Accurate radar refractivity retrievals are critical for quantitative applications, such as assimilating refractivity into numerical models or studying boundary layer and convection processes. However, the technique as originally developed makes some simplistic assumptions about the heights of ground targets (H T ) and the vertical gradient of refractivity (dN/dh). In reality, the field of target phases used for refractivity retrieval is noisy because of varying terrain and introduces estimation biases. To obtain a refractivity map at a constant height above terrain, a 2D horizontal refractivity field at the radar height must be computed and corrected for altitude using an average dN/dh. This is achieved by theoretically clarifying the interpretation of the measured phase considering the varying H T and the temporal change of dN/dh. Evolving dN/dh causes systematic refractivity biases, as it affects the beam trajectory, the associated target range, and the refractivity field sampled between selected targets of different heights. To determine H T and dN/dh changes, a twofold approach is proposed: first, H T can be reasonably inferred based on terrain height; then, a new method of dN/dh estimation is devised by using the property of the returned powers of a pointlike target at successive antenna elevations. The dN/dh obtained shows skill based on in situ tower observation. As a result, the data quality of the retrieved refractivity may be improved with the newly added information of dN/dh and H T .

In Taiwan, comparing with the major hazardous Mei-Yu fronts and typhoons in summer, a synoptic condition with the relatively weak high and low pressure systems at certain locations in the autumn season may bring torrential rainfall to northern Taiwan. A detailed dual-polarimetric/Doppler radar analysis was carried out for one case under this condition to reveal the mesoscale precipitation mechanisms and microphysical characteristics over terrain.The high pressure system moving eastward off China and the low pressure system over the ocean in the southeast of of south-eastern Taiwan formed a convergent zone at the low levels, resulting in a sequence of convective activities. These convective cells moved westward, and became more organized and intense in the Mt. Datun area and the estuary of Tamsui River near the lee side of Mt. Datun.Moderate intensity convective cells were embedded in the wide, long-lasting stratiform regions. In the mountain area with intense precipitation, terrain-induced upward motion of the cells enhanced condensation, significantly increased drop counts, and acted as a feeder. The older cells in this convective system continued to provide lighter hydrometeors in the upper layer and formed a widespread stratiform region as a seeder. The wider spectrum of drop size distribution set the stage for collision and coalescence process, resulted in the larger drops formed at the low level of the mountain area. Along with the increasing concentration of raindrops, the total effect finally caused heavy rainfall over the mountain area.

To properly use radar refractivity data quantitatively, good knowledge of its errors is required. The data quality of refractivity critically depends on the phase measurements of ground targets that are used for the refractivity estimation. In this study, the observational error structure of refractivity is first estimated based on quantifying the uncertainties of phase measurements, data processing, and the refractivity estimation method. New correlations between the time series of phase measurements at different elevation angles and between polarizations are developed to assess the bulk phase variability of individual targets. Then, the observational error of refractivity is obtained by simulating the uncertainties of phase measurements through the original refractivity estimation method. Resulting errors in refractivity are found to be smaller than 1 N-unit in areas densely populated with reliable point-like stationary ground targets but grow as the target density becomes sparse.

Although antennas have well-known power patterns that are commonly used to understand the quality of measurements, they also have phase patterns that are difficult to obtain and are seldom discussed in the radar meteorological community. This study presents the characteristics of the antenna phase pattern of the McGill S-band radar. Phase variations in azimuth and elevation with respect to the main beam axis are obtained using high-resolution scans of an isolated ground target and of an emission source. The two-way phase pattern is relatively constant within the radar main beam, but it changes rapidly at the power minima between the main beam and the first sidelobe. The effects of this phase pattern on ground and weather targets were evaluated and were found to be much more pronounced for point targets than for distributed targets. Nevertheless, proper knowledge of the phase pattern of the radar antenna would enhance the ability to better select ground targets for radar refractivity retrieval and to estimate the quality of radar data.