Comparing meteorological spaceborne and ground-based radars: optimal satellite overpass distance from a ground-based radar site

Abstract

Spaceborne and ground-based radars (GRs) provide a complementary view of the complex precipitation field. As there are significant differences between the two instruments, it is possible to use the peculiarities of one sensor to check the consistency of the results of the other. For instance, the radar measurement is affected by changes in the spatial resolution, which ‘worsens’ with the square of the range. Unfortunately, the GR has to measure rain from small to large distances. In contrast, changes in size of the Tropical Rainfall Measuring Mission (TRMM) radar sampling volume are almost negligible, since its measurements originate from similar distances of 400–420 km. However, the TRMM precipitation radar (TPR) vertical resolution becomes poorer when the angle of incidence increases (i.e. when the distance of the TPR samples from the nadir increases). This article shows that part of this problem, which affects the reference radar (namely the TPR), can be transferred to the radar under investigation (namely the GR), depending on the satellite overpass distance from the GR site. Given the present swath of the TPR (∼240 km), the worst cases occur when the satellite passes close or at the maximum range of the GR. A small influence is found when the satellite passes at approximately half of the GR range. These results, which have been derived for the first-ever spaceborne radar (the TRMM radar), can be easily adapted to future missions (e.g. the global precipitation measuring dual-wavelength radar).

Acknowledgements

The analytical solution presented here was stimulated by the numerical solution provided by Jürg Joss within the EU-funded VOLTAIRE project. The authors thank the anonymous reviewers for their helpful, careful and appropriate comments.