Comparison Of Columnar Water Vapor Determined From Microwave Emmision And Solar Transmission Measurements

Water vapor and liquid water (clouds) play a major role in remote sensing applications as ground-based mm-wave spectroscopy (Peter and Ksmpfer, 1991, Kknpfer et al. 1991) and communications due to their effect on the propagation of electromagnetic waves. Since the columnar water vapor or integrated precipitable water vapor IWV and the liquid water content ILW in the troposphere are highly variable in time and space only continuous measurements of these quantities can provide adequate corrections for remote sensing observations. Dual channel microwave instruments (20/30 GHz) measuring the atmospheric emission, are well known for their capability to measure continuously IWV and ILW also during night and cloudy conditions (Guiraud et al., 1979, Westwater, 1978 and Elgered et al. 1982). The same capability is attributed to photometers measuring the absorption of the radiation of the sun in the range of 350 1000 nm, however only during clear sky conditions. During half a year simultaneous measurements of both types of instruments have been performed in order to validate the accuracy of both instruments as well as to estimate the retrieval quality depending on the used emission (MW) and absorption (IR/Vis) models included in the IWV retrieval algorithms. The retrieved IWV from the water vapor radiometer (WVR) and the sun photometer (SPM) exhibiting a rms scatter of about 9 % (SPM) and 13 % (WVR) show a good agreement with radiosonde data taken a t noon. Between both instruments with independently retrieved IWV, the comparison yielded a mean IWV difference of 1.6 mm (10 %) and a rms scatter of 1.3 mm (8 %) for the whole periode of observation. Thus the rms is lower between the WVR and the SPM than with the radiosonde data, while the difference of the mean between both instruments is higher, reflecting possible calibration offsets or inconsistencies in the MW emission or IR/Vis absorption model. I) Microwave measurements