GROUND-BASED MICROWAVE RADIOMETRY AND LONG-TERM OBSERVATIONS OF ATMOSPHERIC WATER-VAPOR

Microwave radiometer data and radiosonde data from the time period 198 1-1995 have been used to study long-term trends in the integrated prec ipitable water vapor (IPWV). The two instruments have operated 37 km a part on the Swedish west coast. Model parameters are estimated for the entire data sets as well as for subsets of the data. The IPWV model p arameters are a mean value, a linear drift with time, and the amplitud e and phase of an annual component. The radiosonde data, which are uni formly sampled in time, show an increase in the IPWV of 0.03 mm/yr wit h a statistical standard deviation of 0.01 mm. The microwave radiomete r data, which are not at all uniformly sampled in time, show -0.02+/-0 .01 mm/yr. We show that the disagreement is caused by the different sa mpling of the data for the two instruments. When the two data sets are reduced to include only data that are sampled simultaneously, we find an agreement between all estimated model parameters, given their stat istical uncertainties. This suggests that if the microwave radiometer had also been operating continuously over the 15-year period, its data would have implied a linear trend similar to the result obtained from the radiosonde data. The general quality of the data, in terms of the short time scatter, has been improved over the time period. The root mean square (RMS) difference between the IPWV measured by the radiomet er and by the radiosondes was 2.1 mm during the first 5 years and was reduced to 1.6 mm during the last 4 years. These values include the re al difference in the IPWV between the two sites. The bias, radiometer- radiosonde, was 0.1 mm for the whole data set and varied between -0.2 and 0.9 mm for smaller data sets of a few years.