Preliminary Evaluation of the Error Budgets in the TALIS Measurements and Their Impact on the Retrievals

Wenyu Wang,Yongqiang Duan,Zhenzhan Wang

doi:10.3390/rs12030468

Abstract

The THz Atmospheric Limb Sounder (TALIS) is a Chinese sub-millimeter limb sounder being designed by National Space Science Center of the Chinese Academy of Sciences to measure the temperature and chemical constituents vertically in the middle and upper atmosphere, with good precision and vertical resolution. This paper presents a simulation study that assesses the measurement errors and their impacts on the retrievals. Three error sources, including instrument uncertainties, calibration errors and a priori errors, are considered. The sideband weight uncertainty, the local oscillator, the pointing angle offsets and the measurement noise (NEDT), are considered as instrument uncertainties. Calibration errors consist of the hot target offset, the nonlinearity residual of the two-point calibration, use of the Rayleigh–Jeans (R–J) approximation and the choice of the antenna pattern. A priori profile errors of temperature, pressure and species are also considered. The results suggest that the antenna pattern mainly affects the retrievals in the troposphere. The NEDT is a major error source affecting all of the retrievals. The R–J approximation has a great impact upon the retrievals at 643 GHz, and should not be used. The local oscillator offset leads to an obvious error above 50 km. The effect of nonlinearity residuals cannot be neglected above 70 km. The impact of the sideband weight uncertainty and the hot target offset are relatively small. The pointing and the a priori errors can be neglected in most observation regions.

Highlights

Observation of the long-term evolution of the chemical species and the status of Earth’s atmosphere is essential for scientists to understand the physical and chemical processes in the atmosphere and to assess the changes of climate
The error induced by local oscillator offset (LO) is around 5 K above 60 km, since the line width is quite narrow in the upper atmosphere
The THz Atmospheric Limb Sounder (TALIS) is the first microwave limb sounder being developed in China, and will contribute to atmosphere observation in the future

Summary

Introduction

Observation of the long-term evolution of the chemical species and the status of Earth’s atmosphere is essential for scientists to understand the physical and chemical processes in the atmosphere and to assess the changes of climate. Atmosphere Research Satellite (UARS) launched in 1991 provided atmospheric data of temperature and species such as ClO, O3, H2O, SO2, CH3CN and HNO3 [2,3,4]. This information helps scientists understand the physical and chemical processes which lead to the O3 depletion. The Sub-Millimeter Radiometer (SMR) carried by the Odin satellite observed the atmospheric limb for half of the observation time, producing various species profiles in the middle atmosphere such as O3, ClO, N2O, HNO3, H2O, CO and NO, as well as isotopes of H2O, O3 and ice clouds with a high vertical resolution of 1–2 km [5,6,7,8].

Description of TALIS

Instrument Uncertainties

A Priori Errors

Model and Retrieval Method

Results

Conclusions