Combined Space and Ground Radar Observations: An Impact of TRMM PR MDZ

The simultaneous observations of the precipitation from both space and ground based radars, now possible with satellite-borne rain radars as the PR in TRMM, open new perspectives, in particular to resolve complex structures like tropical storms. Work is underway to generate combined TRMM PR and the Bauru, Brazil (22 gr 21'28"S, 49gr 5'37"W) S-band Doppler radar, involving the important issue of the PR sensitivity. This paper presents an initial assessment of the impact of the PR MDZ on the data/applications. For a set of 33 selected rainfall events within 100 km range of the Bauru radar, mostly from the rainy (October-March) period but including a few cases from the dry period, were computed the portions of the ground radar reflectivity values lying below the PR MDZ. In the results, which are compatible with observations in Florida, a degree of seasonal and daily stratification is noted. Also, the amount of reflectivities under TRMM MDZ level followed the decrease of the ground radar own MDZ with range. The distribution of the difference in heigth of the echo tops as estimated by the ground radar with boundaries defined in one case by its MDZ and in the other by the TRMM MDZ, is presented for 4 range rings. There were no noticeable range effects within the maximum range of 100 km utilized in this work. For a subset of 12 storms a comparison of collocated vertical cross-sections from the PR and the Bauru radar was made and differences were verified against the corresponding TRMM lightning data. The lightning activity was compatible with the echo tops indicated by the Bauru radar, while in many cases the PR provided echo tops for the same storm did not characterize severity. An intense storm which delivered damaging hail , situated at about 35 km from the radar with echoes topping at more than 20 km was re-analysed limiting the outermost reflectivity to the PR MDZ of 17 dBZ. The indicated echo top heigth lowered to about 14 km which did not suggest the actual severity of the storm. On the other hand, an indication of the impact of the PR MDZ on the areal delimitation of the precipitation was obtained by following up a relatively widespread rainfall system and comparing that delimitation with and without the PR MDZ cut-off. It is suggested that the rainfall which would go undetected may be significant, for instance, in terms of the effect on the energy flux which would be produced by a latent heat release associated with that rainfall volume.