The subject of this thesis is the application of optical backscatter measurements to locate a special property of the lowest part of the atmosphere -- the mixing height. Mixing height is the altitude of the top of the layer where all the fluxes emitted at the ground become well mixed. Since Holzworth in 1967, the knowledge of this altitude is considered relevant when modeling transport of pollutants or general fluxes originating at the ground. Indirect estimations of the mixing height are possible using atmospheric models, but its accuracy is quite low. Since several institutions are attempting to estimate precise ground fluxes, networks of measurement stations are being created. The correct use of the measured fluxes, in order to estimate the evolution of the air masses, is limited by the accuracy of the localization of this layer. It can be detected in several different ways. Most are related to a direct sounding, performed with meteorological balloons. Remote sensing techniques are also attempted with acoustical or optical instruments. Both optical and acoustical methods have advantages and disadvantages. This work is focused on optical instruments like lidar and ceilometers, which are basically small cost-effective lidar systems.

A novel lidar ceilometer prototype based on divided lens optics has been designed, built, characterised, and tested. The primary applications for this manufacturable ground-based sensor are the determination of cloud base height and the measurement of vertical visibility. First, the design, which was developed in order to achieve superior performance at a low cost, is described in detail, along with the process used to develop it. The primary design considerations of optical signal to noise ratio, range-dependent overlap of the transmitter and receiver channels, and manufacturability, were balanced to develop an instrument with good signal to noise ratio, fast turn-on of overlap for detection of close range returns, and a minimised number of optical components and simplicity of assembly for cost control purposes. Second, a novel imaging method for characterisation of transmitter-receiver overlap as a function of range is described and applied to the instrument. The method is validated by an alternative experimental method and a geometric calculation that is specific to the unique geometry of the instrument. These techniques allow the calibration of close range detection sensitivity in order to acquire information prior to full overlap. Finally, signal processing methods used to automate the detection process are described. A novel two-part cloud base detection algorithm has been developed which combines extinction-derived visibility thresholds in the inverted cloud return signal with feature detection on the raw signal. In addition, standard approaches for determination of visibility based on an iterative far boundary inversion method, and calibration of attenuated backscatter profile using returns from a fully-attenuating water cloud, have been applied to the prototype. The prototype design, characterisation, and signal processing have been shown to be appropriate for implementation into a commercial instrument. The work that has been carried out provides a platform upon which a wide range of further work can be built.

In this study, four and a half months of ceilometer data from Östergarnsholm are used to analyze cloud related to processes in the boundary layer. Measurements are divided into two categories, which are defined by wind direction: a continental and a marine sector. The results show that there are significant differences in the height of the lowest cloud bases detected for the two sectors, where cloud base heights are lower for the marine wind sector during unstable and neutral conditions. The ceilometer’s ability to detect several cloud base heights simultaneously is utilized to test whether a double layer structure (DLS) can be detected. The results of this particular analysis are inconclusive as to whether a DLS has been observed or not. Detected cloud base heights differ greatly from heights suggested by the lifting condensation level (LCL). A new empirical formula for lowest cloud base height is then derived using the measurements. The Ceilometer’s estimations of sky cover are assessed to be of reasonable quality. This is suggested by computed high correlation with incoming shortwave radiation at noon for three months.  However, histograms of cloud cover measurements suggest that the ceilometer tends to probably either overestimate or underestimate cloud cover.  Large differences in cloud cover were observed for the two wind sectors during unstable conditions. For the months of July and August, a diurnal cycle in cloud cover for the continental wind sector was observed which suggest the presence of Stratocumulus. Measurements performed during upwelling conditions closely resemble those of the marine wind sector performed during stable conditions.

The subject of this thesis is the application of optical backscatter measurements to locate a special property of the lowest part of the atmosphere -- the mixing height. Mixing height is the altitude of the top of the layer where all the fluxes emitted at the ground become well mixed. Since Holzworth in 1967, the knowledge of this altitude is considered relevant when modeling transport of pollutants or general fluxes originating at the ground. Indirect estimations of the mixing height are possible using atmospheric models, but its accuracy is quite low. Since several institutions are attempting to estimate precise ground fluxes, networks of measurement stations are being created. The correct use of the measured fluxes, in order to estimate the evolution of the air masses, is limited by the accuracy of the localization of this layer. It can be detected in several different ways. Most are related to a direct sounding, performed with meteorological balloons. Remote sensing techniques are also attempted with acoustical or optical instruments. Both optical and acoustical methods have advantages and disadvantages. This work is focused on optical instruments like lidar and ceilometers, which are basically small cost-effective lidar systems.