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J. Tournadre

Abstract

Since the launch of the Ocean Topography Experiment (TOPEX)/Poseidon in 1992, several studies have demonstrated that dual-frequency altimeter measurements cannot only accurately detect rain events but can also be used to infer quantitative values. The main problems with these techniques are the limited time and space sampling of nadir-looking instruments and the uncertainty in the height of the freezing level necessary to infer the surface rain rate from the measured signal attenuation. In addition to radar altimeters, altimetric satellites carry microwave radiometers designed to correct for atmospheric water effects. Using a radiative transfer model and simplified rainy atmospheres, a method of inversion of the microwave brightness temperatures in terms of freezing level is presented. The surface rain rate is then computed from the altimeter attenuation and the radiometer freezing level. The rain climatology is computed for the three altimeters currently in operation using a mixed lognormal distribution. Comparison with the Global Precipitation Climatology Project and Special Sensor Microwave Imager (SSM/I) climatologies shows that the use of freezing level greatly improves the altimeter climatology, which is of the same quality as that of the SSM/I for annual mean. The merging of the three altimeters is investigated. The resulting monthly mean rain rates are comparable to those derived from SSM/I. The high along-track resolution of altimeters also allows the determination of the length of rain events. The mean length is close to the SSM/I footprint size in the Tropics, but at higher latitude 80% of the rain has length scales smaller than 10 km, which might explain the relative underestimation of the mean rain rate by SSM/I.

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J. Tournadre

Abstract

High-rate (20 Hz) altimeter echo waveforms are particularly sensitive to small-scale changes of surface backscatter and they contain a wealth of information on the sea surface. A detailed analysis of these waveforms often reveals the signatures of objects that emerge from the sea. Such objects, such as beacons, lighthouses, ships, and small islands, have deterministic signatures that can be estimated using simple geometry. Examples of signatures of ships, beacons, and islands in the TOPEX/Poseidon and Jason altimeters waveforms are presented and analyzed. Some potential applications are also presented.

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J. Tournadre
and
S. Bhandari

Abstract

Information on the spatial and temporal variability of rain rate is important not only for meteorology and hydrology but also for the design of remote sensing and in situ measuring or of millimeter wave communication systems. The Ocean Topography Experiment (TOPEX)/Jason Tandem Mission (TM) collocated rain dataset is used in this study to determine the small space-scale (5 km) and time-scale (70 s) rain variability. TOPEX and Jason dual-frequency (Ku and C bands) radar altimeter data have been extensively used during the past decade to detect and study oceanic precipitations. During the TM, designed to intercalibrate and validate the two altimeters, the two satellites were put on the same orbit with a 70-s time separation. With combined use of altimeter and passive microwave radiometers (also available on board altimeter missions), rain intensity, rain attenuation, rain layer height, rain event length, and surface winds can also be estimated and provide valuable coincident geophysical contextual information. The size of the TM collocated rain database (140 000 samples) is large enough to allow a meaningful statistical analysis of the time–space variability of rain over the World Ocean. The analysis of the different terms contributing to the variability of rain attenuation, from which rain rate is inferred, shows that the geophysical and/or instrumental noise is small enough to allow a meaningful estimation of the variability of the measured rain rate. The analysis of the time and space variabilities and their relation reveals two well-defined regimes. The first one, corresponding to convective rain cells (i.e., rain rate greater than 3–4 mm h−1, length smaller than 50 km, and freezing level greater than 3.5 km), is characterized by high temporal and spatial variabilities (greater than 2–3 mm h−1) that increase with increasing rain intensity and decreasing cell length. Horizontal variability is significantly larger than the temporal one and surface wind has a very limited impact. The second regime corresponds to stratiform and/or weak rain cells. The temporal and spatial variabilities are relatively low (on the order of 1–2 mm h−1) and vary little with rain intensity and cell length. The temporal variability increases with surface wind and largely exceeds the spatial variability (ratio of 2 or more); the ratio strongly increases with increasing wind speed.

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