Analysis of Short Space–Time-Scale Variability of Oceanic Rain Using TOPEX/Jason

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 21 , 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 21 ) 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 21 ) 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. * Current affiliation: Ahmedabad,

Improving the altimetric rain record from Jason‐1 and Jason‐2

“…The rms spatial differences for Jason‐1 and Jason‐2 are very similar, which is not surprising given their common design and processing. The values here for Jason‐1 are ∼20% less than found by TB09 [Table 2], which were less than the values noted for TOPEX. In fact for the most common wind conditions, the along‐track differences at K u and C band are both below 0.1 dB, with greatest spatial variability associated with the extremes of σ 0 (see Figures 8a and 8b).…”

“…At both frequencies the temporal change is greatest at the rare conditions of very high σ 0 , with the differences more pronounced at K u band. TB09 agree that the temporal change at C band is less than at K u band for σ C 0 < 16 dB, which corresponds to the majority of the data, but find C band to have the greatest changes in very low winds [TB09, Figure 8b]. As one might expect the longer wavelengths that cause C band scattering to be less changeable, it is likely that their results were affected by the differences in instrumental construction/processing.…”

“…This 6 month period of nearly simultaneous rain attenuation data from Jason‐1 and Jason‐2 affords a wonderful opportunity to review the results of Tournadre and Bhandari [2009, hereafter TB09], contrasting the spatial and temporal scales of rain variability. For their analysis they used data from TOPEX and Jason‐1 during December 2002 to August 2003.…”

“…The temporal changes between Jason‐1 and Jason‐2 observations are approximately 50% of that noted between Jason‐1 and TOPEX [TB09]. This suggests that despite the best endeavors of those authors a considerable part of their temporal changes were due to the difficulties of comparing slightly different altimeters.…”

Metocean Comparisons of Jason-2 and AltiKa—A Method to Develop a New Wind Speed Algorithm

“…The mission plans for Jason-1 and Jason-2 had tandem phases that allowed more than 6 months of observations simultaneous with their predecessors in that particular orbit. This enabled detailed cross-calibration studies between Jason-1 and TOPEX (Quartly 2004;Tournadre 2004;Tournadre and Bhandari 2009) and then between Jason-1 and Jason-2 (Quartly 2009b, Quartly 2010aQuartly 2010b). Although AltiKa occupies the track previously surveyed by ERS-1, ERS-2 and Envisat, there was no overlap in time with Envisat as its altimeter stopped working in April 2012.…”