Stable boundary layer regimes at Dome C, Antarctica E t i e n n - - PowerPoint PPT Presentation

Stable boundary layer regimes at Dome C, Antarctica

E t i e n n e V i g n

• n

, B a s J . H . v a n d e Wi e l , I v

• G

. S . v a n H

• i

j d

• n

k , C h r i s t

• p

h e G e n t h

• n

, S t e v e n J . A . v a n d e r L i n d e n , J . A n t

• n

v a n H

• f

t , P e t e r B a a s , Wi l l i a m Ma u r e l , O l i v i e r T r a u l l é a n d G i a m p i e t r

• C

a s a s a n t a

I n t r

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u c t i

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d a t a C

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c l u s i

• n

s V e r t i c a l p r

• fj

l e s D i s c u s s i

• n

S B L r e g i m e s

Introduction

Stable boundary layers (SBL) in different places of the world have been shown to present different dynamical regimes : Weakly SBL Very SBL

• Windy/cloudy conditions
• Weak stratification
• Fully developed turbulence

that decreases with height

• Weak winds/clear­sky conditions
• Strong stratification, mechanical

decoupling may occur

• Weak/intermittent turbulence

which may increase with height Poorly understood

I n t r

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d a t a C

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c l u s i

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s V e r t i c a l p r

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l e s D i s c u s s i

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S B L r e g i m e s

Aim :

➔ To identify and characterize regimes at a place where the SBL

experiences a large range of static stability : Dome C, east Antarctic Plateau

I n t r

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d a t a C

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c l u s i

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s V e r t i c a l p r

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l e s D i s c u s s i

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S B L r e g i m e s

Data: in-situ measurements at Dome C

D

• m

e C : ( 1 2 3 . 3 ° ,

• 7

5 . 6 ° ) , 3 2 3 3 m a . s . l . , fm a t ( s l

• p

e < 1 ‰) 2 m T e m p e r a t u r e

G e n t h

• n

e t a l 2 1 3

1 m Wi n d

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c l u s i

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s V e r t i c a l p r

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l e s D i s c u s s i

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S B L r e g i m e s

4 5 m 2 m Wind and temperature measurements at :

41 m 32 m 25 m 17 m 10 m 3 m 2 m 1 m

Data: in-situ measurements at Dome C

Only in summer BSRN

+

I n t r

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u c t i

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d a t a C

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c l u s i

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s V e r t i c a l p r

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l e s D i s c u s s i

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S B L r e g i m e s

How to distinguish SBL regimes ?

• SBL regimes can not be distinguished using a local stability parameter (z/L, Ri)

(Van Hooijdonk et al 2015, Monahan et al 2015, our study)

• External forcing (e.g geostrophic wind) not measurable

→ Wind speed at 'crossing point' (Van de Wiel et al 2012) In summer = ? 'diurnal' cycle

Surface radiative cooling starts

In winter, no diurnal cycle (polar night), SBL more stationary → U10m as the control parameter

I n t r

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d a t a C

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c l u s i

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s V e r t i c a l p r

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l e s D i s c u s s i

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S B L r e g i m e s

SBL regimes identification

I n t r

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u c t i

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d a t a C

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c l u s i

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s V e r t i c a l p r

• fj

l e s D i s c u s s i

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S B L r e g i m e s

SBL regimes : wind and temperature

Wind speed ratio with the 2m wind Wind direction difference with the 2m wind Temperature difference with the surface All 2014-2015 data in stable conditions

I n t r

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d a t a C

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c l u s i

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s V e r t i c a l p r

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l e s D i s c u s s i

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S B L r e g i m e s

SBL regimes : wind and temperature

Wind speed ratio with the 2m wind Wind direction difference with the 2m wind Temperature difference with the surface All 2014-2015 data in stable conditions

The temperature inversion does not depend to the wind speed Large scatter due to

• Amount of

incoming radiations at the surface

• Non­stationnary situations

I n t r

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u c t i

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d a t a C

• n

c l u s i

• n

s V e r t i c a l p r

• fj

l e s D i s c u s s i

• n

S B L r e g i m e s

SBL regimes : wind and temperature

Wind speed ratio with the 2m wind Wind direction difference with the 2m wind Temperature difference with the surface All 2014-2015 data in stable conditions

The temperature inversion does not depend to the wind speed Large scatter due to

• Amount of

incoming radiations at the surface

• Non­stationnary situations
• Because dots are organized

along a 'S' curve with 'backfolding'

I n t r

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u c t i

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d a t a C

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c l u s i

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s V e r t i c a l p r

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l e s D i s c u s s i

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S B L r e g i m e s

SBL regimes : wind and temperature

All 2014-2015 data in stable conditions

I n t r

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s V e r t i c a l p r

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l e s D i s c u s s i

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S B L r e g i m e s

SBL regimes : wind and temperature

3 m 17 m 32 m One wind speed threshold for the whole tower (the whole SBL?)

