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 o n , B a s J . H . v a n d e Wi e l , I v o G . S . v a n H o o i j d o n k , C h r i s t o p h e G e n t h o 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 o o n v a n H o o 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 o C a s a s a n t a

Introduction I n t r o d u c t i o n Stable boundary layers (SBL) in different places of the world have been shown to present different dynamical regimes : d a t a Weakly SBL Very SBL S B L r e g i m e s ● Weak winds/clear­sky conditions ● Windy/cloudy conditions ● Strong stratification, mechanical V e r t i c a l ● Weak stratification p r o fj l e s decoupling may occur ● Fully developed turbulence ● Weak/intermittent turbulence that decreases with height which may increase with height D i s c u s s i o n C o n c l u s i o n s Poorly understood

I n t r o d u c t i o n Aim : d a t a S B L r e g i m e s ➔ To identify and characterize regimes at a place where the SBL experiences a large range of static stability : Dome C, east Antarctic Plateau V e r t i c a l p r o fj l e s D i s c u s s i o n C o n c l u s i o n s

Data: in-situ measurements at Dome C I n t r o d u c t i o n d a t a S B L r e g i m e s V e r t i c a l p r o fj l e s D o m e C : ( 1 2 3 . 3 ° , - 7 5 . 0 6 ° ) , 3 2 3 3 m a . s . l . , fm a t ( s l o p e < 1 ‰) 2 m T e m p e r a t u r e 1 0 m Wi n d D i s c u s s i o n C o n c l u s i o n s G e n t h o n e t a l 2 0 1 3

Data: in-situ measurements at Dome C I n t r o d u c t i o n Wind and temperature measurements at : 41 m d a t a 32 m 25 m S B L 4 5 m r e g i m e s 17 m 10 m V e r t i c a l 3 m p r o fj l e s 2 m D i s c u s s i o n 1 m 2 m C o n c l u s i o n s + Only in BSRN summer

How to distinguish SBL regimes ? I n t r o d u c t i o n ● 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 ) d a t a ● External forcing (e.g geostrophic wind) not measurable → Wind speed at 'crossing point' ( Van de Wiel et al 2012 ) S B L r e g i m e s V e r t i c a l In summer p r o fj l e s = ? 'diurnal' cycle D i s c u s s i o n C o n c l u s i o n s Surface radiative cooling starts In winter, no diurnal cycle (polar night), SBL more stationary → U 10m as the control parameter

I n t r o d u c t i o n d a t a S B L SBL regimes identification r e g i m e s V e r t i c a l p r o fj l e s D i s c u s s i o n C o n c l u s i o n s

SBL regimes : wind and temperature I n t r o d u c t i o n d a t a Wind speed ratio with the 2m wind S B L r e g i m e s Wind direction difference with the 2m wind V e r t i c a l p r o fj l e s Temperature D i s c u s s i o n difference with the surface C o n c l u s i o n s All 2014-2015 data in stable conditions

SBL regimes : wind and temperature I n t r o d u c t i o n d a t a Wind speed ratio with the 2m wind S B L r e g i m e s Wind direction difference with the 2m wind V e r t i c a l The temperature inversion does not depend to the wind speed p r o fj l e s Large scatter due to Temperature ● Amount of D i s c u s s i o n difference incoming radiations at the with the surface surface ● Non­stationnary situations C o n c l u s i o n s All 2014-2015 data in stable conditions

SBL regimes : wind and temperature I n t r o d u c t i o n d a t a Wind speed ratio with the 2m wind S B L r e g i m e s Wind direction difference with the 2m wind V e r t i c a l The temperature inversion does not depend to the wind speed p r o fj l e s Large scatter due to Temperature ● Amount of D i s c u s s i o n difference incoming radiations at the with the surface surface ● Non­stationnary situations ● Because dots are organized C o n c l u s i o n s along a 'S' curve with 'backfolding' All 2014-2015 data in stable conditions

SBL regimes : wind and temperature I n t r o d u c t i o n d a t a S B L r e g i m e s V e r t i c a l p r o fj l e s D i s c u s s i o n C o n c l u s i o n s All 2014-2015 data in stable conditions

SBL regimes : wind and temperature I n t r o d u c t i o n 3 m 17 m 32 m d a t a S B L r e g i m e s V e r t i c a l p r o fj l e s D i s c u s s i o n C o n c l u s i o n s One wind speed threshold for the whole tower (the whole SBL?)

