Abstract's details
Towards a unique method for a global and multi-surface Wet Tropospheric Correction retrieval : a 1-D Variational approach
CoAuthors
Event: 2016 Ocean Surface Topography Science Team Meeting
Session: Instrument Processing: Corrections
Presentation type: Type Poster
Contribution: not provided
Abstract:
The ongoing and future altimetry missions (ie Jason-3, Sentinel-3, SWOT) aim to deliver a measurement of the topography at a finer spatial resolution and a higher temporal rate over open ocean and other heterogeneous surfaces, including coastal regions, hydrological targets, ice and sea-ice. In this perspective, the role played by the wet tropospheric correction (WTC) is critical, due to its large temporal and spatial variability and its crucial weight in the final budget error.
Current WTC retrieval algorithms are based on empirical approaches using measured (radiosonde) or modeled (numerical weather prediction analysis) atmospheric profiles; in both cases, a radiative transfer model relates the integrated content of WTC to the top of the atmosphere brightness temperatures and a relation is fitted between the two datasets.
This method is valid for average atmospheric conditions over Open Ocean only, where an open sea emissivity model is available. The performances are then strongly degraded wherever the instrumental measurements are contaminated by other surface types (land, ice, sea-ice); solutions exist to correct for this contamination but will not be able to satisfy the future constraints on the retrieval errors over coastal regions or hydrological surfaces.
To address this issue, a one-dimensional variational approach (1D-VAR) for WTC retrieval is a good candidate to provide a unique method well adapted to all surfaces and different atmospheric conditions.
Where current algorithms directly provide an integrated value of WTC, the latter approach aims to estimate the atmospheric profiles that best explain the TOA TB measurements. The WTC is then computed from integration of the retrieved profiles.
Depending on the surface, the emissivity is provided by a model (Open Ocean) or an emissivity atlas (other surfaces) estimated from TOA measured TBs. In the case of land/ice/sea-ice contamination in the TOA TB measurement, emissivity is calculated from the combination of Open Ocean and land/ice/sea-ice emissivities weighted by the land/ice/sea-ice proportion in the measurement’s footprint.
In direct line with previous work (Desportes et. al., 2010) showing the potential of 1D-VAR over coastal regions, this presentation will focus on the 1D-VAR WTC retrieval in the Mediterranean Sea. Atmospheric profiles and surface parameters from the 2.5 km high horizontal resolution NWP model AROME are used as background and combined to radiometer measurements at low frequencies (classical 18.7, 23.8 and 31.4 GHz). To go further, we will also study the potential of combining low frequencies to other instruments providing radiometric measurements at higher frequencies such as 53.6, 89, 118, 157 and 183 +/- 7 GHz. Coupled to lower frequencies, their finer spatial resolution and relatively higher sensitivity to the atmosphere and cloud liquid water will be used to better address coastal approach, in particular for the estimation of surface emissivity according to land proportion within the measurement footprint, and to possibly enhance WTC retrievals over cloudy situations.
Current WTC retrieval algorithms are based on empirical approaches using measured (radiosonde) or modeled (numerical weather prediction analysis) atmospheric profiles; in both cases, a radiative transfer model relates the integrated content of WTC to the top of the atmosphere brightness temperatures and a relation is fitted between the two datasets.
This method is valid for average atmospheric conditions over Open Ocean only, where an open sea emissivity model is available. The performances are then strongly degraded wherever the instrumental measurements are contaminated by other surface types (land, ice, sea-ice); solutions exist to correct for this contamination but will not be able to satisfy the future constraints on the retrieval errors over coastal regions or hydrological surfaces.
To address this issue, a one-dimensional variational approach (1D-VAR) for WTC retrieval is a good candidate to provide a unique method well adapted to all surfaces and different atmospheric conditions.
Where current algorithms directly provide an integrated value of WTC, the latter approach aims to estimate the atmospheric profiles that best explain the TOA TB measurements. The WTC is then computed from integration of the retrieved profiles.
Depending on the surface, the emissivity is provided by a model (Open Ocean) or an emissivity atlas (other surfaces) estimated from TOA measured TBs. In the case of land/ice/sea-ice contamination in the TOA TB measurement, emissivity is calculated from the combination of Open Ocean and land/ice/sea-ice emissivities weighted by the land/ice/sea-ice proportion in the measurement’s footprint.
In direct line with previous work (Desportes et. al., 2010) showing the potential of 1D-VAR over coastal regions, this presentation will focus on the 1D-VAR WTC retrieval in the Mediterranean Sea. Atmospheric profiles and surface parameters from the 2.5 km high horizontal resolution NWP model AROME are used as background and combined to radiometer measurements at low frequencies (classical 18.7, 23.8 and 31.4 GHz). To go further, we will also study the potential of combining low frequencies to other instruments providing radiometric measurements at higher frequencies such as 53.6, 89, 118, 157 and 183 +/- 7 GHz. Coupled to lower frequencies, their finer spatial resolution and relatively higher sensitivity to the atmosphere and cloud liquid water will be used to better address coastal approach, in particular for the estimation of surface emissivity according to land proportion within the measurement footprint, and to possibly enhance WTC retrievals over cloudy situations.