Aerosol layer heights over Doha and their relation to Direct Normal Irradiance ground measurements

Abstract

Depending on their physical properties, aerosols in the atmosphere can scatter and/or absorb solar radiation and thus reduce the amount of solar radiation reaching the surface. Under cloud free conditions, the extinction of the Direct Normal Irradiance (DNI) is primarily caused by aerosols in the atmosphere. DNI data in locations where ground measurements are not available can be derived or estimated using satellites. However, for areas characterized by high aerosol load like in Qatar, satellite data gives DNI values with high uncertainties related to the inaccurate determination of aerosols present on a particular day. Ceilometer devices, which are based on the Lidar (LIght Detection And Ranging) technique, when operated on a routine basis, are reliable tools for long-term observation and qualitative assessment of the vertical distribution of aerosols in the atmosphere. Indeed, ceilometer measurements combined with specific retrieval software enable the detection of the vertical structure of the atmospheric boundary layer. Based on this data, the height of aerosol layers can be determined. This study describes an analysis over Doha, Qatar (25.33° N, 51.43° E), of day-to-day variability of aerosol layer heights measured by a ceilometer around solar noon under cloudless conditions; the layer heights are compared with day-to-day variability of DNI measured at the same time and same location. Aerosol layer heights are obtained using a CL51 Vaisala ceilometer. Ground measurements of DNI are collected by a pyrheliometer mounted on a high-quality Kipp & Zonen solar radiation measurement station, equipped with a sun tracker. The results of measurements for clear sky conditions over several months during this year (2013) will be presented. The study of the relation between the daily variation of aerosol layer heights and the direct component of the solar radiation is part of the solar resource assessment project within the Qatar Environment and Energy Research Institute and represents a first insight before further investigating the correlation of ceilometer backscatter measurements with ground-measured solar radiation.