Estimates of any precipitation characteristics based on temporally sparse observations entail uncertainty because of the natural variability of rainfall in space and time. This study measures the sampling-related uncertainties of monthly mean reflectivity profile and surface rainfall distribution. Radar and rain gauge data collected during the 1993/94 monsoon season at Darwin, Australia, are used to show the sensitivity of monthly three-dimensional radar-echo and precipitation characteristics to the frequency of observation. The data are partitioned into convective, stratiform, and anvil components according to the horizontal and vertical structure of the echoes. The analyses of this study reveal the expected trend that the uncertainties of estimated precipitation characteristics using infrequent observations scale with rainfall amount. The results have implications for climatological studies using spaceborne observation platforms revisiting a given area intermittently.
The Tropical Rainfall Measuring Mission (TRMM) satellite radar, which will revisit a given 500km by 500km region approximately twice daily, will likely encounter significant problems in estimating the vertical profile of radar reflectivity in the tropics. Monthly mean reflectivity statistics (based on observations within 150km of the Darwin radar) exhibit a sampling-related uncertainty of about 20% in both rain and snow. In addition, the radar signal of the TRMM satellite will be highly attenuated below the 0°C level, and the precipitation radar will be insensitive to reflectivity less than about 20 dBZ. Therefore, the spaceborne radar will have an obscured view of the vertical precipitation structure. Reliable reflectivity statistics based on TRMM satellite radar data may be obtained primarily within an altitude range of about 5-7.5km-an altitude range though that is important for cloud electrification because of the mixed-phase precipitation processes taking place there. The sampling uncertainty, signal attenuation, and radar sensitivity vary with precipitation type. Moreover, estimation of the convective rain fraction will be compromised by uncertainties in the echo classification as well as a choice of Z-R relation. These results imply the importance of information collected by ground validation site radars to improve upon TRMM satellite estimates of precipitation characteristics and the derived vertical profile of latent heating.