Implementation of Radiative Transfer Functions for Microwave Radiometers on Altimetry Satellites Using the Community Radiative Transfer Model

The Microwave Radiometers (MR) onboard the Jason/Sentinel-6 series altimetry satellites measure
atmospheric total water vapor contents to derive the wet tropospheric path delay, which is crucial for
correcting the sea surface height measurements. They typically consist of three channels, around
18GHz, 23GHz, and 34GHz, providing accurate atmospheric path delay correction over the open ocean.
Launched in November 2020, Sentinal-6A Michael Freilich (S6A) is the latest reference altimetry mission.
S6A is equipped with an Advanced Microwave Radiometer for Climate (AMR-C) with a footprint size of
~25 km. It also has an experimental High-Resolution Microwave Radiometer (HRMR) with a high spatial
resolution of around 5 km across three channels at 90 GHz, 130 GHz, and 168 GHz, designed to improve
accuracy over coastal areas. Due to their nadir-only observations and the fewer number of channels
compared to other passive MR instruments (e.g. ATMS and SSMI), the current version of the Community
Radiative Transfer Model (CRTM) does not include the MR sensors from altimetry missions. As a result,
these observations cannot be assimilated into current operational weather and hurricane forecasting
models. However, there is a strong need to incorporate the radiative transfer functions of MR sensors
into CRTM. Firstly, including the function of MR sensors on altimetry satellites could enable the
assimilation of the high spatial resolution MR observations into the weather and hurricane prediction
models, especially with the current implementation of all-sky weather conditions in Numerical Weather
Prediction (NWP) models. Secondly, a radiative transfer model is essential for validating and comparing
measured MR brightness temperatures with simulations using NWP model outputs. Moreover, AMR-C
and HRMR sensors include channels sensitive to atmosphere water vapor, ocean surface winds, rainfall,
and ice/snow coverage, allowing these observations to be used for product retrievals. Therefore,
implementing radiative transfer functions for altimetry MR sensors within CRTM is a critical step for
sensor calibration and validation, future data assimilation in weather and hurricane prediction models,
and snow/ice coverage retrievals.

In this study, we have developed and implemented radiative transfer functions for the AMR-C/HRMR of
S6A within CRTM. The two sensors were combined into a single pseudo sensor, AMRCHRMR_S6A. The
microwave Rosenkranz line-by-line model was used as a reference to generate the atmospheric
transmittance coefficients for CRTM, considering both vertical and horizonal polarizations of the sensors
across all six channels. We have tested its capability for simulating brightness temperatures using
ECMWF and Radio Occulation water vapor products. The primary application of this capability is to
validate the accuracy and long-term stability of the AMR-C and HRMR brightness temperature. We will
report on the comparison between S6A MR brightness temperatures and simulated brightness
temperatures from ECMWF/ERA-5 and other atmosphere water vapor products at collocated locations.
Additionally, this capability will be used to quantify the impact of the wet tropospheric path delay from
the two-sided AMR onboard the SWOT satellite on the swath sea level measurements.

Future missions, including Sentinel-6B/6C, CRISTAL-A/B, and various small satellite missions, will feature
similar instruments, potentially providing larger global spatial coverage as a constellation compared to a
single satellite. Implementing radiative transfer functions for MR sensors on altimetry missions will extend the application of their observations beyond altimetry, contributing to a broader range of
remote sensing research.