Large aperture scintillometry observations in central London during ClearfLo

The NERC funded ClearfLo project, conducted in London was concerned with the interaction of meteorological, air quality and health processes. To obtain turbulent sensible heat fluxes, traditional eddy covariance (EC) measurements were undertaken at several heights above ground level. EC provides point measurements with a source area directly upwind, with a scale in the order of 102 m2. Numerical weather prediction operates at scales of several kilometres (1-5 km). Path averaged sensible heat fluxes representative of a similar scale are useful for model evaluation. These can be obtained from large aperture scintillometers (LAS) by measuring the changes in the refractive index of the atmosphere. Three instrument models were compared: the LAS 150 and LAS MkII manufactured by Kipp & Zonen (KZ, The Netherlands) and the BLS900 from Scintec (Germany). Four such instruments (i.e. one LAS 150, two LAS MkII, one BLS900) were operated over two paths (4.4 km, 1.29 km length) across residential areas in London. The refractive index structure parameters, C_n^2, from the different instruments were analysed in summer and winter at 1 min, 15 min and 30 minute intervals. Direct comparisons of data collected on the same paths have very high correlations, even for different instrument types, with better agreement in summer when turbulent exchanges are larger. The spatial variation of C_n^2 was assessed between LAS systems and also in relation to EC observations. Sensible heat fluxes derived from the LAS and EC systems allow the spatial variability of this flux in the centre of London to be assessed.