observations of boundary-layer turbulence using doppler lidar and surface fluxes
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Observations of boundary-layer turbulence using Doppler lidar and surface fluxes. Robin Hogan Alan Grant, Ewan O’Connor, Anthony Illingworth, Guy Pearson Department of Meteorology, University of Reading. Overview. Boundary-layer observations at Chilbolton - PowerPoint PPT PresentationTRANSCRIPT
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: