright 4km, no diffusion

22 00/XXXX © Crown copyright 2km, no scheme

23 00/XXXX © Crown copyright 2km, no scheme

24 00/XXXX © Crown copyright 2km, no scheme

25 00/XXXX © Crown copyright 2km, no scheme

26 00/XXXX © Crown copyright Scheme and diffusion

27 00/XXXX © Crown copyright Scheme and diffusion

28 00/XXXX © Crown copyright Scheme and diffusion

29 00/XXXX © Crown copyright Scheme and diffusion

30 00/XXXX © Crown copyright Scheme and diffusion

31 00/XXXX © Crown copyright Scheme and diffusion

32 00/XXXX © Crown copyright Summary of results

33 00/XXXX © Crown copyright Summary of results

34 00/XXXX © Crown copyright Summary of results

35 00/XXXX © Crown copyright Summary of results

36 00/XXXX © Crown copyright Summary of results

37 00/XXXX © Crown copyright Summary of results

38 00/XXXX © Crown copyright Summary of results

39 00/XXXX © Crown copyright Summary of results

40 00/XXXX © Crown copyright Can the scheme be modified to be more scale selective? Scales observed in system controlled by CCT? Modify convection scheme –If size of cells related to CAPE limit intensity of the scheme when CAPE is large enough to suggest cells are resolved –Limit mass flux

41 00/XXXX © Crown copyright Summary of results

42 00/XXXX © Crown copyright Summary of results

43 00/XXXX © Crown copyright Summary of results

44 00/XXXX © Crown copyright Summary of results

45 00/XXXX © Crown copyright Summary and conclusions Does the addition of the convection scheme and diffusion have an effect on the scales in the system? –Convection scheme: No significant changes with grid lengths 2km, 4km unless CCT was reduced, in which case larger scales more dominant. –Diffusion: had more impact, damping small scales. –Both schemes: More significant impact.

46 00/XXXX © Crown copyright Do the choices of diffusion coefficient and CAPE closure timescale determine these scales? –CCT? Larger scales observed as CCT decreases (monotonic). –Diffusion coefficient? Smaller scales damped as diffusion increases (monotonic). –Both schemes together: more complex. –More quantitative analysis needed.

47 00/XXXX © Crown copyright Future.. Some type of scheme required. –Modified version of current scheme –Shallow convection scheme –other convection schemes –turbulence scheme Targetted diffusion

48 00/XXXX © Crown copyright

49 00/XXXX © Crown copyright Current UM convection scheme Gregory-Rowntree mass flux scheme aims to represent all types of moist and dry convection. Effects of ensemble of convective plumes characterised by –total mass flux associated with updraughts –average thermodynamic properties Characteristics obtained using cloud model based around parcel theory and modified by entrainment/detrainment. CAPE closure scheme determines amount of convection –mass flux calculated based on removal of CAPE over –mass flux CAPE/ – 30 minutes, 60 minutes

50 00/XXXX © Crown copyright Power spectra of vertical velocity min min

51 00/XXXX © Crown copyright Effects of diffusion

52 00/XXXX © Crown copyright CAPE dependent CCT Function 1 Function 2

53 00/XXXX © Crown copyright Summary of results q cl cmf

54 00/XXXX © Crown copyright CCT=60min and diffusion

55 00/XXXX © Crown copyright CCT=60min and diffusion

56 00/XXXX © Crown copyright CCT=60min and diffusion

57 00/XXXX © Crown copyright CCT=60min and diffusion

58 00/XXXX © Crown copyright CCT=60min and diffusion

59 00/XXXX © Crown copyright half diffusion and scheme

60 00/XXXX © Crown copyright half diffusion and scheme

61 00/XXXX © Crown copyright half diffusion and scheme

62 00/XXXX © Crown copyright half diffusion and scheme

63 00/XXXX © Crown copyright half diffusion and scheme

64 00/XXXX © Crown copyright half diffusion and scheme

65 00/XXXX © Crown copyright half diffusion and scheme

66 00/XXXX © Crown copyright half diffusion and scheme