1 00/xxxx © crown copyright carol roadnight, peter clark met office, jcmm halliwell representing...
TRANSCRIPT
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Carol Roadnight, Peter Clark
Met Office, JCMM
Halliwell
Representing convection in convective scale NWP models : An
idealised study
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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.
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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.
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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.
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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?
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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
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Does the scheme behave in a physically
sensible way? Grid length CCT
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With and without convection scheme
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With and without convection scheme
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With and without convection scheme
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With and without convection scheme
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With and without convection scheme
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With and without convection scheme
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With and without convection scheme
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4km, no diffusion
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4km, no diffusion
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4km, no diffusion
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4km, no diffusion
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4km, no diffusion
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4km, no diffusion
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4km, no diffusion
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2km, no scheme
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2km, no scheme
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2km, no scheme
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2km, no scheme
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Scheme and diffusion
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Scheme and diffusion
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Scheme and diffusion
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Scheme and diffusion
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Scheme and diffusion
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Scheme and diffusion
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Summary of results
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Summary of results
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Summary of results
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Summary of results
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Summary of results
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Summary of results
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Summary of results
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Summary of results
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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
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Summary of results
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Summary of results
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Summary of results
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Summary of results
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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.
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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.
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Future.. Some type of scheme required.
– Modified version of current scheme
– Shallow convection scheme
– other convection schemes
– turbulence scheme
Targetted diffusion
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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
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Power spectra of vertical velocity
60 30
min min
20 10
min min
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Effects of diffusion
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CAPE dependent CCT
LS/totalCCT=60 min 0.32CCT=30 min 0.08CCT=10 min 0.04Function 1 0.92Function 2 0.84
Function 1
Function 2
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Summary of results
qcl cmf
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CCT=60min and diffusion
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CCT=60min and diffusion
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CCT=60min and diffusion
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CCT=60min and diffusion
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CCT=60min and diffusion
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“half” diffusion and scheme
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“half” diffusion and scheme
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“half” diffusion and scheme
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“half” diffusion and scheme
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“half” diffusion and scheme
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“half” diffusion and scheme
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“half” diffusion and scheme
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“half” diffusion and scheme