Abstract:
The earth's climate has undergone significant changes due to the combined effects of natural changes and human activities. To understand the impact of climate change, the most fundamental work is to establish high-quality data required for climate purposes. Currently, the long series observations mainly come from satellites and sites. However, most satellite sensors are designed for short-term and imminent weather monitoring and numerical prediction, rather than long-term climate monitoring. To meet future research needs, more efforts are needed in data reprocessing such as satellite calibration and multi-source data fusion.Global Navigation Satellite System Radio Occultation (GNSS-RO) is a system that carries a receiver on low orbit satellite to receive radio signals transmitted by the global navigation satellite system. GNSS-RO detects the earth's atmosphere in a borderline manner during relative motion. When propagating in non-vacuum atmosphere, radio signals may appear bent or delay due to different atmospheric physical characteristics. After complex processing, physical parameters such as atmospheric temperature, humidity, and density can be inverted. Each receiver observes approximately 500 occultation events per day, which are almost randomly distributed on the earth and not affected by clouds and underlying surfaces. These data provide a source of observational information with high vertical resolution and long-term stability, extending from near surface to upper stratosphere. The original occultation observation is based on time and position measurements, needing no calibration, which has advantages in climate change study.The occultation receiver on FY-3C/3D/3E meteorological satellite can receive GPS and Beidou Navigation Satellite System (BDS) signals, and the records are almost nine years long. To analyze the accuracy and stability of temperature records from multiple radio occultation, the mean and standard deviation of the dry temperature of FY-3C/3D/3E GPS and BDS radio occultation are studied using ERA5 data. It demonstrats that the accuracy of the dry temperature profile is the highest between 200 hPa and 20 hPa and the error characteristics of GPS and BDS radio occultation are similar. The stability of the average temperature deviation of FY-3C GPS for 5-year time series is very good, which is -0.0055 K·a
-1. After several algorithm improvements, the standard deviation of FY-3C GPS dry temperature decreased to about 1 K at the beginning of 2018. BDS radio occultation products are operationally provided since April 2021, and there is a good consistency between FY-3C/3E and between GPS and BDS radio occultation. Due to the algorithm adjustment at the beginning of 2021, the average deviation between FY-3D radio occultation and ERA5 data shows a significant jump. In general, the stability of multiple radio occultation dry temperature records is good and promising for climate change monitoring and research. It is necessary to carry out homogeneity reprocessing.