Example ADA Data
The ADA instrument has been operated on a number of projects often in conjunction with other instruments which also measure droplet size distribution. Presented here is some data from the CLACE-2 experiment at the Jungfraujoch in Switzerland, showing comparisons between ADA data and data from a Forward Scattering Spectrometer Probe (FSSP-100) with the DMT electronics upgrade installed.
The CLACE2 Project
The CLACE2 project was held at the Jungfraujoch High Alpine Research Station Sphinx Laboratory during June and July 2002. This project, to examine cloud – aerosol interactions in the lower free troposphere, involved an extensive range of aerosol and cloud microphysical instrumentation, part of which included an ADA and FSSP mounted in close proximity on the roof of the lab. During the project a wide variety of cloud conditions were experienced, and a considerable amount of in cloud data was obtained. This provided an ideal dataset for the intercomparison and evaluation of the performance of the ADA and FSSP under a wide range of conditions, and in the light of additional information from other instrumentation employed at the site.
Results from ADA and FSSP
A good dataset was obtained from both instruments, however software issues meant ADA was not logged during some cloud events. There were several periods of continuous cloud lasting a number of days, with a wide variety of spectral shapes and mode diameters observed. ADA droplet concentration, was compared with that from the FSSP, and ADA wind speed with the sonic anemometer. Agreement was generally good, as can be seen in figure 1. However, as ADA only measures one component of wind speed, and is not aspirated, it must be pointed into wind to make good measurements.
While there is good agreement for droplet number, and air speed between ADA and the other instruments, the cloud droplet spectra show some significant differences in sizing. ADA consistently reports larger sizes than the FSSP. This is clearly illustrated in figure 3, where it can be seen that there is closer agreement at small sizes, but large discrepancies at larger sizes. Figure 4 shows ADA and FSSP mode diameter plotted against each other, showing that generally ADA reports diameters 1.35 times larger than the FSSP. This would lead to an approximate factor of 2.4 difference in liquid water content.
The time series of FSSP and ADA size distributions shown in figure 2 clearly show that ADA is able to measure the size distribution in much greater detail than the FSSP and often resolves features which cannot be seen at all in the FSSP data, especially the presence of a second mode in the distribution at small sizes.
Please click on the figures to see a larger version and read the full caption.
Conclusions
Good agreement has been observed between ADA and FSSP droplet number concentrations when both instruments are operating well. However, significant differences were observed between the size distributions reported by the ADA and FSSP, with ADA consistently reporting mode diameters a factor of 1.35 larger than the FSSP. This factor is well outside the error on the measurements, and is very significant for volume or liquid water content measurements. Further investigation into the observed differences between the two instruments is needed, including lab studies with monodisperese droplets and calibration particles. In general these results show that if operated correctly for the conditions being measured ADA has the potential to be a powerful new addition in the field of microphysical measurements. ADA’s higher resolution size data, immunity from Mie Scattering problems and coincidence errors, as well as the additional information which can be obtained from the data represent significant advantages over existing technology. However ADA has it’s own problems which need to be fully investigated and characterised, and the significant differences in sizing between ADA and FSSP must be resolved.