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Length Scale analysis of the transition from shallow to deep convection
João Paulo A. Martins(1)
Pedro M. A. Miranda(1)
Pedro M. M. Soares(1)
João Teixeira (2)
(1)University of Lisbon - CGUL/IDL(2) Jet Propulsion Laboratory/California Institute of Technology
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Motivation
• Diurnal cycle of tropical convection:
• Most GCMs (or SCMs) fail to get the right timing of precipitation
• New approaches are needed (e.g. Rio et al 2009)
• EDMF Shallow Convection
• Enhanced Emanuel (1991) Deep Convection Scheme (new trigger functions)
• Cold pool parameterization
Bechtold et al. (2004)
OBS
Rio et al. (2009)
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TRMM-LBA Shallow to deep convection case study
• Grabowski et al. (2006) (GCSS-WG4 Case 4)– SCM/CRM intercomparison - observations made during the TRMM-LBA, Rondonia, Brazil, February 23, 1999.
• The case considered is a daytime convective development over land
• 7h30 to 13h30 LT
Rad Cooling Surface Fluxes
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Using MesoNH as an LES
Grid 600×600×128 240×240×128Horizontal Resolution 100 m 250 m
Start Time 0730 LTEnd Time 1330 LT
Vertical GridAbove 1500 m – stretching of 1.2% from 100m at
1500m to 500 m near model topTime-step 2 s
Boundary Conditions Cyclic
Sub-grid processes parameterizations
TURBULENCE = 3D, 1 and ½ closure (Cuxart et al., 2000)
CLOUD = ICE3 (6 classes of hydrometeors) (Pinty and Jabouille, 1998)
Advection SchemeMomentum – Centered in time, 4th order
Thermodynamic and scalars – Piecewise Parabolic Method
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Results in fair agreement with previous studies of the case
Higher horizontal resolution:•BL is higher •Clouds form earlier but reach less higher altitudes•Precipitation rate is lower
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Vapor Gotículas
Precipitação
Neve
Transition is delayed in higher resolution
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Length Scale growth
ShallowConvectio
n
DeepConvection
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Beginning of precipitation
• Turbulent Integral Length Scale, based on spectral analysis (e.g. Pino et. al, 2006)
Evolution of length scales on z/zi=0.5
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x (km)
y (
km)
θv perturbation (K)
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x (km)
θv perturbation (K)
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Summary• The transition from shallow to deep convection was
simulated by an LES model (two different resolutions)
▫ Precipitation onset is delayed and deep convection is weaker with higher resolution (100 m)
• Length scales are analyzed with an integral spectral method
▫ Vertical velocity lengh scale is proportional to PBL height in time and in altitude
▫ Theta length scales increases significantly in PBL from precipitation onset
▫ Results suggest cold pool dynamics strongly influences length scales of thermodynamic variables inside PBL
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Thank you for your atention!
Photo: João Martins
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