Robin Hogan Department of Meteorology University of Reading Observations of boundary- layer turbulence using Doppler lidar and surface fluxes

Daytime surface energy budget 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. 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

Fluxes on 11 July 2007 Most 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 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

Height Potential temperature Longwave cooling Shortwave heating Cloud Height 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

Conclusions 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 CO 2 measurements?