SBL regimes : turbulence I n t r o d u c t i o n d a t a Variance of the vertical velocity S B L r e g i m e s V e r t i c a l p r o fj l e s December 2009­ January 2010 data D i s c u s s i o n One wind speed threshold for the whole tower (the whole SBL?) C o n c l u s i o n s

I n t r o d u c t i o n d a t a S B L r e g i m e s Vertical profiles of temperature V e r t i c a l p r o fj l e s D i s c u s s i o n C o n c l u s i o n s

Vertical profiles of temperature in each regime I n t r o d u c t i o n d a t a S B L r e g i m e s V e r t i c a l p r o fj l e s U 10m > U threshold U 10m < U threshold D i s c u s s i o n C o n c l u s i o n s

Vertical profiles of temperature in each regime I n t r o d u c t i o n 'Exponential' shape d a t a S B L r e g i m e s V e r t i c a l p r o fj l e s D i s c u s s i o n Radiative SBL Radiative dominated SBL C o n c l u s i o n s

Vertical profiles of temperature in each regime I n t r o d u c t i o n 'Exponential' shape 'S' shape d a t a S B L r e g i m e s V e r t i c a l p r o fj l e s D i s c u s s i o n Radiative SBL Radiative dominated SBL C o n c l u s i o n s

Vertical profiles of temperature in each regime I n t r o d u c t i o n 'Exponential' shape 'S' shape d a t a S B L r e g i m e s V e r t i c a l p r o fj l e s D i s c u s s i o n Radiative SBL Increase of gradient Radiative dominated SBL = non linear process → Turbulent SBL C o n c l u s i o n s

I n t r o d u c t i o n d a t a Discussion S B L r e g i m e s Existence of 2 SBL regimes ? V e r t i c a l Physical meaning of the wind speed threshold ? p r o fj l e s D i s c u s s i o n C o n c l u s i o n s

Discussion : I n t r o d u c t i o n Surface energy budget and SBL regimes d a t a S B L r e g i m e s V e r t i c a l p r o fj l e s D i s c u s s i o n Net radiative cooling / surface sensible heat flux C o n c l u s i o n s

Discussion : I n t r o d u c t i o n Surface energy budget and SBL regimes Regime 1= Mechanical decoupling Regime 2= compensation d a t a Sensible heat flux Radiative heat loss Radiative heat loss S B L = r e g i m e s V e r t i c a l p r o fj l e s D i s c u s s i o n Net radiative cooling / surface sensible heat flux C o n c l u s i o n s

Discussion : I n t r o d u c t i o n Surface energy budget and SBL regimes In agreement with theory of the minimum wind speed for sustainable turbulence (MWST) d a t a (Van de Wiel et al 2012, Van Hooijdonk et al 2015) S B L Є [5 7] m s ­1 r e g i m e s V e r t i c a l p r o fj l e s D i s c u s s i o n Net radiative cooling / surface sensible heat flux C o n c l u s i o n s

I n t r o d u c t i o n d a t a Discussion : S B L r e g i m e s Is the mechanically decoupled regime dynamically stable ? V e r t i c a l (Does the surface temperature decrease to 0 K ?) p r o fj l e s D i s c u s s i o n C o n c l u s i o n s

Lumped parameter theory, Van de Wiel et al, to be submitted I n t r o d u c t i o n during clear­sky nights d a t a S B L r e g i m e s V e r t i c a l p r o fj l e s D i s c u s s i o n C o n c l u s i o n s

Lumped parameter theory, Van de Wiel et al, to be submitted Equilibrium lines I n t r o d u c t i o n d a t a Most adapted for Dome C conditions S B L r e g i m e s V e r t i c a l p r o fj l e s Conceptual modelling Snow heat diffusion + D i s c u s s i o n radiative transfert in the atmosphere C o n c l u s i o n s Isothermal Turbulent radiative heat flux cooling