The precipitation radar (PR) onboard the Tropical Rainfall Measuring Mission (TRMM) provides the 3-dimentional rain structures over the wide area of ocean and land. The derived rain rate is overall in good agreement with the surface measurement. However, there are still some discrepancies between the PR estimate and the rain rate derived from the microwave radiometer on the TRMM satellite. One of the key issues to validate the TRMM PR is a raindrop size distribution (DSD).

Wind profilers have preliminary been developed to measure wind but can also measure vertical profiles of DSD. A vertically pointing wind profiler can detect echoes scattered from both atmospheric refractive index irregularity and raindrops and thereby can measure both the vertical air motion and drop fall speeds. The measured Doppler spectrum is converted into the DSD assuming an appropriate relationship between the raindrop size and its fall speed.

We made simultaneous rain observations using a 443 MHz wind profiler and the PR at Okinawa, Japan when the TRMM satellite overpassed the observation site. Vertical profiles of DSD were derived from the Doppler spectrum measured with wind profiler using the iterative method (Kobayashi and Adachi, 2003 submitted to JGR). This method needs to assume no prior particular shape of DSD and can therefore provide a decrease/increase in drop concentrations at particular size (Kobayashi and Adachi, 2000,J.Atmos.Oceanic.Technol.). Vertical profiles of the reflectivity factor (Z) and rain rate are calculated from DSD. Results show that Z measured from both sensors are generally in good agreement. However, there are sometimes discrepancies for rain rate. In the measurements on 3 Jul 2002, the vertical profiles of the echo power measured with the TRMM PR were in good agreement with that of the wind profiler. This suggests that the both instrument measured the same precipitation. The reflectivity factor measured with the both sensors has two peaks at altitudes of 4.4 and 5 km and has a sharp minimum between the two peaks. The rain rate profile from the TRMM PR has peaks and dips at altitudes corresponding to those of the received power as expected from the TRMM PR algorithm. For the wind profiler, however, no corresponding minimum of rain rate was observed. A change in the DSD with altitude may leads to the discrepancy. The DSD derived from the wind profiler shows increases in the drop concentration of small size and decreases in the large drop concentrations at the altitude of the discrepancies. The rain rate is approximately proportional to ^3 ( the mean diameter> while Z is ^6. Rain rate is therefore more sensitive to drop concentrations of small size. The TRMM PR may not be able to detect small drops which have significant contributions to rain rate but have small contributions to Z in the present case. We recalculate the rain rate by neglecting smaller drops than 0.5 mm in diameter. The minimum value of rain rate appears after this modification. In the TRMM PR observations, the contributions of small drops to rain rate are critical for accurate estimate of rain rate.