Search

The classical rain attenuation correction scheme of Hitschfeld and Bordan (HIBO) and the newer iterative approach by Hildebrand (HL) are reconsidered. Although the motivation for the HL algorithm was an extension into ranges, where HIBO tends to be unstable, it is shown here that the contrary is the case. The finite-range resolution causes an intrinsic instability of HL already at moderate attenuation, where HIBO would still deliver stable results. Therefore, the authors concentrate the further analysis on HIBO, and confirm that the usual implementation of HIBO does not account correctly for finite-range resolution. They suggest a modified scheme that produces exact retrievals in the ideal case of perfect measurements. For vertically pointing Doppler radars a new element is explored in the attenuation correction—namely, calculating rain attenuation κ and rainfall R from Doppler spectra via the raindrop size distributions (RSDs). Although this spectral scheme (SIBO) avoids the uncertainty of Z–R and Z–κ relations, the superiority of this approach is not a priori obvious because of its sensitivity to vertical wind. Therefore, radar rain rates, based on a Z–R relation and on RSDs, respectively, are compared with in situ measurements. The results indicate better agreement for RSD-based retrievals. Because κ is closely correlated with R, the authors assert the advantage of RSD-based retrievals of κ. The application of HIBO and SIBO to real data shows that the uncertainty of standard Z–R relations is the main source of deviation between the two versions. In addition, the comparison of profiles suggests that the parameters of Z–R relations aloft can deviate considerably from near-surface values. Although artifacts cannot be excluded with certainty, there is some evidence that this observation actually reflects microphysical processes.

Data of vertically pointing microrain radars (MRRs), located at various sites around the Baltic Sea, were analyzed for a period of several years. From the Doppler spectra profiles of drop size distributions (DSDs) are obtained. A significant height dependence of the shape of the DSDs—and thus of the Z–R relations—is observed at high rain rates. This implies, for the considered sites, that ground-based Z–R relations lead to underestimation of high rain rates by weather radars.

Humidity spectra obtained with a closed-path optical hygrometer (LI-6262) show an apparent low-pass characteristic with a cutoff frequency varying between 0.01 and 0.1 Hz. Laboratory measurements revealed that contamination of the intake filter is responsible for the damping of high frequencies. It was further found that the effect of the intake filter on the response function has a nonlinear component with the consequence that the cutoff frequency is a function of the humidity variance. The uncontaminated “lifetime” of the intake filter depends on the particular air quality and may be less than 24 h. The usefulness of closed-path hygrometers for long-term eddy covariance measurements of humidity is therefore questioned.

A Doppler sodar and an ultrasonic anemometer–thermometer, which are operated continuously at a nuclear research center in Germany, are described. Mean values and second moments including turbulent surface fluxes recorded over 1 yr are analyzed in order to assess their credibility. Particularly, the effects of rain on the data quality and the statistical bias caused by a reduced data availability—typical for remote sensing measurements—are investigated. Mean profiles of the vertical wind variance for different classes of Monin–Obukhov length, the mean vertical wind component versus wind direction, and the annual variation of 24-h averages of the surface heat flux are presented.