Mixing-layer height retrieval with ceilometer and Doppler lidar: from case studies to long-term assessment

摘要

Aerosol signatures observed by ceilometers are frequently used to derivemixing-layer height (MLH) which is an essential variable for air qualitymodelling. However, Doppler wind lidar measurements of vertical velocity can provide amore direct estimation of MLH via simple thresholding.A case study reveals difficulties in the aerosol-based MLH retrieval duringtransition times when the mixing layer builds up in the morning and when turbulence decays in theafternoon. The difficulties can be explained by the fact that the aerosoldistribution is related to the history of the mixing process and aerosolcharacteristics are modified by humidification. The results of the case studyare generalized by evaluating one year of joint measurements by a VaisalaCT25K and a HALO Photonics Streamline wind lidar.On average the aerosol-based retrieval gives higher MLH than the wind lidar with an overestimationof MLH by about 300 m (600 m) in the morning (late afternoon).Also, the daily aerosol-based maximum MLH is larger and occurs later during the dayand the average morning growth rates are smaller than those derived from thevertical wind.In fair weather conditions classified by less than 4 octa cloud cover the meandiurnal cycle of cloud base height corresponds well to the mixing-layerheight showing potential for a simplified MLH estimation.