Robin HoganDepartment 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

Daytime surface energy Daytime surface energy budgetbudget

• Updrafts (w’>0) tend to be warmer (T’>0) and moister (q’>0) than average

• Downdrafts (w’<0) tend to be cooler (T’<0) and dryer (q’<0) than average

• Hence the temperature flux ( ) and water vapour flux ( ) are both positive in the day

• They can be measured with fast-response w, T and q.

' 'w T

Shortwave

Longwave

Warm, moist surface

“Net radiation”Sensible heat flux:heat transported to air by turbulence and convection

Latent heat flux:loss of energy by evaporation Ground heat flux

Height

Temperature or mixing ratio

' 'w q

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)

Surface heating leads to convectively generated

turbulence

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

when sensible heat flux goes negative at 1800

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

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 cloud top: normal convection and positive skewness

ConclusionsConclusions• Potential for lots of new boundary-layer science to be done

with such instruments• Possible applications:

– Evaluation of boundary-layer schemes, e.g. Met Office scheme (Adrian Lock) which predicts the depth to which surface- and cloud-top-driven turbulence will penetrate: can infer directly from skewness

– Test the inferences of turbulence intensity using tethered balloon– Study turbulence in the poorly understood and poorly modelled

nocturnal boundary layer– Combine Doppler lidar with RAMAN humidity to get profiles of

latent heat flux

– Potential for Chilbolton to become a “FLUXNET” site with its CO2 measurements?