I n t r

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s V e r t i c a l p r

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l e s D i s c u s s i

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S B L r e g i m e s

SBL regimes : turbulence

December 2009­ January 2010 data

Variance of the vertical velocity

One wind speed threshold for the whole tower (the whole SBL?)

I n t r

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u c t i

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d a t a C

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s V e r t i c a l p r

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l e s D i s c u s s i

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S B L r e g i m e s

Vertical profiles of temperature

I n t r

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d a t a C

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s V e r t i c a l p r

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l e s D i s c u s s i

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Vertical profiles of temperature in each regime

U10m > U threshold U10m < U threshold

I n t r

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s V e r t i c a l p r

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Vertical profiles of temperature in each regime

Radiative dominated SBL 'Exponential' shape Radiative SBL

I n t r

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l e s D i s c u s s i

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Vertical profiles of temperature in each regime

Radiative dominated SBL 'S' shape 'Exponential' shape Radiative SBL

I n t r

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l e s D i s c u s s i

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Vertical profiles of temperature in each regime

Radiative dominated SBL Radiative SBL 'S' shape 'Exponential' shape Increase of gradient = non linear process → Turbulent SBL

I n t r

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s V e r t i c a l p r

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l e s D i s c u s s i

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Discussion Existence of 2 SBL regimes ? Physical meaning of the wind speed threshold ?

I n t r

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l e s D i s c u s s i

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Discussion : Surface energy budget and SBL regimes

Net radiative cooling / surface sensible heat flux

I n t r

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Discussion : Surface energy budget and SBL regimes

=

Regime 1= Mechanical decoupling Regime 2= compensation

Radiative heat loss Radiative heat loss Sensible heat flux

Net radiative cooling / surface sensible heat flux

I n t r

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s V e r t i c a l p r

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l e s D i s c u s s i

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Discussion : Surface energy budget and SBL regimes

In agreement with theory of the minimum wind speed for sustainable turbulence (MWST) (Van de Wiel et al 2012, Van Hooijdonk et al 2015) Є [5 7] m s­1

Net radiative cooling / surface sensible heat flux

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Discussion : Is the mechanically decoupled regime dynamically stable ? (Does the surface temperature decrease to 0 K ?)

Lumped parameter theory, Van de Wiel et al, to be submitted

I n t r

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u c t i

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d a t a S B L r e g i m e s V e r t i c a l p r

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l e s D i s c u s s i

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C

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c l u s i

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s during clear­sky nights

Lumped parameter theory, Van de Wiel et al, to be submitted

I n t r

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d a t a S B L r e g i m e s V e r t i c a l p r

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l e s D i s c u s s i

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C

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c l u s i

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s Turbulent heat flux Isothermal radiative cooling

Conceptual modelling

Most adapted for Dome C conditions

Snow heat diffusion + radiative transfert in the atmosphere Equilibrium lines

Lumped parameter theory, Van de Wiel et al, to be submitted

I n t r

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u c t i

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d a t a S B L r e g i m e s V e r t i c a l p r

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l e s D i s c u s s i

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C

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c l u s i

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s Turbulent heat flux Isothermal radiative cooling

Conceptual modelling

Lumped parameter theory, Van de Wiel et al, to be submitted

I n t r

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d a t a S B L r e g i m e s V e r t i c a l p r

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l e s D i s c u s s i

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C

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c l u s i

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s Snow heat diffusion + radiative transfert in the atmosphere Isothermal radiative cooling

Conceptual modelling

I n t r

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s V e r t i c a l p r

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l e s D i s c u s s i

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Conclusions

• 2 well distinct regimes, U10m threshold for the whole tower
• Abrupt transition
• 2 types of vertical temperature profiles ('exp' and 'S' shape) associated to the

2 regimes

• U10m threshold

→ MWST theory

• ΔT10m vs U10m : 'S' shape with two 'stable' branches

→ 2 regimes are dynamically stable

• Explanation : 'lumped parameter' theory :
• ne turbulent regime, one diffusive regime

I n t r

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u c t i

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d a t a C

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c l u s i

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s V e r t i c a l p r

• fj

l e s D i s c u s s i

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Conclusions

• 2 well distinct regimes, U10m threshold for the whole tower
• Abrupt transition
• 2 types of vertical temperature profiles ('exp' and 'S' shape) associated to the

2 regimes

• U10m threshold

→ MWST theory

• ΔT10m vs U10m : 'S' shape with two 'stable' branches

→ 2 regimes are dynamically stable

• Explanation : 'lumped parameter' theory :
• ne turbulent regime, one diffusive regime
• Challenge models to represent the 2 regimes
• Vignon et al 2016, submitted to Q.J.R.M.S, Van de Wiel et al, to be submitted

C

• l

u m b u s 1 5 3 k m

T h a n k y

• u

f

• r

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• u

r a t t e n t i

• n

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