Abstract's details
Harmonizing the Jason-1, Jason-2, Jason-3 Time Series of Altimeter Rain Flags
CoAuthors
Event: 2019 Ocean Surface Topography Science Team Meeting
Session: Quantifying Errors and Uncertainties in Altimetry data
Presentation type: Type Oral
Contribution: not provided
Abstract:
In this presentation, we evaluate the time series of altimeter rain flags provided on the Jason-1 (version E), Jason-2 (version D), and Jason-3 (version D) geophysical data records, and provide recommendations to mitigate inconsistencies.
We first select measurements where the altimeter rain flag is set, only considering measurements over ocean as determined by both the altimeter and radiometer surface type flags on the data products (= 0), low latitudes (< 50 degrees N/S), altimeter echo type (= 0), and radiometer brightness temperature interpolation flag (not 3). For each exact repeat cycle, 5,000 – 40,000 measurements are flagged as rain. While the cycle-to-cycle variability is high, we observe two major discontinuities: 1) an increasing trend in the number of rain flagged measurements on the Jason-1 GDR-E products, from ~5,000 measurements at the beginning of the mission to ~30,000 at the end, and 2) a consistently higher number of measurements flagged as rain on the Jason-3 products, by about 40%, compared to Jason-2.
We have used the well-documented rain flag algorithm to explore the source of these discrepancies and to provide possible solutions for improved consistency. For the trend in Jason-1 rain flags, we recommend the computation of the rain flag using sigma0 instead of AGC, due to the significant drift in the Jason-1 AGC through its lifetime. For the bias in number of rain-flagged measurements in Jason-2 vs Jason-3, we recommend using updated Jason-3 C- and Ku-band MLE-3 sigma0 calibrations of -0.100 and -0.270 dB, respectively. These calibrations represent an alignment of sigma0 values to a reference table used in the rain flag algorithm. These calibrations are slightly different from the values used to generate the rain flag on the Jason-3 GDR-D products, of -0.012 and -0.231 dB, respectively. We show the influence of this new rain flag on cycle-averaged statistics for various parameters such as SSHA, SWH, wind speed, etc.
Finally, we compare the rain flags from the Jason series of products to observations from independent precipitation missions (TRMM and GPM).
We first select measurements where the altimeter rain flag is set, only considering measurements over ocean as determined by both the altimeter and radiometer surface type flags on the data products (= 0), low latitudes (< 50 degrees N/S), altimeter echo type (= 0), and radiometer brightness temperature interpolation flag (not 3). For each exact repeat cycle, 5,000 – 40,000 measurements are flagged as rain. While the cycle-to-cycle variability is high, we observe two major discontinuities: 1) an increasing trend in the number of rain flagged measurements on the Jason-1 GDR-E products, from ~5,000 measurements at the beginning of the mission to ~30,000 at the end, and 2) a consistently higher number of measurements flagged as rain on the Jason-3 products, by about 40%, compared to Jason-2.
We have used the well-documented rain flag algorithm to explore the source of these discrepancies and to provide possible solutions for improved consistency. For the trend in Jason-1 rain flags, we recommend the computation of the rain flag using sigma0 instead of AGC, due to the significant drift in the Jason-1 AGC through its lifetime. For the bias in number of rain-flagged measurements in Jason-2 vs Jason-3, we recommend using updated Jason-3 C- and Ku-band MLE-3 sigma0 calibrations of -0.100 and -0.270 dB, respectively. These calibrations represent an alignment of sigma0 values to a reference table used in the rain flag algorithm. These calibrations are slightly different from the values used to generate the rain flag on the Jason-3 GDR-D products, of -0.012 and -0.231 dB, respectively. We show the influence of this new rain flag on cycle-averaged statistics for various parameters such as SSHA, SWH, wind speed, etc.
Finally, we compare the rain flags from the Jason series of products to observations from independent precipitation missions (TRMM and GPM).