Miklós Szakáll

A Wind Tunnel Study on the Shape, Oscillation, and Internal Circulation of Large Raindrops with Sizes between 2.5 and 7.5 mm

Abstract Precipitation prediction using weather radars requires detailed knowledge of the shape parameters of raindrops falling at their terminal velocities in air. Because the raindrops undergo oscillation, the most important shape parameters from the radar prediction point of view are the equilibrium drop shape, the time-averaged axis ratio, and the oscillation frequency. These parameters for individual water drops with equivalent diameter from 2.5 to 7.5 mm were investigated in a vertical wind tunnel using high-speed video imaging. A very good agreement was found between the measured and the theoretically determined raindrop shape calculated by a force balance model. A new method was developed to determine the equivalent drop diameter with the help of the oscillation frequency. The drop size determination by means of the frequency method was found to be three times more precise than by volumetric methods. The time-averaged axis ratio was found to be equal to the equilibrium axis ratio in the investigat...

Diode laser based photoacoustic humidity sensors

Abstract Various diode laser based photoacoustic (PA) systems applied in water vapour detection are presented. The dependence of the systems’ performance on the physical and chemical properties of the measured gas is discussed. It was found that the pressure dependence of the measured PA signal is different for amplitude and wavelength modulated lasers. This difference is explained. Linearity in a concentration range of more than four orders of magnitude in combination with typical accuracy of a few percentages was proved.

External Cavity Diode Laser Based Photoacoustic Detection of CO2 at 1.43 μm: The Effect of Molecular Relaxation

Photoacoustic spectroscopy, based on an external cavity diode laser operating at 1431 nm, was used for measuring CO2 concentration as a minority component in a gas mixture. By using N2 as a buffer gas, a molecular relaxation effect was observed, which influenced both the amplitude and the phase of the measured photoacoustic signal and consequently reduced the sensitivity of the PA system. This molecular relaxation effect could be suppressed by adding water vapor of a constant and relatively high (∼4%) concentration to the gas sample. In parallel with this, the arising spectral interference between H2O and CO2 necessitated the development of a simple yet efficient signal analysis method, which increased the sensitivity of the system by more than one order of magnitude and accordingly reduced the minimum detectable CO2 concentration down to ∼1000 ppm.

A high-sensitivity, near-infrared tunable-diode-laser-based photoacoustic water-vapour-detection system for automated operation

A photoacoustic sensor system for automatic detection of low concentrations of water vapour is described in this paper. A Littman-configuration external-cavity diode laser operating at 1125 nm was used as a light source in combination with a high-sensitivity measuring photoacoustic cell, a reference photoacoustic cell and PC-controlled electronics. The system was calibrated with synthetic gas samples and a detection limit of 13 µmol per mol of water vapour was determined. Adsorption/desorption phenomena at the walls of the measuring photoacoustic cell were found to be an important limiting factor for the sensitivity of the system.

Diode laser based photoacoustic water vapor detection system for atmospheric research.

A wavelength modulated, distributed feedback diode laser based photoacoustic water vapor mixing ratio measuring system for atmospheric research applications is presented. Laser modulation parameters were optimized either at 180 or 500 mbar total pressure to enhance the systems sensitivity for low or high pressures (upper troposphere/lower stratosphere or biosphere exchange layer), respectively. A wavelength locking method was developed that ensured sub-picometer absolute (5 × 10−7 relative) wavelength stability of the laser while consuming minimum additional measurement time. At the calibration of the system, correction factors for the pressure- and temperature-dependence of the photoacoustic signal were determined, which were in turn applied to the calculation of the water vapor mixing ratio from the measured signal during the test operation of the system. The introduced features resulted in reliable, sub-ppm-level water vapor detection even under abrupt gas pressure or temperature variations typical in open atmospheric applications.

A Comprehensive Observational Study of Graupel and Hail Terminal Velocity, Mass Flux, and Kinetic Energy

AbstractThis study uses novel approaches to estimate the fall characteristics of hail, covering a size range from about 0.5 to 7 cm, and the drag coefficients of lump and conical graupel. Three-dim...

Chemistry of riming: the retention of organic and inorganic atmospheric trace constituents

During free fall in clouds ice hydrometeors such as snowflakes and ice particles grow effectively by riming, i.e., the accretion of supercooled droplets. Volatile atmospheric trace constituents dissolved in the supercooled droplets may remain in ice during freezing or may be released back to the gas phase. This process is quantified by retention coefficients. Once in the ice phase the trace constituents may be vertically redistributed by scavenging and subsequent precipitation or by evaporation of these ice hydrometeors at high altitudes. Retention coefficients of the most dominant carboxylic acids and aldehydes found 5 in cloud water were investigated in the Mainz vertical wind tunnel under dry growth (surface temperature < 0 C) riming conditions which are typically prevailing in the mixed phase zone of convective clouds (i.e., temperatures from 16 to 7 C and a liquid water content of 0.9± 0.2 g cm 3 ::::::::::::: 0.9± 0.2 gm 3). The mean retention coefficients of formic and acetic acids are found to be 0.68±0.09 and 0.63±0.19. Oxalic and malonic acids as well as formaldehyde show mean retention coefficients of 0.97± 0.06, 0.98± 0.08 and 0.97± 0.11, respectively. Application of a semi-empirical model on the present and earlier wind 10 tunnel measurements reveals that retention coefficients can be well interpreted by the effective Henry’s law constant accounting for solubility and dissociation. A parameterization for the retention coefficients has been derived for substances whose aqueous phase kinetics are fast compared to mass transport timescales. For other cases, the semi-empirical model in combination with a kinetic approach is suited to determine the retention coefficients. These may be implemented in high resolution cloud models.

Gas-detection instrument based on external-cavity diode lasers and photoacoustic detectors

Among the numerous detection methods applicable in spectroscopic gas detection systems photoacoustics is probably the simplest, most cost effective, yet a highly sensitive one. By using a properly designed photoacoustic cell, equipped with a commercial microphone and electronics for phase sensitive detection, absorbed electromagnetic radiation as low as 10 nW can be easily detected. The choice of a suitable light source for PA gas detection, however, is crucial in order to preserve the simplicity and practical advantages of the combined gas detection system. In the photoacoustic system presented here, therefore, external cavity diode lasers have been adopted. Contrary to ordinary diode lasers, external cavity diode lasers are ideal sources for spectroscopic studies as they have a narrow linewidth and ar continuously tunable in a relative wide wavelength range. External cavity diode lasers operating in different wavelength range have been developed and concentration determination of water vapor gaseous mixtures are presented. Sensitivity of the system presented here could reach the ppb. level under optimal conditions.

Development and application of external-cavity diode laser systems for photoacoustic gas detection

Two external cavity diode laser systems operating at 860 nm and at 1125 nm have been developed and assembled into photoacoustic gas detection systems. Both laser systems have been found to be suitable for water vapor detection applications. Comparison of the two systems has been made regarding their stability, mode structure, tunability etc. Their performance has been compared with other type of diode lasers as well.

Possible application areas of a diode-laser-based photoacoustic gas detection method

Due to its simplicity, high sensitivity, reliability and low price diode laser based photoacoustic gas detection can be expected to gain more and more widespread applications in the close future. Examples of the possible application areas such as environmental monitoring, agriculture, medical science chemical process control etc. are discussed here.