Comparison of Convective Mixing Layer Heights Retrieved by Ceilometer and Doppler Lidar

Boundary layer mixing height strongly influences air quality since concentrations at the surface decrease with the amount of mixing taking place (Holzworth, 1967). A variety of remote sensing instruments have been used to profile the boundary layer (see Barlow et al 2011 for an overview of remote sensing approaches applied in the urban context). This paper is concerned primarily with comparing two types of instrument, Doppler lidar and ceilometer, with a view to evaluating the capabilities of an automated ceilometer mixing layer height retrival.

Doppler lidar systems have been used to profile boundary layer meteorology by monitoring aerosol backscatter and vertical velocity of aerosols and cloud particles (Pearson et al 2008, O'Connor et al 2010, Barlow et al 2011, Harvey et al 2013). Laser ceilometers have been used to deduce the convective mixing layer height through detection of negative gradients in aerosol returns (Munkel and Rasanen, 2004, Wiegner et al, 2006, de Haij et al, 2009, Haeffelin et al, 2012). Ceilometer retrievals of mixing height can be difficult to obtain in some conditions, for example during stable night time conditions when it can be difficult to discern mixed from residual layers by backscatter gradients alone (Haeffelin et al 2012), or when aerosol loading is very low. However, ceilometers are low-cost instruments deployed in large networks around the world and therefore have strong potential to provide continuous monitoring of the boundary layer not feasible with traditional techniques such as radiosonde profiling.

Here, operational retrievals of mixing layer height from a Campbell Scientific CS135 ceilometer platform (Campbell Scientific, Ltd, 2014) running a retrieval algorithm based on the gradient method (de Haij et al 2009) are compared with mixing layer heights derived from backscatter and vertical velocity data from the HALO Photonics Doppler Lidar at Chilbolton observatory in Hampshire, UK. The accuracy of the mixing layer heights output by the ceilometer algorithm are assessed for a variety of atmospheric conditions, and a new quality factor flagging system is proposed.