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

Abstract

As well as range, the AltiKa altimeter provides estimates of wave height, H_s and normalized backscatter, σ⁰, that need to be assessed prior to statistics based on them being included in climate databases. An analysis of crossovers with the Jason-2 altimeter shows AltiKa H_s values to be biased high by only ˜0.05m, with a standard deviation (s.d.) of ˜0.1m for seven-point averages. AltiKa's σ⁰ values are 2.5–3 dB less than those from Jason-2, with a s.d. of ˜0.3 dB, with these relatively large mismatches to be expected as AltiKa measures a different part of the spectrum of sea surface roughness. A new wind speed algorithm is developed through matching a histogram of σ⁰ values to that for Jason-2 wind speeds. The algorithm is robust to the use of short durations of data, with a consistency at roughly the 0.1 m/s level. Incorporation of H_s as a secondary input reduces the assessed error at crossovers from 0.82 m/s to 0.71 m/s. A comparison across all altimeter frequencies used to date demonstrates that the lowest wind speeds preferentially develop the shortest scales of roughness.

Keywords:

• AltiKa altimeter
• K_a-band
• sigma0
• wind speed
• wave height
• histograms

Funding

This work was carried out as part of the KaNUTE project, and was supported through the PML research programme Altika—Innovative Meteorological Studies.