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

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)

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

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

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

Vapor Gotículas

Precipitação

Neve

Transition is delayed in higher resolution

Length Scale growth

ShallowConvectio

n

DeepConvection

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

x (km)

y (

km)

θv perturbation (K)

x (km)

θv perturbation (K)

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

Thank you for your atention!

Photo: João Martins