Range-adjustment for ground-based radar derived with TRMM radar: analyses in Israel confirm previous findings

With the introduction of weather radar onboard the Tropical Rainfall Measuring Mission (TRMM) satellite, meteorological radar applications have been successfully extended to a global scale. Another important characteristic (pregio) of the TRMM-Precipitation-Radar (TPR) is represented by its long-term, continuously monitored electronic stability. The calibration factor is assumed to have a remarkable accuracy: its uncertainty is smaller than 1 dB. Consequently, TPR provides the possibility of assessing the average bias of ground-based radars around the world. Quantitative assessments of this kind have been performed, e. g., in Florida, Colorado, Texas, Marshall Islands, Australia and Cyprus. In this last case, a novel comparison between the TPR and the ground-based radar (GR) was also suggested, based on the fact that the GR and the TPR provide a complementary view: the (GR) measures rain from a lateral direction, while the space-borne radar sees it from the top. The lateral GR measurements are used for quantitative precipitation estimation at distances between 10 km (or even less) and 100 km (or even more). Because of this large ratio of distances, the scattering volume changes by a factor of over 100, since the volume increases with the square of the distance. One effect of the beam divergence is a 1/r2 range-dependence that is already corrected by the radar equation. A second phenomenon is the influence of non-homogeneous beam filling in combination with the average decrease of the vertical reflectivity profile with height, which is the focus of this paper. As an example, at longer ranges of the GR, the lower part of the volume could be in rain, whereas the upper part of the same pulse could be filled with snow, or even be without an echo. This influence becomes more important at longer ranges, since the scattering volume increases in size. On the other hand, the scattering volume of TPR has a similar size in all the locations. Its size is not correlated to the distance between the echo and the GR. This advantage of TPR stimulated the idea of using TRMM radar to estimate the influence of sampling volume of the ground-based radar. The analysis was based on the average, linear radar reflectivity, in circular rings around the GR site as a function of the range, D, from the GR site. The lateral GR measurements are acquired, in the present case, at distances between 10 and 120 km. For both radars, we compute the average Z in the same circular ring. We use 8 rings “centered” at 25, 50, 65, 75, 85, 95, 105, 115 km. Rings are 10 km wide, but the 1st and 2nd ones that are 30 and 20 km wide. Hence, the volumes used to determine the averages are large, even much larger than the rather coarse TPR pulse volume resolution. The large sampling volume reduces mismatches caused by different beam widths and by changes in the weather in space and time. Let and be values of average reflectivity, averaged in azimuth at distance D from the GR for both the GR and the TPR. These two variables show similar behavior, except for the decreased sensitivity of the GR with distance. Factor F(D) = ()/() is statistically explained in this paper using a regression between Log(F(D)) and Log(D). The slope in this regression reflects the deviation of the radar sensitivity from the common 1/r2 law (i.e., it reflects the rate of change of the calibration with distance). Negative values can be expected and were in fact found in Cyprus, since the sampling volume of the GR increases with the square of the distance. Here the analysis has been repeated in Israel. Also in this case the sensor is a C-band radar. The radar site coordinates are: Latitude: 31.99°; Longitude: 34.90°; altitude: 60 m). Reflectivity echoes acquired at the lowest elevation have been compared with the lowest TPR radar reflectivity values, the so-called NearSurfZ. Just like happened with the Cyprus radar, also with the Israeli radar a negative slope is found: in Cyprus it was of the order of –10 dB/decade; in Israel it is found to be approximately –7 dB/decade. This means a significant apparent decrease of sensitivity with range of the GR. The negative value of this range-effect reflects the fact that the GR underestimates precipitation at longer distances. This fact has long been known: in literature, the range-dependence of GR-observations has been extensively investigated using in situ rain gauges. In Cyprus, for the first time, the TPR has been used. In this paper, the exercise has been repeated in Israel with similar findings: the range dependence of the GR/TPR ratio is significant. This is interpreted to be caused by the increasing sampling volume of the GR with range, combined with non-homogeneous beam filling. We conclude that the TRMM radar offers the unique opportunity of validating Ground-based Radars (GR). The algorithm developed, allows a quantitative comparison between TRMM radar and any ground-based radar worldwide. The concept allows not only quantifying the average bias of the GR with respect to the TPR, but also establishing the “real” dependence of the GR data on the range. It is extremely valuable in a Global Precipitation Measuring perspective.