Robin HoganAlan Grant, Ewan O’Connor,

Anthony Illingworth, Guy PearsonDepartment of Meteorology, University of Reading

Observations of boundary-Observations of boundary-layer turbulence using layer turbulence using

Doppler lidar and surface Doppler lidar and surface fluxesfluxes

OverviewOverview• Boundary-layer observations at Chilbolton

– Boundary-layer height mapping (Cyril Morcrette)– Humidity convergence from refractivity (John Nicol)– Evaluation of boundary-layer cloud forecasts (Andrew Barrett)– Boundary-layer cloud microphysics (Ewan O’Connor)

• Properties of turbulence from Doppler lidar and surface fluxes– Variance and skewness of vertical wind– The nature of turbulence driven by cloud-top radiative cooling– Potential for evaluating boundary layer parameterizations

Mapping BL top with Mapping BL top with radarradar

Cyril Morcrette, MWR 2007

Radar sees clear-air scattering from refractive index inhomogeneities at boundary-layer top

Radar BL top

Mesoscale model BL top

RefractivityRefractivity• Low-level atmospheric refractivity can be derived by

measuring the phase shift between pieces of “ground clutter” with a scanning radar (higher refractivity usually means higher humidity)

• Animation of refractivity during the passage of a sea breeze from south:

• Met Office working to assimilate it to provide information on humidity convergence as a precursor to convection

John Nicol

Diurnal cycle composite of Diurnal cycle composite of cloudsclouds

Andrew Barrett, submitted to GRL

Meteo-France:Local mixing scheme: too little entrainment

Swedish Met Institute:Prognostic TKE scheme: no diurnal evolution

All other models have a non-local mixing scheme in unstable conditions and an explicit formulation for entrainment at cloud top: better performance over the diurnal cycle

Radar and lidar provide cloud boundaries and cloud properties above the site

Fluxes on 11 July 2007Fluxes on 11 July 2007Most of incoming solar energy used for evaporation

and transpiration

Photosynthesis

Turbulence

Input of sensible heat “grows” a new cumulus-capped boundary layer

during the day (small amount of stratocumulus in early

morning)

Insects carried in updrafts to above the boundary layer top Convection is “switched off”

when sensible heat flux goes negative at 1800

Surface heating leads to convectively generated

turbulence

Comparing variance to previous Comparing variance to previous studiesstudies

New contribution to variance

• Lidar temporal resolution of ~30 s– Smallest eddies not detected, so add them using -5/3 law– Only valid in convective boundary layers where part of the inertial

subrange is detected

• Normalize variance by convective velocity scale– Reasonable agreement with Sorbjan (1980) and Lenschow et al. (1989)

SkewnessSkewness

• Skewness defined as – Positive in convective daytime boundary layers– Agrees with aircraft observations of LeMone

(1990) when plotted versus the fraction of distance into the boundary layer

• Useful for diagnosing source of turbulence

Skewness in convective BLsSkewness in convective BLs• Both model simulations and laboratory visualisation show

convective boundary layers heated from below to have narrow, intense updrafts and weak, broad downdrafts, i.e. positive skewness

Courtesy Peter Sullivan NCAR

Narrow fast updrafts

Wide slow downdrafts

Why is skewness positive?Why is skewness positive?• Consider TKE budget:

• Vertical flux of TKE is– So skewness positive when TKE transported upwards by turbulence

2 2 3' ' ' ' ' ' / 2w e w u w v w

TKE generated by shear and buoyancy in lower BL

TKE destroyed by dissipation & buoyancy suppression at BL top

TKE transported upwards by turbulence

Transport

Stull

...a alternative related ...a alternative related explanationexplanation

• Updraft regions of large eddies have more intense small-scale turbulence than the downdraft regions

• This leads to an asymmetric velocity distribution

• Will test this later...

Large eddies only

All eddies

Heig

ht

Potential temperature Potential temperature

Longwave cooling

Shortwave heating

Cloud

Heig

ht

Negatively buoyant plumes generated at cloud top: upside-down convection and negative skewness

Positively buoyant plumes generated at surface: normal convection and positive skewness

Stratocumulus cloud

11 April 11 April 20072007

CospectrumCospectrumVertical wind from 11 April 2007 at time of transition

• The cospectrum, Cww2:– Defined as the complex

conjugate of FFT of w’ multiplied by FFT of w’ 2

• Can be thought of as a spectral decomposition of the third moment:

• Hunt et al. (1988) showed that it goes as freq-2 in intertial subrange Skewness dominated by larger

(20 minute timescale) eddies

Closed-cell stratocu.Closed-cell stratocu.• Previous studies have shown

updraft/downdraft asymmetry in stratocumulus at the largest scales

• Puzzle as to why aspect ratio of cells is as much as 30:1

Shao & Randall (JAS 1996)

Comparison of top-down and Comparison of top-down and bottom-upbottom-up

• Variance of vertical wind– Good agreement with previous

studies if H = 30 W m-2 – Variance peaks in upper third of BL:

agrees with Lenschow et al.’s fit provided theirs is inverted in height

• Skewness– Very good agreement with

the fit of LeMone (1990) to aircraft data provided hers is inverted in sign and height

Aircraft vs LESAircraft vs LES

LES: Moeng and Wyngaard (1988)

Aircraft observations: LeMone (1990)

Upside-down Carson’s modelUpside-down Carson’s model• If all the physics is the same

but inverted, we can apply Carson’s model to predict the growth of the cloud-top-cooling driven mixed layer

• With longwave cooling rate of H = 30 W m-2 and lapse rate of = 1 K km-1, we estimate growth of 1.1 km in 3 hours, approximately the same as observed

Very little turbulence

Lidar sees continuous cloud

Turbulence not reaching ground

Negative skewness

Lidar sees continuous cloud

Turbulence throughout BL

Skewness changes sign

Potential to evaluate the Lock BL Potential to evaluate the Lock BL schemescheme

Lidar sees continuous cloud

Turbulence with a minimum

Skewness changes sign

Lidar sees Cu below Sc

Turbulence with a minimum?

Skewness of both signs

Lidar sees Cu Turbulence below Cu base

Negative skewness

Potential to evaluate the Lock BL schemePotential to evaluate the Lock BL scheme

ConclusionsConclusions• Many ways in which the Chilbolton instruments can be used to

investigate the properties of the boundary layer• Variance and skewness from 2.5 years of continuous Doppler

lidar data could be used to evaluate the categories predicted by the Lock boundary-layer scheme in the Met Office model

• More details here: