Drop size distribution measurements in convective storms with a vertically pointing 35‐GHz Doppler radar

Measurements at vertical incidence in convective storms with a 35-GHz pseudonoise coded high-resolution radar at Johannesburg, South Africa, are discussed. The radar is bistatic and has the capability to measure targets at very short ranges (≈ 19 m). This capability is used to derive an accurate experimental mean velocity-reflectivity ( v―-Z) relationship in convective storm systems. It is shown that at an altitude of 19 m the standard deviation of the data points from the mean v―-Z relationship is about 0.25 m s−1, indicating that the contribution of the vertical air velocity at the low altitude of 19 m is less than about ±0.5 m s−1 (95% confidence limit). The experimental v―-Z relationship is then used to derive drop size distributions from the measured Doppler spectra in convective rainfall with high time resolution (10 s). Some of the measured drop size distributions are discussed and related to specific kinematic features of the storm under study. For example, drop size distributions measured in a severe squall line show a large abundance of smaller drops (< 1.5 mm in diameter) in regions of wind shear near the edges of the main downdraft core. A possible explanation of this large concentration of smaller drops is sorting of the hydrometeors caused by shear in the horizontal wind that is generated by the divergence of the downdraft at the ground. These observations show the usefulness of this radar measurement technique to detect features of drop size distributions in convective systems and relate them to storm structure and kinematics.