Characteristics of vertical profile of rainfall parameters in a Baiu frontal rainfall using 2D-Video distrometer, COBRA, and 400MHz Wind Profiler

To improve the accuracy of space-borne and ground-based radar estimated rainfall intensity and rain retrieval algorithms for space-borne microwave radiometers, the vertical profile of rainfall parameters, especially raindrop size distribution, are very important. Intensive field observations for subtropical precipitation using COBRA (C-band polatimetric radar), 400MHz Wind Profiler (WPR), and two types of ground-based disdrometer were carried out in June 2006 at Okinawa Island, Japan. These instruments are part of the Okinawa Sub-tropical Environment Remote Sensing Center, operated by the National Institute of Information and Communication Technology (NICT). The ground-based DSD was observed by a 2D-Video distrometer and a Joss-type disdrometer. The vertical profile of DSD was estimated from Doppler spectrum observed by the 400MHz WPR. The data were then used to analyze the vertical variations of rainfall parameters that include raindrop size distribution (DSD), mass weighted drop diameter (Dm), normalized number concentration (N0*), radar reflectivity (Z), differential reflectivity (ZDR), and correlation coefficient (ρHV). The radar ZDR, and Dm were also derived from the 400MHz WPR. Inter-comparisons among these rainfall parameters obtained from the various instruments were also conducted. All radar rainfall parameters (Z, ZDR, and ρHV) were averaged over the three rainfall periods. To characterize the vertical variations in the rainfall parameters for the various rainfall types during the Baiu front event of 2 June 2006, the data were separated into the following categories: (a) convective rainfall (CV), (b) stratiform rainfall before the CV (Before-CV), and (c) stratiform rainfall after the CV (After-CV). We focused on the differences in rainfall parameters between the ‘Before-CV' and the ‘After-CV' rain events. From an examination of the ground-based measurements of the DSD, we find that the size distributions of the ‘Before- CV' rain are generally broader and less peaked than the ‘After-CV' rain events. These differences in the DSD during the two stratiform periods of rain are reflected in changes in the Z-ZDR and Z-R relationships for the COBRA radar data where the values of ZDR were typically larger during the ‘Before-CV' rain than in the ‘After-CV' rain. This result was confirmed from the radar ZDR estimated from the 400MHz WPR data. Using the COBRA data, the location of the bright band (BB) in the ‘Before-CV' period was found to be lower than the BB location during ‘After-CV' period. This result was confirmed by the Doppler spectrum observed by the 400MHz Wind Profiler.