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

Abstract

[1] Dual-frequency rain flagging has long been a standard part of altimetric data analysis, both for quality control of the data and for the study of rain itself, because altimeters can provide a finer spatial sampling of rain than passive microwave instruments can. However, there have been many varied implementations, using different records of the surface backscatter and different thresholds. This paper compares four different measures available for the recently launched Jason-2. The evaluation compares these measures against clearly desired properties, finding that in most cases the adjusted backscatter and that from the ice retracker perform much better than the one recommended in the users' handbook. The adjusted backscatter measure also provides a much better link to observations from Jason-1, opening up a much longer period for consistent rain investigations and enabling greatly improved analysis of the short-scale variability of precipitation. Initial analysis shows that although the spatial and temporal gradients of backscatter increase at very low winds, the spatial gradients in rain attenuation are concentrated where rainfall is greatest, while the temporal changes have a simple broad latitudinal pattern.

Investigating Short-Wavelength Correlated Errors on Low-Resolution Mode Altimetry

“…5c), that is, in zones where the altimeter waveforms are more likely to integrate substantial inhomogeneities in backscatter strength over the footprint. Indeed, the map for the red population is quite consistent with geographical distributions observed by Quartly et al (1996), Quartly (2010), and Tran et al (2005) for altimeter rain flags, and by Mitchum et al (2004) and Thibaut et al (2010) for major sigma0 blooms (TOPEX and Envisat, respectively). This is coherent with the simulations from section 2: these regions are also where the retracker's model is more likely to be insufficient to describe the measured waveform, resulting in a spatially coherent error similar to Fig.…”

“…In our simulation, the bloom is limited to a very small zone of the ocean (i.e., a Dirac with respect to the altimeter resolution). While the approach is simpler than the circular-patch model developed by Tournadre et al (2006), or the simulations with square facets from Quartly (1998), the principle is the same.…”

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.…”

Jason-1/Jason-2 Metocean Comparisons and Monitoring

“…3c), but the variations with wave height are less coherent. [Note that the Jason-2 GDRs also contain an alternative definition of σ 0 based on a sea-ice model; the offset between the ocean and ice definitions of σ 0 vary slightly with wave height (Quartly 2009c). ] Quartly 2009c) noted that the standard deviation of the σ 0 mismatch between instruments (after bulk rescaling) is less than the r.m.s.…”